Travel health: schistosomiasis

July 1, 8:08 PMInfectious Disease ExaminerRobert Herriman

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The schistosomes or blood flukes are parasites that have been around since antiquity. It was described in writings in the Gynacological Papyrus of Kahun from 1900 BC. Calcified ova have been found in Egyptian mummies from 1250 BC. In other words, schistosomes can be associated with great civilizations of the past.

Unfortunately they are not just part of history, but they are a serious plague on certain parts of the world, places where Americans may travel for fun, adventure, military or volunteer work today.

Schistosomiasis is an acute or chronic disease, produced by parasites called Schistosoma. It is not a single disease, but a disease complex initiated by several different species of schistosomes. The three most important human species are Schistosoma mansoni, S. haematobium and S. japonicum.

It is 2nd only to malaria in terms of socioeconomic & public health importance, with 200 million people infected in approximately 75 countries.

It is also a “man-made” disease, frequently associated with the development of irrigation and hydroelectric projects (like the Aswan Dam ) in developing countries.

Geographically, the 3 species are found in Africa (S.mansoni and S. haemotobium), the Middle East (S. haemotobium), the Arabian Peninsula, Brazil, Suriname, Venezuela and the Caribbean (S. mansoni) and China, the Philippines and parts of Indonesia (S. japonicum).

The schistosomes are found in fresh water. This water gets contaminated by infected people working in the rice field, fisherman in the lake or children playing who indiscriminately defecate or urinate in the water.

Schistosomes have a very complicated life cycle. The eggs in the feces or urine hatch and the ciliated miracia swim to the specific snail species where it penetrates and goes through a couple of stages in the snail. After a period, thousands of the infective stage (cercariae, below) are released and swim around looking for a human to infect.

The free-swimming cercariae are capable of penetrating the unbroken skin of the human host.

In the human the adult schistosomes (see video below) eventually end up in the blood vessels of the intestines (S. mansoni and S. japonicum) or bladder (S. haemotobium). Here they produce eggs which are the cause of the disease.

The problems with schistosomiasis are many and can include; S. mansoni and S. japonicum: Katayama fever, hepatic perisinusoidal egg granulomas, Symmers’ pipe stem periportal fibrosis, portal hypertension, and occasional embolic egg granulomas in brain or spinal cord. People, especially children are characterized with a large distended abdomen due to enlarged liver and spleen (below right).

Pathology of S. haematobium schistosomiasis includes: hematuria (blood in the urine), scarring, calcification, squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.

How would you be at risk for schistosomiasis? The Centers for Disease Control and Prevention (CDC) state that exposure to schistosomiasis is a health hazard for persons who travel to disease-endemic areas.

Outbreaks of schistosomiasis have occurred among adventure travelers on river trips in Africa, as well as among resident expatriates, such as Peace Corps volunteers in high-risk areas. Those at greatest risk are travelers who wade, swim, or bathe in freshwater in areas where sanitation is poor and the snail hosts are present.

Schistosomiasis can be diagnosed by identifying the characteristic eggs in stool samples, biopsy of tissue, rectal (all) or bladder (S. haemotobium only). If eggs cannot be found in stool samples, antibody detection tests are available.

The drug of choice is praziquantel for infections caused by all Schistosoma species.

There is not a vaccine or any other prophylaxis available for prevention of schistosomiasis. Prevention is by avoiding fresh water. According to the CDC, because there is no practical way for the traveler to distinguish infested from noninfested water, travelers should be advised to avoid wading, swimming or other contact with freshwater in disease-endemic countries.

Untreated piped water coming directly from canals, lakes, rivers, streams or springs may contain cercariae, but heating bathing water to 50° C (122° F) for 5 minutes or filtering water with fine-mesh filters can eliminate the risk of infection. If such measures are not feasible, travelers should be advised to allow bathing water to stand for 2 days, because cercariae rarely remain infective longer than 24 hours.

There are non-human schistosomes found here in the U.S. These are typically caused by schistosomes of birds or mammals, which can be found in fresh or salt water.

They penetrate the human skin and cause a dermatitis better known as “swimmer’s itch”. This has been reported in lakes of several states including Florida.

Other than the rash, it causes little other disease due to the fact it cannot complete its life cycle in humans.