The filariasis causing worms are unique in that they create microfilaria instead of the traditional eggs. These are basically motile eggs - they aren't yet larva, but they do move around. They can be sheathed or unsheathed, and they can display periodicity which corresponds to the preferred intermediate host. (Pretty cool!) They tend to be circulating in the blood during these periods, but can be completely undetectable when they're not in this period.
Wucheria bancrofti live in the tropical and sub-tropical regions of the world. Humans are the only known definitive host, which means if we get rid of it in humans it won't come back! However, this is kind of hard, because not everyone who has this infestation shows symptoms, or the microfilaria might not be present when a blood sample is taken.
The adult forms of Wucheria bancrofti lives in the lymph nodes. They tend to like those closest to the groin, which will become important in a minute. The microfilaria migrate into the blood, where they display a nocturnal periodicity. It is hypothesized that they go to the lungs when they're not in the blood.
Most often Wucheria bancrofti is transmitted by Culex quinqefasciatus mosquitoes, though several other species can transmit it. The mosquito takes a blood meal, ingesting the microfilaria. These mature to infective larva inside the mosquito. When the mosquito takes a meal from a human, these wiggle their way down the proboscis of the mosquito, infecting the human. They migrate to the human lymph nodes, where they mature.
The adults can live up to 5 years in the body. The microfilaria can live 1-2, during which time the person is actively infectious.
The most common pathology is asymptomatic microfilaremia. This means that the microfilaria are just chilling out and circulating in the blood. They'll show up on a slide, but won't really cause any problems. People can develop lymphangitis. However, in cases of long infections or large infections, people can develop chronic lymphadema and eventual elephantiasis (the most well known characteristic of Filariasis). Wucheria bancrofti in particular can cause hydrocele and scrotal elephantiasis, which I'm not going to put a picture of, but if you're interested I suggest looking up.
What tends to happen is adults living in the lymphatic channels cause dilation and chronic edema due to gravity. As the worms die, collagen builds around them, forcing the lymph nodes open even farther. This causes the lymph to not drain properly, which makes it collect in the extremities. Unfortunately, even if the worms are taken care of (with DEC), this process is irreversible, as extensive damage has been done to the lymph nodes.
Diagnosis is done through looking at the blood in a lab (which must be done over several hours, as the microfilaria could show up at different times), or through using an ultrasound to examine the lymph nodes. They create a characteristic "Filaria Dance Sign", which can be seen in this video: http://www.youtube.com/watch?v=d3KWh6xqQm0
Brugia malayi is very similar to Wucheria bancrofti, but its geographic distribution is limited to Southeast Asia. Differentiation is done based on the microfilaria, as the adults look quite similar. It has reservoir hosts in macaques and leaf monkeys. It creates similar pathology, except that hydrocele is not seen. It's also primarily transmitted by Mansonia species of mosquitoes.
Photo Credit: Stanford.edu
On the left: Wucheria bancrofti microfilaria
On the right: Brugia malayi microfilaria
These are nicely stained so they look really different. They usually don't.
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