High Temps, Lactation Tied to STEC Shedding in Cattle

Cristina Venegas-Vargas, a former graduate student and lead author of the study, poses in front of dairy cattle.

Dairy cattle under stress are significantly more likely to shed Shiga toxin-producing E. coli (STEC) in their feces, according to a recent study conducted in Michigan. The stress causing this shedding of STEC was linked to higher temperatures, the animal’s first lactation, and the animal’s first month of milk production.

The number of cattle shedding STEC in their feces varied across the several herds sampled on Michigan farms, and the fecal shedding frequencies of STEC serotypes other than 0157 exceeded the frequencies of STEC 0157, says Shannon Manning, PhD, a molecular biologist at Michigan State University and principal investigator of the study published in the online edition of Applied and Environmental Microbiology.

“We think that stress is important for pathogen shedding levels in cattle as well as other animals, as it has previously been suggested to cause a negative energy balance, although the mechanism is not fully understood,” she says. Some studies have linked heat stress to Salmonella shedding in cattle, swine, and poultry, she adds.

Cattle are a common carrier of STEC, and food or water contaminated with cattle feces is a common source of E. coli infections in humans. Dr. Manning says that a greater understanding of the specific factors that increase the risk of STEC contamination and shedding in different types of cattle should result in new management strategies of such high-risk animals.

Isolation of dairy cattle during their first lactation, for example, would limit the pathogens from colonizing cattle and entering the food supply, she says. “Other practices might involve ways to reduce stress from heat, or limiting contact with other animals in extreme temperatures to prevent transmission to susceptible animals. These are the types of practices that a farmer could apply to the entire herd and may result in a decrease in transmission and prevalence of STEC.”

The research was conducted by a multidisciplinary team at Michigan State, which included farmers from six dairy farms and five feedlots in the state. Participating farmers completed questionnaires and interviews about their cattle herds and management practices. Researchers collected fecal and blood samples from each animal, as well as ruminal fluid for use in future studies. The samples were screened for STEC using multiple microbiological and molecular tests, and the data analyses focused on identifying predictors of STEC shedding in both dairy and beef cattle herds.

The samples showed a higher prevalence of STEC in beef (21 percent) versus dairy (13 percent) cattle. Dairy cattle were significantly more likely to shed STEC when the average temperature was higher than 28.9 degrees Celsius (84 degrees Fahrenheit) one to five days prior to sampling, during first lactation, and when the animal was fewer than 30 days in milk.