The Norway rat as a reservoir host of Cryptosporidium parvum

The potential of Norway rats (Rattus norvegicus) to spread the parasite Cry ptosporidium parvum was investigated by examining parasite prevalence in re lation to the structure and movements of three permanent rat populations li ving on farmland in Warwickshire (UK) from October 1994 to March 1997. One population lived among a group of farm buildings housing cattle, while the other two had no contact, with livestock, one living around a pond and its outflowing stream and the other on a rubbish tip. Overall, parasite occurre nce was 24% (n = 4.38), but it varied according to body weight (age) with 4 0% of juveniles (less than or equal to 100 g) infected decreasing to 12% fo r adults >400 g, suggesting that actively breeding populations are potentia lly more likely to spread the parasite than non-breeding populations. There was no difference in prevalence between the three populations. The parasit e was detected in more males (29%) than females (19%). Seasonally, on the l ivestock farm, prevalence was significantly lower in autumn (10%), but vari ed little (31-36%) from winter to summer. In contrast, on the arable farm, prevalence peaked in summer (50%) with a trough in winter (6%). Infection i n rats appeared to last <67 days. Rats living on the livestock farm had hom e ranges largely confined to the cattle sheds, thereby maintaining a potent ial source of infection for livestock if rodent control was not part of a d econtamination program. Equally, rats living around the pond on the arable farm provided a source of oocysts to contaminate the pond water, as well as being able to carry the parasite to nearby farm buildings or even to neigh boring farms.