Schistosomiasis (bilharzia) risks increased by HIV

A Kenyan study reported online in The Lancet has found that HIV-positive men with low CD4 counts, due to HIV, are at higher risk of re-infection with the worms that cause schistosomiasis than adults with undamaged immune systems. Taken with other evidence, including resistance to re-infection that appears with age and following drug treatment that kills the worms, the researchers argue that a preventive vaccine may be possible. However, more research is needed to understand the immune responses leading to resistance (Karanja).

An ongoing study from Zimbabwe, reported at the XIV International Conference on AIDS in Barcelona, has failed to confirm previous reports by Karanja and others that people with HIV were less likely to shed schistosomes in their urine than people without HIV (Kallestrup).

Both studies reflect a growing recognition of the importance of interactions between widespread, mostly tropical, diseases and HIV. It is still uncertain whether schistosomiasis promotes HIV disease, although there is some evidence that other helminths increase viral load and may bias immune responses in a way that makes it harder for the body to control HIV. Treatment of helminths has been shown to reduce viral load and may also be important for effective immune responses to future vaccines against HIV, according to Israeli professor Zvi Bentwich, who is a pioneer of research in this field.

The Kenyan team studied 96 male car-washers in Kisumu from 1995 to 1999 who were routinely exposed to the worms in the shallow waters of Lake Victoria where they stood while washing down vehicles. Careful records were kept of the number of cars each man had washed, as a basis for payment. These turned out to be a good measure of the level of risk of reinfection with schistosomiasis.

All men had blood taken for CD4 counts and stool and urine samples taken for schistosomiasis, which was treated as soon as eggs were found. Of around 100 men recruited into the study, 30% were HIV positive to begin with and the rate of new infections was above 8% per person per year. This allowed a comparison to be made of reinfection rates among groups of men with different CD4 counts, HIV-positive and HIV-negative. Once other factors were taken into account, they estimated that HIV-positive men with a CD4 count below 100 cells per mm³ had a 1.37 times higher re-infection rate than HIV-negative men.

Background: schistosomiasis

Schistosomiasis – also known as bilharzia - is a debilitating parasitic disease caused by five different kinds of flatworm or blood fluke (helminth). 500 to 600 million people are at risk worldwide, in 74 countries, of whom 20 million are severely ill, another 120 million have some symptoms and another 60 million or more are infected. Its economic impact is second only to malaria, in reducing productivity at work and in limiting children’s ability to learn.

It is common in large parts of Africa with two main kinds – S. mansoni (whose eggs are shed into faeces) and S. haematobium (whose eggs are shed into urine). Eggs pass into water where they rest on water plants, until eaten by snails where they grow into larvae that are then shed into fresh water. These larvae then enter the bodies of people who enter the water.

The standard treatment for schistosomiasis in Kenya is a drug called praziquantel, which is highly effective and costs US 25 cents per treatment for adults, less for children. Oxamniquine is also used to treat intestinal schistosomiasis and metrifonate is used to treat urinary infections with S. haematobium.