Dirofilaria repens is a filarial nematode that affects dogs and other carnivores such as cats, wolves, coyotes, foxes, and sea lions, as well as muskrats. It is transmitted by mosquitoes. Although humans may become infected as aberrant hosts, the worms fail to reach adulthood while infecting a human body. Epidemiology It is most often found in the Mediterranean region, sub-Saharan Africa, and Eastern Europe. Italy bears the highest burden of European dirofilariasis cases in humans : (66%), followed by France (22%), Greece (8%), and Spain (4%). In Europe, the parasite has spread as far north as Estonia. Lifecycle The lifecycle of D. repens consists of five larval stages in a vertebral host and an arthropod (mosquito) intermediate host and vector. In the first stage, mated adult female worms produce thousands of microfilariae (larvae) into the circulation daily, which are ingested by mosquitoes in a blood meal. Larvae develop into infective larvae within the mosquito over the next 10–16 days, depending on environmental conditions, before being reintroduced back into a new host. Microfilariae undergo secondary developmental changes in the insect. For the final two stages of development, third-stage larvae are inoculated back into a vertebral host during an act of feeding. The adults of D. repens reside in the subcutaneous tissues of dogs and cats, where they mature in 6–7 months. Adult worms are 1–2 mm in diameter (females are 25–30 cm in length, the males being shorter). Humans are accidental hosts because adult worms cannot reach maturity in the heart or in the skin. Most infective larvae introduced into humans are thought to die; therefore, infected individuals usually are not microfilaremic. Human disease is amicrofilaremic. Infections in humans Infections in humans usually manifest as a single subcutaneous nodule, which is caused by a macrofilaria that is trapped by the immune system. Subcutaneous migration of the worm may result in local swellings with changing localization. In addition, rare cases of organ manifestation have been reported, affecting the lung, male genitals, female breast, or the eye. The latter is found in particular during the migratory phase of the parasite. D. repens occurs more commonly in adults (aged 40–49 years). The only exception is in Sri Lanka, where children younger than nine years are most likely to be infected. The youngest individual reported was aged four months. Diagnosis Final diagnosis is established by microscopic examination of the excised worm. Making a definite species