Examination of Heat Stress and Stage of Lactation (Early versus Late) on Fecal Shedding ofE. coliO157:H7 andSalmonellain Dairy Cattle

Abstract

Mature, healthy lactating dairy cattle were sampled on two farms in the southwestern United States to examine the effects of heat stress (Experiment I) and stage of lactation (Experiment II) on the fecal shedding of E. coli O157:H7 and Salmonella. To examine the effects of heat stress, fecal samples were collected from 45 cows at 7:00 AM (coolest part of the day) and 5:00 PM (hottest part of the day) in August 2002 on a 250 cow dairy. The study was replicated one month later (n = 170 total samples). A temperature-heat index (THI) was calculated for each sampling time. In Experiment II, stage of lactation was examined by sampling lactating dairy cattle early [< 60 days in milk (DIM)] and late (> 150 DIM) in the lactation cycle in the summer of 2001. The study was replicated the following summer (60 cows/group/replicate; n = 240 total samples). For Experiment I, THI averaged 75 and 82 for the AM and PM samplings, respectively, indicating the cows were beginning to experience heat stress in the morning and by afternoon were in severe heat stress. The shedding of E. coli O157:H7 tended to be higher (p = 0.09) in the afternoon sampling of the first replicate, however was not different in the second replicate or when both replicates were pooled (p > 0.10). Salmonella shedding was not different (p > 0.10) at any sampling time with nearly 100% of the cows positive. Stage of lactation had no effect on the number of cows shedding E. coli O157:H7 (p > 0.10). Salmonella shedding tended to be higher (p = 0.09) in early lactation cows in the first replicate, while in the second replicate more late lactation cows were shedding Salmonella (p < 0.05); however, there were no differences due to stage of lactation when replicates were pooled (p > 0.10). While further research is needed, results of this research highlight the variability in pathogen shedding in healthy dairy cattle and indicate that environmental factors and/or production demands may influence shedding patterns of E. coli O157:H7 and Salmonella.

Factors Associated with Shiga Toxin-Producing Escherichia coli Shedding by Dairy and Beef Cattle

“…With regard to age at the time of calving, a German study found that animals undergoing first lactation had a higher risk of STEC shedding than older cows (44), while a longitudinal study in the United Kingdom reported the prevalence of STEC O157 to be the highest in cows 2 years of age, the typical age at first calving (43). Although the multitude of factors that could contribute to the association between lactation number and STEC are not completely understood, they could be related to stress and the energy requirements needed by cows in the first lactation (45), in addition to decreased immune function. A negative energy balance, which is common during the first lactation, has been suggested to alter the digestive microbiota composition (45), which could potentially favor STEC colonization and shedding.…”

“…Although the multitude of factors that could contribute to the association between lactation number and STEC are not completely understood, they could be related to stress and the energy requirements needed by cows in the first lactation (45), in addition to decreased immune function. A negative energy balance, which is common during the first lactation, has been suggested to alter the digestive microbiota composition (45), which could potentially favor STEC colonization and shedding.…”

“…During early lactation, significant physiological and metabolic changes occur with high metabolic demand, which may favor intestinal STEC colonization and shedding (46). Two additional studies, however, found no association between STEC and stage of lactation, yet these studies also focused only on STEC O157 and were conducted in the southwestern United States (45,47).…”

“…Heat stress has been suggested to impact STEC O157 shedding and was found to be important for Salmonella shedding as well (45). Not all studies, however, have observed differences associated with ambient temperature (45,49,50), which varies considerably by geographic location.…”

“…Heat stress has been suggested to impact STEC O157 shedding and was found to be important for Salmonella shedding as well (45). Not all studies, however, have observed differences associated with ambient temperature (45,49,50), which varies considerably by geographic location. The lack of positive associations between STEC shedding and the degree of animal contact and the number and proportion of lactating cattle provides support for the hypothesis that specific stressors may enhance host susceptibility and contribute to shedding.…”