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Abstract & Commentary
Synopsis: A newly identified, apparently pathogenic species of cryptosporidium, C meleagridis, is described.
Source: Pedraza-Diaz S, et al. Cryptosporidium meleagridis from humans: Molecular analysis and description of affected patients. J Infect. 2001;42:243-250.
Several groups worldwide have been diligently studying the genus Cryptosporidium. Infection is usually due to the species C parvum, and gained notoriety as an opportunistic infection in patients with HIV infection. Large outbreaks have occurred in recent years due to contaminated urban water supplies. The infection often produces diarrhea and has become more than just a pest. The prospect of additional pathogenic species implies how little we know about this protozoan.
This current work from England, a large cooperative endeavor, features 19 patients with diarrhea from whose stools a cryptosporidium was detected. Because this organism cannot be isolated with routine microbiological methods, its presence is detected as in this study by modified microscopy and immunoflourescence staining. Two anticryptosporidial monoclonal antibodies, 1 made "in house" and 1 commercially available, were used for immunoflourescence. The study also involved extensive analysis by polymerase chain reaction (PCR) amplification and sequencing of 8 different gene fragments including a heat shock protein, ß-tubulin, and 18S rRNA genes.
The results taken together suggested that a species other than C parvum was associated with diarrhea. The molecular analysis—in particular the DNA sequence data—showed a genetically homogenous group (except for 1 sample). Five of the genes analyzed had sequences that were identical and different for those reported previously for C parvum. Sequences for oocyst wall proteins, and 18S rDNA and HSP70, were identical or very similar to those reported for C meleagridis from turkeys.
The tubulin gene PCR produced products only in 3 samples. Genetic analysis of the tubulin sequences produced dendograms (species similarity plots) showing identity for a first group called C parvum genotype 1, C parvum genotype 2, and a third group of C meleagridis that is genetically related to C parvum genotype 2 but only distantly related to genotype 1.
Fecal samples were collected for more than 2000 patients. Only brief clinical data were reviewed for patients with diarrhea and that brief clinical data from the 19 patients with C meleagridis showed that 6 had recent foreign travel, 4 with sea or swimming pool bathing, and 2 with HIV underlying infection. Four of the patients were coinfected with Giardia and 1 with C parvum. Two patients reported contact with a sick pet, 1 with a dog, and 1 with a cat. Cases dated back to 1995 (stools had been frozen) and were as recent as July 2001. Three cases were part of drinking water outbreaks in which C parvum was causal in most other patients.
Comment by Joseph F. John, MD
Very little is known about the natural reservoirs for cryptosporidia. C parvum has become a scourge for companies who provide potable water for communities since they know it is almost impossible to remove every last ooycyst. C parvum has been highly implicated as a pathogen in urban water outbreaks. The team of McLauchlin and colleagues (including Pedraza-Diaz from this current paper) has been central to defining the expanding epidemiology of cryptosporidia.1 Livestock and, now in this paper, poultry, seem to be important reservoirs. The mechanism of potable water contamination remains a much bigger question.
Since there are only brief clinical data on the 19 patients presented here, we cannot glean much about the severity of their disease or response to therapy. More important is the fact that at least 1 patient with HIV became infected, having no other intestinal pathogens detected.
This article highlights the discrimination of modern molecular techniques for characterizing microbial pathogens, in this case even a nonculturable one. Five gene loci had identical DNA sequences suggesting identity among the isolates that could be placed in the meleagridis species.
In this study, patients were identified by the presence of oocysts consistent with cryptosporidium morphology, one that cannot be used for species differentiation. So, it is possible that the prevalence of C meleagridis, may be grossly underestimated. Persons infected with Giardia may also harbor C meleagridis, detectable only by DNA amplification of cryptosporidium loci; examples of this were used in this study.
Pedraza-Diaz and colleagues are to be commended for their crafty use of molecular techniques to define the range of Cryptosporidium species that may infect humans. Much more needs to be discovered about the host range of this group of parasites and the mechanism of their transmission.
Dr. John, Professor of Medicine and Microbiology, University of Medicine & Dentistry—New Jersey, Robert Wood-Johnson Medical School, is Co-Editor of Infectious Disease Alert.
1. McLauchlin J, et al. Molecular epidemiological analysis of Cryptosporidium spp. in the United Kingdom: Results of genotyping Cryptosporidium spp. in 1705 fecal samples from humans and 105 fecal samples from livestock animals. J Clin Microbiol. 2000;38: 3984-3990.