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| | [[Image:dirofilariasis 2.jpg|right|thumb|125px|<small><center>'''Dirofilariasis'''. Courtesy of T. Scase</center></small>]] | | [[Image:dirofilariasis 2.jpg|right|thumb|125px|<small><center>'''Dirofilariasis'''. Courtesy of T. Scase</center></small>]] |
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| − | Dirofilaria is a filarial worm. Female worms measure up to 30cm in length and males up to 15cm. The worms have a life-span of 5-7 years. Up to 250 worms may establish in the heart and [[Respiratory Parasitic Infections - Pathology#Dirofilaria immitis|pulmonary arteries]] of dogs and cats.
| + | Heartworm (HW) infection is caused by a filarial organism, Dirofilaria immitis . At least 70 species of mosquitos can serve as intermediate hosts; Aedes , Anopheles , and Culex are the most common genera acting as vectors. Patent infections are possible in numerous wild and companion animal species. Wild animal reservoirs include wolves, coyotes, foxes, California gray seals, sea lions, and raccoons. In companion animals, HW infection is seen primarily in dogs and less commonly in cats and ferrets. HW disease has been reported in most countries with temperate, semitropical, or tropical climates, including the USA, Canada, and southern Europe. In companion animals, infection risk is greatest in dogs and cats housed outdoors. Although any dog, indoor or outdoor, is capable of being infected, most infections are diagnosed in medium- to large-sized, 3- to 8-yr-old dogs. |
| − | Dirofilaria worms produce microfilariae, not eggs.
| + | Infected mosquitos are capable of transmitting HW infections to humans, but there are no reports of such infections becoming patent. Maturation of the infective larvae may progress to the point where they reach the lungs, become encapsulated, and die. The dead larvae precipitate granulomatous reactions called “coin lesions,” which are medically significant because radiographically they appear similar to metastatic lung cancer. |
| | + | HW infection rates in other companion animals such as ferrets and cats tend to parallel those in dogs in the same geographic region, but usually at a lower prevalence. No age predilection has been reported in ferrets or cats, but male cats have been reported to be more susceptible than females. Indoor and outdoor ferrets and cats can be infected. Other infections in cats, such as those caused by the feline leukemia virus or feline immunodeficiency virus, are not predisposing factors. |
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| − | '''Microfilariae''':
| + | ==Life Cycle== |
| − | *in peripheral circualtion
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| − | *periodicity - maximum numbers in blood evening/night
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| − | *greater than 300µm long
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| − | *life-span 2years
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| − | *present in approximately 60% of infected dogs
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| − | *microfilariae are absent from the circulating blood if:
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| − | **only immature worms present
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| − | **only one worm present
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| − | **only one sex
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| − | **microfilariae killed by immune response (in 15% of dogs)
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| − | **females sterilised by chemotherapy (e.g. ivermectin).
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| − | '''Intermediate hosts''':
| + | ==Pathogenesis== |
| − | *many, but not all, species of mosquito.
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| − | '''In mosquito''':
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| − | *microfilariae → L1 → L2 → infective L3
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| − | *this takes 1week at 30°C, or 4weeks at 18°C - there is no development below 14°C.
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| − | *when mosquito next feeds:
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| − | **L3 moves to mouthparts
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| − | **up to 12 L3 deposited on skin
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| − | **enter body via puncture wound.
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| − | '''Local Epidemiology''':
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| − | *determined by feeding preferences of local species, and population density.
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| − | *up to 45% of non-protected dogs infected in some parts of USA.
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| − | '''Zoonotic hazard''':
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| − | *human infection can occur, but few cases are diagnosed
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| − | *this usually happens when a radio-opaque plaque is detected in the lung, and further investigation shows it to be caused by a trapped ''D. immitis'' larva.
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| − | === Feline Heartworm Disease=== | |
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| − | *A canine parasite - see under Dog Nematodes for life-cycle etc.
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| − | *Cats are abnormal hosts, and so ''D. immitis'' is not very infective for cats.
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| − | *Nevertheless, feline infection is common (up to 25%) in some heavily endemic areas.
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| − | *But only small numbers of adult worms (1-3) establish.
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| − | *The prepatent period is longer (approximately 8months) than in the dog.
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| − | *Few, if any, microfilariae are produced (<20% of cases positive).
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| − | *The life-span of the worm is shorter (2-3years).
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| − | *However: one dead adult → acute pulmonary crisis (thromboembolism).
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| − | *Lung pathology similar to dog, but little heart pathology.
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| − | *Coughing starts 4-6months post-infection.
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| − | *Antibody-detection ELISA used for diagnosis, but false positives occur (antigen ELISA cannot be used as antigen rarely expressed in cats).
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| − | *There is no licensed adulticidal therapy, and treatment may be fatal for the cat as well as the worm.
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| − | *Ivermectin or selamectin can be used for prevention.
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| | ==Signalment== | | ==Signalment== |