Thailand’s Disease Control Department investigated an unusual outbreak of eye infections linked to a Bangkok condominium complex in 2024, highlighting the importance of maintaining water system safety. Public health officials emphasize that proactive monitoring and swift responses are crucial for preventing and controlling infectious disease outbreaks.
On March 21, 2026, the Epidemiology Bureau of the Department of Disease Control shared details of the investigation, which began when multiple residents of a Bangkok condominium reported experiencing similar symptoms of conjunctivitis, commonly known as pinkeye.
The case, dubbed “The Red Eye” by investigators, initially involved a tiny number of individuals. However, the number of affected residents quickly climbed to nearly one hundred, prompting questions about a common source of infection.
The investigation team began by interviewing patients, who described symptoms including eye pain, excessive tearing, light sensitivity, and blurred vision – symptoms severe enough to disrupt daily life for some.
Even as viral and bacterial infections are typical causes of conjunctivitis, testing revealed the presence of Microsporidia, a parasite typically found in soil, water, and waste. Infections with this parasite are uncommon and usually require direct contact with the organism, making the outbreak particularly puzzling.
What initially struck the team was the lack of typical risk factors among the patients. None of the affected residents reported contact with soil, and none used contact lenses while swimming – common pathways for Microsporidia infection.
This led investigators to focus on the environment within the condominium complex. The team systematically examined common areas, including the swimming pool, before turning their attention to the building’s water system.
Testing revealed that the building’s water supply was cloudy and contained sediment, with chlorine levels falling below acceptable standards in several locations. Further investigation revealed that the water tank had only been cleaned annually and chlorine was not consistently replenished.
This discovery transformed the investigation from a search for individual risk factors to a focus on a systemic issue. Officials believe the compromised water system created an environment conducive to the growth and spread of Microsporidia.
This may have been the first large-scale outbreak of Microsporidia keratitis in Thailand.
Once the source was identified, control measures were implemented immediately. The water tank was thoroughly cleaned, and a chlorine shock treatment was administered to restore proper disinfection levels. The number of new cases began to decline rapidly, and the outbreak subsided.
The case illustrates the challenges and rewards of field epidemiology, where answers are often found not in laboratories or textbooks, but in real-world investigations.
Detailed findings from the investigation of the Microsporidia keratitis outbreak at a Bangkok condominium in May-July 2024 are available in the Weekly Epidemiological Surveillance Report (WESR) https://he05.tci-thaijo.org/index.php/WESR/article/view/7544.
