Heartworm
Encyclopedia
Heartworm is a parasitic
roundworm (Dirofilaria immitis) that is spread from host
to host through the bites of mosquito
es. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog
but it can also infect cat
s, wolves, coyote
s, fox
es and other animals, such as ferret
s, sea lion
s and even, under very rare circumstances, human
s. The parasite is commonly called "heartworm"; however, that is a misnomer because the adult actually resides in the pulmonary arterial system (lung arteries) for the most part, and the primary insult to the health of the animal is a manifestation of damage to the lung vessels and tissue. Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infections. Heartworm infection
may result in serious disease for the host.
, heartworm has now spread to nearly all locations where its vector, the mosquito, is found. Transmission of the parasite occurs in all of the United States (cases have even been reported in Alaska
), and the warmer regions of Canada
. The highest infection rates are found within 150 miles of the coast from Texas
to New Jersey
, and along the Mississippi River
and its major tributaries
. It has also been found in South America
, southern Europe
, Southeast Asia
, the Middle East
, Australia
, and Japan
.
Heartworms go through several life stages
before they become adults infecting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle. The rate of development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the heart takes 6 to 7 months in dogs and is known as the "prepatent period".
After infection, the third stage larva
l heartworms (L3) deposited by the mosquito grow for a week or two and molt to the fourth larval stage (L4) under the skin at the site of the mosquito bite. Then they migrate to the muscle
s of the chest
and abdomen
and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next 3 to 4 months they increase greatly in size. The female adult worms are about 30 cm in length, and males are about 23 cm with a coiled tail. By approximately 6.5 to 7 months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilaria
e.
The microfilariae circulate in the bloodstream for as long as two years, waiting for the next stage in their life cycle in the gut of a bloodsucking mosquito. When ingested by a mosquito, the microfilariae undergo a series of molts
to the infective third larval stage and then migrate to the salivary glands of the mosquito, where they wait to infect another host. The incubation period required to reach the stage where the microfilariae become transmittable to another host can be as little as two weeks or as long as six weeks, depending on the warmth of the climate, and the larval life cycle ceases entirely if the ambient temperature drops below 14° Celsius (57° Fahrenheit).
e or antigen
s cannot detect prepatent infections. Rarely, migrating heartworm larvae get "lost" and end up in unusual sites such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizure
s and lameness. But normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.
Many dogs will show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially on exercise and early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood and, finally, congestive heart failure.
pipientis is an intracellular bacterium that is an endosymbiont
of Dirofilaria immitis. It is thought that all heartworms are infected with Wolbachia to some degree. Research indicates that the inflammation that occurs at the die-off of adult heartworms or larvae is in part due to the release of Wolbachia bacteria or protein into the tissues. This may be particularly significant in cats, in which disease seems to be more related to larval death than living adult heartworms (see below). Pre-treating heartworm positive animals with an antibiotic such as doxycycline
to remove Wolbachia may prove to be beneficial, but further studies are necessary.
Microfilarial detection. This was accomplished most commonly in the past by the microscopic identification of microfilariae on a direct blood smear, above the buffy coat
in a microhematocrit tube, using the modified Knott test, or after millipore filtration. The accuracy of these tests, typically used for routine screening or diagnosis of heart worm infection, is improved by multiple testing. The modified Knott test and millipore filtration are more sensitive because they concentrate microfilariae, improving the chance of diagnosis. The direct smear technique allows examination of larval motion, helping in the distinction of D. immitis from Acanthocheilonema reconditum. This distinction is important because the presence of the latter parasite does not require expensive and potentially harmful therapy. However, the potential for amicrofilaremic infections is 5-67%. The number of circulating microfilariae does not correlate with the number of adult heartworms and is therefore no indicator of disease severity.
Antigen test. In most practices, microfilarial detection has been supplanted by or supplemented with antigen testing. Combining the microfilaria and adult antigen test is most useful in dogs receiving diethylcarbamazine
or no preventative (as macrolides as for example ivermectin or moxidectin
typically render the dog amicrofilaremic). Up to 1% of infected dogs are microfilaria positive and antigen negative.
Immunodiagnostics
(ELISA
, lateral flow immunoassay, rapid immunomigration techniques) to detect heartworm antigen in the host's blood are now regularly used. The weakness of these tests is that they only detect the antigens released from the adult female worm's reproductive tract and hence will produce negative results during the first 5–8 months of infection. The specificity of these tests is close to 100 percent and the sensitivity is more than 80%. A recent study demonstrated a sensitivity of only 64% for infections of only one female worm, but improved with increasing female worm burden (85%, 88%, and 89% for two, three and four female worms, respectively). Specificity in this study was 97%. False negative
test results can be due to low worm counts, immature infections and all male infections.
Radiography
. X-rays are used to evaluate the amount of lung damage caused by the presence of heartworms.
-based compound. The currently approved drug in the US, melarsomine dihydrochloride, is marketed under the brand name Immiticide. It has a greater efficiency and fewer side effects than previously used drug (thiacetarsamide sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage infections.
After treatment, the dog must rest (restricted exercise) for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, when the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure
and death. According to the American Heartworm Society, use of aspirin
in dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit and may be contraindicated. It had previously been recommended for its effects on platelet adhesion and reduction of vascular damage caused by the heartworms.
The course of treatment is not completed until several weeks later when the microfilariae are dealt with in a separate course of treatment. Once heartworm tests are negative, the treatment is considered a success.
Surgical removal of the adult heartworms is also a treatment that may be indicated, especially in advanced cases with substantial heart involvement.
Long term monthly administration of ivermectin year round at three times the dose normally used for heartworm prevention (see "Prevention") will eventually kill adult heartworms. However, this is not the treatment of choice for removal of adult heartworms for two reasons. First, this treatment is not as effective as melarsamine. More importantly, adult heartworms do not begin to die until 18 months of treatment have elapsed, which is not acceptable for dogs with high-volume infections. Long term treatment over a year with doxycycline daily and heartguard plus has been shown to be effective in early heartworm who are asymptomatic.
From time to time various homeopathic, natural or organic
products are used as preventives for heartworm disease. However, such products have never been proven effective by rigorous scientific methods, and the choice of using them should be evaluated carefully.
(Interceptor Flavor Tabs and Sentinel Flavor Tabs
) and moxidectin
(ProHeart) administered as pills or chewable tablets. Moxidectin is also available in both a 6-month and 12-month sustained release injection, ProHeart 6, ProHeart 12, administered by veterinarians. The injectable form of moxidectin was taken off the market in the United States due to safety concerns in 2004 but the FDA returned a newly formulated ProHeart 6 to the market in 2008. ProHeart 6 remains on the market in many other countries including Canada and Japan. Its sister product, ProHeart 12 is used extensively in Australia and Asia as a 12-month injectable preventive. Topical treatments are available as well. Advantage Multi (imidacloprid + moxidectin) Topical Solution, which utilizes moxidectin for control and prevention of roundworm
s, hookworm
s, heartworms, whipworm
s, as well as imidacloprid
to kill adult flea
s. Selamectin
(Revolution), is a topical preventive that is likewise administered monthly and also controls flea
s, tick
s, and mite
s.
Preventive drugs are highly effective and when regularly administered will protect more than 99 percent of dogs and cats from heartworm. Most compromises in protection result from failure to properly administer the drugs during seasonal transmission periods. In regions where the temperature is consistently above 14 °C (57 °F) year round, a continuous prevention schedule is recommended.
It has been shown that lapses of up to 4 months between doses of Ivermectin-based products still provides 95% protection from adult worms. This 'safety net' period is called the 'Reach Back Effect'. Annual heartworm testing is highly recommended for pet owners who choose to use minimal dosing schedules.
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) Topical Solution.
The infection rate in cats is 1 to 5 percent of that in dogs in endemic
areas. Both indoor and outdoor cats are infected. The mosquito vector is known to enter homes.
or allergic bronchitis.
Obstruction of pulmonary arteries due to emboli
from dying worms is more likely to be fatal in cats than dogs because of less collateral circulation
and fewer vessels.
An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to 3 months. The antibody test is more sensitive than the antigen test but it does not provide direct evidence of adult infection. It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for HARD.
X-rays
of the chest of a heartworm infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography
is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.
s a few days after treatment. The effects of melarsomine are poorly studied in cats. Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended and there are no drugs approved in the US for use in cats.
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short term corticosteroid
. Surgery has also been used successfully to remove adult worms. Three drugs are approved for use in cats in the US: ivermectin, milbemycin, and selamectin. The prognosis for feline heartworm disease is guarded.
, forms around the dead worm as it is being killed and absorbed. If an infected person happens to have a chest X-ray following granuloma formation, the nodule may be large enough to resemble lung cancer
on the X-ray and require a biopsy
for a pathologic assessment. This may well be the most significant medical consequence of human infection by the dog heartworm.
At one time it was thought that the dog heartworm infected the human eye, with most cases reported from the southeastern United States. However, these cases are now thought to be caused by a closely related parasite of raccoon
s, Dirofilaria tenuis
. Several hundred cases of subcutaneous infections in humans have been reported in Europe, but these are almost always caused by another closely related parasite, Dirofilaria repens
, rather than the dog heartworm.
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
roundworm (Dirofilaria immitis) that is spread from host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
to host through the bites of mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
es. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
but it can also infect cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s, wolves, coyote
Coyote
The coyote , also known as the American jackal or the prairie wolf, is a species of canine found throughout North and Central America, ranging from Panama in the south, north through Mexico, the United States and Canada...
s, fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
es and other animals, such as ferret
Ferret
The ferret is a domesticated mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur...
s, sea lion
Sea Lion
Sea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...
s and even, under very rare circumstances, human
Human
Humans are the only living species in the Homo genus...
s. The parasite is commonly called "heartworm"; however, that is a misnomer because the adult actually resides in the pulmonary arterial system (lung arteries) for the most part, and the primary insult to the health of the animal is a manifestation of damage to the lung vessels and tissue. Occasionally, adult heartworms migrate to the right heart and even the great veins in heavy infections. Heartworm infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
may result in serious disease for the host.
Distribution
Although at one time confined to the southern United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, heartworm has now spread to nearly all locations where its vector, the mosquito, is found. Transmission of the parasite occurs in all of the United States (cases have even been reported in Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
), and the warmer regions of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. The highest infection rates are found within 150 miles of the coast from Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
to New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
, and along the Mississippi River
Mississippi River
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
and its major tributaries
Tributary
A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...
. It has also been found in South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, southern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, Southeast Asia
Southeast Asia
Southeast Asia, South-East Asia, South East Asia or Southeastern Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia. The region lies on the intersection of geological plates, with heavy seismic...
, the Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
.
Course of infection
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
before they become adults infecting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle. The rate of development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the heart takes 6 to 7 months in dogs and is known as the "prepatent period".
After infection, the third stage larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l heartworms (L3) deposited by the mosquito grow for a week or two and molt to the fourth larval stage (L4) under the skin at the site of the mosquito bite. Then they migrate to the muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s of the chest
Chest
The chest is a part of the anatomy of humans and various other animals. It is sometimes referred to as the thorax or the bosom.-Chest anatomy - Humans and other hominids:...
and abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next 3 to 4 months they increase greatly in size. The female adult worms are about 30 cm in length, and males are about 23 cm with a coiled tail. By approximately 6.5 to 7 months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilaria
Microfilaria
The microfilaria is a stage in the life cycle of certain parasitic nematodes in the family Onchocercidae. In these species, the adults live in the circulatory system of vertebrates while the early larval stages develop in blood-feeding arthropod vectors...
e.
The microfilariae circulate in the bloodstream for as long as two years, waiting for the next stage in their life cycle in the gut of a bloodsucking mosquito. When ingested by a mosquito, the microfilariae undergo a series of molts
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
to the infective third larval stage and then migrate to the salivary glands of the mosquito, where they wait to infect another host. The incubation period required to reach the stage where the microfilariae become transmittable to another host can be as little as two weeks or as long as six weeks, depending on the warmth of the climate, and the larval life cycle ceases entirely if the ambient temperature drops below 14° Celsius (57° Fahrenheit).
Hosts
Hosts of Dirofilaria immitis include:- dogDogThe domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
- catCatThe cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
- wolf
- coyoteCoyoteThe coyote , also known as the American jackal or the prairie wolf, is a species of canine found throughout North and Central America, ranging from Panama in the south, north through Mexico, the United States and Canada...
- foxFoxFox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
- ferretFerretThe ferret is a domesticated mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur...
- sea lionSea LionSea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...
- African LeopardAfrican LeopardThe African Leopard is a leopard subspecies occurring across most of sub-Saharan Africa. In 2008, the IUCN classified leopards as Near Threatened, stating that they may soon qualify for the Vulnerable status due to habitat loss and fragmentation. They are becoming increasingly rare outside...
Panthera pardus pardus - humanHumanHumans are the only living species in the Homo genus...
- rarely.
Clinical signs of infection
Dogs show no indication of heartworm infection during the 6-month-long prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of microfilariaMicrofilaria
The microfilaria is a stage in the life cycle of certain parasitic nematodes in the family Onchocercidae. In these species, the adults live in the circulatory system of vertebrates while the early larval stages develop in blood-feeding arthropod vectors...
e or antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
s cannot detect prepatent infections. Rarely, migrating heartworm larvae get "lost" and end up in unusual sites such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizure
Seizure
An epileptic seizure, occasionally referred to as a fit, is defined as a transient symptom of "abnormal excessive or synchronous neuronal activity in the brain". The outward effect can be as dramatic as a wild thrashing movement or as mild as a brief loss of awareness...
s and lameness. But normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.
Many dogs will show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially on exercise and early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood and, finally, congestive heart failure.
Role of Wolbachia
WolbachiaWolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...
pipientis is an intracellular bacterium that is an endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
of Dirofilaria immitis. It is thought that all heartworms are infected with Wolbachia to some degree. Research indicates that the inflammation that occurs at the die-off of adult heartworms or larvae is in part due to the release of Wolbachia bacteria or protein into the tissues. This may be particularly significant in cats, in which disease seems to be more related to larval death than living adult heartworms (see below). Pre-treating heartworm positive animals with an antibiotic such as doxycycline
Doxycycline
Doxycycline INN is a member of the tetracycline antibiotics group, and is commonly used to treat a variety of infections. Doxycycline is a semisynthetic tetracycline invented and clinically developed in the early 1960s by Pfizer Inc. and marketed under the brand name Vibramycin. Vibramycin...
to remove Wolbachia may prove to be beneficial, but further studies are necessary.
Diagnosis
Three methods can be used for the diagnosis:Microfilarial detection. This was accomplished most commonly in the past by the microscopic identification of microfilariae on a direct blood smear, above the buffy coat
Buffy coat
The buffy coat is the fraction of an anticoagulated blood sample after density gradient centrifugation that contains most of the white blood cells and platelets.-Description:...
in a microhematocrit tube, using the modified Knott test, or after millipore filtration. The accuracy of these tests, typically used for routine screening or diagnosis of heart worm infection, is improved by multiple testing. The modified Knott test and millipore filtration are more sensitive because they concentrate microfilariae, improving the chance of diagnosis. The direct smear technique allows examination of larval motion, helping in the distinction of D. immitis from Acanthocheilonema reconditum. This distinction is important because the presence of the latter parasite does not require expensive and potentially harmful therapy. However, the potential for amicrofilaremic infections is 5-67%. The number of circulating microfilariae does not correlate with the number of adult heartworms and is therefore no indicator of disease severity.
Antigen test. In most practices, microfilarial detection has been supplanted by or supplemented with antigen testing. Combining the microfilaria and adult antigen test is most useful in dogs receiving diethylcarbamazine
Diethylcarbamazine
Diethylcarbamazine is an anthelmintic drug that does not resemble other antiparasitic compounds. It is a synthetic organic compound which is highly specific for several parasites and does not contain any toxic metallic elements.-Uses:...
or no preventative (as macrolides as for example ivermectin or moxidectin
Moxidectin
Moxidectin is a parasiticide used for the prevention and control of heartworm and intestinal worms, and can be found in treatments prescribed for animals such as dogs, cats, horses, cattle and sheep. It is also used for the prevention of red mite in caged birds...
typically render the dog amicrofilaremic). Up to 1% of infected dogs are microfilaria positive and antigen negative.
Immunodiagnostics
Immunodiagnostics
Immunodiagnostics is a diagnostic methodology that uses an antigen-antibody reaction as their primary means of detection. The concept of using immunology as a diagnostic tool was introduced in 1960 as a test for serum insulin...
(ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
, lateral flow immunoassay, rapid immunomigration techniques) to detect heartworm antigen in the host's blood are now regularly used. The weakness of these tests is that they only detect the antigens released from the adult female worm's reproductive tract and hence will produce negative results during the first 5–8 months of infection. The specificity of these tests is close to 100 percent and the sensitivity is more than 80%. A recent study demonstrated a sensitivity of only 64% for infections of only one female worm, but improved with increasing female worm burden (85%, 88%, and 89% for two, three and four female worms, respectively). Specificity in this study was 97%. False negative
Type I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
test results can be due to low worm counts, immature infections and all male infections.
Radiography
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
. X-rays are used to evaluate the amount of lung damage caused by the presence of heartworms.
Treatment
If an animal is diagnosed with heartworms, treatment may be indicated. Before the worms can be treated, however, the dog must be evaluated for heart, liver, and kidney function to evaluate the risks of treatment. Usually the adult worms are killed with an arsenicArsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
-based compound. The currently approved drug in the US, melarsomine dihydrochloride, is marketed under the brand name Immiticide. It has a greater efficiency and fewer side effects than previously used drug (thiacetarsamide sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage infections.
After treatment, the dog must rest (restricted exercise) for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, when the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure
Respiratory failure
The term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
and death. According to the American Heartworm Society, use of aspirin
Aspirin
Aspirin , also known as acetylsalicylic acid , is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It was discovered by Arthur Eichengrun, a chemist with the German company Bayer...
in dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit and may be contraindicated. It had previously been recommended for its effects on platelet adhesion and reduction of vascular damage caused by the heartworms.
The course of treatment is not completed until several weeks later when the microfilariae are dealt with in a separate course of treatment. Once heartworm tests are negative, the treatment is considered a success.
Surgical removal of the adult heartworms is also a treatment that may be indicated, especially in advanced cases with substantial heart involvement.
Long term monthly administration of ivermectin year round at three times the dose normally used for heartworm prevention (see "Prevention") will eventually kill adult heartworms. However, this is not the treatment of choice for removal of adult heartworms for two reasons. First, this treatment is not as effective as melarsamine. More importantly, adult heartworms do not begin to die until 18 months of treatment have elapsed, which is not acceptable for dogs with high-volume infections. Long term treatment over a year with doxycycline daily and heartguard plus has been shown to be effective in early heartworm who are asymptomatic.
From time to time various homeopathic, natural or organic
Organic product
An organic product is made from organic raw materials. It also requires Organic certification.-Food:*Fruit and vegetables*Meat, dairy, eggs*Nuts and seeds*Flour and Grains*Herbs- Processed foods :*Organic juice*Canned goods*Frozen vegetables...
products are used as preventives for heartworm disease. However, such products have never been proven effective by rigorous scientific methods, and the choice of using them should be evaluated carefully.
Prevention
Prevention of heartworm infection can be obtained through a number of veterinary drugs. The drugs approved for use in the US are ivermectin (sold under the brand names Heartgard, Iverhart, and several other generic versions), milbemycinMilbemycin
The milbemycins are a group of macrolides chemically related to the avermectins and were first isolated in 1972 from Streptomyces hygroscopicus.They are used in veterinary medicine as antiparasitic agents against worms, ticks and fleas....
(Interceptor Flavor Tabs and Sentinel Flavor Tabs
Sentinel Flavor Tabs
The combination milbemycin oxime/lufenuron is a parasite control drug in which the active ingredient, milbemycin oxime, eliminates fleas and worms, while a second active ingredient, lufenuron, arrests the development of eggs and larvae, preventing them from maturing and continuing the infestation...
) and moxidectin
Moxidectin
Moxidectin is a parasiticide used for the prevention and control of heartworm and intestinal worms, and can be found in treatments prescribed for animals such as dogs, cats, horses, cattle and sheep. It is also used for the prevention of red mite in caged birds...
(ProHeart) administered as pills or chewable tablets. Moxidectin is also available in both a 6-month and 12-month sustained release injection, ProHeart 6, ProHeart 12, administered by veterinarians. The injectable form of moxidectin was taken off the market in the United States due to safety concerns in 2004 but the FDA returned a newly formulated ProHeart 6 to the market in 2008. ProHeart 6 remains on the market in many other countries including Canada and Japan. Its sister product, ProHeart 12 is used extensively in Australia and Asia as a 12-month injectable preventive. Topical treatments are available as well. Advantage Multi (imidacloprid + moxidectin) Topical Solution, which utilizes moxidectin for control and prevention of roundworm
Toxocaridae
Toxocaridae is a zoonotic family of parasitic nematodes that infect canids and felids and which cause toxocariasis in humans...
s, hookworm
Hookworm
The hookworm is a parasitic nematode that lives in the small intestine of its host, which may be a mammal such as a dog, cat, or human. Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus. A. duodenale predominates in the Middle East, North Africa, India...
s, heartworms, whipworm
Whipworm
The human tapworm is a roundworm, which causes trichuriasis when it infects a human large intestine. The name whipworm refers to the shape of the worm; they look like whips with wider "handles" at the posterior end.-Life cycle:The female T. trichiura produces 2,000–10,000 single celled eggs per day...
s, as well as imidacloprid
Imidacloprid
Imidacloprid is a nicotine-based, systemic insecticide, which acts as a neurotoxin and belongs to a class of chemicals called the neonicotinoids. Although it is now off patent, the primary manufacturer of this chemical is Bayer CropScience,...
to kill adult flea
Flea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...
s. Selamectin
Selamectin
Selamectin is a topical parasiticide and antihelminthic used on dogs and cats, distributed by Pfizer. It prevents heartworms, fleas, ear mites, sarcoptic mange , and certain types of ticks in dogs, and prevents heartworms, fleas, ear mites, hookworms, and roundworms in cats...
(Revolution), is a topical preventive that is likewise administered monthly and also controls flea
Flea
Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...
s, tick
Tick
Ticks are small arachnids in the order Ixodida, along with mites, constitute the subclass Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians...
s, and mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
s.
Preventive drugs are highly effective and when regularly administered will protect more than 99 percent of dogs and cats from heartworm. Most compromises in protection result from failure to properly administer the drugs during seasonal transmission periods. In regions where the temperature is consistently above 14 °C (57 °F) year round, a continuous prevention schedule is recommended.
It has been shown that lapses of up to 4 months between doses of Ivermectin-based products still provides 95% protection from adult worms. This 'safety net' period is called the 'Reach Back Effect'. Annual heartworm testing is highly recommended for pet owners who choose to use minimal dosing schedules.
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) Topical Solution.
Feline heartworm disease
While dogs are a natural host for D. immitis, cats are an atypical host. Because of this, there are significant differences between canine and feline heartworm disease. The majority of heartworm larvae do not survive in cats, so unlike in dogs, a typical infection in a cat is 2 to 5 worms. The life span of heartworms is considerably shorter in cats, only two to three years, and most infections in cats do not have circulating microfilariae. Cats are also more likely to have aberrant migration of heartworm larvae, resulting in infections in the brain or body cavities.The infection rate in cats is 1 to 5 percent of that in dogs in endemic
Endemic (epidemiology)
In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...
areas. Both indoor and outdoor cats are infected. The mosquito vector is known to enter homes.
Pathology
The vascular disease in cats that occurs when the L5 larvae invade the pulmonary arteries is more severe than in dogs. A syndrome related to this inflammatory reaction has been identified in cats: heartworm-associated respiratory disease (HARD). HARD can occur 3 to 4 months after the initial infection and is caused by the presence of the L5 larvae in the vessels. The subsequent inflammation of the pulmonary vasculature and lungs can be easily misdiagnosed as feline asthmaFeline asthma
Feline asthma is a common allergic respiratory disease in cats, affecting at least one percent of all adult cats worldwide. It is a chronic progressive disease for which there is no cure. Common symptoms include wheezing, coughing, labored breathing and potentially life-threatening...
or allergic bronchitis.
Obstruction of pulmonary arteries due to emboli
Embolism
In medicine, an embolism is the event of lodging of an embolus into a narrow capillary vessel of an arterial bed which causes a blockage in a distant part of the body.Embolization is...
from dying worms is more likely to be fatal in cats than dogs because of less collateral circulation
Collateral circulation
Collateral circulation is when an area of tissue or an organ has a number of different pathways for blood to reach it. This is often as a result of anastamoses - branches formed between adjacent blood vessels....
and fewer vessels.
Signs and symptoms
Acute heartworm disease in cats can result in shock, vomiting, diarrhea, fainting, and sudden death. Chronic infection can cause loss of appetite, weight loss, lethargy, exercise intolerance, coughing, and difficulty breathing. The signs of HARD can persist even after complete elimination of the heartworm infection.Diagnosis
Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If there are only male worms, the test will be negative. Even when there are female worms, an antigen test will usually only become positive seven to eight months post-infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. HARD can be found in cats that never develop adult heartworms and therefore never have a positive antigen test.An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to 3 months. The antibody test is more sensitive than the antigen test but it does not provide direct evidence of adult infection. It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for HARD.
X-rays
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
of the chest of a heartworm infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography
Echocardiography
An echocardiogram, often referred to in the medical community as a cardiac ECHO or simply an ECHO, is a sonogram of the heart . Also known as a cardiac ultrasound, it uses standard ultrasound techniques to image two-dimensional slices of the heart...
is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.
Treatment and prevention
Arsenic compounds have been used for heartworm adulticide treatment in cats as well as dogs but seem more likely to cause pulmonary reactions. A significant number of cats develop pulmonary embolismPulmonary embolism
Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
s a few days after treatment. The effects of melarsomine are poorly studied in cats. Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended and there are no drugs approved in the US for use in cats.
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short term corticosteroid
Corticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...
. Surgery has also been used successfully to remove adult worms. Three drugs are approved for use in cats in the US: ivermectin, milbemycin, and selamectin. The prognosis for feline heartworm disease is guarded.
Human health considerations
The dog heartworm is of negligible public health risk, because it is unusual for humans to become infected. Additionally, human infections usually are of little or no consequence, although rarely an infected human may show signs of respiratory disease. In most cases, however, the heartworm dies shortly after arriving in the human lung, and a nodule, known as a granulomaGranuloma
Granuloma is a medical term for a tiny collection of immune cells known as macrophages. Granulomas form when the immune system attempts to wall off substances that it perceives as foreign but is unable to eliminate. Such substances include infectious organisms such as bacteria and fungi as well as...
, forms around the dead worm as it is being killed and absorbed. If an infected person happens to have a chest X-ray following granuloma formation, the nodule may be large enough to resemble lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
on the X-ray and require a biopsy
Biopsy
A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...
for a pathologic assessment. This may well be the most significant medical consequence of human infection by the dog heartworm.
At one time it was thought that the dog heartworm infected the human eye, with most cases reported from the southeastern United States. However, these cases are now thought to be caused by a closely related parasite of raccoon
Raccoon
Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...
s, Dirofilaria tenuis
Dirofilaria tenuis
Dirofilaria tenuis is a species of nematode, and a known parasite of raccoons....
. Several hundred cases of subcutaneous infections in humans have been reported in Europe, but these are almost always caused by another closely related parasite, Dirofilaria repens
Dirofilaria repens
Dirofilaria repens is a filarial nematode that affects dogs and other carnivores such as cats, wolves, coyotes, foxes, muskrats and sea lions. Although humans may become infected as aberrant hosts, the worms fail to reach adulthood while residing in a human body.-Epidemiology:It most often found in...
, rather than the dog heartworm.
External links
- Traversa D., Cesare A. Di & Conboy G. (2010). "Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated". Parasites & VectorsParasites & VectorsParasites & Vectors is a peer-reviewed open-access medical journal published by BioMed Central. The journal publishes articles on the biology of parasites, parasitic diseases, intermediate hosts, vectors and vector-borne pathogens. Parasites & Vectors was established in 2008 as a merger of Filaria...
3: 62. doi:10.1186/1756-3305-3-62.
American Heartworm Society
- American Heartworm Society Founded in 1974, the American Heartworm Society is internationally recognized as the definitive authority with respect to heartworm disease in dogs and cats.
Resources
- Preventing Heartworm Infection in Dogs (VeterinaryPartner.com)
- Overview and main concepts of Dirofilaria immitis (heartworm) infection (MetaPathogen.com)
- Moxidectin Facts, Usage, and Safety Information
- Feline Heartworm Disease (from Auburn University)
- Heartworm Topics (from the University of Pennsylvania)
- Mosquito-borne Dog Heartworm Disease (University of Florida Extension Bulletin)
- What You Should Know About Heartworm Disease (from the American Veterinary Medical Association)
- Case Study of Canine Heartworm Disease (from the University of California, Davis)
- Case Study of Feline Heartworm Disease (from the University of California, Davis)
- Case Study of Canine Heartworm Disease (from the University of California, Davis)
- Heartworm Prevention Resources for Dog Owners (from Drs. Foster & Smith's Veterinary Staff)
- Heart Worm Medicine for Dogs
- Heartworm in Europe (From Clinica Veterinaria Pet Vet Kamu)
Pet owners
- What Is Heartworm?
- Frequently Asked Questions
- Heartworm Disease in Dogs
- Heartworm Disease in Cats
- Seasonal Timing Schedule for Heartworm Prevention
- Heartworm Prevention In Dogs
- Dealing with Your Dog Heartworm