Salmonellosis in a herd of beef cows

Abstract

Two postparturient beef cows in a herd of 30 developed acute enteritis, with pyrexia and bloody diarrhea containing intestinal casts. Salmonella typhimurium phage type 66 was isolated from feces of both animals; both recovered after treatment with tetracycline. A third cow had died without treatment after showing similar signs.

Salmonellosis was diagnosed in a herd of beef cattle in May 2001, in Renfrew County, Ontario, where beef farms are either a secondary source of income or hobby farms. The affected herd of 30 cows was pastured during the summer; in the winter, it was fed free-choice hay in a sheltered paddock. During the calving season (April and May), the cows were housed in sheltered, bedded pens, with constant access to fresh water and feed.

The herd was first examined because of the sudden onset of bloody diarrhea in a 4-year-old cow that had just calved (Cow #1). On physical examination (day 1), the cow was lethargic and had a rectal temperature of 40°C, respiratory rate 36 breaths/min, and heart rate 80 beats/ min (bpm). The cow was not clinically dehydrated, but had pale mucous membranes. She became very anxious during rectal palpation, and feces collected consisted of intestinal casts, blood clots, and watery diarrhea. One week earlier, another cow showing similar clinical signs had died without receiving veterinary care. Differential diagnoses included acute enteritis caused by Salmonella serovars, Clostridium spp., coccidiosis, or bovine viral diarrhea virus (BVDV).

The first cow (cow #1), was isolated with her calf in a quiet area of the barn to reduce stress and possible spread of a pathogenic agent. She was treated with a single dose of oxytetracycline (Liquamycin LP; Pfizer, London, Ontario), 13.2 mg/kg bodyweight (BW), IV; a nonsteroidal anti-inflammatory agent, ketoprofen (Anafen; Merial, Baie d'Urfé Quebec), 3.0 mg/kg BW, IM, q24h for 2 d; tetracycline HCI (Tetrabol; P.V.U, Calgary, Alberta), 8 g, PO, q12h for 2 d; and antacid-astringent boluses (Astrival; Vetoquinol, Lavaltrie, Quebec), 2 boluses, PO, q12h for 2 d. Cow #1 recovered without complications within 5 d.

On day 8, a second cow (cow #2) had bloody diarrhea after calving. On physical examination, this excitable cow had a heart rate of 88 bpm, a rapid respiratory rate, and a rectal temperature of 41.5°C. Rectal palpation revealed odoriferous, bloody feces containing intestinal casts. Differential diagnoses were extended to include Johne's disease. A blood sample was submitted to the Animal Health Laboratory (University of Guelph, Guelph, Ontario) for testing for BVDV and for antibodies to Mycobacterium avium paratuberculosis, and a fecal sample was submitted for fecal flotation, culture, and sensitivity testing. Cow #2 was treated with a single dose of oxytetracycline, (Liquamycin LP), 6.6 mg/kg BW, IM; tetracycline HCI (Tetrabol), 8 g, PO, q12h for 2 d; astringent-antacid boluses (Astrival), 2 boluses PO, 2q12h for 2 d; and an intestinal adsorbent (Kaopectate Suspension, Pharmacia Animal Health, Orangeville, Ontario) for 2 d. This cow also recovered without complications within 5 d.

The fecal sample from Cow #2 was negative for parasites, and serological tests for BVDV and M. avium paratuberculosis were also negative. However, fecal culture was positive for S. typhimurium phage type 6. The isolate was resistant to spectinomycin, but sensitive to ampicillin, cephalothin, enrofloxacin, neomycin, sulbactam/ ampicillin, sulfisoxazole, tetracycline, and trimethoprim-sulfa. The owner of the cow was notified of the potential human health risks associated with Salmonella serovars, as well as the important prevention and control measures to consider when handling affected animals.

The genus Salmonella belongs to the family Enterobacteriaceae. These organisms are gram-negative, nonspore-forming, facultative anaerobes that exhibit up to 3 different types of antigens: the cell wall-associated somatic (O) antigen; the flagellar (H) antigen; and the capsular (Vi) antigen, which is not found in all salmonellae. The agglutinating properties of the antigens are used to differentiate the more than 2200 distinct Salmonella serovars (2).

Animals are infected via contaminated feeds or fomites. The oral route is the most common portal of entry. After ingestion, the organism is either destroyed by the host's immune defenses and expelled, or it establishes itself within the host, with a predilection for the mucosal lining of the ileum. Once colonization occurs, the host animal is said to be infected (2). Salmonellosis may be manifest as either a chronic or an acute disease. The chronic or enteric form occurs as a localized disease affecting the gastrointestinal tract, initiated when the organisms migrate to the lamina propria through the microvilli and between the tight junctions of the mucosal cells. The mucosal cells are damaged, and fibronecrotic plaques form on the intestinal wall. Loss of mucosal integrity results in malassimilation of digesta and loss of body fluids (2). The acute or septicemic form of salmonellosis occurs as a generalized disease affecting several body systems (1,2). Secretion of enterotoxins by salmonellae may cause diarrhea independent of the mucosal damage caused by migrating organisms. However, the infection may be disseminated systemically from the intestine when the inflammatory response, initiated by invasion into the lamina propria, results in phagocytosis of the organism by macrophages and neutrophils (2). The organisms may then be transported throughout the body, survive as facultative intracellular parasites, and eventually cause embolic lesions (2).

The signs of salmonellosis usually reflect the location of the disease. Signs of localized gastrointestinal disease include profuse, watery, fetid diarrhea. However, if endotoxins and other toxic materials have crossed the intestinal barrier, diarrhea is accompanied by fever, anorexia, depression, and shock. Bacteremia and embolism may cause dyspnea, abortion, and sudden death (1,2).

Consumption of animal products contaminated with S. typhimurium represents an important public health problem in Canada and the United States. In a recent study in Prince Edward Island, the prevalence of Salmonella serovars in slaughtered beef cattle was 4.5% (3). The isolation of S. typhimurium as the predominant serovar in that study suggests that carrier animals could be a potential source of contamination, both to the environment and to beef carcasses. According to an epidemiological study (4), salmonellosis continues to be an important cause of food-borne illness in the United States. In fact, it is estimated that of the 1.5 million reported cases of human illness in the United States due to Salmonella serovars, 95% were food borne (4). Salmonellosis also accounted for 17% of all hospitalizations due to food-borne illness, and Salmonella serovars were among the leading agents implicated in fatal food-borne illness (4). The public must be diligent in handling food products, including beef, because of potential exposure to salmonellae and other pathogenic organisms.

Development of drug resistence by salmonellae raises further public health concerns. In fact, the S. typhimurium phage type 66 isolated in this case was resistant to spectinomycin, a drug commonly used in prevention of neonatal diarrhea in piglets and fowl cholera in turkeys (5). Of particular concern, however, is the existence of S. typhimurium definitive type 104 (DT 104), which exhibits a resistance pattern to multiple antibiotics including ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline (6). This serovar is most commonly found in cattle, but it has also been isolated from sheep, pigs, goats, domestic fowl, turkeys, cats, food products, and humans (6,7). Serovar DT 104 is considered to be very dangerous because it readily accepts resistance plasmids from Escherichia coli into its own genome (7). With extended use of antibiotics, organisms with resistance plasmids persist and proliferate. Serovar DT 104 demonstrates the link between antimicrobial use in animal production and the evolution of antimicrobial resistance in bacteria that can cause serious illness (7). Veterinarians must use care when selecting antimicrobials to treat infection with Salmonella serovars and other enteric pathogens.

Prevention and control of salmonellosis is a major problem because of carrier animals and contaminated feedstuffs. The principles of control include preventing the introduction of carrier animals by maintaining a closed herd or by purchasing from a herd of a known health status. During an outbreak, such as in this case, animals should be isolated and treated (1). Furthermore, affected animals should be tested by multiple fecal cultures after the cessation of clinical signs to ensure that a carrier state has not developed (8). Contaminated buildings must be cleansed, and because of the zoonotic potential of salmonellosis, humans working with the affected animals must maintain a high level of personal hygiene (1).