Elephantiasis

Lymphatic filariasis is caused by infestation by one of three nematodes (Wucheria bancrofti, Brugia malayi, or Brugia timori). These nematodes are spread to humans through the bite of mosquitoes. The mosquitoes are considered vectors, meaning that they spread the nematode, and therefore the disease. Humans are considered hosts, meaning that actual reproduction of the nematode occurs within the human body.

Elephantiasis. Photograph by C. James Webb. Phototake NYC. Reproduced by permission.

The nematode has a rather complicated lifecycle. The larval form lives within the mosquito, and it is this form which is transmitted to humans through the bite of an infected mosquito. The larvae pass into the human lymphatic system, where they mature into the adult worm. Adult worms living within the human body produce live offspring, known as microfilariae, which find their way into the bloodstream.

Microfilariae have interesting properties which cause them to be released into the bloodstream primarily during the night; this property is called nocturnal periodicity. Therefore, the vectors (carriers) of filariasis, which deliver the infective worm larvae to the human host, tend to be the more nocturnal (active at night) species of mosquito.