Meningitis – more than a Streptococcal Infection
Autumn and winter is the time of year we tend to see more meningitis cases on our pig farms. This is not specifically because the temperature drops, but due to the fact that the pigs are under more stress.
Streptococcus suis (the bacterial agent which causes the disease) is considered a normal inhabitant of the upper respiratory tract (especially non-harmful strains) and can be easily found in tonsils. This means that healthy pigs can carry many different strains of the bug. The mechanisms that enable S. suis to spread throughout the animal are not well understood. What is known is that the bacterium is able to spread from the nasopharynx, occasionally resulting in septicaemia and death. The tonsils are potential portals of entry for S. suis, leading to subsequent spread in the blood or the lymph.
Piglets become infected with S. suis from vaginal secretions during parturition and while suckling. Asymptomatic carriers (pigs which carry the disease, but don’t display symptoms) serve as a source of infection for their pen mates after they are mixed in the nursery, when maternal antibodies are no longer present. Clinical infections are seen mainly in weaners (2–5 wk after weaning) and growers. Transmission between herds occurs by the movement and mixing of healthy pigs and carrier pigs.
The introduction of a highly infective strain into a naive herd may result in subsequent onset of disease in weaned and/or growing pigs, but also older pigs and suckling piglets. However, some herds with animals harbouring these strains but not showing illness may suddenly develop serious clinical disease in the presence of other predisposing factors such as overcrowding, poor ventilation, excessive temperature fluctuations, mixing of pigs with an age spread of >2 wk, and infections with other pathogens. Disease outbreaks due to S suis infection have been frequently reported with infections of other disease, particularly viruses such as PRRS. S suis might also be transmitted via fomites and flies, although probabilities are low.
Diagnosis is frequently on the back of clinical and post mortem signs, but lab cultures can be used for confirmation. Response to treatment with penicillin, amoxicillin or ampicillin is usually satisfactory. Vaccination with commercial vaccines has generally had poor results, due to the non-specific nature of the vaccines. Autogenous vaccines, where a vaccine is constructed around the specific strain found on your farm, are available and should have good results.
It is worth noting that transmission to people can occur via contamination of skin wounds or mucous membranes by blood or secretions from infected pigs or by consuming raw meat or blood. The disease is considered to be underdiagnosed and underreported in several countries. Care should be taken if you have broken skin and are handling suspect animals or carcasses.