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Salmonellosis

(452) There are many serotypes of salmonella but the ones that are most likely to cause clinical disease in pigs are Salmonella choleraesuis, and Salmonella typhimurium and to a lesser extent Salmonella derby. Other "exotic" salmonella serotypes may infect pigs and be shed in the faeces for limited periods but they usually remain sub-clinical. S. choleraesuis is the specific host-adapted pig serovar and can cause major generalised disease. S. typhimurium and S. derby are more likely to cause a milder disease the main sign of which is usually diarrhoea. Pigs may become long-term sub-clinical carriers of .S choleraesuis the organism surviving in the mesenteric lymph nodes draining the intestine. Many such carriers do not shed the bacteria in faeces unless they are stressed. Some however, may become sub-clinical carriers of S typhimurium, S. derby and other serotypes. They may be intermittent or continuous faecal shedders but the carrier state is usually relatively short, weeks or a few months and it is self limiting.

If a pig is infected with a large dose of S. choleraesuis it is likely to develop severe generalised clinical disease starting with a septicaemia (blood infection) followed by severe pneumonia and enteritis. Subsequently the organism may settle out in a variety of tissues including the central nervous system, resulting in meningitis, and the joints resulting in arthritis. S. typhimurium and S. derby may also cause septicaemia, become generalised and involved in pneumonia but in most pigs these are transient and enteritis is the only persistent manifestation.

Salmonellosis can occur at any age but is most common in growing pigs over eight weeks of age. Severe S. choleraesuis infection occurs typically at around 12 to 14 weeks.

Clinical signs

Acute septicaemia and pneumonia result in fever, inappetence, respiratory distress and depression. The skin of the extremities (i.e. tail, ears, nose and feet) becomes blue. On the ears, in S. choleraesuis infection, there is often a clear line of demarcation between the blue and normal skin. Foul-smelling diarrhoea which may be blood stained, is a common feature. Jaundice (yellowing) may result from liver damage and lameness from arthritis. Meningitis results in nervous signs. If untreated, mortality may be high.

Diagnosis

It is necessary to submit to the laboratory either fresh faecal samples from untreated pigs, or where available a dead or live untreated pig. This is essential to demonstrate the presence of the organism and differentiate it from other causes.

The post-mortem lesions are strongly suggestive of S. choleraesuis, particularly the generalised pneumonia, the appearance of the lining of the small and large intestine, the congested spleen and multiple small haemorrhages.

Similar diseases

Severe salmonellosis caused by S. choleraesuis can occur alone but it also commonly occurs at the same time as classical swine fever (hog cholera) in those countries in which this disease still occurs. In such countries it is important to ensure by serology and laboratory tests that swine fever is not the primary cause (NB. swine fever usually also affects sows and sucking piglets and also causes mummified litters and abortions).

Severe PRRS in herds with endemic EP may give the appearance of salmonellosis, however PRRS also causes abortions, stillbirths and precipitates scouring in piglets.

Treatment

  • The response to the treatment of salmonella infections is often poor. It is necessary to determine the serotype, the antibiotic sensitivity and treat individual pigs at a very early stage.
  • Preventive medication or strategic medication both in-feed or in the water are important and the medicines used are dependant on the bacterial sensitivity.
Management control and prevention It is most important point to realise that the severity of clinical salmonellosis is dose dependant. This is true to a greater or lesser extent of all infectious diseases but is particularly so of salmonellosis. The overall aim, therefore, is to get the levels in the environment down to below the disease-producing threshold. The second aim is to reduce the spread of infection.
  • Improve hygiene by frequent and thorough removal of waste.
  • Optimise stocking density. Overcrowding predisposes.
  • Reduce stress by reducing moving and mixing.
  • Develop an all-in all-out system with cleaning and disinfection between batches.
  • Avoid the movement of pig faeces between one batch and another.
  • Vaccines against S. choleraesuis are available in some countries and can be used to help in elimination programmes if this type is present. The principle is to vaccinate the sow to produce high maternal antibody and then carry out segregated early weaning or segregated disease control programmes.
  • Disinfect boots between houses.
  • Control vermin and flies.
  • Prevent contamination of feed by birds, rats and mice.
  • Monitor raw feed ingredients and final product.
  • Cleanliness and hygiene are important.
  • Organic acids added to feed also help.
  • In some countries formalin is added instead.
Denmark, Sweden and Eire have national control policies and other countries are considering developing them. The Swedes monitor herds mainly by culturing faeces. They try to eliminate salmonella from all herds. The Danes were the first to use an Elisa to test blood or meat juice for antibodies to the commonest salmonella serotypes in pigs. On the basis of the results they calculate a score for each herd. Herds with high scores are penalised and made to adopt control measures until their scores come down to an acceptable level.

Zoonoses

It is important to remember that salmonella organisms are transmissible to people and are one of the commonest causes of food poisoning. This has two implications for you as a pig farmer.

First, you should ensure that everybody working with the pigs adopts a high standard of personal hygiene so that they do not become infected. Second, it is important that pig farmers and pig meat products have a high public image for safety and do not get linked with outbreaks of human disease. It is therefore imperative that you get salmonella under strict control by the particular use of all-in all-out procedures.

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