Dirofilariasis is an infection by parasites in the Dirofilaria genus. Human dirofilariasis typically manifests as either subcutaneous nodules or lung parenchymal disease, in many cases asymptomatically. The zoonotic filariae of Dirofilaria immitis and Dirofilaria (Nochtiella) repens have become increasingly recognized worldwide as inadvertent human pathogens, with the usual hosts of these infective nematodes being domestic and wild carnivores. The main causative agents are Dirofilaria immitis, Dirofilaria repens, Dirofilaria tenuis, Dirofilaria ursi. Dirofilariasis in humans is caused by Dirofilaria roundworms. The main natural hosts for the three Dirofilaria species that most frequently cause disease in humans are dogs and wild canids (such as wolves and foxes) and raccoons. Humans are infected with Dirofilaria larvae through mosquito bites. Infection can result in nodules under the skin or conjunctiva and lung granulomas (small nodules formed by an inflammatory reaction) that appear as coin lesions (small, round abnormalities) on x-rays, leading to diagnostic procedures to exclude more serious diseases. In the United States, canine dirofilariasis has been reported from all states, and D. tenuis in raccoons is common in many areas where raccoons are found. Canine and human dirofilariasis are most prevalent in eastern and southeastern states, although rates are increasing rapidly in a number of western states. D. immitis is the Dirofilaria species most commonly reported to cause dirofilariasis in humans in the United States. D. repens is the Dirofilaria species most commonly reported to cause dirofilariasis among humans in Europe. One or both of these species have been found to cause human dirofilariasis in other parts of the world. Dirofilaria immitis is cosmopolitan in dogs. Dirofilaria repens infects dogs and cats in the Old World, while D. tenuis infects raccoons in North America. Dirofilaria ursi and D. subdermata are also North American, infecting bears and porcupines, respectively. Dirofilaria striata is a parasite of wild felids in North, Central, and South America.
The prevalence of dirofilariasis in domestic dogs and cats varies by state and geographic region throughout the United States. All states except Nevada have reported D immitis infection in dogs, with the prevalence varying from low levels in some states (0.3%, 0.5%, and 1.2% in Colorado, Washington, and Montana, respectively) to as high as 40% in Florida and South Carolina. Of stray cats in Michigan, 3% were found to have adult D immitis worms at autopsy. In general, the infection is more prevalent in dogs than in cats, in which prophylaxis is not routinely recommended. A similar variation in prevalence of D immitis in dogs and cats occurs elsewhere in the world, except Japan, where as many as 10% of stray cats are infected. In Europe, the recorded prevalence rate of D immitis in dogs is as follows: Greece - 5-11%, Barcelona, Spain - 13%, Northeastern Italy - 44-55%, Canary Islands - 59%.
The definitive treatment of Dirofilaria infection in humans is surgical removal of lung granulomas and nodules under the skin, this treatment is also curative. In many cases, no treatment with medicines is necessary. 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 embolisms 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 no drugs are 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. The severe symptoms caused by the parasite can be avoided by cleansing the skin, surgery, or the use of therapeutic drugs, such as diethylcarbamazine (DEC), ivermectin, or albendazole. The drug of choice, however, is DEC, which can eliminate the microfilariae from the blood and also kill the adult worms with a dosage of 6 mg/kg semiannually or annually. A polytherapy treatment that includes ivermectin with DEC or albendazole is more effective than each drug alone. Protection is similar to that of other mosquito-spread illnesses; one can use barriers both physical (a mosquito net), chemical (insect repellent), or mass chemotherapy as a method to control the spread of the disease. Mass chemotherapy should cover the entire endemic area at the same time. This will significantly decrease the overall microfilarial titer in blood in mass, hence decreasing the transmission through mosquitoes during their subsequent bites. Antibiotic active against the Wolbachia symbionts of the worm have been experimented with as treatment.