By Caroline Wanja
Current public efforts are targeted to malaria control, but the six scientists in a paper, highlight aedes aegypti and chikungunya, and other arboviruses, as potential emerging public health threats in subSaharan Africa (SSA).
Six scientists say that the direct effects of warming temperatures are likely to promote greater environmental suitability for dengue and other greater arbovirus transmission by aedes (Ae) aegypti and reduce transmission for malaria transmission by anopheles gambiac.
According to Eric Modercai, Sadie J. Ryan, Jamie Caldwell, Melissa M. Shah and Desiree LaBeaud, in a paper published by the Lancet, environmentally driven changes in disease dynamics, will be complex and multi-faceted, but given that current public efforts are targeted to malaria control, they highlight aedes aegypti and chikungunya, and other arboviruses, as potential emerging public health threats in sub-Saharan Africa (SSA).
Malaria has placed a major burden on mobility and morbidity on SSA, of 228 million cases and 405,000 deaths as of 2018, despite efforts at curbing the spread of the diseases in the last 20 years.
At the same time, there are many vector-borne diseases such as Rift Valley fever, dengue, chingunyuka, yellow fever, Zika, o’nyong’nyong, West Nile fever, leishmansis, onchocersiasis and African tryponasomiasis, circulate regularly in humans, livestock and wildlife in SSA, though their burden is well-characterised than malaria.
“As an example, there have been over 27,000 cases of arbovirus transmitted in West Africa by aedes mosquito since 2007. The direction and magnitude of the effects of climate change on the transmission of specific water-borne diseases will differ across the regions,” state the scientists in their paper titled: “Climate change could shift disease burden from malaria to arboviruses in Africa” Dengue, chikungunya, and their Ae aegypti mosquito vector are already widespread but under-recognised in Africa, based on studies of vector abundance, human serology, and acute infections from across the continent.
The scientists argue that as climate suitability increases for arboviruses, these diseases could expand and overtake the public health burden of malaria.
The scientists further argue that climate change and urbanization could further drive a shift across most countries in the SSA, from climates that are most suitable to the transmission of malaria, to climates that are almost suitable to dengue and other arboviruses, with major consequences for public health and diseases control strategy.
The scientists draw their conclusions from three lines of evidence: transmission models fit from lab thermal performance data; independent data from human infections; and widespread infections of aedes aegypti, dengue and chingunyuka in SSA.
A study of dengue in 20 cities in Colombia showed a unimodal relationship between incidence and weekly average temperature. There were multiple time windows and lags which were explored.
They peaked at a mean temperature of 28 degrees Celsius, supporting the model-predicted optimum for dengue transmission of 29 degrees Celsius. Other studies have predicted effects of temperature on transmission, peaking at 25 degrees Celsius for malaria (supported by continental-scale data on entomological inoculation rate in Africa) and 29 degrees Celsius for dengue, chikungunya, and Zika viruses (based on human incidence data from Latin America and the Caribbean).
“The dynamics of vector-borne diseases are multi-faceted and involve human mobility and rainfall, water storage practices, urbanization and other practices,” said the six scientists.
Climate change will affect vector-borne disease transmission because temperature changes will affect vector size population size, survival, biting, pathogen incubation rates, and vector competence, rainfall and humidity.
The physiological effects of temperature on the vector are well-established from laboratory experiments and field studies. Multiple vector and parasite traits cause differences in thermal rates of transmission across species.
Field studies from both mosquito-based metrics of transmission risk and human incidence at local and international scales strongly support the non-linear effects of temperature on transmission predicted from laboratory studies and mathematical models.
As climate change leads to warming temperatures, the intermediate thermal optima for vector transmission have two immediate implications, argue the six scientists. First for all vector-borne diseases, climate change drive will increase in some regions and decreases in others, depending on current and future local climates relative to the thermal optima for disease transmission.
Secondly, the relative suitability for different vector-borne diseases will shift: the climate could simultaneously become more suitable for some diseases and less suitable for some diseases.
Specifically, the highest density of people exposed to optimal temperatures for disease transmission, (the so-called risk hotspot) for malaria is expected to shift towards higher elevations such as the Albertine Rift Region in Central Africa and higher latitudes in Southern Africa. The risk hotspot for dengue, chingunya and other ae egypti-transmitted arboviruses is predicted to expand from West Africa throughout SSA
