A potential new Crypto source
New research suggests that migratory birds are a source of Cryptosporidium in drinking water.
It is not the deadliest of waterborne pathogens, but Cryptosporidium spp can infect thousands and kill scores by causing diarrhea. Since the 1993 outbreak in Milwaukee, Wis., which infected 400,000 people and killed 104, researchers and water-quality regulators have tried to keep a strict eye on the deadly protozoa in drinking water. Now, research published today on ES&T's Research ASAP website (DOI: 10.1021/es0626842) reveals six novel genetic variants and a new potential source of Cryptosporidium—migratory birds.
When study leader Kristen Jellison, now an assistant professor at Lehigh University, began her research as a graduate student at the Massachusetts Institute of Technology (MIT) in 1997, she wanted to identify the main source of Cryptosporidium in drinking-water supplies in the Boston area. "Cryptosporidium was found in cattle and herd animals," says Jellison. "And agricultural runoff was considered to be a very big contributor to surface-water contamination."
To determine whether there were additional sources of Cryptosporidium in these waters, Jellison sampled two brooks—Brook SF and Brook JF—that were downstream from agricultural sites and suspected of agricultural contamination. Both of the brooks drain into the Wachusett Reservoir, the primary source of drinking water for Boston and surrounding areas.
The team monitored the two brooks every month from June 2001 to May 2002 for the presence of Cryptosporidium oocysts (the hard-walled infective stage). To isolate the oocysts from the water, they used a standard immunomagnetic separation method. They then amplified the DNA from the oocysts to identify the pathogens.
Although Brook SF appeared to be free of the pathogen, Brook JF showed six new genetic variants or genotypes of Cryptosporidium. The oocysts were found only from June to November, when migratory birds such as Canada geese (Branta Canadensis) are known to stop in the region.
The authors also analyzed oocysts from a Canada goose in New York and one in Illinois. Comparison of the sequences revealed that one genotype from Brook JF was identical to the one in the New York goose, and yet another was very similar. A third genotype was closely related to oocysts in the Illinois goose, whereas the remaining four seemed unrelated to other known genotypes of Cryptosporidium. The observed genotypes were not related to any found in cattle. Taken together, the results suggest that in this agricultural watershed, "birds were more of a source than herd animals," concludes Jellison.
Of all the known species and genetic variants of Cryptosporidium, only two—C. parvum and C. hominis—are known to cause cryptosporidiosis in humans. "There's now growing recognition that some populations, probably mostly immunocompromised individuals and malnourished children in particular, are susceptible for infection with some of the species that we hadn't previously considered to be causes of human infections," says coauthor David Schauer of MIT. Despite the fact that the results generate no immediate public-health concern, the genetic variants described in this study might one day be found to be pathogenic to humans, especially in susceptible populations, he says. So, there is a need to "build up a good picture of, or good database of, what the patterns of human infections are with these genotypes as well," adds Schauer.
Microbiologist Joan Rose of Michigan State University agrees. Cryptosporidium, she says, reproduces both sexually and asexually, which makes "the emergence of more pathogenic or virulent forms" very easy. Knowing the genetic diversity and distribution of the pathogen helps in understanding the evolution of virulent forms from nonvirulent ones. The results are especially important in the wake of the recent concern over avian flu, because "we know there is a role for birds in moving things around," she adds.
