Streptococcus Suis Infection

Streptococcus Suis Infection

Streptococcus suis infection (S. suis infection) is a zoonotic infection transmitted from pigs to humans through direct contact or consumption of raw or undercooked pork products. Once the bacteria enter the body, they can rapidly enter the bloodstream and disseminate to vital organs, with a particular predilection for the meninges (the protective membranes surrounding the brain) and the auditory system. If the infection involves the inner ear or auditory nerve, it can trigger an intense inflammatory response, leading to sudden sensorineural hearing loss and permanent deafness.

Illustration of Streptococcus suis Bacteria

What is Streptococcus suis?

Streptococcus suis is a bacterium commonly found in pigs. It primarily colonizes the upper respiratory tract, particularly the tonsils and nasal cavities, as well as the gastrointestinal and reproductive systems of healthy swine. In many instances, the bacterium colonizes the host without causing overt clinical disease, resulting in asymptomatic carriers that can transmit the pathogen to other pigs and, occasionally, to humans.

However, under conditions of stress, such as environmental changes, transportation, overcrowding, or co-infection with certain viruses, Streptococcus suis can shift from a commensal organism to an opportunistic pathogen. This transition may result in severe disease in swine, including meningitis, arthritis, septicemia, and sudden death.

How does Streptococcus suis infection spread to humans?

Humans can contract Streptococcus suis through direct contact with infected pigs or contaminated pork products via two primary routes:

• Consumption of raw or undercooked pork: This is the primary cause of outbreaks in Thailand, particularly through the consumption of undercooked pork, blood, or offal in traditional dishes such as larb dib, luu, and koi. This cultural dietary practice is the most significant risk factor for widespread transmission.
• Exposure through open wounds: The bacteria can enter the body through cuts, abrasions, or the conjunctiva while handling infected pigs or raw pork. Individuals at higher risk include pig farmers, slaughterhouse workers, and meat vendors.

At present, there is no confirmed evidence of human-to-human transmission. Although infections are typically limited to individuals with direct exposure, the disease can be severe and potentially life-threatening, particularly in those with compromised immune systems.

What are the symptoms of Streptococcus suis Infection?

Streptococcus suis Infection typically progresses rapidly and can be severe. Most patients present with acute symptoms that may progress to serious complications within a short period.

• Acute high fever with chills 
• Severe headache and fatigue 
• Nausea and vomiting 
• Nuchal rigidity (neck stiffness), confusion, or decreased level of consciousness, suggestive of meningitis 
• Rapid hearing loss, tinnitus (ringing in the ears), or dizziness

What are the potential complications?

• Meningitis: A common complication that may cause severe neurological impairment. 
• Sepsis (bloodstream infection): It can lead to dangerously low blood pressure and progress to septic shock. 
• Permanent hearing loss: A hallmark feature of the disease, which may occur early in the course of infection.
• Neurological complications: Including seizures and loss of consciousness. 
• Balance disorders: Resulting from damage to the inner ear (vestibular system).

Why is hearing loss permanent?

Hearing loss caused by Streptococcus suis infection results from an intense inflammatory process that destroys critical structures within the inner ear, particularly the cochlea, the primary organ responsible for sensing sound and converting it into neural signals.

1. Acute inflammatory phase

When Streptococcus suis enters the cochlea, the body responds by mobilizing white blood cells and pro-inflammatory mediators to the site. Although this is a natural defense mechanism, severe inflammation can rapidly damage hair cells, which transduce sound waves into nerve impulses.

2. Fibrotic phase

If the inflammation persists, it can lead to the formation of fibrous tissue (fibrosis) within the inner ear. This scarring interferes with the circulation of endolymph and perilymph, the fluids within the cochlea that are vital for sound transmission.

3. Ossification phase (Labyrinthitis Ossificans)

In the subsequent stage, the body attempts to repair the damaged tissue with calcifications, leading to new bone formation within the cochlea. This process permanently destroys the delicate internal architecture of the inner ear, resulting in profound and irreversible deafness.

What are the risk factors of Streptococcus suis infection?

Dietary and behavioral factors

• The cultural practice of consuming raw or undercooked pork dishes, such as laab dib, luu, koi, nam dib, and soi ju, remains a major driver of outbreaks in Thailand.
• Drinking alcohol while consuming raw or undercooked meat may impair gastrointestinal immune defenses and mask early symptoms, potentially delaying recognition of illness and timely medical care.
• Unhygienic practices during eating grilled or hot pot meals, such as using the same chopsticks or utensils to pick up both raw and cooked foods or placing raw meat next to fresh vegetables, increase the risk of cross-contamination.

Occupational factors

• Individuals in professions requiring direct contact with pigs or pork products may contract the infection through wounds, abrasions, or the conjunctiva. 
• Swine farmers and breeders.
• Slaughterhouse workers.
• Meat vendors and butchers.
• Veterinarians and laboratory personnel who handle swine carcasses or perform diagnostic tests.

Health factors

• Immunocompromised individuals and patients taking immunosuppressive medications.
• Patients with chronic diseases, such as diabetes, kidney disease, heart disease, or hypertension.
• Individuals with asplenia, because the spleen is a critical organ for clearing this specific type of bacteria from the bloodstream.
• Chronic alcohol consumption or cirrhosis impairs the natural bodily defense mechanisms against infection.

What are the diagnostic methods for Streptococcus suis infection?

• Medical history and physical examination: Because early symptoms often resemble those of influenza, a thorough clinical assessment is essential. Doctors will specifically inquire about recent exposure risks, including the consumption of raw or undercooked pork and direct contact with pigs in the preceding 14 days.
• Blood and cerebrospinal fluid (CSF) cultures: The test can confirm Streptococcus suis infection.
• Polymerase chain reaction (PCR) testing: It is a highly sensitive molecular diagnostic method that can detect bacterial DNA, even in patients who have started antibiotic therapy.
• MRI: Magnetic resonance imaging of the brain and inner ear can assess the extent of meningitis and identify abnormalities within the cochlea.
• Audiometry: It evaluates and assesses the degree of hearing loss.

What are the treatment modalities for Streptococcus suis infection?

The treatment for Streptococcus suis infection) requires immediate medical attention. The primary treatment involves intravenous (IV) antibiotics under the close supervision of medical professionals.

• Antibiotic therapy
  • Ceftriaxone: The standard medication for treating meningitis. It is typically administered intravenously at a dosage of 2 grams every 12 hours for approximately 14–21 days, depending on the severity of the infection and the doctor’s discretion.
  • Penicillin G: This may be considered for certain patients, particularly when the bacteria are highly sensitive to the drug. The use of this antibiotic is dependent on lab test results and the attending doctor's discretion.
• Adjuvant corticosteroids
  • Dexamethasone: Administering dexamethasone with antibiotics can help reduce the risk of hearing loss. Doctors may give an intravenous dose of 10 mg of dexamethasone either before or with the first dose of antibiotics. After the initial dose, dexamethasone is continued every 6 hours for a total of 4 days.

Rehabilitation and alternative treatments

For patients with permanent hearing loss, cochlear implantation is a key rehabilitative option that can help restore auditory perception and significantly improve quality of life.

Hyperbaric oxygen therapy (HBOT) has also received attention as an adjunctive treatment. However, current evidence regarding its effectiveness in restoring hearing remains inconclusive. The doctor will consider its use on a case-by-case basis.

What are the preventive measures and proper hygiene practices?

Food preparation and cooking

• Cook pork and other meat thoroughly at a minimum of 70°C for 10 minutes. 
• Separate kitchen utensils for raw and cooked foods, such as cutting boards and knives, to prevent cross-contamination. 
• Purchase pork from reputable sources that meet hygienic standards and are certified by the Department of Livestock Development. The meat should be pale pink, fresh-smelling, and show no signs of spoilage.

Preventive measures for individuals in contact with pigs

• To reduce the risk of bacterial entry through the skin, wear protective gear like boots, rubber gloves, and protective clothing when handling pigs or raw pork. 
• If there are any cuts or open wounds on the hands, they should be securely covered and thoroughly washed with soap and water after any contact with animals or raw pork. 
• Pig farm operators should strictly follow veterinary disease control measures to prevent and limit the spread of infection within swine herds.

Streptococcus suis infection in Thailand (as of 28 April 2026)

The Department of Disease Control (DDC) reported that between 1 January and 24 April 2026, there were 241 identifiable cases of swine streptococcal disease, corresponding to an incidence of 0.371 per 100,000 population. DDC expects the number of cases to increase during the second quarter (April–June), which coincides with warmer weather and a rise in consumption of raw or undercooked pork.

Age-stratified data indicate that the majority of cases occur in individuals aged 60 years and older, followed by those aged 50–59 years and 40–49 years. Geographically, ongoing case reports come from the upper central, lower northern, and lower northeastern regions of Thailand.

Most outbreaks remain closely associated with indigenous social and cultural practices, particularly religious gatherings, merit-making events, and community celebrations where consuming raw or undercooked pork dishes is common. These cultural activities are a key driver of disease transmission.

Common misconceptions about food preparation in Thailand

Myth: Adding lime or lemon juice “cooks” raw meat and makes it safe to eat.

Although citric acid in lime juice has some antibacterial activity, it is ineffective in killing Streptococcus suis in raw meat or blood. The color change after adding lime juice is simply a chemical reaction between the acid and the meat proteins. It does not connote effective pathogen inactivation, like with high-temperature cooking.

Myth: Drinking alcohol with raw food can kill gastric bacteria.

The amount and concentration of alcohol typically consumed are insufficient to decimate the pathogens. Moreover, ingested alcohol may impair immune function and judgment in food selection, thereby increasing the risk of infection.

Myth: Freezing pork can kill bacteria.

Streptococcus suis can survive low temperatures. While freezing may slow bacterial growth, it does not effectively eliminate the pathogen. Thorough cooking with adequate heat remains the only reliable method to inactivate the bacteria.

Myth: Fresh-looking pork is free from infection.

Pigs that appear healthy and pork that looks fresh may still harbor Streptococcus suis. Visual inspection alone cannot ensure safety. Therefore, you must cook the pork well before consumption.

A note from MedPark’s doctors

Streptococcus suis infection can progress rapidly in severity. Without prompt diagnosis and timely treatment, it can lead to serious complications, including meningitis, sepsis, and permanent hearing loss. However, this disease is entirely preventable by refraining from consuming raw or undercooked pork and maintaining proper hygiene practices, including thorough handwashing, using separate utensils for raw and cooked foods, and ensuring that all pork is cooked thoroughly before consumption.