Salmonella are Gram negative, facultatively anaerobic bacilli (family Enterobacteriaceae). Over 1800 serovars are known and are currently considered to be separate species. Standardized classification schemes for Salmonella have not been completely adopted and several synonyms may be used for the same serovar.
Non-typhoidal salmonellosis can result from infection by numerous Salmonella serovars. Common serovars in domestic animals include S. typhimurium, S. dublin and S newport in cattle; S. typhimurium, S. dublin, S. anatum and S montevideo in sheep; S typhimurium and S. choleraesuis in pigs; and S. typhimurium, S. anatum, S. newport, S. enteritidis and S. arizonae in horses. Pullorum disease (caused by S. pullorum) and fowl typhoid (caused by S. gallinarum) are found in chickens. Avian paratyphoid can be caused by a number of different species of Salmonella, including S. typhimurium, S. enteritidis and S. heidelberg. Some serovars tend to produce a particular syndrome: for example, in pigs S. choleraesuis is usually associated with septicemia and S. typhimurium with enteric salmonellosis.
Salmonella can be found worldwide, although the distribution of serovars may vary. Salmonellosis seems to be more common where livestock are intensively farmed.
Transmission usually occurs by the fecal-oral route; bacteria are shed in the feces. Animals can become infected through contaminated feed, pasture, water or close contact with an infected host. Carnivores, including humans, are also infected through meat, milk, eggs and other animal products that are not thoroughly cooked. Salmonella can be spread by fomites, rodents and wild birds, but vectors are not required. Animals may become carriers for months to years.
Some Salmonella can persist for months or years in the environment, particularly in wet, warm environments. S. typhimurium remains viable for seven months in soil, water or feces or on pasture. S. dublin can remain infective for more than a year. S. choleraesuis can survive in pig meat for up to 450 days and for several months in feces or slurry. Salmonella survives for less than one week in composted cattle manure.
Salmonella are susceptible to a variety of disinfectants including 1% sodium hypochlorite, 70% ethanol, 2% glutaraldehyde, iodine-based disinfectants, phenolic disinfectants and formaldehyde. The organism can also be inactivated by moist heat (121° C for a minimum of 15 min) or dry heat (160-170° C for at least an hour).
Symptoms of gastroenteritis usually begin 6 to 72 hours after the ingestion of bacteria.
Salmonella species can cause gastroenteritis, enteric fevers, septicemia and focal infections in humans. Host factors and the virulence of the isolate influence both the severity of the disease and the form it takes. More severe disease is seen in infants, the elderly, and individuals with debilitating illnesses.
More than 2000 serovars can cause gastroenteritis in humans. Approximately half of all cases are caused by S. enteritidis and S. typhimurium. In humans, Salmonella infections most often result in self-limiting gastroenteritis, characterized by nausea, diarrhea (sometimes bloody), vomiting, abdominal cramps and sometimes fever and chills.
Enteric fevers can be caused by S. typhi (typhoid fever) or other Salmonella species. These life-threatening illnesses are characterized by fever, anorexia, headache, myalgia and constipation, and may be preceded by gastroenteritis. Septicemia can also occur without intestinal symptoms or bacteria in the feces. Occasionally, localized infections such as septic arthritis develop. Asymptomatic carriers also occur.
Yes. Salmonella are shed through the course of infection, for several days to several weeks. Humans can become temporary carriers for several months after recovery; 1% of adults and 5% of children shed bacteria for more than a year. Antibiotics can prolong shedding.
Salmonellosis is diagnosed by isolating the causative organism from feces, blood or other specimens. Salmonella species are identified by culture and biochemical testing. Under the microscope, they are Gram negative short rods. Colonies on nutrient agar are grayish, moist, translucent to opaque, and smooth. Broth cultures are turbid and may contain a pellicle or sediment. Selective media are available. Identification can be confirmed by serologic analysis of O and H antigens.
Treatment and Vaccination
Salmonella is susceptible to a variety of antibiotics; resistance is sometimes seen. Antibiotics are given for enteric fevers, septicemia and focal infections. They are not generally recommended for uncomplicated gastroenteritis as they prolong shedding of bacteria without shortening the illness. Supportive therapy, including fluid replacement, is sometimes necessary. Vaccines are not available for non-typhoidal salmonellosis.
Morbidity and Mortality
An estimated 1.4-3 million cases and more than 500 deaths occur yearly in the United States. Salmonellosis is particularly common in infants and young children. Large outbreaks are sometimes seen in restaurants as well as hospitals, nursing homes and other institutions. Most cases of gastroenteritis in healthy adults are self-limiting and resolve without complications; infections may be more serious in young children, the elderly and those with debilitating illnesses.
Clinical salmonellosis can occur in all species of domestic animals. It is most common in cattle, sheep, pigs and horses, with infrequent disease in dogs and cats. Reservoirs include poultry, pigs, cattle, sheep, rodents, horses, tortoises, turtles, cats and dogs. Some serovars are associated with specific animal reservoirs, including poultry with S. enteritidis and pigs with S. choleraesuis. Other serovars infect a wide variety of animals as well as humans. Carriers can occur in all species.
The incubation period for Salmonella gastroenteritis varies with the dose of bacteria and the form of the disease. In horses, severe infections can develop acutely, with diarrhea appearing after 6 to 24 hours. Similarly, humans develop symptoms of gastroenteritis 6 to 72 hours after ingesting bacteria. Long incubation periods are also seen; animals with asymptomatic infections can develop overt disease when stressed.
Infections in Large Animals
Infections in healthy animals may be asymptomatic. Symptomatic infections are often precipitated by stressors such as transport, drought, malnutrition or food deprivation, crowding and some drugs. Clinical disease is common in horses after major surgery. Symptomatic infections may result in several syndromes: acute septicemia, acute enteritis, subacute or chronic salmonellosis, and abortion.
Acute septicemia is usually seen in newborn calves, lambs and foals. It also occurs in pigs up to 6 months of age. Typically, a high fever and severe depression develop acutely, often followed by death within 24 to 48 hours. Pigs and calves may have pneumonia or neurologic signs including nystagmus and incoordination. Pigs often develop a dark red or purple skin discoloration, most often on the ears and abdomen. Animals can be found dead without signs of diarrhea.
Acute enteritis is the most common form in adult animals and older calves. Symptoms may include fever, diarrhea, dehydration, tenesmus, abdominal pain, a drop in milk production and sometimes dysentery. The fever may disappear before diarrhea appears. The feces are foul smelling and contain mucus, shreds of mucus membrane or casts of intestinal mucosa, and sometimes large blood clots. Horses often have abdominal pain and severe dehydration and may die within 24 to 48 hours. Sheep sometimes develop “snoring” respiratory sounds associated with regurgitation of the ruminal contents. Surviving animals may become emaciated and remain unthrifty. Calves can develop other complications including joint infections, pneumonia, and gangrene at the tips of the ears and tail or below the fetlock.
Subacute enteritis occurs in adult sheep, horses and cattle. Symptoms may include a mild fever, inappetence, soft feces and dehydration. In cattle, fever and abortion may be seen, followed by diarrhea several days later. Chronic enteritis is mainly seen in older calves, adult cattle and growing pigs. Infected animals have persistent diarrhea, with progressive emaciation, a low-grade intermittent fever and anorexia. The feces are typically scant and may be normal or contain mucus, casts or spots of blood. Rectal strictures sometimes develop in growing pigs. This form of salmonellosis may follow an episode of acute enteritis.
Abortions can occur after acute or chronic enteritis, or without other clinical signs. S. dublin is often associated with abortions in cattle and Salmonella abortus ovis with abortions in sheep. These two infections can occur without enteritis. Abortions in pregnant ewes may be followed by a fetid, dark red vaginal discharge and sometimes death.
Infections in Small Animals and Birds
Salmonellosis is relatively rare in dogs and cats. In these species, acute diarrhea is typical, either with or without septicemia. Pneumonia or abortion may be seen and cats sometimes develop conjunctivitis. Rats and mice may also develop enteritis or septicemia. The clinical signs in rodents include anorexia, weight loss, conjunctivitis and a rough coat, with sporadic deaths. Avian paratyphoid mainly occurs in hatchling chickens, turkeys and other birds, and is rare in older birds. Symptoms may include somnolence, anorexia, watery diarrhea and increased thirst.
Yes. Organisms are shed in the feces and animals may become carriers for months to years.
Enteric infections are diagnosed by clinical signs and the isolation of Salmonella from the feces. Isolation alone is unreliable; Salmonella can be found in the feces of healthy animals and in animals ill from other causes. Blood may be cultured from animals with septicemia, egg contents from poultry, and fetal stomach contents, fresh placenta and vaginal swabs after abortions. Heart blood, bile, liver, spleen and mesenteric lymph nodes are usually sampled at necropsy. Environmental samples, including feed, water, and feces from wild rodents and birds, may be helpful.
Salmonella species are identified by culture and biochemical testing. Under the microscope, they are Gram negative short rods. Colonies on nutrient agar are grayish, moist, translucent to opaque, and smooth. Broth cultures are turbid and may contain a pellicle or sediment. Selective media are available. Identification can be confirmed by serologic analysis of O and H antigens.
Serology on acutely ill animals is rarely diagnostic; agglutinins do not appear until 2 weeks after an infection. However, herd sampling may be helpful. Serologic tests include the whole blood test for the rapid diagnosis of S. pullorum and S. gallinarum in poultry and the tube agglutination test for all species of farm animals. Enzyme-linked immunosorbent assays (ELISAs) are also available for some serovars.
Treatment and Vaccination
Septicemia can be treated with a variety of antibiotics; treatment of gastroenteritis is controversial as these drugs may prolong fecal shedding and alter the intestinal flora. Fluid therapy and other supportive care may be indicated. Commercial killed vaccines or autogenous bacterins are sometimes used in outbreaks, particularly when pregnant cattle are involved.
Morbidity and Mortality
Acute septicemia in very young animals can result in morbidity and mortality rates up to 100%. Mortality in horses may also be very high. Enteric infections are often self-limiting, although animals can become chronically infected or remain unthrifty.
Post-mortem lesions usually include signs of necrotizing fibrinous enteritis or septicemia. In animals with enteritis, the intestine contains mucosal erosions; these lesions are most apparent in the lower ileum and colon. Hemorrhages and fibrin strands are common and the lumen may contain blood. Similar lesions may be seen in the abomasum. Extensive diphtheritic membranes are sometimes found in the intestines and inflammation may be noted in the wall of the gall bladder. The mesenteric lymph nodes are usually edematous and hemorrhagic. Other lesions may include fatty degeneration in the liver, bloodstained fluid in the serous cavities, and petechial hemorrhages under the epicardium or other serous membranes. In animals with acute septicemia, there are usually extensive submucous and subserous petechial hemorrhages.
In cattle with chronic salmonellosis, discrete areas of necrosis are usually found in the cecal and colonic mucosa. The lesions are characterized by necrotic material over a red granular surface, in a thickened intestinal wall.
“Bacterial infections caused by Gram-negative bacilli. Enterobacteriaceae.” In The Merck Manual, 17th ed. Edited by M.H. Beers and R. Berkow. Whitehouse Station, NJ: Merck and Co., 1999. 8 Nov 2002 <http://www.merck.com/pubs/mmanual/section13/chapter157/157d.htm>.
Euzéby, J.P. “List of bacterial names with standing in nomenclature. Salmonella nomenclature.” July 2000. 24 October 2002 <http://www.bacterio.cict.fr/Salmonellanom.html>.
Giannella, R.A. “Salmonella.” In Medical Microbiology. 4th ed. Edited by Samuel Baron. New York; Churchill Livingstone, 1996. 23 Oct 2002 <http://www.gsbs.utmb.edu/microbook/ch021.htm>.
“Material Safety Data Sheet – Salmonella choleraesuis.” Canadian Laboratory Centre for Disease Control, May 2001. 15 October 2002
“Material Safety Data Sheet – Salmonella spp. (excluding S. typhi, S. choleraesuis, and S. paratyphi).” Canadian Laboratory Centre for Disease Control, March 2001. 15 October 2002
“Salmonella.” Animal Health Australia. The National Animal Health Information System (NAHIS), 1996. 18 Oct 2002
“Salmonellosis.” Centers for Disease Control (CDC), Dec 2001. 24 Oct 2002 <http://www.cdc.gov/ncidod/dbmd/diseaseinfo/salmonellosis_t.htm>.
“Salmonella.” In Schnierson’s Atlas of Diagnostic Microbiology, 9th ed. Abbott Park, IL: Abbott Laboratories, 1984, pp. 24-5.
“Salmonellosis.” In The Merck Veterinary Manual, 8th ed. Edited by S.E. Aiello and A. Mays. Whitehouse Station, NJ: Merck and Co., 1998, pp. 120-3;241-3;251-2;1321;1947-9.
“Salmonellosis” In Manual of Standards for Diagnostic Tests and Vaccines. Paris: Office International des Epizooties, 2000. 25 Oct 2002 <http://www.oie.int/eng/normes/mmanual/A_00114.htm>.
“Salmonellosis (Avian).”Animal Health Australia. The National Animal Health Information System (NAHIS). 18 Oct 2002
“Salmonellosis (Pigs).” Animal Health Australia. The National Animal Health Information System (NAHIS). 18 Oct 2002
Copyright 2003, ISU
Center for Food Security and Public Health
Iowa State University College of Veterinary Medicine
Ames Iowa USA 50011
Phone: 515 294 7189
Fax: 515 294 8259