Streptococcus suis

Clinical importance

Streptococcus infections can cause systemic disease in all ages of pigs with clinical presentations ranging from septicemia and neurological symptoms to sudden death. There are 35 identified strains of Streptococcus suis, serotypes 1, 1/2, 4, and 3 being the most prevalent in the USA.

This disease is considered an emerging zoonosis since the number of human cases reported has risen in recent years, especially in Asian countries. Infection of humans typically occurs through skin lesions and results in septicemia leading to meningitis and polyserositis. People handling pigs for work are more at risk but handwashing with soap is often enough to prevent the infection.

Streptococcus suis is a commensal bacterium from the upper respiratory tract of pigs. Non-virulent strains can easily be isolated from the nostrils and the tonsils of healthy pigs. Streptococcosis is a common occurrence in any country raising pigs. Disease prevalence is low, usually 5% of the pigs are affected even though there may be 100% carriers. Mortality is variable and can reach 20% if pigs are infected by virulent strains and left untreated.

Etiology and transmission

Streptococcus suis is an encapsulated gram positive bacteria that is a commensal of the upper respiratory tract of pigs. Most often, piglets get contaminated by the sow in the birth canal with non-virulent strains. Few pigs will carry the virulent strains but when they do, transmission occurs through different routes, the most common being by direct contact, nose-to-nose. Aerosol transmission is less frequent but can happen. Other vectors such as birds and flies are also suspected to aid transmission between farms.

Associated symptoms

Infected pigs may not show clinical signs, but stresses usually related to poor management practices can result in a higher chance of developing disease. Co-infections, especially with PRRSV lead to a more severe clinical picture. Clinical signs most often occur in nursery-age pigs between 5 and 10 weeks of age but can appear pre-weaning as well.
There are three different presentations of the disease: peracute, acute, and chronic.

Peracute form

No clinical signs are seen in the peracute form. Pigs are found dead with no precursor sign.

Acute form

The first symptom is fever followed by the anorexic and depressed behaviors associated with it. Quickly, affected pigs present signs of meningitis, such as squinting eyes and flattened ears. Depression, paddling, ataxia, nystagmus and opisthotonos are very often associated with Streptococcus suis infections.

Chronic form

Polyarthritis is a common symptom of the infection and is accompanied by swollen joints and lameness. Labored breathing or thumping can be seen in chronic cases due to polyserositis. Additionally, Streptococcus suis can be a secondary and opportunistic pathogenic bacterium in pneumonia cases.

Associated lesions

Macroscopic lesions

With pigs that die suddenly due to peracute infection, clouding of the brain tissue is the most common macroscopic lesion. In the less acute forms, Streptococcus suis causes polyserositis, therefore fibrin can be found in the thorax, abdomen and joints of the pigs during necropsy. Swelling of the spleen, a characteristic sign of septicemia is also typically found. Valvular vegetative endocarditis can be found in more chronic cases. Swollen joints with fibrinopurulent content are seen in cases of lameness.

Microscopic lesions

Microscopically, the clouding seen in peracute cases is caused by fibrinopurulent meningitis. Common lesions of non-peracute case are inflammation and edema of the brain. The heart can also be affected, with epicarditis being another common microscopic lesion.

Diagnosis

Clinical signs and macroscopic lesions seen during necropsy are very indicative of Streptococcus suis infection. However, a definitive diagnosis requires the isolation of the bacteria through culture. In cases with neurological signs, the brain always needs to be sent to the diagnostic laboratory, sometimes with a swab of the meninges. In lameness cases, a swab of the inside of the joint(s) is useful. Heart, lungs and spleen are also valuable since the disease is septicemic. A positive result on culture of the tonsils or the upper respiratory tract is not conclusive considering the high rate of healthy carriers. Serotyping can be valuable for two reasons. First, due to the many types of strains, it is important to identify the virulence of the strain to determine if Streptococcus suis is truly the causative agent of the symptoms. Second, monitoring the strain(s) found in a herd over time would help identifying any new introductions. This is important information to know when choosing vaccination and treatment options.

Differential diagnosis

The differential for neurological signs in weaned pigs includes Glaesserella parasuis, F18 E.coli, and salt toxicity.

Treatment, Prevention and Control

Antibiotics are the best treatment against Streptococcus suis infection. A susceptibility test should be run first to ensure the proper treatment is applied. In general, amoxicillin and ampicillin are the best treatment options whereas tetracycline tend to be the least effective.
Because Streptococcus suis is present in most herds, preventing the development of the disease is the main goal. Common risk factors are large temperature swings, high stocking density, and poor air quality. Streptococcus suis is not very resistant in the environment and is very susceptible to most disinfectants so a thorough cleaning and disinfection protocol should prevent the disease from spreading between groups. Autogenous vaccines are commonly used to prevent severe clinical signs but they do not protect from infection. No commercial vaccine is currently available.