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U.S. Department of Health and Human Services

Animal & Veterinary

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by Kathy Hollinger, D.V.M., M.P.H.
FDA Veterinarian Newsletter September/October 1998 Volume XIII, No V

In 1994-1995 a case control study was performed by the Ministry of Agriculture Fisheries and Food (MAFF) Central Veterinary Laboratory (CVL) to assess risk factors in cattle herds for isolation of the multi-drug resistant pathogen Salmonella typhimurium DT104(StmDT104).

The Recurrent Clinical Disease Study was proposed in 1996 by the Epidemiology Department at CVL to assess the extent of recurrent episodes of S. typhimurium DT104 in affected cattle herds in England and Wales. Information from the 1994-1995 case control study indicated the potential persistence of the organism on cattle farms, for up to 18 months after the first episode on more than 75 percent of 25 farms studied. This led epidemiologists to believe that recurrence of disease and subsequent environmental amplification of the organism from clinically affected animals would lead to the establishment of this organism as an endemic pathogen in cattle herds. The purpose of the study was to assist in the development of a "Code of Practice for the Prevention and Control of Salmonella in Cattle Herds," to be disseminated for use by farmers. In 1997 CVM sent Dr. Kathy Hollinger to CVL to work with Dr. John Wilesmith and Dr. Sarah Evans to develop a protocol for the StmDT104 Recurrent Clinical Disease Case Control Study. Dr. Hollinger designed a study, developed the protocol and tested the survey instrument for the study. Two different test mailings were distributed. The second test mailing included a handwritten note encouraging the participation of the farmers, in an effort to improve the response rate which subsequently increased by almost 40 percent. Dr. Hollinger developed a second survey to describe the education, experience and animal health information to which farmers with herds affected by StmDT104 are most commonly exposed. This was important as CVL wished to return the information to farmers in a format and a resource that would be widely disseminated to the target audience. Other areas of research conducted during her stay in England included researching the origins of the epidemic multi-drug resistant strain, assessing the epidemiology of the epidemic strain from the statutory reporting under the zoonoses order and compiled the StmDT104 susceptibility monitoring surveillance data from 1980 to the present for food animal species and researched additional losses in naladixic acid and trimethoprim susceptibility.

The two questionnaires developed for the study were mailed to farmers in October and November of 1997 from CVL. Follow-up reminders were sent out at one-week intervals from mailing the first questionnaire. A thank-you letter and an information sheet describing risk factors for salmonellosis in cattle were mailed to farmers upon receipt of completed questionnaires. Data was entered at CVL by the Epidemiology Department and checked through double entry procedures. Response rate to the Recurrent Clinical Disease (RCD) questionnaire was approximately 70 percent, very close to the targeted response rate. This was due to a combination of factors. First, the inclusion of a handwritten note, second, notification of the Veterinary Investigation Centers, the branch of MAFF that performs the diagnostic laboratory functions for the farmers and their veterinarians, third, reminder mailings that included the questionnaire and, finally, follow-up telephone reminders to those farmers who had not returned the questionnaire within a month of the first mailing.

In late June 1998 Dr. Hollinger returned to CVL to begin data manipulation and analysis of the two study questionnaires. Activities which Dr. Hollinger carried out during the three-week stay at CVL included review of the data from the Respondent Profile Survey, performing the descriptive and univariate statistical analysis assessing the farmer’s sources of food animal health information and geographic, age and educationally related trends. The results of this analysis were drafted into an article to be submitted for publication in the near future. The data from the RCD survey were manipulated and prepared for analysis. Descriptive analysis was begun looking at risk factors for disease and the evaluating time to recurrent disease comparing vaccinated and non-vaccinated herds. A dataset for multi-variable analysis was prepared. Another data set was prepared to compare data from the 1994-1995 Case Control study and the 1997 RCD study.

In other work, continued research into the origins of the StmDT104 epidemic using Sentest database and other information from imported bird licenses was obtained. Dr. Hollinger reviewed the antimicrobial susceptibility data and emerging trends in StmDT104, tabulated the data, and drafted an article for publication. The data analysis is ongoing and expected to be completed in October. These findings will be prepared for publication.