June 14, 2021
A veterinarian investigated a case of acute respiratory distress and unexpected sudden death in a farm with 2,000 45-pound pigs. The farm was losing two to seven pigs daily, with some showing respiratory distress immediately prior to death. Other unaffected pigs in the same pen had a normal appearance. The pigs had been regularly vaccinated for PRRSV and medicated for Coccidiosis prevention. A diet change was implemented about 10 days prior.
On necropsy, the lungs were heavy and wet with foam in the trachea, suggesting pulmonary edema. The muscles were pale, and the stomachs were mostly empty. Tissues were collected and submitted to our diagnostic laboratory for further investigation. Regular multiplex real-time PCR for respiratory pathogens ruled out PRRSV, swine influenza, circovirus, and Mycoplasma hyopneumoniae. The microscopic examination result was not specific. There was a subjectively increased number of neutrophils in the pulmonary septa (suggesting septic pneumonia), in the hepatic sinusoids, and the renal glomerular capillaries. The pathologist reading the slides was considering the septicemia, but regular cultures were overgrown by contaminants. Because septic pneumonia was suspected, PCR testing for Salmonella was requested and was positive. Subsequently, bacterial culture with enrichment was performed, with Salmonella isolated. Antimicrobial resistance analysis demonstrated resistance to ampicillin, streptomycin, sulfonamide, and tetracycline (ASSuT resistance). The isolate was serotyped as Salmonella enterica subsp. enterica serotype I 4,,12:i:-.
Salmonella enterica ser. 4,,12:i:− is genetically and antigenically similar to S. enterica ser. Typhimurium. Both serotypes are likely indistinguishable clinically. In a current study, S. enterica ser. 4,,12:i:- was strongly correlated with necrotizing enterocolitis in pigs . Nowadays, S. enterica ser 4,,12:i:- is much more frequently isolated from pig samples submitted to our and others’ diagnostic laboratories [1, 2]. In addition, S. enterica ser. 4,5,12:i:− was one of the most common serotypes causing gastroenteritis in humans, with contaminated pork products deemed to be the main source of human infection . In this case series, S. enterica ser. 4,,12:i:- caused septic pneumonia without obvious diarrhea. Although the histopathology examination of the intestine was obscured by severe autolysis, we believe the infection started from the intestine, and the bacteria and bacterial endotoxin subsequently disseminated systemically through the bloodstream.
What do we learn from this case series?
1. Diagnostic challenges:
Indeed, Salmonellosis is a well-known disease in pigs. However, the diagnosis sometimes can be challenging because the clinical and pathologic expression varies from localized enterocolitis to septicemia. In this case, many infectious diseases, as well as environmental, physical, and nutritional issues were initially considered as the cause of unexpected sudden death. The clinical expression of septic pneumonia, when presented, may be confused with respiratory syndromes caused by other typical respiratory viral and bacterial pathogens. Based on our experience, Salmonellosis is frequently dropped off from the list of differential diagnoses of pneumonia. It is not uncommon that only a piece of lung, but not intestine and other visceral organs, is submitted to the laboratory for investigation. In cases with septicemia and potentially some fever, decay of the tissues after death is very rapid. The clinical usage of antibiotics and overgrowth of postmortem bacterial contamination are possible reasons for failing to isolate Salmonella. Therefore, timely necropsy to obtain fresh diagnostic samples and comprehensive sample collection are important.
2. Pigs with Salmonella can get sick and die
In general, the outcome of Salmonella infection in pigs is highly dependent on the infecting serovar and age of the pigs. As an emerging serotype, much of the pathology of S. enterica ser. 4,,12:i:- is still unknown at this stage, with research ongoing. Similar to infections with S. enterica ser. Typhimurium, fever and diarrhea are the typical clinical signs. Environmental stress such as pen movements, and immunocompromise are predisposing factors. Events that disrupt the balance of gut microbes (dysbiosis), such as changing diets and improper usage of antibiotics, are important factors to trigger these diseases. Gut health in pigs provides more benefits than might be expected. In this case, S. enterica ser. 4,,12:i:- was deemed to be a main cause of illness. On the other hand, S. enterica ser 4,,12:i:- and other serotypes can play as one of the multiple pathogens in pigs. Pigs with concurrent debilitating diseases, such as E. coli and rotaviral enteritis or porcine respiratory disease complex, always have higher illness and death rates. Without early and proper intervention, the situation can progress into a disaster on the farm. Once again, good hygiene and biosecurity are important to prevent and control Salmonellosis and other infectious diseases. Proper use of antibiotics and routine vaccines are also important.
3. Risk for our pork consumers
Another important impact of Salmonella infections is their zoonotic potential. Pigs can be infected with a wide range of Salmonella serotypes and many of them, including S. enterica ser. 4,,12:i:-, are also isolated from human patients with gastroenteritis. Many of them isolated in our laboratory show resistance to a broad range of antibiotics. Remember pigs can serve as an asymptomatic Salmonella carrier or develop only transient, sporadic diseases, which may be neglected. However, they often shed the bacteria in their feces for a long period. Therefore, Salmonella can potentially contaminate pork products and pose a threat to human health, particularly these strains with multiple antimicrobial resistance. Awareness and surveillance of Salmonella in pig farms is critical to public health.
Many veterinary diagnostic laboratories have bacteriology and food safety sections providing regular tests for Salmonella and other pathogens at affordable costs. For more information on this case, contact Dr. Chun-Ming Lin, pathologist/assistant professor at the Animal Disease Research and Diagnostic Laboratory at South Dakota State University.
Sources: Chun-Ming Lin, South Dakota State University, who are solely responsible for the information provided, and wholly own the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
1. Arruda BL., et al. Salmonella enterica I 4,,12:i:- Associated with Lesions Typical of Swine Enteric Salmonellosis. Emerging Infectious Diseases. 2019;25(7):1377-1379.
2. Naberhaus SA., et al. Emergence of Salmonella enterica Serovar 4,,12:i:- as the Primary Serovar Identified from Swine Clinical Samples and Development of a Multiplex Real-Time PCR for Improved Salmonella Serovar-level Identification. Journal of Veterinary Diagnostic Investigation. 2019;31(6): 818–827.
3. Soyer Y. et al., Salmonella enterica Serotype 4,5,12:i:−, an Emerging Salmonella Serotype That Represents Multiple Distinct Clones. Journal of Clinical Microbiology. 2009;47(11): 3546–3556.
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