A concerning rise in drug-resistant bacterial infections is being linked to strains of Streptococcus suis (S. suis) found in pigs, with researchers warning of potential increases in human infections. A study conducted by a team from Kasetsart University’s Faculty of Public Health has identified specific strains prevalent in Thailand that exhibit high virulence and resistance to common antibiotics.
Understanding Streptococcus Suis (S. suis)
Streptococcus suis is a bacterium that can infect both pigs and humans. In humans, it can cause serious illnesses such as meningitis, sepsis (a life-threatening bloodstream infection), and infective endocarditis (inflammation of the heart lining). The primary route of transmission to humans in Thailand is through the consumption of raw or undercooked pork products.
The research focused on identifying and characterizing strains of S. suis circulating within Thailand. Researchers collected bacteria from infected patients and analyzed their virulence, with a particular emphasis on strains dominant in the country. These dominant strains include ST 233 and ST 104, which are part of Serotype 2, a group responsible for a significant majority – 93.4% – of S. suis cases identified nationwide.
High Virulence and Mortality Rates
To assess the potential danger posed by these strains, the researchers conducted experiments involving mice. Bacterial sub-strains, specifically ST 233 and ST 104, were introduced to the animals. The results were stark: these strains led to mortality rates between 80% and 100% within just 48 hours, primarily due to the development of septicaemia. This indicates a potent ability of these particular S. suis strains to overwhelm the host’s system rapidly.
Further testing revealed that other serotypes also pose a threat. S. suis Serotype 14 demonstrated similar high mortality rates, while Serotype 31 resulted in intermediate mortality levels after approximately 42 hours. These findings underscore the broad spectrum of risk associated with different S. suis strains circulating in the pig population.
Historical Context and Prevalence
The first documented case of S. suis infection in Thailand was recorded in 1987, involving two patients in Bangkok. A significant outbreak occurred in 2000 in Lamphun province, leading to 10 fatalities. Since these initial detections, a total of 1,798 human infections have been recorded up to the time of the study. The overall disease prevalence is estimated at approximately 0.97 cases per 100,000 individuals.
While Serotype 2 remains the most common cause of human infections, other serotypes are also implicated. Serotype 4 accounts for 5.2% of human cases, with other serotypes contributing smaller percentages. The research also highlighted a dynamic shift in the prevalence of different strains over time, suggesting an evolving landscape of S. suis epidemiology.
Emergence of Drug Resistance
A critical finding of the study is the emergence of drug resistance within certain S. suis strains. The research established a link between specific strain clusters, namely CC233 and CC1688, and resistance to widely used antibiotics such as penicillin and ceftriaxone. This development is particularly alarming as these drugs are often the first line of treatment for bacterial infections.
Furthermore, the study identified associations between particular strains (ST1, ST104, ST233, and ST1688) and severe clinical outcomes, including meningitis, sepsis, and infective endocarditis. The combination of high virulence and antibiotic resistance in these strains presents a significant public health challenge.
Implications and Recommendations
The researchers emphasized the need for continued investigation into why Thailand appears to be a significant hub for S. suis infections. However, they stressed that the current findings serve as a crucial step in raising public awareness regarding the risks associated with consuming raw or undercooked meat. They reiterated that thorough cooking, which effectively destroys the bacteria through heat, is the most effective preventive measure for consumers.
Beyond consumer awareness, the study’s findings are expected to inform and encourage pig farmers to adopt more hygienic farming practices. Implementing stringent biosecurity measures and sanitation protocols can help prevent the transmission of S. suis from animals to humans, thereby reducing the overall burden of infection. Further research into the genetic mechanisms of resistance and the development of alternative treatment strategies may also be warranted to combat the growing threat of drug-resistant S. suis infections.
