Nervous and Locomotor Systems
Salmonella is a well-known genus of bacteria, mostly because it is a zoonosis causing food poisoning outbreaks, widely reported by the media. Among the 2,400 Salmonella serotypes, three infect pigs: Salmonella choleraesuis, Salmonella typhimurium and Salmonella derby. This chapter will focus on Salmonella choleraesuis, which typically affects growing pigs less than 5 months of age. Common in the USA in the 1990s, salmonellosis due to Salmonella choleraesuis is less common today due to effective vaccination protocols but still causes sporadic issues.
Etiology and transmission
Salmonella choleraesuis is a gram negative bacteria that colonizes the intestinal tract of pigs, more particularly the colon. The main route of transmission is fecal-oral, as the bacteria is shed rapidly upon infection and up to 5 months post-infection. Additionally, the bacteria is resistant in the environment and can survive up to 6 months in dry fecal matter. Therefore, infection through contaminated environment or fomites should be considered as a possible route of transmission. Other transmission routes include nose-to-nose and short distance aerosol. Unlike S. typhimurium, S. choleraesuis does not infect any species other than swine.
Salmonella choleraesuis causes septicemia in weaned pigs, less than 5 months of age. Occasionally other ages of pigs can be affected. Duration of the disease is highly variable. The first symptom seen in infected pigs is fever, accompanied by lethargy and anorexia. While the morbidity rate is low (<10%), cases tend to be fatal. Sudden death is therefore quite common with pigs showing signs of cyanosis on the extremities: blue ears are often seen.
In the next days following infection, yellow diarrhea and coughing may be seen. Since the bacteria becomes septicemic, it may also cross the blood brain barrier and infect the central nervous system. In this case, the pig will display signs related to meningitis but this clinical presentation is quite rare.
Just like S. typhimurium, the most common clinical signs associated with salmonellosis due to S.choleraesuis is yellowish diarrhea in growing pigs. True or False?
Cyanosis of the skin, especially on the ears, feet and abdomen, is commonly found on pigs that died unexpectedly. Upon necropsy, look for swelling of the gallbladder, lymph nodes, spleen and liver. The liver may also display small areas of necrosis, leading to varying degrees of icterus severity. In pigs that have displayed coughing, typical lesions associated with bacterial pneumonia such as consolidation of the cranioventral lobes, can be seen. If pigs have been sick for several days and shown signs of diarrhea, intestinal lesions similar to the ones seen with S.typhimurium infections are observed: pseudomembranes on the ileum and button ulcers in the colon.
Due to the septic nature of this disease, microscopic lesions can be found in many locations. On organs there will be small areas of necrosis infiltrated with white blood cells. Endothelial cells of all affected organs and reticular cells of the spleen will display hyperplasia. Intestine will display lesions typical of S. typhimurium.
What macroscopic lesion is most indicative of salmonellosis due to S.choleraesuis?
- Blue ears, feet, and abdomen
- Digested blood in the jejunum
- Polyserositis with vegetative valvular endocarditis
- Thickened ileum looking like a garden hose
Clinical signs associated with salmonellosis can be indicative: sudden death with cyanosis of extremities, and yellowish diarrhea but they are not definitive. Lesions found during the necropsy, such as cyanosis, swollen spleen and gallbladder, pseudomembranes in the ileum or button ulcers in the spiral colon should be indicative of salmonellosis. Isolation of the bacteria along with clinical signs and lesions is the only way to get a definitive diagnosis. Salmonella can easily be cultured from colon samples. Herd monitoring is difficult due to the prevalence of infection and the intermittent shedding of chronically infected pigs.
Causes of sudden death in nursery pigs should include Streptococcus suis, salt toxicity, F18 E.coli and Glaesserella parasuis; in growing pigs, intestinal torsion, ulcers or hemorrhagic bowel syndrome should be considered.
If diarrhea is present, the differential is similar to S.typhimurium: Rotavirus, coronavirus, E.coli, ileitis and swine dysentery must be considered.
Treatment, Prevention and Control
Like S.typhimurium it is almost impossible to prevent infection by S.choleraesuis. However, clinical signs severity and duration can be decreased through aggressive antimicrobial therapy early in the onset of the disease. Typically drugs such as apramycin, ceftiofur, trimethoprim-sulfonamide, or gentamicin can be used. It is also important to monitor for antimicrobial resistance, as this is common in all Salmonella strains. Modified-live vaccine are effective and have decreased the disease prevalence since the 1990s. Good management practices, such as providing a clean, dry, and well-ventilated environment can help reduce severity of symptoms.
The prevalence of Salmonella choleraesuis cases has decreased in the past 20 years due to:
- Eradication protocols in sow farms around the country
- Effective vaccination protocols protecting not against infection but against clinical signs
- Cross-protection between Salmonella typhimurium and Salmonella choleraesuis