Not all microsporidiosis causes chronic diarrhoea according to study from Peru

Not all infections with the protozon Enterocytozoon bieneusi, which causes the intestinal disorder known as microsporidiosis, are associated with serious illness in HIV-positive individuals, according to a large epidemiological study from Lima, Peru, published in the May 15^th issue of the Journal of Infectious Diseases.

Chronic diarrhoea was caused only by certain genotypes of the parasite, infecting about one third of all the patients with microsporidiosis in the study. Of note, despite screening for a wide variety of potential sources, the only risk factor that investigators could identify for contracting these more serious strains was eating watermelons.

Microsporidiosis

Microsporidiosis is caused by protozoan parasites called microsporidia, the most common of which is E. bieneusi. A number of studies have shown that E. bieneusi infections of the small intestine are an important cause of chronic diarrhoea in people with AIDS.

Efforts to find an effective treatment of microsporidiosis (aside from antiretroviral therapy) have been disappointing. Although open-label studies have reported some success in treating the infection with a variety of anti-parasitic drugs, randomised controlled trials have invariably failed to show that these treatments are significantly better than placebo at actually stopping diarrhoea.

For patients who cannot yet access antiretroviral treatment, strategies to avoid the infection could be extremely useful. However, little is known about how the infection is actually transmitted.

Some small studies have linked transmission of microsporidia to unprotected sexual activity, eating food contaminated with microsporidia, and drinking or swimming in contaminated water.

The study in Peru

In the largest study of microsporidiosis yet, researchers in Peru evaluated how often intestinal microsporidiosis causes chronic diarrhoea, the risk factors associated with the infection, and the influence of intestinal microsporidiosis on survival in 2652 HIV-positive patients.

Patients were drawn from two Lima hospitals or self-referral (participants benefited by getting free CD4 cell count tests). About two-thirds of the population were male, 6% received antiretroviral therapy (ART), and the median CD4 cell count at study entry was 131 cells/mm³.

Each patient was asked to provide three stool specimens on three separate days. The specimens were examined for microsporidia, as well as for a number of other causes of diarrhoea including Cryptosporidium, Isospora, Cyclospora, ova and parasites.

Patients were considered to have diarrhoea if they had three or more loose or liquid stools within a 24-hour period. Chronic diarrhoea was defined as diarrhoea lasting for at least 28 days.

Microsporidiosis was diagnosed in 67 of the patients in the cross-sectional study (3%) and E. bieneusi was found in stool samples of 56. When these samples were sent to the United States Centers for Disease Control for genetic analysis, eleven different E. bieneusi genotypes were identified. The two most common genotypes, designated Peru-1 and Peru-2, were not associated with significant increases in serious illness.

But the other genotypes were associated with a fourfold increased risk of chronic diarrhoea. It is interesting to note that the association of these E. bieneusi genotypes with chronic diarrhoea was more statistically significant than the one seen for cryptosporidiosis or other intestinal infections.

Patients were questioned at study entry about a large number of risk factors that could possibly expose them to E. bieneusi infection. These included:

• Person-to-person contact (including 15 variables related to the presence of children in the home and contact with persons with diarrhoea and 27 variables related to sexual practices).
• Waterborne (twelve variables related to drinking-water source, treatment, and storage,contact with surface water and swimming).
• Food-borne (33 variables related to the consumption of high-risk foods, such as food from street vendors or fresh raw fruits and vegetables — high risk because they could have been grown in manure treated soil, or washed with contaminated water).
• Zoonotic (presence of 15 specific domestic farm and pet animals in the home and contact with droppings from each of these animals).

Zoonosis is any infectious disease that can be transmitted from animals, both wild and domestic, to humans. The word is derived from Greek words zoon (animal) and nosos (disease). Many serious diseases fall under this category

Low CD4 cell counts (below 100 cells/mm³) were consistently associated with developing microsporidiosis, with 90% of patients with microsporidiosis having a CD4 lymphocyte count below 100 cells/mm³. There was no increased risk associated with sexual orientation, specific sexual behaviours, contact with nappy-age children with or without diarrhoea, or swimming.

The risk factors for transmission that were observed depended upon genotype. Contact with duck or chicken droppings and lack of running water, flushing toilet, or rubbush collection were associated with genotype Peru-, a genotype that did not appear to actually cause chronic diarrhoea.

But the only risk factor significantly associated with infection by any of the other genotypes was eating watermelons.

Why watermelons?

The study authors note that “watermelon, already cut and often doused with water of questionable cleanliness, is often sold on the street in Lima; however, a number of other fruits (e.g., mango and cucumber) are sold the same way” with no increased risk of E. bieneusi infection. However, previous studies have reported “outbreaks of salmonellosis associated with melons and subsequent microbiologic studies have suggested contamination in the field and the adherence of bacteria to the rind.” The same may be true for microsporidiosis.

Survival impact

Chronic diarrhoea has consistently been found to decrease survival in patients with AIDS. In this study, the researchers also conducted a nested sub-analysis which found that cryptosporidiosis (observed in 9% of the population) was associated with a decrease in survival. Still, they concluded that microsporidiosis, even caused by the genotypes associated with chronic diarrhoea, had no significant impact on survival.

However, the authors do not present the survival data for those disease-causing genotypes, and as the number of samples was small, there is a chance that the sub-analysis could have been underpowered to reach this counter-intuitive conclusion.

Implications

Some of these findings may not extend beyond Peru. For example, E.bieneusi genotypes in other parts of the world may be transmitted in different ways. However, the association of watermelon eating with microsporidiosis warrants closer inspection in other settings.

The finding that some E. bieneusi genotypes do and others do not cause chronic diarrhoea could help explain the conflicting results seen in many of the clinical trials of anti-parasitic drugs, such as nitazoxanide, for microsporidiosis. Where possible (if stool samples have been properly stored), those trials may need to be reassessed in light of E. bieneusi genotypic data.