Question

An outbreak of salmonellosis occurred in a large university veterinary teaching hospital. During the first 7 weeks of the outbreak, Salmonella infantis was isolated from 35 animals, including 28 horses, 4 cows, a camel, a goat and a dog. Cultures of fecal samples collected at the time of admission were all negative for S. infantis. During the course of the outbreak, several infected horses developed fever and diarrhea and some veterinary students felt that they might have been infected.

   A total of 148 environmental samples were collected for culture during weeks 1-7 of the outbreak and S. infants were obtained from rectal thermometers, the rubber mat flooring of a horse stall and from the hands of one hospital worker.

   The large-animal part of the hospital was closed. The facility was then cleaned with a high-pressure power washer and disinfected with quaternary ammonium products. Surgical recovery stall mats were cleaned a second time with hydrogen peroxide. Individual thermometers were stored in 0.5% chlorhexidine solution. Salmonella was not isolated upon re-sampling of the cleaned facility and the hospital was reopened.

   The first 2 animals admitted after reopening, a horse and a cow, were found to be positive for S. infants in the feces after only a few days. Environmental samples were again positive. A second, worse outbreak occurred and over 80% of the animals had fever or diarrhea.

1) Why do you think the sanitizing and disinfection failed to control the Salmonella?

2. Explain 2 ways that the animals had become infected.

Answer 1

1. The disinfection failed due to the difficulty associated with disinfection of rough surfaces like concrete blocks and porous mats. Moreover, disinfection never ensures 100% removal of pathogens. Due to the nature of the large surface area that had been disinfected, there always exists chances that pathogens might escape in the crevices and pores.

2. The possible ways that the animals had become infected are

a. run-off water that is used to clean the area might be one source of contaminated water

b. Persistence of pathogens in the rough-concrete surfaces even after disinfection

c. Salmonella is present in carrier organisms for weeks together without showing any symptoms. The carriers spread the pathogen