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A Survey For Salmonella in Swine On - Farm Occurrence of Salmonella in By Isabel Turney Harris, DVM, Ph.D. This is a report describing the results of a nationwide on-farm survey of the occurrence of Salmonella in swine feed and feed ingredients. Salmonellosis is a zoonotic disease; that is, one affecting both animals and humans. It is caused by infection with Salmonella, a group of Gram negative facultative intracellular bacteria. Their occurrence is very widespread in animal species, approximately 2400 different serotypes have been described. Consideration of salmonellosis in swine means recognition of two different medical entities. One is the swine disease of salmonellosis caused by the host - adapted serotypes, primarily Salmonella choleraesuis and Salmonella typhimurium. Transmission of the disease is thought to occur primarily from pig to pig through ingestion or inhalation of infected fecal material or nasopharyngeal secretions, with the inapparently affected but shedding animal (carrier pig) an important source of this agent. Additionally, the swine raising environment is thought to play a role in maintenance and transmission of the bacteria with birds, rodents, and contaminated feed or water as possible sources of infection. The other medical impact of Salmonella is the foodborne disease of salmonellosis in humans caused by the ingestion of animal meat contaminated by any of a number of Salmonella spp. bacteria. Of the foodborne diseases caused by bacterial pathogens reported to the Centers for Disease Control for the period 1973-1987, Salmonella accounted for 42 percent of the outbreaks and 51 percent of the cases. Other studies have reported that pork was responsible for 11 percent of the Salmonella outbreaks traced to ingestion of meat. The Food Safety Inspection and Service considers that any Salmonella found in pig feed could cause infection in humans. The Food and Drug Administration has a goal of zero contamination of Salmonella in animal feeds and feed ingredients. In 1993, the FDA conducted a survey of animal and plant protein processors in order to determine the prevalence of Salmonella in those feed ingredients. They reported that Salmonella was detected in 56.4 percent of the samples of animal protein and 36 percent of the vegetable protein products. Of the animal protein processors, 62 percent had a Salmonella positive product as did 37 percent of the vegetable protein processors. It would appear that the potential for Salmonella to enter the swine raising environment from feed exists. Clearly, further efforts in defining the existence of Salmonella contamination in the swine raising environment is important in developing methods to reduce the probability of the bacteria ending up in the food chain. This study was initiated to determine if Salmonella were present at detectable levels in swine feed and feed ingredients on the farm. Information was gathered as well to aid in assessing factors which may be associated with finding Salmonella in such an environment. Survey Methods The survey was designed in order to obtain samples of swine feed and feed ingredients on farms of different types and sizes from a wide geographical area of the United States. The sampling protocol was designed to detect Salmonella on the farm if the incidence was at least 10 percent . An attempt was made to sample those feeds and ingredients which were more likely to harbor Salmonella (protein sources of animal or vegetable origin), and those areas of the feeding system where Salmonella contamination from the pigs or environment may occur in the feed ( i.e. feed taken from the floor, troughs, feeders, and bulk storage areas). Farms may or may not have had a history of salmonellosis. Feeds could be either on-farm produced, purchased complete, or both. Sampling techniques, forms and questionnaires were tested by a preliminary collection of samples from two farms. Herds were selected by veterinarians who responded to our request on the American Association of Swine Practitioners - PIC electronic mail system. A sampling "kit" was sent for each herd. This kit contained cups and bags for collecting samples, instruction sheets on what to sample, and a herd questionnaire for collecting data on the type and size of herd, number and composition of rations used, and management, sanitation, and health security measures employed on the farm. We requested that fifty 100g samples of feeds and feed ingredients be collected. Current Findings - As of March 22, 1995, we have obtained feed and ingredient samples from 23 farms in eight different states. - A total of 950 samples have been cultured for the presence of Salmonella. The organism has been isolated from 29 ( 3.1 percent ) of these samples. Of the 23 farms which cultures have been obtained, 11 ( 48 percent ) have been positive for the presence of Salmonella. Preliminary results are shown in Tables 1- 3. From information obtained, it appears that Salmonella may be a common inhabitant of the swine raising environment. To date, the Salmonella isolated from finished feeds have been those found in either complete ground or pelleted feed from feeders, troughs, the floor, or bulk storage areas. Salmonella isolated from feed ingredient samples were obtained from bagged storage with the exception of one grain sample (corn) from the feedmill floor. Salmonella isolated from samples taken from feeders, troughs, the floor or bulk storage areas could certainly have originated from sources other than the feed or ingredients. Other possibilities include rodents, birds, the pigs themselves, humans, or some other environmental source on the farm. Those isolated from the bagged feed ingredients, fishmeal and grower-finisher premix, clearly originated from those components. A comparison of the serotypes of Salmonella obtained from this study, with a list of those most commonly isolated from swine or human sources is shown in Table 4. The serotypes S. agona, S. derby, S. anatum, S. worthington, S. infantis, and S. montevideo, which were found in this study, are also found on either the swine or human list. Salmonella typhimurium and S. choleraesuis have not yet been isolated from feed or ingredients in this study. The largest volume of feed and ingredients submitted from any one farm was 5000 grams (50 samples X 100g/sample), or about nine pounds. Of that 5000 grams, only one-tenth (500 g) was actually placed into culture media. Therefore, considering the volume of finished feed and feed ingredients which may be on a farm at any one time, the culturing of only about 500 grams (approximately one pound) indicates: 1) that a negative result could be a false-negative finding and 2) a positive result means that there could be a large number of Salmonella present either in the feed, ingredients, and/or farm environment. As Salmonella is probably only a sporadic contaminant of feed, either the level of organisms must be relatively high for them to be detected, or the sample taken must by chance have been more heavily contaminated than the larger volume from which it came. Increasing the number of samples and the volume of samples, or by pooling samples, would increase the probability of detecting Salmonella if present. It is not clear what the significance of finding Salmonella in the feed related environment of the pig means as to swine disease prevention or to food safety issues for humans. However, this area must certainly be considered when formulating on-farm Salmonella reduction schemes. TABLE 1 - PRELIMINARY FINDINGS Herd Results: Negative Herds Positive Herds Total 12 ( 52%) 11 ( 48%) 23 Positive = Salmonella isolated from at least 1
feed or feed ingredient on the farm Sample Results: Negative Samples Positive Samples Total 921 ( 96.9%) 29 ( 3.1%) 950 Positive = Salmonella isolated by culture TABLE 2 FEED AND FEED INGREDIENTS Type of Sample Total No. No. Positive % Positive Complete
Feeds
793
25
3.2 Grain
41
1
2.4 Table 3. Salmonella Serotypes Isolated from Feed and Feed Ingredients
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