Monday, October 1, 2012

Blood flukes on an island

Schistosoma haematobium
Schistosomiasis is a parasitic disease caused by trematodes of the genus Schistosoma. There are four main species that infect humans. One of them is Schistosoma haematobium that causes urinary schistosomiasis.

Urinary schistosomiasis is often chronic and can cause pain, secondary infections, kidney damage, and even cancer. It has been infecting humans for at least 4000 years and had its own specific hieroglyph in ancient Egyptian. In the time before treatments were widely available, it was still so prevalent in Egypt that boys were traditionally expected to go through a “male menarche”—sometime during adolescence, it was normal for them to urinate blood. The infections continue to be a significant public health problem in much of Africa and the Middle East, second only to malaria among parasitic diseases. 

Bulinus sp.
The life cycle of the parasite involves an intermediate snail host (species of the genus Bulinus). Four weeks after the initial penetration into the snail the larval form - the cercariae - begin to be released. The free swimming  cercariae burrow into human skin when it comes into contact with contaminated water. They enter the blood stream of the host where they travel to the liver to mature into adult flukes. In order to avoid detection by the immune system inside the host, the adults have the ability to coat themselves with host antigens. After a period of about three weeks the young flukes migrate to the bladder to copulate. The female fluke lays as many as 3,000 eggs per day which migrate to the urinary bladder and ureters to be eventually released back into the water.

A parasitological survey was conducted on the Tanzanian island Mafia. Prior to the study a survey found that 7.5% of the School children had experienced symptoms of urinary schistosomiasis. However, parasitologically proven cases were lacking and earlier malacological surveys found only a few potential host species. Combined parasitological and malacological surveys were supplemented with observations on the compatibility of local Bulinus species with Schistosoma haematobium. The researchers used DNA Barcoding to identify both snail and schistosome. They found no evidence to suggest that Schistosoma haematobium is being actively transmitted on Mafia Island. With the substantial travel to and from the island, the refractory nature of local snails and evidence from DNA barcoding of schistosomes and snails, the authors conclude that the cases of urogential schistosomiasis are the result of imported infection.

I guess this is good news as the low prevalence allows for better control and chances are high that this parasite will not be established on the island.

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