Streptococcus suis infection

Occurrence: Worldwide, some herds free.
Age affected: Piglets pre-weaning, weaners, growers/ finishers, human risk.
Causes: Bacterium - Streptococcus suis; poor hygiene; poor housing and ventilation; wounds.
Effects: Arthritis, nervous signs, stunting, fever, death.

Causes

Streptococcus suis is a streptococcal bacterium which has a distinctive boat shape, possesses a capsule to resist phagocytosis (digestion by white cells in the body) and toxins. 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 facilities are not widely available, although PCR testing is now more widely avaiable. S. suis is sensitive to a wide range of antimicrobials and farm disinfectants but survives 512 days on the tonsils in carrier animals and 25 days at 9°C in dust.

S. suis infects pigs at or shortly after birth from the sow or by aerosol or contact within 5-25 days of mixing with carrier animals. It 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 and pneumonia also occur. Immunity develops but is type specific. S. suis, particularly serotypes 2 and 14 can infect man causing deafness followed by meningitis and death.

Mode of transmission

Transmission is primarily from pig to pig by the oronasal route. Infection can take place from the sow’s vagina at birth and from her tonsils. Infection then spreads from pig to pig especially after weaning and mixing. Infection can also follow contact with infected dust and by aerosol, by contact with contaminated surfaces, implements, feeders, drinkers and clothing. The organism can infect other species, including humans, and be carried by flies, but the part played by reservoirs of infection in other species on farm is not clear and is much less important than direct transmission. The same applies to transmission from farm to farm.

Clinical signs

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, bind 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 a 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, tremor, 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% morbidity.

Serotype 1 infections in piglets are suggested by the clinical signs of polyarthritis, meningitis and death in pigs aged 10-21 days.  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 long after any clinical signs have subsided. Isolated may be identified to species using biochemical tests. Typing is carried out by reference laboratories as it requires specialist serological reagents.

Postmortem lesions

Purulent meningitis, joints inflamed and swollen by mucoid synovial fluid and valvular endocarditis are frequently present at the post-mortem examination of piglets. S. suis may be demonstrated by culture and in smears, confirming the cause of the condition. The post-mortem findings in older pigs consist of a reddened carcase, enlarged reddened lymph nodes, fine strains of fibrin in the peritoneal and pleural cavities and pneumonia. Oedema and congestion of the brain may be visible when this is exposed and the cerebrospinal fluid may be cloudy.

Purulent arthritis may be present in young pigs and in some infections in older animals. The presence of meningitis can be confirmed microscopically. S. suis infection can be confirmed by culture and by PCR. Serotyping of isolates and genetic characterisation are important in monitoring the sources of infection and for confirming the routes of spread on farms.

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, trimethoprim sulphonamide and other antimicrobials can be sued 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 re-assessed 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.

Control of the disease in piglets may be possible by farrowing sows in isolation and by fumigating the accommodation between farrowings. Infection of piglets at birth may be prevented by treating the sow with long-acting penicillin may eliminate the infection from the sow and prevent disease in the piglets.

Control of the disease in older pigs can be by medication during the incubation period using ampicillin or amoxicillin in water for 7 days prior to expected disease, in feed with phenoxymethyl penicillin at 75-100 g/tonne or procaine penicillin at 200-300 g/tonne. It may be possible to prevent or reduce infection using autogenous vaccines, but no vaccines effective against all strains and variations have yet reached the market.

All-in, all-out husbandry methods reduce spread. Medicated early weaning has eradicated disease from a herd but slaughter, disinfection and repopulation is the most reliable method of eliminating infection.

Special note

As S. suis can infect and kill humans, staff should be aware of this and take precautions to avoid infection. This applies particularly during post mortem examinations. Family members should be made aware of the early signs of the disease such as deafness, headaches and other nervous signs and inform the physician of their suspicions.