DISEASES AND EPIDEMIOLOGY

Disease occurs as peracute septicemia and acute, subacute, or chronic enteritis. Acute disease caused by some serovars (such as Salmonella Typhimurium) is characterized by high morbidity, and low mortality, manifesting as depression, diarrhea, and fever. Dehydration and electrolyte imbalance may be sufficiently severe, especially when accompanied by septicemia, to be fatal in infection by other serovars (Salmonella Dublin in calves and Salmonella Choleraesuis in piglets). Young animals are often more severely affected than adults, and shipping, concurrent infections, treatment with immunosuppressive drugs, and oral antibiotics are risk factors.

Outbreaks of disease in animals and humans have been associated with Salmonella Newport since the 1970s. The organism has emerged as an important cause of diarrhea outbreaks in horses and dairy cattle, with high mortality in periparturient and neonatal animals. The serovar has consistently been ranked among the 10 salmonellae most commonly isolated from human food-borne infections, having been recovered from hamburger, chicken, roast beef, potato salad, pork, alfalfa sprouts, and seafood. Multidrug resistance is a major problem, with resistance described to chloramphenicol, sulfamethoxazole, tetracycline, strep tomycin, and most recently to cephalosporins.

Serovars commonly affecting swine are Typhimurium, Copenhagen, Derby, Newport, Agona, and Choleraesuis var. Kunzendorf. Typhimurium causes enteritis, whereas Choleraesuis is swine adapted, causing systemic disease with high mortality and low morbidity in young pigs. Asymptomatic carriage and sometimes sporadic shedding by recovered or subclinically affected animals maintains the infection in herds. A classic experimental epidemiologic study examined Salmonella shedding in “joyriding pigs”; the authors demonstrated that culture-negative pigs, loaded onto a truck and driven a few hundred miles, were then detected as shedders. It is also not uncommon for pigs to develop severe enteritis during shipping, creating challenges for shippers and packers, through contamination of equipment and pens and transmission to previously uninfected animals.

Calves 1 to 2 weeks of age usually develop enteric symptoms when infected with S. Typhimurium and septicemia with S. Dublin. Most calves recover and become carriers, but dehydration and severe pneumonia contribute to mortality; some calves experience polyarthritis, osteomyelitis, or meningoencephalitis. Salmonellosis is rarely life threatening in adult cattle, but short- or long-term recovered carriers may expose others in the herd and contaminate milk. Salmonella Typhimurium and Salmonella Dublin are the leading entries on an extensive list of infecting serovars. Salmonella Dublin and Salmonella Muenster also cause abortion in dairy cattle.

Salmonella Gallinarum and Salmonella Pullorum cause septicemic disease in turkeys (fowl typhoid) and chickens, respectively, but these birds are infected with a wide variety of serovars. Young chicks often develop fatal septicemia (Figure 16-1), and Salmonella Enteritidis phage type 4 causes a lethal infection in older chickens (Figure 16-2). Salmonella enterica ssp. arizonae is a common problem in turkeys. Even where the traditional avian pathogens S. Gallinarum and S. Pullorum have been eliminated, poultry are frequently colonized with one or more serovars, including Typhimurium, Enteritidis, Heidelberg, Infantis, Montevideo, and Anatum, any of which may be transmitted to humans. Vertical transmission, especially of S. Enteritidis, S. Typhimurium, and Salmonella Heidelberg, makes eggs a common vehicle for human infection.

FIGURE 16-1 Systemic lesions of Salmonella infection in a cockatiel.

(Courtesy Raymond E. Reed.)

FIGURE 16-2 Avian salpingitis as a result of Salmonella infection.

(Courtesy Raymond E. Reed.)

Equine salmonellosis is an uncommon but nonetheless important problem, especially as a nosocomial infection in veterinary teaching hospitals. Many strains isolated from this setting are resistant to multiple antimicrobials. The infectious dose is apparently quite small in horses that have been exposed to the stress of transportation and are affected by underlying illness. Salmonella Typhimurium is encountered most frequently, but others implicated in disease include Agona, Krefeld, Anatum, and Saint-Paul. Foals are more susceptible than adults, often developing septicemic disease.

Clinical salmonellosis is a common problem in companion animals. The likelihood that a dog will be a carrier is higher in dogs from contaminated environments (such as dogs working cattle). Frequent infection of racing greyhounds is apparently related to contaminated meat used as food.

Salmonella enterica ssp. arizonae occasionally causes fatal infections in chicks, turkey poults, humans, dogs, and cats, and is found frequently in snakes and lizards.

Abortion is a manifestation of Salmonella infection in several species. Salmonella Abortusovis is commonly associated with ovine abortion (without enteritis) and S. Dublin may also be isolated (usually from animals with enteritis). Salmonella Abortusequi infection causes equine abortion.

Salmonellae reside in the normal vertebrate gastrointestinal tract, and asymptomatic carriers among domestic and wild animals and birds introduce the infection to and maintain it in herds and flocks (Figure 16-3). Feeds of animal origin (fish meal, bone meal, and meat meal) and contaminated water are also common sources of infection. Long-term survival in manure, soil, and other aspects of the environment facilitate transmission. Turtles, lizards, and snakes, as well as sick and recovering humans may shed organisms. Other sources are whole eggs (duck eggs may have a higher prevalence of infection) and egg products, meat and meat products, contaminated water, contaminated equipment and utensils, fertilizers, and animal feeds prepared from bones, fish meal, and meat. Biosecurity is also often compromised by Salmonella-carrying rodents and wild birds.