Zoonotic disease with a mortality rate of 20-40% - human infection with Streptococcus suis

One day, Lao Zhang, a farmer who had raised pigs for decades, suddenly had a fever, headache, and joint pain. After taking cold medicine for a few days, his condition worsened and he began to experience tinnitus and blurred vision. After his family rushed him to the hospital, an experienced clinical doctor suspected that Lao Zhang had human swine streptococcal disease, so he contacted the local CDC to collect samples for testing. It was later confirmed that Lao Zhang was indeed infected with swine streptococci. Facing Lao Zhang's worried and somewhat puzzled family, the doctor explained that this bacteria mainly infects pigs, but if you are not careful, it can also be transmitted to humans through pigs. Lao Zhang may have been infected by contacting pigs' blood or body fluids while taking care of sick pigs.

Streptococcus suis is commonly found in the upper respiratory tract (especially the tonsils and nasal cavity), intestines and blood of pigs. For workers in pig farms and slaughterhouses, this bacterium can pose a fatal health hazard. Once the best time for treatment is delayed due to misdiagnosis, the mortality rate can be as high as 20-40%. Fortunately, due to timely and accurate diagnosis and treatment, Mr. Zhang's condition was effectively controlled and he was eventually discharged from the hospital.

What is Streptococcus suis?

Streptococcus suis is a Gram-positive coccus with a capsule, belonging to Lancefield group D Streptococcus. According to the difference in its capsular antigen (CPS), Streptococcus suis is divided into 35 serotypes (types 1 to 34, type 1/2), of which types 1, 2, 7, and 9 are pathogenic bacteria of pigs. In pig herds, especially those raised in densely populated areas, Streptococcus suis sometimes causes pneumonia, meningitis, and sepsis. For humans, Streptococcus suis is an important zoonosis pathogen. So far, 10 serotypes that can infect humans have been found, namely serotypes 2, 4, 5, 7, 9, 14, 16, 21, 24, and 31 [1], with type 2 being the most common. Close contact between humans and sick or dead pigs is the main cause of human infection with Streptococcus suis. Streptococcus suis can enter the human body through skin wounds and cause infection. It can also be transmitted by eating foods such as uncooked pork or pig blood contaminated with Streptococcus suis. Since 2019, the number of human cases infected with Streptococcus suis in my country has shown a clear increasing trend [2], and the public health threat of Streptococcus suis cannot be ignored.

Image 1: Colony morphology of Streptococcus suis on OXOID selective medium (light red colonies of 0.5-1 mm in size)

Transmission routes and symptoms of Streptococcus suis

The transmission of Streptococcus suis among the population is relatively low, but when handling, slaughtering or processing pork, people with wounds on their hands may come into contact with the bacteria and become infected. Patients infected with Streptococcus suis mainly present with meningitis accompanied by permanent deafness. Patients also often experience symptoms such as sepsis, endocarditis, pneumonia, and arthritis. In severe cases, toxic shock syndrome or even death may occur [3-4].

Picture 2: Possible symptoms of human infection with Streptococcus suis

Risk factors for human infection with Streptococcus suis

Risk factors for human infection with Streptococcus suis include a variety of factors, usually related to exposure to infectious agents, environmental factors, and individual health conditions. The following are some of the main risk factors:

1. Close contact with pigs:

Farming and slaughtering industries: People who raise pigs, slaughter, process or handle pork are at the highest risk, especially without proper protection.

Farm visitors: Visits to pig farms or contact with sick pigs may also increase the risk of infection, risk level.

2. Environmental sanitation:

Poor hygiene: Unclean working environment or pig house may lead to the spread of bacteria, risk level.

Cross contamination: During slaughter and processing, if handling tools and equipment are not properly disinfected, it may cause infection and risk level.

3. Individual health status:

Weakened immune system: People with underlying diseases, immunodeficiency or who are receiving immunosuppressive therapy are at higher risk of infection, risk level.

Wounds or skin injuries: People with wounds or cuts on their skin are more susceptible to infection when handling pigs, and the risk level is higher.

4. Geographical and economic factors:

Areas with developed pig farming: In areas with developed pig farming, the risk of infection is generally higher, and the risk level is higher.

Low-income areas: In areas with insufficient resources and poor sanitary conditions, the risk of infection is also higher, and the risk level is higher.

5. Food safety:

Raw or undercooked pork: Eating pork products that are not thoroughly cooked may result in infection, risk level.

6. Occupational exposure:

Veterinarians: Veterinarians who work directly with infected pigs face a higher risk, risk level.

How to prevent human infection with Streptococcus suis?

For people who come into contact with pigs and their products, such as breeders, slaughterers and veterinarians, the following are some important measures to prevent Streptococcus suis infection:

1. Vaccinate pigs

On July 16, 2021, the Ministry of Agriculture and Rural Affairs approved six new veterinary drugs, including the bivalent inactivated vaccine for swine streptococcosis and contagious pleuropneumonia.

2. Use of Personal Protective Equipment (PPE)

Wear protective clothing, gloves, masks and goggles: When in contact with pigs and their products, you need to wear protective clothing, gloves, masks (such as N95) and goggles to prevent direct contact of the skin and mucous membranes with possible sources of infection.

Non-slip shoes or boots: Wear waterproof, non-slip boots to avoid being splashed with blood or body fluids, especially when working in a slaughterhouse.

3. Maintain good hygiene habits

Wash your hands frequently: Wash your hands thoroughly with soap and water after contact with pigs, their secretions or products, and use alcohol-based disinfectants for further disinfection.

Avoid touching your face with your hands: Avoid touching your eyes, nose and mouth with your hands while working to reduce the risk of infection.

Clean and disinfect wounds promptly: Any skin damage should be disinfected and covered immediately to avoid direct contact with the source of infection.

4. Standardized workflow

Strictly follow operating procedures: Especially when slaughtering, processing and handling pig products, the operating procedures should comply with national biosafety regulations to reduce the risk of infection.

Keep the working environment clean: regularly clean and disinfect slaughterhouses, farms and veterinary laboratories to reduce pathogens in the environment

5. Health monitoring and early identification

Regular physical examinations: People engaged in breeding and slaughtering should undergo regular physical examinations, especially paying attention to areas prone to infection such as the skin, respiratory tract and ears.

If you discover symptoms of infection early, such as fever, headache, vomiting, skin rash, etc., you should seek medical attention as soon as possible to prevent the disease from worsening.

Vaccination: Although there is currently no widely used human vaccine, progress has been made in the development of specific vaccines for high-risk groups.

6. Avoid unnecessary exposure

Minimize unnecessary contact: Reduce the frequency and duration of direct contact when handling pigs or products.

Avoid working when you are sick: If you have broken skin, weakened immunity, etc., you should avoid direct contact with pigs and their products to reduce the risk of infection.

7. Maintain a healthy lifestyle

A balanced diet, a regular schedule and adequate exercise can boost your immunity and help fight infection.

Picture 3: Pig farm workers wearing protective gear correctly

Laboratory Detection of Streptococcus suis

The laboratory uses advanced technologies such as culture analysis, mass spectrometry, and gene sequencing to quickly and accurately detect Streptococcus suis. These technologies can help researchers identify different strains of Streptococcus suis, evaluate their drug resistance, and develop appropriate treatment plans. Currently, this type of research is widely used in the pork processing and breeding industries to help control the spread of diseases among pigs and humans.

Image 4: Laboratory testing of Streptococcus suis

Although small, Streptococcus suis poses a hidden danger that cannot be underestimated. Through in-depth understanding of it, promotion of health and safety awareness and correct protective measures, we can better protect breeding and processing personnel and food safety, and effectively prevent the spread of infection by this type of bacteria.

References:

[1] LIU ZY, XU QH, LIANG PJ, PENG ZR, YAO HC,ZHENG H,WU ZF. The characteristics of population structure and antimicrobial resistance of Streptococcus suis serotype 8, a non-negligible pathotype[J]. Transboundary and Emerging Diseases, 2022, 69(5):e2495-e2505.

[2] GOYETTE-DESJARDINS G,AUGER JP,

[3]Heidt MC, Mohamed W, Hain T, et al. Human infective endocarditis caused by Streptococcus suis serotype 2[J].J Clin Microbiol, 2005, 43(9):4898-4901.

[4] Leelarasamee A, Nilakul C, Tien-Grim S, et al. Streptococcussuis Toxic-shock Syndrome and Meningitis[J]. J Med AssocThai,1997,80(1):63-68.

Source: Chongqing Science Writers Association

Author: Zhu Yong, deputy chief laboratory technician, Xie Jiawei, deputy chief technician, and Li Ling, deputy chief technician, Chongqing Rongchang District Center for Disease Control and Prevention; Zou Jingbo, chief laboratory technician, Chongqing Yongchuan District Center for Disease Control and Prevention

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