CLIMATE and weather conditions exert a range of more indirect effects through wider effects on the natural environment and on human systems.
For example, as drought may affect water-storage, land-use, irrigation practices and population movement, in turn, affecting vector ecology and human exposure to infection.
For example, a recent World Health Organisation (WHO) report summarising the importance of vector-borne diseases states that previously relatively stable geographical distributions are now changing owing to a range of factors – including climate change, intensive farming, dams, irrigation, deforestation, population movements, rapid unplanned urbanisation and phenomenal increases in international travel and trade.
Given the strength and range of these connections, it is not surprising that there is abundant observational evidence of the effects of meteorological factors, from seasonal and inter-annual patterns of disease incidence in specific locations, to the sturdy explanatory power of climate variables when accounting for the geographical distribution of most, if not all, vector-borne diseases.
These influences may reinforce each other. For example in locations where the temperature increases associated with El Niño events are superimposed on long-term increase in temperature, or may oppose each other, for example as changes in global temperature over the past decade or so appear to have currently damped the longer-term upward temperature trends – a phenomenon currently occurring in Zimbabwe and many other parts of southern Africa.
The effect of climate change, particularly of increasing temperatures in tropical, zones may be deleterious to some species, adversely affecting habitat suitability and forcing certain tick species to colonise new areas.
In South Africa, for example, it has been predicted that increasing the temperature by 2°C will decrease habitat suitability for four tick species — the African blue tick rhipicephalus decoloratus, the South African bont tick amblyomma hebraeum, the brown ear tick rhipicephalus appendiculatus and the small smooth bont-legged tick hyalomma truncatum.
Another study suggests that the progressive increase in temperatures seems to be forcing the dispersion of tropical bont tick amblyomma variegatum towards areas outside of zones that have a prolonged dry period such as in Zimbabwe. The study of tick activity, reproduction and survival depend on several factors which, in turn, have a direct impact on tick distribution and abundance.
These include vegetation coverage, host availability, moisture, temperature conditions and human activities.
A complete understanding of all aspects involved in the transmission dynamics of tick-borne pathogens is possibly beyond current human capabilities.
Additional knowledge on ticks, animals, pathogens and their interactions with the whole ecosystem will be needed for a comprehensive understanding.
Climate is an important influence on vector-borne disease transmission, and there is evidence that ongoing climate change is affecting, and will continue to affect, the distributions and burdens of these infections.
There has been a marked increase in research output on climate change and health in recent years, with vector-borne diseases among the better-represented subject areas.
In adjunct to the World Health Assembly Resolution on climate change and health in 2008, the African Regional Health and Environment Ministerial process has identified strengthening health protection from climate change as a top priority for the region.
In their 2008 and 2010 conferences, health and environment ministers from across the region expressed their concern that Africa is already experiencing the effects of climate change, which are likely to become more severe, exacerbate environmental risk factors to human health and undermine Africa’s progress towards the Millennium Development Goals (MDGs).
Consequently, they committed to work across sectors to manage environmental and health risks related to climate variability and change.
There is convincing evidence indicating the direct or indirect effects of global changes on tick-borne diseases.
Importantly, it is impossible to disconnect the mutual influences of global changes such as deforestation, land use change and climate change on tick-borne pathogen transmission systems, as several of these factors may act synergistically on hosts, vectors, pathogens and humans themselves.
Many recent studies have investigated the influence of climate change on tick-borne diseases which occur in different parts of the world.
Zimbabwe must not be found wanting, especially if the national herd is at risk.
Dr Tony Monda is a writer, an agro-researcher, critic and Heritage Studies consultant. He holds a PhD in Art Theory and Philosophy, and a Doctorate in Business Administration and Post-Colonial Heritage Studies.