Tap water biggest source of cryptosporidium for people with AIDS

Tap water is the most likely source of cryptosporidiosis, an infection that can cause massive and often untreatable diarrhoea in people with AIDS, according to a study published this week in the online journal

BioMedCentral Public Health.

The study found that people with AIDS in San Francisco who drank tap water were 23 times more likely to develop cryptosporidiosis than people who drank bottled water.

In the past there has been uncertainty over the source of cryptosporidial infection in people with HIV. Some have blamed contact with animals, unpasteurised milk, swimming in contaminated water or sexual contact with a carrier of cryptosporidium. It has also been suggested that bottled water is unsafe.

The San Francisco study set out to assess the relationship between these potential routes of exposure and the development of cryptosporidiosis in people diagnosed with AIDS between May 1996 and September 1998 in San Francisco.

Forty-nine cases of cryptosporidiosis were identified and confirmed by laboratory testing of stool samples, and were matched with 99 controls with similar CD4 cell counts, age, sex, ethnicity and date of AIDS diagnosis. Controls were defined as uninfected with cryptosporidium after three negative stool samples. Ninety-eight per cent of subjects were male and 70% were white.

Participants in the study were asked about their patterns of water consumption.

• People who always drank tap water at home were 23 times more likely to have been diagnosed with cryptosporidiosis than people who avoided it entirely.
• People who occasionally drank tap water at home were seven times more likely to have been diagnosed with cryptosporidiosis than people who avoided it entirely.
• People who only drank bottled water at home were 91% less likely to have been diagnosed with cryptosporidiosis, and a habit of drinking only bottled water at home was strongly correlated with avoiding it elsewhere.
• Sexual activity, contact with faecal matter and sex with a partner with diarrhoea were not associated with a diagnosis of cryptosporidiosis.

Based on these associations, the authors estimate that up to 85% of San Francisco’s cryptosporidiosis cases in people with AIDS were caused by drinking contaminated tap water. Filtering of the water supply did not appear to be protective.

However, the study did not control for the use of HAART, so it is unclear if, in patients with matched CD4 counts, Highly Active Antiretroviral Therapy was protective against cryptosporidiosis.

During the period under investigation cryptosporidiosis cases declined from 126 in 1995 to 32 in 1998, and only 13 were reported in 2001, suggesting that at a population level, HAART is protective due in part to immune restoration (cryptosporidium rarely causes a lengthy or life-threatening illness at CD4 cell counts above 150-200 cells/mm³).

However, in individuals who have CD4 cell counts that remain low despite HAART, the risk of cryptosporidiosis remains unclear. The authors note that whilst cryptosporidiosis is associated with foreign travel amongst immunocompetent California residents, this study found that it was associated only with drinking contaminated tap water in people with compromised immunity, suggesting that precautions remain prudent for those with low CD4 cell counts. Furthermore, the design of the study selected cryptosporidiosis cases only in those already diagnosed with AIDS or in those who experienced it as an AIDS-defining illness, implying advanced immune suppression.

Two precautions have been recommended in the past: drinking bottled water, or boiling tap water before use. This study did not distinguish between people who boiled tap water and those who did not, so a cautious interpretation would be to consume bottled water to minimise the risk of cryptosporidiosis.

Another precaution, switching to bottled water if warned of a cryptosporidiosis outbreak, may be ineffective. No outbreak of waterborne cryptosporidiosis was detected during the period of the study in the general population, yet at least 72 cases occurred in people with AIDS.

Further information

Cryptosporidiosis - review of symptoms, epidemiology and treatment - a summary of previous research.