Two federal agencies Thursday warned that drinking tap water could be fatal to people with weakened immune systems and suggested that they may want to take precautions, such as boiling water before consuming it.
The Centers for Disease Control and Prevention and the Environmental Protection Agency issued a report saying boiling water for a minute is the surest way to eliminate the chance of contracting cryptosporidium, which can cause severe diarrhea, nausea and stomach cramps.
"We do not know if the level of the organisms in the water poses a public health threat," said Dennis Juranek, associate director of the division of parasitic diseases at the CDC. "But we cannot rule out that there will be a low level of transmission of the bacteria" to people who consume water directly from the tap, he added.
A study published in March found that cryptosporidium is much more infectious than researchers previously believed. Volunteers became infected after ingesting as few as 30 organisms.
The parasite is sometimes found in water supplies that have been contaminated with sewage or animal waste.
It is not usually life-threatening except for people with weakened immune systems, such as those who have AIDS, or cancer and transplant patients taking drugs that suppress the immune system.
Alternatives to boiling include using a water filter or drinking bottled water, the agencies said.
Only filters that remove particles measuring one micrometer or less should be considered, said CDC spokesman Tom Skinner.
Among the most effective are those labeled "reverse osmosis" or "absolute" one-micrometer filters, as well as those certified by the National Sanitation Foundation International.
Bottled water treated by distillation or reverse osmosis is safe, the report said. And bottled water from protected wells and springs is safer than water from other sources.