Human Dirofilariasis in Japan

Human filariasis is mainly caused by the parasites, Wuchereria bancrofti and Brugia malayi, whose adults live in the lymphatic vessels of humans. In Japan, bancroftian filariasis was once endemic, but has been completely eradicated from the country. Although imported cases of filariasis are occasionally reported [1,2,3], no autochthonous case has been identified in recent years. By contrast, more than 10 cases of filariasis of animal origin are diagnosed annually in Japan. The most important parasite responsible for zoonotic filariasis in Japan is Dirofilaria immitis, the canine heartworm. The adult worms reside in the pulmonary arteries and the right ventricle, resulting in severe heart failure, which may cause sudden death of the affected dog. Humans can also be infected with D. immitis by a mosquito bite, but the larvae are unable to reach maturity in humans or primates, which are unsuitable hosts. Infected people present either pulmonary infarct or a subcutaneous nodule. The parasite is also occasionally observed in a deep inner organ. Hence, it is frequently confused with malignant tumor. Human dirofilariasis, therefore, can be categorized into two groups: pulmonary and extra-pulmonary dirofilariasis. Extra-pulmonary dirofilariasis is classified further into four groups: cardiovascular, subcutaneous, visceral, and ophthalmic dirofilariasis. In this article, we focus on the studies of zoonotic filariasis that have been carried out by Japanese researchers in Japan.