Like it or not, fighting climate change is critical to controlling the spread of the Zika virus.
News of the Zika virus outbreak in Brazil has prompted me to consider two environmental aspects affecting the control of infectious disease. Firstly, how changes in climate are influencing the abundance and seasonality of disease vectors which place humans at greater risk of infection. Vectors are animals such as mosquitoes that transmit viruses to humans or other animals they come into contact with. Secondly, how much we have learned from recent history given that the link between vector borne infection and climatic conditions has been studied for at least the last two decades, based on my review of the available scientific findings. My findings emphasise the importance of combating global climate change as a crucial factor in controlling the spread of the Zika virus, and other vector borne diseases in the future.
In 2000, the World Health Organisation (WHO) Bulletin published an article entitled ‘Climate change and vector-borne diseases: a regional analysis’ discussing the latest research at the time, looking at the role rising temperatures play in increasing the transmission of vector-borne diseases. This analysis focused on a number of regions around the world including Africa, South and North America, Asia, Australia and New Zealand.
The WHO Bulletin noted that mosquito species including Aedes aegypti are responsible for transmitting most vector borne diseases reported to medical authorities – including the Zika virus, which is related to dengue fever, yellow fever and the West Nile virus. These species are very sensitive to temperature changes, both as larvae in aquatic environments and as adults. Higher water temperatures mean larvae will mature quicker and more offspring may be produced during the transmission period. In warmer climates, female adult mosquitoes digest blood faster and feed more frequently which means increased breeding rates and therefore higher mosquito populations. Higher breeding rates and larvae maturing faster mean there are more disease carrying mosquitoes that can transmit the disease to humans and animals that come into contact with humans.
Changing rainfall patterns have short and long term effects on vector disease carrier’s habitats. Higher rainfall can increase the number and quality of breeding sites for mosquitoes, ticks and snails which can transmit viruses to humans and other animals they come into contact with. Patterns of human settlement can also influence trends in disease transmission. In South America, over 70% of the population live in urban areas so they are less likely to be exposed to disease carrying mosquitoes. In Africa, more than 70% of the population live in rural areas and come into closer contact with larval breeding sites. Their risk of exposure to disease carrying mosquitoes is higher.
Looking at this research, I was particularly interested in the WHO’s analysis of the influence of temperature ranges on growth and development stages of the mosquitoes that transmit diseases such as the Zika virus. They noted that for many diseases, incubation in vectors is interrupted at 14-18°C as this temperature is too low to drive the required biological processes. As temperatures rise, malaria parasites and viruses complete their incubation period in female mosquitoes faster. At 35-40°C disease transmission may decline due to higher vector death rates which reduces the number of breeding age mosquitoes.
The WHO report also pointed out that tropical and temperate climates in Asia provide ideal climatic conditions for the spread of a number of diseases. These include malaria, dengue fever, dengue haemorrhagic fever and schistosomiasis. Research has shown that El Niño and other weather events may play a significant part in making conditions more favourable to the establishment and maintenance of mosquito populations. The incubation period of dengue 2 virus in the Aedes aegypti mosquito could be reduced from 12 days at an ambient temperature of 30°C to 7 days at 32-35°C.
In additional research, September 2013 saw a research team from the University of Arizona (UofA) School of Geography and Development investigate the abundance and seasonality of West Nile virus vectors in their article entitled ‘Regional and seasonal response of a West Nile virus vector to climate change’. Their article was published in the Proceedings of the National Academy of Sciences.
The UofA research team investigated on the vector Culex quinquefasciatus because it feeds on bird and human hosts, is found in urban environments, and is suspected to be the primary vector of the West Nile Virus in many southern US states. As changes in climate can differ between locations and over time, mosquito sensitivity to these changes is dependent on the seasonal timing and intensity of rainfall. Warmer temperatures can extend the period of time that mosquito populations are active. Therefore, as virus incubation periods decrease, they can spread within the mosquito population faster, meaning that viruses can ultimately be transmitted faster.
These researchers concluded that climate change will modify seasonal mosquito population levels across the US, and thus public health interventions must be robust enough to account for site-specific climate changes in order to implement the most effective strategies to control the transmission of the West Nile virus to humans. Further research was needed to better understand the response of birds carrying the West Nile virus to climate change. The implications of changes in land cover and land use also needed to be investigated as these changes can influence how resistant the land is to drying. If land is more resistant to drying, more water can collect, which can facilitate the establishment of mosquito populations.
Based on what has been reported in the media, the Zika virus is clearly one that we need to pay close attention to from both human health and environmental perspectives. The health consequences are clearly horrendous and can’t be ignored. Based on the scientific literature, I would suggest that the evidence of a link between changes in climate and the spread of vector borne diseases is also one that can’t be ignored.
As a scientist I would like to think that the Zika virus outbreak is the incident that awakens the world from inaction on climate change as we realise that is critical to controlling the spread of these infectious diseases. As a realist though, I fear that the next time a vector borne disease outbreak occurs, we will give due attention to the symptoms however we may not pay equal attention to fighting climate change which is critical to controlling the spread of the Zika virus and other similar viruses.