Two of the five main debilitating diseases transmitted by insects are caused by nematodes, namely filarial worms. The diseases are bancroftian and brugian filariases, commonly termed elephantiasis and onchocerciasis (or river blindness). Other filariases cause minor ailments in humans, and Dirofilaria immitis (canine heartworm) is one of the few significant veterinary diseases caused by this type of parasite. These filarial nematodes are dependent on Wolbachia bacteria for embryo development and thus infection can be reduced or eliminated with antibiotics (see also section 5.10.4).
Bancroftian and brugian filariasis
Two worms, Wuchereria bancrofti and Brugia malayi, are responsible for over a hundred million active cases of filariasis worldwide. The worms live in the lymphatic system, causing debilitation, and edema, culminating in extreme swellings of the lower limbs or genitals called elephantiasis. Although the disease is less often seen in the extreme form, the number of sufferers is increasing as one major vector, the worldwide peridomestic mosquito, Culex quinquefasciatus, increases.
The cycle starts with uptake of small microfilariae with blood taken up by the vector mosquito. The microfilariae move from the mosquito gut through the hemocoel into the flight muscles, where they mature into an infective larva. The 1.5 mm long larvae migrate through the hemocoel into the mosquito head where, when the mosquito next feeds, they rupture the labella and invade the host through the puncture of the mosquito bite. In the human host the larvae mature slowly over many months. The sexes are separate, and pairing of mature worms must take place before further microfilariae are produced. These microfilariae cannot mature without the mosquito phase. Cyclical (nocturnal periodic) movement of microfilariae into the peripheral circulatory system may make them more available to feeding mosquitoes.
Onchocerciasis actually kills no-one directly but debilitates millions of people by scarring their eyes, which leads to blindness. The common name of “river blindness” refers to the impact of the disease on people living alongside rivers in West Africa and South America, where the insect vectors, Simulium black flies (Diptera: Simuliidae), live in flowing waters. The pathogen is a filarial worm, Onchocerca volvulus, in which the female is up to 50 mm long and the male smaller at 20–30 mm. The adult filariae live in subcutaneous nodules and are relatively harmless. It is the microfilariae that cause the damage to the eye when they invade the tissues and die there. The major black-fly vector has been shown to be one of the most extensive complexes of sibling species: “Simulium damnosum” has more than 40 cytologically determined species known from West and East Africa; in South America similar sibling species diversity in Simulium vectors is apparent. The larvae, which are common filter-feeders in flowing waters, are fairly readily controlled, but adults are strongly migratory and re-invasion of previously controlled rivers allows the disease to recur.