Crohn’s disease was identified as an emerging disease in humans in the 1940’s and the disease has been associated with human populations in developed nations with intensive farming practices (Hermon-Taylor and Bull 2002). In recent years, the bacterium Mycobacterium avium subspecies paratuberculosis (Map) has been implicated as a possible causative agent in Crohn’s disease and Inflammatory Bowel Disease (Mishina et al 1996; Collins et al 2000). In the ruminant animal, Map is known to cause Johne’s disease, which shares similar etiology to Crohn’s disease. In ruminants, Johne’s disease is a disease typically associated with intensive cattle management practices where fecal-oral transmission is high (Chiodini et al 1984; Cocito et al 1994). In this chronic infection, Johne’s disease eventually results in extreme weight loss and poor animal performance. Diagnosis of Johne’s disease in ruminants may only occur when extreme symptoms appear, but Map may be carried and shed in feces for many months prior to any diagnosis.
In cattle, Map infection appears to occur in the neonatal calf via ingestion from a contaminated environment. This contamination can occur from feces of cattle or free-ranging animals infected and shedding Map. Incubation of the pathogen can occur asymptomatic in the bovine host 18-48 months (Grant et al 2002) and, as a consequence, transmission of Map to humans from production animals may occur unbeknownst to producers. As a result of Johne’s disease in dairy cattle, Map has been monitored in dairy cattle as part of a National Animal Health Monitoring System (NAHMS) study. In the NAHMS 1996 study report, Map infection was found in 10% or more of the animals in approximately one-quarter of the dairy herds tested (NAHMS 1997). Contaminated bovine milk has been suggested as a potential route of transmission to humans, since Map has been demonstrated to be resistant to pasteurization (Grant et al 2002).
Map are very slow growing bacteria and culture identification of Map in the human food supply has been problematic. Recent adaptation of molecular techniques that specifically target genetic elements in this bacterium’s chromosome has dramatically improved detection and time of analysis (Ellingson et al 2003).
At this time, little information exists on prevalence of Map in slaughtered cattle in the United States. Feedlots are intensive management systems that concentrate animals and offer an opportunity for cross-contamination and prolonged incubation prior to slaughter. In addition, meat from culled dairy and beef cows is predominantly processed into ground beef. In either fed or cull cattle, contamination of feces, hide or carcasses may be potential routes for transmission to beef products, but has not been determined. Determining the prevalence of Map in cattle using molecular techniques will provide information that determines if and where interventions to decrease Map transmission need to be applied.
In this project, we screened 3368 samples from 601 animals to determine the prevalence in feces, on hides, on carcasses pre-evisceration, and on post-intervention carcasses using PCR technology. Enrichments of samples taken from carcass swabs were performed to confirm PCR results as numbers of Map in these samples may be below detectable limit of the methodology. Animals were sampled from slaughter plants in Eastern, Midwestern and Western states the United States, and included cull cows and fed cattle. The project provides information on Map prevalence in cattle and identifies possible points for intervention.
The stated objectives for this work were:
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