Larval mites, known as “chiggers,” from the family Trombiculidae are vectors of the bacteria that causes scrub typhus. The infection has been implicated as a common cause of acute encephalitis syndrome in India, where public health professionals are looking to better understand the variety of mite species present on rodents and their rates of infection. (Photo credit: Hansell F. Cross, Georgia State University, Bugwood.org)

For more than 30 years, the Gorakhpur region of Uttar Pradesh in India has seen recurring outbreaks of acute encephalitis syndrome. Mostly affecting children age 14 and under and occurring during monsoon season, AES is frequently fatal (20-30 percent of cases). For many years, a primary suspected cause was Japanese encephalitis virus, spread by infected Culex mosquitoes. But research in the last few years has identified another, also entomological, cause: scrub typhus, which results from bacteria transmitted by larval mites in the family Trombiculidae.

In a report published last week in the Journal of Medical Entomology, Manoj Murhekar, M.D., of India’s National Institute of Epidemiology and colleagues at BRD Medical College and the Indian Council of Medical Research, outline the latest understanding of the causes of AES in India and highlight areas of future research, including entomological studies on the mite species involved.

“In addition to vaccination for Japanese encephalitis virus and appropriate treatment, vector control could be an important strategy for prevention and control for these diseases,” Murhekar says. “Also, for scrub typhus, additional studies are needed to understand the transmission dynamics. In view of this, it is essential to share the etiological analysis of Gorakhpur AES with the entomological community.”

Researchers began to suspect the involvement of scrub typhus in recent years after the broad adoption of a vaccine for Japanese encephalitis virus (JEV) failed to significantly reduce occurrence of AES. Further testing showed approximately 60 percent of AES patients in Gorakhpur in 2014 and 2015 had evidence of infection by the bacteria that causes scrub typhus, Orientia tsutsugamushi. Meanwhile, during that same time, only about 10 percent of AES cases were linked to JEV.

Scrub typhus is itself an infectious disease characterized by fever, headache, muscle pain, and rash, according to the U.S. Centers for Disease Control and Prevention. Infected larval mites, commonly known as “chiggers” and primarily of the genus Leptotrombidium, can transmit the bacterial pathogen, O. tsutsugamushi, when they bite.

Public health authorities in India are working to step up treatment of AES patients with azithromycin and scrub typhus patients with doxycycline, Murhekar says. Meanwhile, studies are underway to index mite species present on rodents and understand their seasonality and to document levels of O. tsutsugamushi infection in rodents and mites.

“Besides these studies, feasibility of different vector control strategies in the area also needs to be assessed,” says Murhekar.