Masters Theses

Date of Award

8-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

Alan G. Mathew

Committee Members

Kelly Robbins, Ann Draughon

Abstract

Two experiments were conducted to determine: 1) the effects of different antibiotic regimens on the shedding patterns of pigs infected with Salmonella typhimurium; and 2) whether the development of antibiotic resistance plays a role in shedding patterns. In experiment 1, 36 50-day-old pigs were orally and intranasally challenged with 3 mL (1.2 X 109 CFU/mL) of Salmonella typhimurium obtained from the USD A Animal Disease Control Center (Ames, lA). Pigs were blocked by litter and separated into four treatment groups including: 1) i.m. injection of ceftiofur sodium (cef) for 3 days followed by inclusion of oxytetracycline (otc) at lOOg/ton of feed, 2) apramycin (apr)(150g/ton of feed) for 14 days followed by otc (100g/ton), 3) carbadox (car)(50g/ton) followed by otc (100g/ton), 4) no antibiotics (control). Treatments were initiated 48 hours post inoculation and continued until pigs reached market weight. Rectal swab samples were collected pre-inoculation, 48 and 72 hours post-inoculation, and at weekly intervals thereafter. At P < .05, the overall incidence of salmonella shedding was reduced in pigs receiving cefrotc and apr/otc treatments when compared with the control group. Pigs receiving the car/otc treatment shed the salmonella in the same amount as control pigs. The second experiment involved 48 50-day-old pig. Pigs were orally and intranasally inoculated with 3 mL (1.3 x 109 CFU/mL) of a Salmonella typhimurium isolate containing a nalidixic acid resistance marker. Pigs were blocked by litter and received the same treatments as in experiment one. At P < .05 the overall incidence of shedding was reduced in pigs receiving the apr/otc treatment; however, no differences were observed between antibiotic treatments. Positive samples from five pigs from each treatment in experiment two were analyzed via disc-diffusion on 2, 4, 7, 21, 42 and 70 days post-inoculation. A time effect was detected indicating that the proportion of isolates resistant to at least one of the test antibiotics varied over time. Resistance was measured at 47% at 2d PI and reached a pinnacle of 89% at 70d PL Treatment * time interactions were detected and in each treatment the percentage of isolates resistant to at least one of the test antibiotics increased significantly over time; again reaching a pinnacle at 70d PI. Similar antibiotic * time interactions were detected as the percentage of isolates resistant to each of the specific antibiotics increased over time. These data indicate that the use of antibiotics during Salmonella typhimurium infections results in increased proportions of antibiotic resistant Salmonella typhimurium isolates. Such resistance, however, does not result in increased shedding of the organism and in the case of apr/otc, while the proportion of resistant isolates increased, fecal shedding decreased.

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