Salmonellosis

Occurrence: Worldwide.
Age affected: All ages (especially growers / finishers), human risk.
Causes: Bacteria of Salmonella family; stress, poor hygiene.
Effects: Death, fever, diarrhoea, nervous signs.

Causes

Salmonellosis is caused by serovars of Salmonella enterica, a bacterium which grows readily in culture. Antigenic factors present on their cell walls (‘O’ antigens) and on their flagellae (‘H’ antigens) are used to name serovars which can be subdivided by phage typing, plasmid profiling, restriction endonuclease analysis, resistance pattern and whole or partial genome sequencing and allow the origin of infections to be traced. S. enterica serovars Typhimurium, Derby, Saintpaul, Infantis, Heidelberg, Typhisuis and Choleraesuis may all occur in pigs. A common serovar is Typhimurium of which Definitive (phage) Type (DT) 104 and the recent monophasic variant 4.5.12i are multi-antibiotic resistant.

Salmonella infection is oronasal and results in three types of infection:

  1. septicaemia which may precede or follow enteritis;
  2. enteritis with some degree of local invasion; and
  3. colonisation of the intestine or its contents alone.

All may be followed by the persistence of small numbers of organisms in the gut mucosa, mesenteric lymph nodes or tonsils in clinically-normal carrier animals. Choleraesuis is a pig pathogen, infecting pig herds for long periods and causes septicaemia, while Typhimurium causes enteritis with local invasion but can cause septicaemia. Infection is followed by the development of systemic antibodies from day 7 post-infection, peaking at 28 days post-infection and declining but still demonstrable 80 days post-infection.

Mode of transmission

Salmonellas are shed in the faeces of infected pigs and infection  results from direct exposure to faeces from infected pigs or consumption of food or water contaminated with the organism. The organism survives drying for months and some infections could arise from dust containing dried organisms. Protein sources in feed may also be contaminated and unusual serotypes of S. enterica may give rise to transient infections. Transfer from pen to pen is usually by the movement of infected pigs, but infection may be spread on boots, clothing and implements.

In some cases, salmonellas are spread by rodents, insects, flies and by birds within a piggery. Infection is usually introduced to a farm with infected pigs, but may arrive in feed in untreated water, on boots or transport, by contact with infected birds, rats or mice and from other farm animals.

Clinical signs

Salmonellosis should be suspected where there is a history of infection on a farm and where pigs die of acute septicaemia. The presence of fever and diarrhoea suggests salmonellosis and necrotic material in the faeces also suggest salmonellosis. The isolation of the organism from the faeces in profuse culture confirms the diagnosis.

Past infection may be confirmed by identifying serum antibody. ELISA tests using ‘O’ antigens may be carried out on serum and on meat juices. The Danish meat juice/serum ELISA is designed to detect a wide range of serovars using ‘O’ factors 1, 4, 5, 6, 7 and 12 in the LPS antigen. It is a herd test and detects exposed pigs at slaughter.

Salmonellosis should be suspected where there is a history of infection on a farm and where pigs die of acute septicaemia. The presence of fever and diarrhoea suggests salmonellosis and necrotic material in the faeces also suggest salmonellosis. The isolation of the organism from the faeces in profuse culture confirms the diagnosis.

Past infection may be confirmed by identifying serum antibody. ELISA tests using ‘O’ antigens may be carried out on serum and on meat juices. The Danish meat juice/serum ELISA is designed to detect a wide range of serovars using ‘O’ factors 1, 4, 5, 6, 7 and 12 in the LPS antigen. It is a herd test and detects exposed pigs at slaughter.

Postmortem lesions

At post-mortem examination of pigs with the septicaemic form, the carcase is in good condition, there are multiple haemorrhages, enlarged haemorrhagic lymph nodes and an enlarged spleen. In the enteric form, the presence of small intestinal inflammation, enlarged mesenteric lymph nodes, necrosis of Peyer’s patches, typhlocolitis or rectal stricture may suggest salmonellosis.

Confirmation is by isolation of salmonellae from all parts of the carcass, especially spleen, lung, intestine and mesenteric lymph node. Small numbers of S. enterica in faeces, slurry, caecal contents, tonsil or lymph nodes suggest carriage rather than active salmonellosis.  

Treatment and prevention

Affected animals may be injected with tetracyclines, streptomycin, apramycin, neomycin, ampicillin, amoxicillin, spectinomycin, trimethoprim:sulphonamide, enrofloxacin, danofloxacin or ceftiofur.

Tetracyclines; streptomycin; neomycin; apramycin ampicillin; amoxicillin, trimethoprim: sulphonamide, enterofloxacin, danofloxacin can be used in water for 3-5 days or until 2 days after the cessation of clinical signs. Feed medication can be used, but inappetent animals must be treated individually. Disinfection should accompany courses of treatment. Failure to respond to antimicrobial treatment may suggest that the organism responsible is resistant to the antimicrobial used.

Isolation, the use of salmonella-free stock, the use of clean, heat-treated feed and water, rodent and fly control and netting against birds may all help prevent the introduction of salmonellosis to a farm. Control on infected farms may require medication when disease is expected and should be accompanied by disinfection. All-in, all-out housing, coupled with isolation of batches by disinfectant barriers and thorough cleaning and disinfection between batches may reduce the spread. Numbers of salmonellas present may be reduced by the inclusion of organics acids in feed or water.

Vaccination is possible. Live attenuated Chloeraesuis and Typhimurium vaccines have been described and the latter are available in Europe. Killed vaccines are also protective. Eradication of the organism has been achieved by complete depopulation, or by treatment of a whole herd, coupled with cleaning, disinfection and a 3-week empty period.

Special note

Salmonellas are an important cause of human food-borne infection, and control programmes are in force in many countries and throughout the European Community. Salmonella isolates from both pigs and humans are typed and recorded. Salmonella monitoring takes place systematically in abattoirs by the culture of carcasses, lairages and the environment. In some countries, a meat juice ELISA has been used to detect antibody in carcases at the herd level and the results used to reduce salmonella infection on the farms concerned and thereby to reduce the number of infected pigs being presented for slaughter. Some Nordic countries are completely free from salmonella infection in their pigs.