Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Veterinary Microbiology and Preventive Medicine

First Advisor

George W. Beran


The effects of environmental stress on the antimicrobial drug resistance of E. coli of the intestinal flora of swine from a farm where no antimicrobials are supplemented in feed were studied. Initially the effects of cold stress were studied. Fecal samples were initially collected from animals of different age groups (growers, finishers, gilts, and sows). Afterwards only finishers were sampled over a period of two years. Samples were collected over periods considered seasonally normal and stable (baseline) as well as during drastic drops (>15°C) in environmental temperature (cold stress). Baseline prevalence of ampicillin and tetracycline resistance for younger pigs (growers) was significantly higher (P < 0.05) than for older pigs. Also when animals were exposed to cold stress, a significant increase in ampicillin and tetracycline resistance was observed for animals of all age groups. In a second phase comprised of two studies (1) and (2), the effects of heat stress were studied. In study 1, ten finisher hogs were heat stressed (34°C) for 24 hours. Percentages of antimicrobial drug resistance after heat stress were significantly higher when compared with pre-stress levels for amikacin, ampicillin, cephalothin, neomycin, and tetracycline. This high prevalence of resistance persisted up to slaughter of animal 10 days after stress. In study 2 samples of different sections of the gastrointestinal tract were collected after heat stress and compared with control animals. Results obtained indicated that E. coli that colonized ileum and cecum had a higher prevalence of resistance for ampicillin and tetracycline than the ones that colonized colon and rectum. When animals were exposed to heat stress, prevalence of resistance to ampicillin and tetracycline increased (P < 0.05) to levels similar to those observed in the ileum and cecum. Gastrointestinal motility was artificially induced with a cholinergic drug (Neostigmine) during the third phase of the study. Samples of ileum, cecum, colon, and rectum were collected from treated and control animals. Results obtained were very similar to those obtained during the heat stress study. To test the hypothesis that environmental stress induces a reduction in intestinal transit time an experiment was designed in which two groups of animals received a liquid marker (Chromium-EDTA), following which swine in one of the groups were heat stressed. Results obtained showed a reduction in the intestinal transit time for the stressed group. This finding sustained the initial hypothesis that an outflow of resistant organisms from the upper segment of the intestinal tract (ileum and cecum) moves to the lower tract (colon and rectum) when animals are environmentally stressed.



Digital Repository @ Iowa State University,

Copyright Owner

Manuel Humberto Moro



Proquest ID


File Format


File Size

98 pages