Xunde Li
Western Institute for Food Safety and Security, University of California Davis

Cryptosporidium and Giardia are protozoan parasites infect a wide range of vertebrates including farm animals, wildlife, and humans. These parasites cause waterborne and foodborne zoonosis by contaminating water and foods. Wildlife is increasingly considered as potential sources that transmit pathogens to food animals and contaminate pre-harvest produce. We hypothesize that certain species of wild rodents in pre-harvest food production region carry Cryptosporidium and Giardia. The objective of this work was to identify the role of wild rodents in Cryptosporidium and Giardia dissemination in produce-production environment.
Between 2009 and 2010, fecal samples were collected from wild rodents in fields adjacent to cattle rangelands and leafy green blocks in one of California’s major produce production region. Rodents were live-trapped and released after collecting defecated fecal samples. Samples were screened for Cryptosporidium oocysts Giardia cysts using fluorescent microscopy. PCR products from microscopy-positive samples were sequenced and analyzed. Total 265 fecal samples were obtained from 10 species of wild rodents. Cryptosporidium and Giardia were detected in 73 (27.5%) and 65 (24.5%) of the samples. For specific rodent species, Cryptosporidium and Giardia were detected in 0% (0/3) and 0%(0/3) of Brush mouse(Peromyscus boylii); 33.3% (1/3) and 33.3%(1/3) of California ground squirrels(Spermophilus beecheyi); 10.5% (4/38) and 13.2% (5/38) of California parasitic mouse(Peromyscus californicus); 0% (0/2) and 0% (0/2) of California pocket mouse(Chaetodipus californicus); 32.8% (61/186) and 26.9% (50/186) of Deer mouse(Peromyscus maniculatus); 16.7% (1/6) and 16.7% (1/6) of Dusky-footed wood rat(Neotoma fuscipes); 0% (0/1) and 0% (0/1) of Harvest mouse(Reithrodontomys megalotis); 0% (0/2) and 50% (1/2) of House mouse(Mus musculus); 33.3% (1/3) and 33.3% (1/3) of Kangaroo rat(Dipodomys deserti); 0% (0/4) and 0% (0/4) of Meadow vole(Microtus californicus); and 29.4% (5/17) and 40% (6/17) of undetermined species. DNA sequencing analysis revealed that Giardia in this rodent population is composed of assemblages B, C, and E and Cryptosporidium is C. parvum-like. Deer mouse (Peromyscus maniculatus) is the dominant rodent species in this produce-production region. The arithmetic mean intensity of fecal shedding in positive samples was 471,426 oocysts and 2,975,623 cysts per gram of feces. If all deer mice were averaged, the overall arithmetic mean intensity was 152,073 oocysts and 799,898 cysts per gram of feces. Assuming the deer mouse population density are the average of reported range of 10-25 adults per hectare, for example 15 mouse, there would be approximately 5 mouse shedding Cryptosporidium oocysts and 4 deer mouse shedding Giardia cysts per hectare. We have determined the average daily fecal shedding is 1.48 gram per adult mouse. Calculated by number of positive animals, daily fecal shedding, and arithmetic mean of (oo)cysts per gram of feces, the daily Cryptosporidium and Giardia loading from deer mouse alone would be approximately 3.5 × 106 oocysts and 1.7 × 107cysts per hectare respectively.
The high incidence of Cryptosporidium and Giardia exist in the wild rodent population adjacent to rangeland and produce area underlines the potential risk of transmission to food animals and contamination pre-harvest produce. Findings suggested the need of developing strategies preventing environmental dissemination of Cryptosporidium and Giardia by wild rodents.