Streptococcus suis

Clinical importance

Streptococcus infection can cause systemic disease in all ages of pigs with clinical presentations ranging from septicemia to central nervous system symptoms. 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 bacteria 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 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. Clinical signs occur in nursery-ages pigs between 5 and 10 weeks of age.
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 signs.

Acute form

The first symptom is fever and the anorexia and depressed behavior associated with it. Quickly, affected pigs present signs of meningitis, such as squinting eyes and flattened ears. Depression, paddling, ataxia, nystagmus and head tilts 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. Streptococcus suis may be a cause of pneumonia, but that is highly debated. Most often, Streptococcus suis is a secondary and opportunistic pathogenic bacteria in pneumonia cases.

Associated lesions

Macroscopic lesions

With pigs that die suddenly due to peracute infection, clouding of the brain tissues is the most common macroscopic lesion. In the less acute forms, Streptococcus suis caused polyserositis, therefore fibrin can be found in the thorax, and abdomen of the pigs at 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 CNS 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 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 2 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, the herd may be infected with multiple strains and the type of strain could change over time due to the introduction of a new strain. 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 options whereas erythromycin and tetracycline tend to be the least effective.
Because Streptococcus suis is in most herds, preventing the development of the disease is the main goal. Common risk factors are large temperature variances, 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. Vaccines are commonly used to prevent onset of disease but they do not protect from infection. In this case, autogenous vaccines are most often implemented. Vaccines should not be given early in the piglets’ life due to maternal immunity interference.