Salmonellosis - Unravelling Some of the Mysteries
David G S Burch  BVetMed DECPHM MRCVS
Veterinarian, Octagon Services Ltd

(Published in Pig World, July 2007)

Following the meeting 'Serious about Salmonella' and other interviews in Pig World about the vagaries of the ZAP scoring scheme, I thought it opportune to comment on some of the findings and try to put them into context.

The ZAP score in England has not declined over the last 4 years and still remains around 30% positive. It is expected, that if we continue to use this scoring system then the EU will put a requirement to reduce the score next April 2008 and bring us eventually into line with the rest of Europe. Producers have seen large variations in their scores, almost batch by batch, but some are high at over 80% casting them into the Grade 3 category. This long term could have disastrous consequences on their future business survival, if the future EU restrictions are severe.

What is happening when the ZAP scores soar to 80%? Salmonella enterica Typhimurium accounts for 67% of isolations from pigs and is considered highly invasive after colonisation in the gut. As well as being found in the gut contents it can also be found in the lymph nodes and occasionally it will cause a septicaemia and death. Using a Danish model for isolation and sero-conversion to S. Typhimurium, it can be seen (see Graph 1) that when a severe challenge occurs in the finishing shed (dotted lines) then there is a strong likelihood that the seroconversion will be almost 100% at slaughter.

Graph 1. S. Typhimurium isolation and seroconversion model (DK)

S. Typhimurium isolation and seroconversion model - Denmark
(Source: based on Kranker et al 2002)

If there is a moderate challenge in weaner pigs with S. Typhimurium, by the time they get to slaughter the antibody response has dropped to about 20%. The meat-juice ELISA test used for ZAP monitoring is based on antibodies found in meat juice. Interestingly, the faecal culture test is less sensitive than the serology test, so fewer pigs are shown to be positive, but in fact the serology test would require an infection to cause seroconversion. In the UK, we have a higher incidence of solid floor systems and this predisposes pigs to a higher risk of challenge if Salmonella are brought into the finishing house. We often bring in pigs from a number of nurseries and the chances are they will be of mixed Salmonella status. Those that have had an earlier infection as weaners, may still be shedding the organism and challenge possibly na´ve pigs giving them a high level of exposure in the finishing shed and as a result, a high serological response. Many farms do not clean out between batches of pigs, or do not have time to as the next batch is waiting to come in as the others leave.

In a good review paper on 'Risk factors associated with Salmonella prevalence on swine farms', Funk and Gebreyes (2004) examined some of the factors involved: -

Human vectors - Good biosecurity practices for keeping unnecessary people out and giving personnel access to changing and toilet/hand washing facilities before they come in was helpful.

Flooring types - slatted floors reduced infection, solid floors increased infection.

Housing contamination - Salmonella can survive for 6 years in the environment. Cleaning and disinfection were not always effective. Reliance on disinfectants and ineffective cleaning may be the reason. Recent Scottish information showed that most disinfectants were poorly effective against S. Typhimurium, therefore improved cleaning and possible use of detergents to remove organic matter is more important.

Pig flow management - Just all-in/all-out management was not sufficient but there is a need to supply changes of clothes, washing facilities and equipment between houses. Mixed source batches are likely to increase the chances of infection.

Sow to pig transmission - More research is required here. The VLA is carrying out work in this area and the results may be available in the next 6 months. Often serotypes found in sows are not the same as in the piglets, which possibly reflects the importance of housing contamination subsequently as the pigs go through the system.

Transmission from other animals - Rodents, cats, dogs, wild birds and wildlife including foxes have all been incriminated. Flies and beetles may also be physical carriers.

Feed - Feed seems to be considered low risk. In the UK it is often the more exotic salmonella that are recovered as contaminants than S. Typhimurium, although it can occur. Feed may also be a Salmonella reducer, especially if it is coarse ground, contains acids or acid-based growth promoters, whey based and even fermented. It is thought that the acidification enhances the killing of Salmonella in the stomach but it is not absolute. It is also preventative not curative, so when they are taken out, breakdowns will occur.

Stocking densities - there is a trend for higher salmonella levels with higher stocking densities probably due to higher pig to pig and faecal contact and higher stress.

Herd health status - High health status appears to be related to lower salmonella infection in herds. This may be possibly due to the whole context of keeping the herd clean and biosecure and keeping such diseases as PRRS and PMWS (Circovirus) out.

They did not review the different products, such as vaccines, antimicrobials and competitive exclusion products that have been used to reduce Salmonella infections. Ongoing investment and research on behalf of BPEX is looking at these. They did not examine the exposure of pigs either during transport or at the lairage or during the slaughter process. From a hazard analysis and critical control point (HACCP) system, processing would appear to offer a quick control of the potential transmission of salmonella to man via pork. The use of hot water sprays to reduce contamination have been shown to be highly effective in Denmark and are under consideration in the UK by the FSA, but who is going to pay for this and how, has not been determined.

BPEX will be under pressure to get a reduction in the ZAP score in England and there will be a tightening up of the scheme shortly. Whether they will go for the Scottish approach where farms that are above the average ZAP score are encouraged to be inspected to help reduce their score, I am not sure. The Pig Veterinary Society is pleased to help producers but at the end of the day it will be the producers that have to implement any new procedures



Funk, J. and Gebreyes, W.A. (2004) Risk factors associated with Salmonella prevalence on swine farms. Journal of Swine Health and Production 12, 5, 246-251.

Kranker, S, Alban, L., Boes, J. And Dahl, J. (2002) Longitudinal investigation of Salmonella Typhimurium in integrated swine herds. Proceedings of the 17th IPVS Congress Ames, Iowa, USA, Vol 1, p 317.


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*Pig Diseases & Medication:  Pig Technical Reviews
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