Age affected: All ages.
Causes: Bacteria - Streptococcus suis, Haemophilus parasuis, Salmonella etc.; viral. infections; poor ventilation; overcrowding; stress.
Effects: Inappetence, trembling, twitching, abnormal eye movements, convulsions, death.
Meningitis is inflammation of the membranes which surround the brain (meninges). It is commonly caused by the bacteria Streptococcus suis (streptococcal meningitis) and Haemophilus parasuis (Glässer's disease). S. suis is a streptococcus which is a distinctive boat shape, possesses a capsule to resist phagocytosis (digestion by white cells in the body) and a toxin. At least 35 capsular types have been distinguished and some of these produce particular forms of disease (serotype 1 in baby pigs, serotype 2 in meningitis in older pigs, serotype 14 in arthritis). The organism is easily cultivated in the laboratory, but full typing requires special facilities. S. suis is sensitive to a wide range of antimicrobials and farm disinfectants but survives 512 days on the tonsils and 25 days at 9˚C in dust. S. suis multiplies in the tonsillar crypts, is taken up by white blood cells and is distributed in the blood to the brain and joints. It can cause septicaemia within a few hours which may kill the animal or cause meningitis which is frequently fatal. Arthritis also occurs. Immunity develops but is type specific.
Mode of transmission
S. suis infects piglets from the sow at or shortly after birth; by aerosol or by contact within 5-25 days of mixing with carrier animals. Indirect infection from pen furniture and dust can occur. Infection is usually transmitted to an uninfected farm in carrier pigs, but contaminated transport, flies and even birds may transmit infection mechanically. More than one serotype may be present on a farm.
Serotype 1 causes disease in piglets of 10-14 days of age. Affected animals cease to thrive, become rough-coated, develop fever (40.6-41.1°C, 105-106°F) and may die. Enlarged, hot, painful swollen joints develop later or animals become stiff, blind or show muscular tremors ending in ataxia or death. Sudden death may occur in endocarditis due to streptococcal infection. Typically, up to two thirds of the litter develop some form of the disease. Serotype 2 causes disease in pigs aged 3-12 weeks or more with an incubation period of 24 hours to 2 weeks. Outbreaks often begin with the death of a pig in good condition. Fever of 40.6-41.7°C, 105-107°F and flushing of the skin may occur in live pigs. Nervous signs such as incoordination, tremors, paralysis, paddling, opisthotonus and tetanic spasms develop in that order. Death can occur within 4 hours of the onset of the clinical signs. Animals developing meningitis have a glassy stare, flushed skin and unsteady gait. Head tilt is sometimes present. Arthritis may occur in younger pigs and occasionally in gilts and sows. Bronchopneumonia may occur. Mortality varies from 1-50% in any batch of pigs and, in herds with enzootic infection, may be 0.5% with 1% clinically affected.
Serotype 1 infections in piglets are suggested by the clinical signs and the age of the affected pigs (10-21 days). S. suis may be demonstrated by culture, in direct smears and by molecular means, confirming the condition and differentiating it from H. parasuis infections. S. suis serotype 2 infections in older pigs should be suspected when pigs are seen with head tilt, a glassy stare, flushed skin and unsteady gait followed by nervous signs such as incoordination, tremor, paralysis, paddling, opisthotonus and tetanic spasms and by death within hours. The organism can be isolated from the tonsils of carrier pigs. Isolates may be identified to species using biochemical tests. Typing is carried out by reference laboratories.
The post-mortem findings in serotype 1 infection in piglets include purulent meningitis, inflamed joints swollen by mucoid synovial fluid and valvular endocarditis. In weaned pigs infected by serotype 2, there is a a reddened carcase, enlarged reddened lymph nodes, fine strands of fibrin in the peritoneal and pleural cavities and pneumonia. Oedema and congestion of the brain may be visible and the cerebrospinal fluid may be cloudy. Purulent arthritis may be present in young pigs. The presence of meningitis can be confirmed microscopically. S. suis infection can be confirmed by culture, and the condition differentiated from H. parasuis infection. Isolates can be serotyped by specialist laboratories.
Treatment and prevention
Affected pigs should be treated by injection with antimicrobial for 3-5 days. Penicillin is the drug of choice although amoxicillin, ampicillin, cephalosporins, treimethoprim sulphonamide and other antimicrobials can be used, but not aminoglycosides or the tetracyclines. Affected pigs should be removed to a quiet pen and given water and food, manually if necessary. Paralysed animals should be rehydrated using saline given per rectum. Animals with advanced septicaemia, meningitis and arthritis rarely respond and all cases should be reassessed 3 days after the onset of treatment. Paralysed animals should be killed humanely. Pigs in the same pen as affected animals should be treated in feed or in water, as infection is widespread and the incubation period is so short. Control of the disease in piglets may be possible by farrowing sows in isolation and by fumigating the accommodation between batches. Injection of piglets at birth with long-acting penicillin may treat the disease before it becomes clinical. As there is no universally – effective commercial vaccine at present, control of the disease in older pigs can be by medication of the water with ampicillin or amoxicillin for 7 days prior to expected disease, in feed with phenoxymethyl penicillin at 75-100 g/tonne or using procaine penicillin at 200-300 g/tonne. All-in, all-out husbandry methods reduce spread. Medicated early weaning has eradicated disease from a herd but slaughter, disinfection and repopulation is most reliable. It is possible to vaccinate using autogenous vaccine produced from the farm’s own strains, but these can alter with time and may not always be protective.
S. suis can infect humans to cause a meningitis which is sometimes fatal, so care should be taken when handling living cases and also at post-mortem examination.