Researchers predict Tularemia outbreaks as climate changes

By Dr. Ananya Mandal, MDOct 15 2019

Stockholm University researchers have found that the impact of climate change of disease incidence of tularemia could be statistically predicted. In high-latitude regions, where the disease was less common, global warming could lead to increasing number of cases of the disease they wrote.

The researchers chose tularemia as a study model because, “tularemia, (is) one of the most studied diseases in high-latitude regions, which is likely to be impacted by large regional hydroclimatic changes,” they wrote. The study titled, “Potential for Hydroclimatically Driven Shifts in Infectious Disease Outbreaks: The Case of Tularemia in High-Latitude Regions,” was published in the latest issue of the journal International Journal of Environmental Research and Public Health.

The team of researchers wrote that with climate change there is a slow and steady rise in the temperatures of the high latitude regions of the world. This leads to a change in the rain, snow and water flow patterns they wrote. Living conditions and insect populations including vector populations are also affected with the changes in the climate, they explained. They wrote, “Hydroclimatic changes may be particularly pronounced in high-latitude regions and can influence infectious diseases, jeopardizing regional human and animal health.” They hydroclimatic changes, they write include, “runoff, evapotranspiration, and often both”.

The team from the Department of Physical Geography at Stockholm University explain that diseases such as tularaemia are carried by small rodents and also affect humans. With changes in climate and water conditions there is a rise in the vectors of this infection and thus the number of cases is also on the rise. A record number of cases of tularaemia for example have been reported from Sweden of late, they wrote. The authors add, “tularemia, caused by the arthropod-borne pathogen Francisella tularensis, affects wild animals and humans, and is one of the most researched high-latitude vector-borne diseases likely to be impacted by hydroclimatic change.”

Bacteria Francisella tularensis, Gram-negative pleomorphic bacteria which cause zoonotic infection tularemia - Illustration Credit: Kateryna Kon / Shutterstock

According to the authors the mechanism behind rise in such cases are due to several reasons including, “abundance of vectors, such as mosquitoes and ticks, pathogen survival outside the host, host-pathogen interactions that are closely related to community ecology and biodiversity, dampening of host immunity, exposure to water-borne pollution and associated infections, including from remobilization of previously frozen pathogens through permafrost thaw under warming and deterioration in health status, e.g., malnutrition and disruption of health systems associated with extreme hydroclimatic events, such as droughts and floods.”

Gia Destouni, co-author of the study, in a statement said, “tularemia is fatal to rodents and a serious disease in humans. We have used a tested statistical disease model that has, for example, been able to predict six out of seven years with a relatively large number of tularemia cases in humans in Dalarna County between 1981 and 2007. This type of tested statistical disease model is a valuable tool that enable us to predict outbreaks in the future under altered climactic and water conditions.”

The disease spreads from rodents to humans from bites of mosquitoes is considered to be the commonest transmission mode Destouni says. The vast areas of forests in Sweden, Finland, Russia and Alaska have a different eco system that is fast changing she said. She added, “Northern communities and ecosystems are particularly vulnerable to climate change, which is greater here than the global average. For this study, we chose to look at tularemia because it is a well-studied disease with tested statistics on the spread of the disease to humans at these latitudes.” Speaking on their study model she said, “We were then able to use tularemia as a model disease for the development of a new methodology for predicting how changed temperatures and water flows can lead to new circumstances for the spread of the disease to humans in northern communities. In future research, similar methodology can be used for similar studies of the effects of climate change on different infectious diseases and in different parts of the world.”

The results of their study showed a validated statistical model for “quantifying possible hydroclimatically driven shifts in outbreak conditions”. Their work revealed, “high sensitivity of tularemia outbreaks to certain combinations of hydroclimatic variable values.” The authors wrote, “The methodology developed also facilitates relatively simple identification of possible critical hydroclimatic thresholds, beyond which unacceptable endemic disease levels may be reached.” Authors wrote in conclusion, “The generality and spatiotemporal transferability of statistical disease models, such as that for tularemia used in this study, also need to be investigated in further research.”