Outbreak of salmonellosis in pigs with postweaning multisystemic wasting syndrome

POSTWEANING multisystemic wasting syndrome (PMWS) is an infectious viral disease cause by porcine circovirus type 2 (PCV-2) (Allan and Ellis 2000). It is characterised by progressive weight loss, respiratory signs and jaundice (Clark 1997, Chae 2004). The disease occurs in herds that are usually otherwise in good health. PMWS is endemic in many pigproducing countries and continues to be a major cause of wasting disease in pigs. In the Republic of Korea, PMWS is commonly associated with respiratory diseases (Kim and others 2003). However, secondary infections with bacterial pathogens, particularly Salmonella Typhimurium and Salmonella Choleraesuis, typically occur concurrently with PMWS in many herds. Most herds undergo severe devastating diarrhoea. This short communication describes an outbreak of salmonellosis in pigs in herds with PMWS in the Republic of Korea. Between January 2003 and April 2004, there were 37 cases of severe diarrhoea and retardation of growth from 24 pig farms. Twenty-two cases were from 2003 and 15 cases occurred in 2004. All 37 cases were weaned pigs, aged from 28 to 63 days. Morbidity was 10 to 20 per cent but mortality was 70 to 80 per cent in all of the herds affected. The diarrhoea was initially yellowish in colour but progressed to black; antibiotic therapy was ineffective in treatment of the diarrhoea in all cases. Postmortem examinations were performed on the 37 pigs and samples of lung, liver, kidney, small and large intestines, spleen, tonsil, and inguinal lymph nodes were collected in neutral buffered formalin. Small and large intestines from all of the pigs were examined for viral pathogens such as PCV-2 and classical swine fever (CSF) virus, and for bacterial pathogens such as Salmonella species, Brachyspira pilosicoli and Brachyspira hyodysenteriae. Homogenates of the small and large intestines were inoculated into cultures of PCV-free porcine kidney 15 cells, MARC-145 cells, swine testicular cells, primary porcine fallopian tube cells and Vero cells. In situ hybridisation and immunohistochemistry were performed for PCV-2 and CSF virus, respectively, as described by Kim and Chae (2001) and Choi and Chae (2003). Positive and negative control tissues of each virus were included for each in situ hybridisation and immunohistochemistry procedure (Kim and Chae 2001, Choi and Chae 2003). PCR was carried out as previously described for Lawsonia intracellularis, B pilosicoli, B hyodysenteriae and salmonellae in porcine intestinal specimens (Elder and others 1997, Kim and others 1998, Choi and others 2002). The most consistent and predominant histopathological feature seen was lymphoid depletion in the lymphoid follicle and the paracortical zone of the lymph nodes, with the lymphoid follicles and the paracortical zone being replaced by fibrovascular stroma and occasionally macrophages. Multinucleated giant cells were scattered between stromal components, and grape-like clusters of intensely basophilic intracytoplasmic inclusion bodies appeared in these areas (Fig 1). Depletion of the medullary cords and empty medullary sinuses were also observed. A strong hybridisation signal for PCV-2 was detected in the cytoplasm of macrophages in lymph nodes, spleen and Peyer’s patches (Fig 2) in all 37 pigs tested. In the colon, necrosis frequently extended to involve the lamina propria (Fig 3), submucosa, muscularis mucosa and lymphoid follicles. Necrotic tissues were separated from the healthy mucosa by a marginal zone of leucocytes, and the deeper tissues were heavily infiltrated by neutrophils. The lamina propria and submucosa contained numerous neutrophils and macrophages. Salmonellae were detected in the small and large intestines from all 37 cases by PCR. Three pigs were also positive for L intracellularis and two pigs were also positive for B pilosicoli. B hyodysenteriae was not detected in the small and large intestines by PCR. S Typhimurium was isolated from 23 pigs and S Choleraesuis was isolated from two pigs. No other bacterium was isolated from the small and large intestines. Cytopathic viruses were not detected after five passages.