Dirofilaria immitis

Geographic Range

Dirofilaria immitis is found in Southern Europe, India, China, Japan, Australia and North and South America. (Aranda, et al., June 1998; Soulsby, 1968)


Dirofilaria immitis is found in many tropical, subtropical and temperate regions of the world, particularly humid areas and river valleys where environmental conditions harbor the breeding of mosquito vectors.

The intermediate host may belong to several species of mosquitos such as the Aedes, Anopholes, and Culex. The primary definitive host of D. immitis is the dog. However, other abnormal definitive hosts are cats (produces a different syndrome than in the dog), fox, coyote, wolf, sea lions, harbor seals, laboratory ferrets, horses, bears, raccoons, wolverines, muskrats and red pandas. Larvae do not grow to adults in humans. (Aranda, et al., June 1998; Sloss and Bendbrook, 1961)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools
  • coastal

Physical Description

Dirofilaria immitis is a cylindrical, slender white worm. As a nematode, it has a cuticle with three main outer layers made of collagen and other compounds. The outer layers are non-cellular and are secreted by the epidermis. The cuticle layer protects the nematodes so they can invade the digestive tracts of animals.

Nematodes have longitudinal muscles along the body wall. The muscles are obliquely arranged in bands. Dorsal, ventral and longitudinal nerve cords are connected to the main body of the muscle.

Both sexes are different. The adult male, measuring 12-16 cm, is smaller than the adult female, which is 25-30 cm. The male has a posterior end spirally coiled and a tail with many alae, which are thickenings of the cuticle. The female posterior is straight. Both sexes have a mouth, a filariform esophogus, anal pore, excretory pore and a nerve ring. The male has a seminal vesicle and testis while the female bears an ovary and oviduct.

The larvae, called microfilariae, are 307-322 micrometers long and 6.7-7.1 micrometers wide. They have a straight posterior end regardless of the sex and a tapered anterior end. They have no cephalic hook and are not ensheathed. (Barnes, 1987; Brusca and Brusca, 2003; Soulsby, 1968)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    12 to 30 cm
    4.72 to 11.81 in


The mosquito ingests Dirofilaria immitis larvae, microfilariae (first stage juvenile, L1), after a blood meal from a dog or other infected host. The first stage larvae (L1) develop into second stage larvae (L2) in the malpighian tubes. Then the third stage larvae (L3) develop and enter the body cavity, the hemoceol. Development from the microfilariae to the third larval stage takes 15-16 days. The L3 escape in a pool of hemolymph as the mosquito inserts its labium into the definitive host, which is usually a dog. If its stylets are withdrawn, the L3 enter through the puncture wound made by the mosquito. The L3 will then molt into the fourth larval stage (L4) in the definitive host 0-14 days after infection. The L4 migrate to submuscular membranes and subcutaneous tissue and remain dormant. They molt into the fifth larval stage (L5) and migrate through venule walls and end up in the pulmonary arterioles and right heart of the definitive host. The L5 will mature into adults that will further migrate to the right ventricle or pulmonary artery 85-120 days after infection. The adult will reach maturity in a further 2-month period and shed microfilariae in the blood to begin the cycle all over again.

The adults can remain in the heart or artery for up to 7 years. Microfilariae can remain in the circulation of the mosquito for up to 2 years. In humans the parasites can not reach the adult stages and remain in the larval stages. No microfilariae are ever present in the blood of humans because the parasites can never fully develop to shed the microfilariae into the blood. (Muro, et al., September 1999; Soulsby, 1968)


Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella. (Barnes, 1987; Brusca and Brusca, 2003)

  • Parental Investment
  • pre-fertilization
    • provisioning


The Dirofilaria immitis microfilariae are more motile and more numerous than D. reconditum, a closely related species whose microfilariae are found in dogs and are often confused with D. immitis. The D. immitis microfilariae are in the blood of the definitive host at all times but they do exhibit periodicity. In Europe there was a minimum observance of microfilariae at 8:00 AM and at 8:00 PM there was a maximum observance of microfilariae. This trend is an adaptation to the feeding time of their mosquito vectors. The Culex species is an important vector of D. immitis and it is a nocturnal feeder. The microfilariae are most numerous in the blood of the definitive host at night, increasing the chances of infecting the intermediate host. (Aranda, et al., June 1998; Soulsby, 1968)

Communication and Perception

Nematodes within the Secernentea have phasmids, which are unicellular glands. Phasmids likely function as chemoreceptors. Females may produce pheromones to attract males.

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Brusca and Brusca, 2003)

Food Habits

The developing Dirofilaria immitis larvae feed on the cells of the malpighian tubes in the intermediate host, the mosquito. The adult D. immitis feed on blood and nutrients in the circulation of the dog or other host. Pharyngeal glands and intestinal epithelium produce digestive enzymes to feed on the hosts’ body fluids. Extracellular digestion begins within the lumen and is finished intracellularly. (Barnes, 1987; Brusca and Brusca, 2003; Soulsby, 1968)

  • Animal Foods
  • body fluids


As parasites, these animals are not usually preyed on directly, but are ingested from host to host. (Barnes, 1987; Brusca and Brusca, 2003)

Ecosystem Roles

The mosquito ingests Dirofilaria immitis larvae, microfilariae (first stage juvenile, L1), after a blood meal from a or other infected host. Larvae are then passed back to dogs where they develop into adults.

Species Used as Host

Economic Importance for Humans: Negative

Dirofilaria immitis is of veterinary importance because it threatens the health of dogs, cats and other animals, including humans. The dog heartworm causes pulmonary dirofilariosis in humans when immature worms accidentally infect humans and begin developing in nodules in the lungs or other subcutaneous tissue. In heavy infections of dogs, cats and other wildlife, the worms can cause circulatory distress, interference with functions of the heart valves and cause congestion of the right side of the heart. Cirrhosis of the liver and endarteritis can become apparent after 9-10 months if no treatment is administered. Initially in dogs the clinical signs include decreased exercise endurance, chronic cough and collapse after excercise. In humans the clinical symptoms are less apparent including lack of stamina. Diagnosis in animals includes testing the blood for microfilariae and performing a thoracic radiography. Administering a lung biopsy and chest x-ray is the primary diagnostic tool in humans. In order to temporarily control this disease in humans it is important keep dogs indoors during peak mosquito biting hours, which is at night. This will control the infection of the reservoir host. Also, spraying to abate mosquitoes would control the disease by decreasing the vector population. The adaptability of some vector mosquito species particularly in the genus Culex, plays an important role in the spread of infection. To control the transmission of D. immitis in domestic animals, preventative techniques administered by regular veterinary visits can drastically reduce the risk of infection. For example, Selamectin is a liquid applied topically on a dog or cat that safely prevents the transmission of the adult heartworm. Dogs living in open air and poor conditions seem to be more prone to becoming infected. (Aranda, et al., June 1998; Cancrini, et al., September 2000; Muro, et al., September 1999; Soulsby, 1968)


Renee Sherman Mulcrone (editor).

Shelby Freda (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.


an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.


This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.


found in the oriental region of the world. In other words, India and southeast Asia.

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reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


chemicals released into air or water that are detected by and responded to by other animals of the same species


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scrub forest

scrub forests develop in areas that experience dry seasons.


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


living in residential areas on the outskirts of large cities or towns.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


living in cities and large towns, landscapes dominated by human structures and activity.


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Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Cancrini, G., E. Allende, G. Favia, F. Bornay, F. Anton. September 2000. Canine dirofilariosis in two cities of southeastern Spain. Veterinary Parasitology, 92(1): 81-86.

Muro, A., C. Genchi, M. Cordero, F. Simon. September 1999. Human Dirofilariosis in the European Union. Parasitology Today, 15(9): 386-389.

Ohio State University, 2001. "Dirofilaria immitis (canine or dog heartworm)" (On-line). Parasites and Parasitological Resources. Accessed September 23, 2004 at http://www.biosci.ohio-state.edu/~parasite/dirofilaria.html.

Settles, E. 1993. "Canine Heartworm Disease" (On-line). Pets and Companion Animal Publications. Accessed September 23, 2004 at http://muextension.missouri.edu/explore/agguides/pets/g09930.htm.

Sloss, M., E. Bendbrook. 1961. Clinical Parasitology. Iowa: Iowa State University Press.

Soulsby, E. 1968. Helminiths, Arthropods and Protozoa of Domestic Animals. Baltimore: Baltimore Publishers.