Climate change and infectious disease

WRITER: ABIGAIL BUTCHER, ABUTCHER@UGA.EDU CONTACT: COURTNEY MURDOCK, CMURDOCK@UGA.EDU

Can global climate change have a positive impact on the transmission potential of vector-borne diseases like malaria? Dr. Courtney Murdock, of UGA’s College of Veterinary Medicine and the Odum School of Ecology, suggests that recent research conducted in the field of thermoecology disrupts the common narrative regarding the correlation between rising temperatures and mosquito metabolic rates. Murdock credits this finding to her non-siloed research approach and the cross-disciplinary support her lab receives from modelers and field researchers alike. She intends to utilize these theoretical and pedagogical methods in her upcoming research on the Zika virus in coordination with an NSF initiative announced earlier in the year.

To address the emerging problems related to infectious diseases like malaria or Zika, Dr. Murdock works across biological scales. Her interest in host processes and how they influence individual variation in phenotypes extends to how that can affect population dynamics. Her current work on malaria illustrates this fluid research methodology, which looks mechanistically at how temperature influences the mosquito-malaria interaction. Murdock’s lab uses RNA sequencing to generate transcriptional profiles of both the mosquito and the malaria parasite to see how temperatures affect the physiology of those organisms as well as the interaction between the two – called a “G by G by E interaction” due to the interacting effects of two different genotypes (the mosquito and the parasite) and an environmentally mediating factor. Murdock plans to link that work to phenotypic measurements her team and collaborators are currently collecting to better understand how temperature affects things like the proportion of the mosquito population that will become infectious and the rate at which a parasite develops to infectiousness.

This work on the effects of temperature on physiology has caused Murdock to take issue with a dogma regarding climate change and its relationship to the transmission of vector-borne infectious diseases. Up until fairly recently, the common belief was that climate change would increase the global mean temperature, extending the season mosquitoes have for transmission. The assumption was that by increasing mosquito metabolic rates (reducing development time), the potential for transmission would increase. However, the literature supporting this position employed models which presumed that temperature possessed a fixed linear relationship to mosquitoe and parasite traits associated with transmission.

Sensing an oversight, Dr. Murdock turned to the research of her collaborator Dr. Erin Mordecai, an n ecologist who built models with non-linear relationships between temperature and the mosquito and parasite traits that are important for transmission. With these relationships incorporated, Mordecai’s group made a thermally dependent model which illustrated the nonlinear relationship of malaria transmission potential.

Inspired by this approach, Dr. Murdock’s latest work explores the consequences of these nonlinear thermal relationships. While there is good evidence to suggest that warming in cool and marginal regions for the parasite results in increased malaria transmission, less is known on how future warming in already highly permissive areas for the parasite will affect transmission. In laboratory experiments, she simulated warming scenarios above the parasite’s predicted thermal optimum (from around 25 degrees C) by placing infected mosquitoes at 27 C, 30 C, and 33 C in an effort to understand its affect on transmission rates in warmer climates. Murdock also incorporated diurnal temperature variation to replicate conditions in the field. What she found was that transmission potential decreases significantly (by 60 to 85%) with warming temperatures. Additionally, increases in temperature fluctuation exacerbate the effect of warming temperature. This also results in decreases in transmission potential. Her findings suggested that climate change could result in a decrease in malaria transmission potential in some places.

Murdock’s success using nonlinear thermal dynamics to understand mosquito-borne disease transmission will play a critical role in future research concerning the Zika virus. As the transmission of Zika is not currently well understood, her lab experiments will focus on an analysis of mechanistic and population level questions while focusing on temperature as a large driver in the dynamics of vector-borne diseases in general. Her emphasis on thermal performance will also generate critical data, which will then be used to construct mathematical and statistical models aimed at improving prediction. Such a model could be used for real time forecasting, in addition to providing insight into the effectiveness of different strategies for mitigating Zika transmission.

Not one to be siloed in the hard sciences, Dr. Murdock is actively coordinating with faculty inside and outside of the University of Georgia to better understand how socio-demographic predictions of human exposure integrate with environmental variables to influence disease risk. Such work will help determine and address the impact these factors have on people across the socioeconomic spectrum.