Dirofilaria is a genus of roundworm (nematodes). These worms cause Dirofilariasis in humans defined as infection by filarial nematodes of the genus Dirofilaria.
Two subgenera, Dirofilaria (Dirofilaria) species and Dirofilaria (Nochtiella) species are recognized. Dirofilaria (Dirofilaria) immitis is the most frequent agent of pulmonary dirofilariasis. Although D. (Dirofilaria) immitis primarily causes lung lesions, it has, on rare occasion caused intra-abdominal infection and even subcutaneous nodules. Approximately 20 species of Dirofilaria are in the subgenus Nochtiella; of these, Dirofilaria (Nochtiella) repens, Dirofilaria (Nochtiella) tenuis, and Dirofilaria (Nochtiella) striata have produced human infection, while Dirofilaria (Nochtiella) ursi–like infections that represent either D. (Nochtiella) ursi or Dirofilaria (Nochtiella) subdermata, or both, rarely can cause human infections. Most human infections by members of the subgenus Nochtiella are D. (Nochtiella) tenuis in the western hemisphere and D (Nochtiella) repens in the eastern hemisphere.
These worms are natural parasites of other, non-human, mammals. In humans, there are two forms of the disease (a) pulmonary dirofilariasis caused primarily by Dirofilaria immitis (the dog heartworm), and (b) subcutaneous dirofilariasis caused primarily by D. tenuis and D. repens, parasites of the raccoon and of dogs and cats respectively; and by D. ursi from the bear. Humans are poor hosts for all Dirofilaria species such that in humans, the worm usually dies before reaching sexual maturity and does not release viable microfilariae.
Dirofilaria immitis is a common parasite of dogs (and some other carnivores) in many parts of the world. It is a considerable veterinary problem, particularly in the USA and Japan. In the dog, adult worms lie coiled in tangled masses in the right ventricle of the heart. The microfilariae circulate in the blood and are transmitted by mosquitoes.
Humans are aberrant hosts for D. immitis, who probably become infected via contaminated mosquitoes. The parasites locate in the right ventricle but fail to mature and die. They are then swept into the pulmonary arteries where they impact and form the nidus of a thrombus. Most human cases are reported from the USA, Japan, and Australia. There is no proof that mature worms of D. immitis or their microfilariae ever develop in humans.
D. tenuis is a subcutaneous parasite of the raccoon. Humans are an abnormal host, in whom gravid female worms have, on rare occasions, been found in the subcutaneous tissue, although without viable microfilariae outside the worm.
D. repens is a subcutaneous parasite of dogs and cats in Europe, Africa, and Asia and probably accounts for incidental infections in humans in these areas.
Both D. tenuis and D. repens are spread by mosquitos. Both parasites present as subcutaneous nodules which develop over several weeks and are variously tender, painful, red, and sometimes migratory. They develop in the conjunctiva, the eyelid, scrotum, breast, arms, or legs. Rarely, D. repens produces pulmonary lesions similar to D. immitis.
From the Latin dīrus (“fearful” or “ominous”) + fīlum (“thread”), Dirofilaria is a genus of nematodes of the superfamily Filarioidea. The first known description of Dirofilaria may have been by Italian nobleman Francesco Birago in 1626 in his Treatise on Hunting: “The dog generates two worms, which are half an arm’s length long and thicker than a finger and red like fire.” Birago erroneously identified the worms as a larval stage of another parasite, Dioctophyme renale. The dog heartworm was named Filaria by American parasitologist Joseph Leidy in 1856, and the genus was renamed Dirofilaria by French parasitologists Railliet and Henry in 1911.
This article uses public domain text from the CDC as cited
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