Increase in Typhus Witnessed in Laos Following Changing Weather Patterns

A new study reveals that the changing weather patterns in Laos could be the reason behind the increase in bugs responsible for the neglected tropical diseases murine typhus and scrub typhus.

Scrub typhus, also called bush typhus, is caused by the Orientia tsutsugamushi bacterium and is spread by the bites of infected chiggers (larval mites). Flea-borne (murine) typhus is the result of Rickettsia typhi bacterium and is spread to people by fleas via animals such as cats, rats or opossums.

The study’s lead researcher Tamalee Roberts, from Lao-Oxford-Mahosot-Hospital-Wellcome Trust Research Unit in Vientiane, Laos, says that little is known about what causes the spread of these common but severely ignored causes of fever. Scrub typhus is found in the Asia-Pacific region and South America while murine typhus is a worldwide disease.

The study, reported in PLOS, included samples for scrub typhus and murine typhus collected between 2003 and 2017, which were then given to the Mahosot Hospital. Examination of patient data together with meteorological and environmental data revealed 17% of patients tested positive for either murine typhus (1,283 of 7,552 patients tested) or scrub typhus (1,337 of 8,150 patients tested).

Roberts stated that the scientists learned that scrub typhus was very seasonal, with cases two times more likely to emerge in the wet season months of July to September compared to the dry season, while murine typhus increases in the dry season.

It was discovered that the incidence of scrub typhus was related to fluctuations in relative humidity whereas murine typhus was related to variation in temperature, Roberts explains. Patients with scrub typhus disease were more likely residents of villages with higher levels of vegetation and surface flooding in the 16 days before the diagnosis.

As cities grow, so will high-risk zones for murine typhus, declares Roberts.

With global heating and risks of attendant higher precipitation, the data suggest that the incidence and spatial distribution of both murine typhus and scrub typhus will increase.

Tamalee Roberts, Study Lead Researcher, Lao-Oxford-Mahosot-Hospital-Wellcome Trust Research Unit

She also emphasizes that additional work is required to evaluate whether the results can be replicated anywhere else.

These results can be used within the region to help predict changes in the distribution and seasonal timing of these diseases, to inform strategies to reduce their incidence and impact.

Tamalee Roberts, Study Lead Researcher, Lao-Oxford-Mahosot-Hospital-Wellcome Trust Research Unit

When weather patterns change, they may also impact the lifecycle of fleas and chiggers with heat causing faster breeding time for fleas and higher amounts of rain and surface water risking higher chigger density and, therefore, increasing the danger of scrub typhus, Roberts notes.

Roberts explains that “the evidence from this research suggests that we will see an increase of both these diseases not only in Laos but in other countries as well. The diseases may also spread to areas where they have not previously been found as temperatures rise.”

Thomas Weitzel, a physician and faculty member at the Clinica Alemana de Santiago, Chile, and part of the Chilean Rickettsia and Zoonosis Working Group, conveys to SciDev.Net that the study is of high significance because both infections are a part of a group of vector-borne diseases, which have been ignored over the past decades.

Mites and fleas as vectors are often overlooked and our knowledge gaps are much larger in comparison to diseases transmitted by mosquitoes and ticks. Therefore, we often underestimate the burden of mite- and flea-borne rickettsioses, which cause important clinical problems (morbidity and mortality) and are easily treatable.

Thomas Weitzel, Physician and Faculty Member, Clinica Alemana de Santiago, Chile

Weitzel adds the research outcomes show that climate will impact the epidemiology of these diseases.

Our understanding of the complex ecology, however, including the interactions of bacterial pathogen, arthropod vectors, animal reservoirs, and human behaviour is still limited, and it is, therefore, too early to draw concrete conclusions.

Thomas Weitzel, Physician and Faculty Member, Clinica Alemana de Santiago, Chile

Journal Reference:

Roberts, T., et al. (2021) A spatio-temporal analysis of scrub typhus and murine typhus in Laos; implications from changing landscapes and climate. PLOS. doi.org/10.1371/journal.pntd.0009685.