Filariasis

Filariasis is an infection caused by worms. Not just any worm: they are filiform nematodes, meaning round and as thin as a thread. They are carried by biting insects such as mosquitoes and black flies.

In the lymphatic form, the one that attacks the body's drainage system, the mosquito leaves larvae in the skin when it bites. Those larvae work their way into the lymphatic system, the network of vessels that collects fluid from the tissues, and there they grow into adults and reproduce.

The damage has two sources. One is mechanical: the adult worm injures the vessel simply by being there. The other is immune: when the worm dies it releases antigens, substances the body recognizes as foreign and attacks with inflammation. Over time those vessels become obstructed, like pipes clogging from the inside, and edema, meaning swelling, and fibrosis, which is scar tissue, appear.

It is usually acquired in childhood, and it can go years without showing itself. Over time, repeated episodes of inflammation can end in chronic lymphedema and in elephantiasis, the enormous thickening that gave the disease its common name.