Salmonella newport - An emerging disease in dairy cattle
Overview: Salmonella enterica serovar newport has recently been named an emerging disease by the American Association of Veterinary Laboratory Diagnosticians (AAVLD). Salmonell-osis, due to any of 2200 Salmonella serotypes (serovars) is one of the few diseases that is increasing in prevalence. While most serotypes are potential human and animal pathogens, only 10 serotypes are responsible for most disease in cattle. The National Veterinary Services Laboratories (NVSL) listed S. newport as one of the top ten most frequently identified Salmonella serotypes from U.S. cattle from July 1998 through June 1999. Nontyphoidal salmonellosis is an infection estimated to cause over 1 million cases of illness and 500 deaths in humans annually in the United States. Cull (market) dairy cows account for a large amount of beef, especially ground beef. Of 58 serotypes isolated by culture from culled dairy cows in five regional market cow establishments in the U.S., S. newport was among the 30 most prevalent serotypes.
Salmonella newport causes significant clinical disease in livestock, particularly cattle, in humans, and in other animal species. Multiple antimicrobial resistant strains of S. newport have been recorded in the U.S. and Canada. All of these strains are resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides and tetracycline (ACSSuT). In addition, many of these strains show intermediate or full resistance to third-generation cephalosporins, kanamycin, potentiated sulphonamides and gentamicin.
Clinical signs: As with most serotypes, Salmonella spp. Infections can cause a variety of clinical signs, and infections in cattle are most commonly subclinical. Salmonella newport has been implicated most commonly with adult dairy cow diarrhea and chronic weight loss or poor production. When clinical signs are present, the most common are fever and diarrhea, although weakness, dyspnea and sudden death would also be consistent with a diagnosis of suspected salmonellosis. Diarrhea may vary from watery to mucoid with fibrin and blood. Bacteremia may occur rapidly, especially in calves under two months of age. In adult dairy cattle, septicemia, diarrhea or abortion may occur. Outbreaks in older animals must be differentiated from bovine viral diarrhea, winter dysentery, and feed-induced indigestion.
Etiology: Salmonella is a gram-negative, facultative intra-cellular bacterium which may penetrate ocular, nasal, oral or intestinal mucous membranes. Infection is most often transmitted by fecal-oral contamination from livestock or rodents or by feeding contaminated feed. Approximately 40% of animal by-products in the U.S., such as fish meal, meat meal, bone meal, or feather meal, are contaminated. Forages or plant proteins such as soybean or cottonseed have also been sources of an outbreak.
Infection in a disease may occur as a cyclic endemic disease. Bacteria are rapidly spread among livestock and into the environment, potentially causing prolonged illness within the herd. Disease is maintained by carrier animals, infected animals, rodents, and environmental contamination, making it difficult to eliminate. It is estimated that >31% of dairy herds in Ohio have at least one infected cow, and that approximately 6% of cows are shedding Salmonella spp, of one or more serotypes, at a point in time.
The group C Salmonella sp. , particularly S. newport and S. montivideo tend to become endemic for years on a dairy farm following a prolonged course of herd illness, as was demonstrated by several dairies in California recently. Cultures taken from calves brought in for necropsy showed that the percentage of dairies in the Tulare area from which S. newport could be isolated increased from 10% in 1985-1986 to 36% in 1987-1988. During both periods, S. newport was second only to S. dublin , which was isolated from 78% of farms in 1985-1986 and 53% in 1987-1988. Thus, the incidence of Salmonella serotypes on a farm can quickly change with the introduction and persistence of new serotypes.
Diagnosis: As gross lesions may vary greatly depending on the course and extent of infection, definitive diagnosis requires culture of the organism from feces, blood, or tissues. Local and regional laboratories can often identify an isolate to its group (e.g. B, C1, C2, C3), but final confirmation of serotypes comes from the National Veterinary Services Laboratory in Ames, Iowa. Confirmation of the development of anti-Salmonella antibodies by serology is useful, but not commonly performed and is not available in most laboratories.
Control: Salmonella has traditionally been a difficult disease to treat and control, and requires an integrated herd approach. Herd serologic profiling using ELISA can be used for all serotypes except S. dublin. Suspect animals should be culled or tested with serology or by multiple fecal or milk cultures. Five samples at weekly intervals is the current recommendation. Positive animals should be culled. Sanitation and disinfection of the environment may be monitored by frequent swab cultures of the pens. Preventing the introduction of exotic serotypes by culturing feeds before use on a farm is currently the only means of protection from an outbreak.
Prompt treatment and isolation or separation of affected calves is also essential for control. Historically, Salmonella spp. were sensitive to florphenicol, 3rd generation cephalo-sporins, trimethoprim sulfa, gentamicin, amikacin, and fluoroquinolones. However, as stated previously, the presence of multiple resistant strains of S. newport makes antimicrobial sensitivity testing increasingly important.
-by Sara Clark, Class of 2004
-edited by Dr. Leon Thacker, ADDL Director
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