Salmonellosis remains a primary disease in weaned pigs.
One hundred and twenty years ago, a disease of swine that was described as hog cholera was thought to be due to Salmonella cholerasuis. Later on, it was learned that a virus caused hog cholera; salmonella was often considered a secondary invader of pigs infected with this virus.
But when hog cholera was eradicated, salmonella infections persisted.
Today the bacteria continue to cause the swine industry health concerns, both as a primary infection of pigs and as a cause of food-borne illness in humans.
There are over 2,000 distinct serotypes of salmonella, but only two are common in swine: Salmonella choler — asuis and Salmonella typhimurium.
S. cholerasuis can result in generalized infection, pneumonia, enterocolitis and meningitis. This serotype rarely causes human infections.
S. typhimurium, on the other hand, can result in human illness and in pigs causes an enterocolitis, observed clinically as watery diarrhea.
Outbreaks of salmonellosis are often associated with other stressors such as transport, mixing, temperature extremes and infection with porcine reproductive and respiratory syndrome (PRRS) virus.
Case Study No. 1
A 500-sow, farrow-to-finish producer in central Indiana decided to convert his facilities to nursery-to-finish production. The existing herd was infected with Actinobacillus pleuropneumonia Type 7, Mycoplasmal pneumonia and PRRS.
Farrowing rooms were converted to nurseries. Existing nursery rooms can hold 600 weaned pigs every three weeks. Breeding-gestation was converted to finishing. Nursery and finisher rooms are on the same site, connected by hallways.
The source of new weaner pigs was a single sow herd that was PRRS negative. Against veterinary advice, some old stock finishers remained on site when the first weaned pigs were purchased and introduced to the cleaned and disinfected nurseries.
Within six months after the change in production flow, death loss increased in the 11- to-12-week-old pigs. There were thumping pigs, poor-doing pigs, and some pigs with cyanosis (purple discoloration) of the extremities. Tissue samples sent to the diagnostic lab tested positive for Salmonella cholerasuis.
Avirulent vaccines can be used in the water to immunize pigs against salmonella. Make sure, however, that feed-grade or water-based antibiotics (that would impact the avirulent salmonella organisms) are not given to the pigs for 3-4 days prior to and after vaccine administration.
When nursery groups were vaccinated, salmonella problems disappeared and vaccination was discontinued. PRRS seroconversion was later demonstrated in the herd, which may have exacerbated salmonella problems.
Case Study No. 2
A 1,200-sow, farrow-to-finish herd was experiencing PRRS-circovirus mortality in early finishing. Because the sow herd was PRRS-stable, it was decided to empty nursery and finishing sites to reduce the interaction of these two organisms.
Some 1,500 nursery pigs, 8-9 weeks of age, were moved from two nursery sites to an empty, off-site contract finisher barn over a three-week period. Three weeks after the first pigs were moved, the fieldman reported that mortality increased significantly.
Numerous pigs in the first two of three rooms exhibited lethargy, weight loss, thumping, pallor, diarrhea and generalized to localized cyanosis. There was excess feed in the bottom of the feeders due to poor feed consumption. Mortality was already at 6%. Pigs were on tilmicosin in the feed and had not responded to injections of ceftiofur hydrochloride.
Postmortems showed pneumonia, enlarged lymph nodes and pale to jaundiced subcutaneous tissues. Salmonella was isolated from the liver and lung of three pigs. Microscopic lesions in liver, lung and kidney were due to S. cholerasuis infection.
Immunohistochemistry was positive for porcine circovirus-associated disease (PCVAD) with mild lesions of this virus; primary lesions were due to salmonella.
Feed-grade medication was changed to carbadox and water medication was initiated, but mortality continued. This group was not closed out at the time this summary was written.
Since previous groups through this barn and in the farrow-to-finish site did not have problems with salmonella, it is assumed that low-level carriers are present. Perhaps moving pigs, the presence of PCVAD and PRRS virus aided the development of salmonellosis.
With concerns about PCVAD, and the possibility that salmonellosis could be confused with PCVAD, this case demonstrates the importance of early and accurate diagnosis of finisher problems.