BY DR TONY MONDA
The impact of climate change on biodiversity and on tick-borne diseases is unpredictable. Human behaviour and awareness is also a strong determinant of environmental health, animal health and human health.
The world over cattle and other livestock are experiencing both, ecological and biological stress due to climate change, which could affect the wellbeing of livestock globally and ultimately of man. The scientific evidence for rapid climate change is compelling and alarming.
Climate change will have profound consequences on livestock production across Africa in multiple ways, both directly and indirectly.
The most important impacts will be experienced in animal health and productivity as well as yields of forages and feed crops. Vector-borne livestock diseases and the duration of severe heat stress are both projected to become more prevalent under global warming.
With temperature increases above 2°Celsius of global warming, distribution and seasonal transmission of vector-borne diseases are expected to increase, exposing tens of millions more people, mostly in West, East and Southern Africa to hunger and to loss of livelihoods.
The burden of vector-borne and climate-sensitive diseases is greatest for the poorest populations, especially in Africa. According to the World Health Organisation (WHO), many cattle herds in Africa are depleted annually from vector-borne diseases such as Theileriosis, an important disease of cattle.
Vector-borne diseases are among the most well studied of the diseases associated with climate change, owing to their large disease burden, widespread occurrence and high sensitivity to climatic factors. A keynote talk given in 2015, in Spain, on climate change, biodiversity, ticks and tick-borne diseases, on the Impact of Environmental Changes on Infectious Diseases (IECID), revealed the inter-dependency of climate and the ecology and how they impact on tick-borne diseases.
Climate and weather conditions also exert a range of more indirect effects, through wider effects on the natural environment and on human systems; for example, drought may affect water-storage, land-use and irrigation practices, and population movement, and in turn, affected vector ecology, and human exposure to infection.
A WHO report summarizing 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 urbanization 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.
The effect of climate change – particularly of increasing temperatures in tropical zones may be detrimental to some species, adversely affecting habitat suitability and forcing certain tick species to colonize new areas. In Southern Africa, for example, it has been predicted that increasing the temperature by 2°C will decrease habitat suitability for four tick species – i.e. the African blue tick Rhipicephalus decoloratus, the Southern African bon tick Amblyommahebraeum, the brown ear tick Rhipicephalus appendiculatus and the small smooth bont-legged tick Hyalommatruncatum.
Another study suggests that the progressive increase in temperatures seems to be forcing the dispersion of tropical bont tick Amblyommavariegatum 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 and temperature conditions, and human activities.
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 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 and Zimbabwe must not be found wanting. In the meantime, due to the high costs of theilericidal drugs and the high costs of treatment of Theilerial infection, prevention is the best means to control the vector ticks. Manual removal of ticks is a common practice, but must be done correctly
Dr.Tony M. Monda BSc, DVM, DPVM, is conducting Veterinary epidemiology, disease control, agro-climatology and agro-economic research in Zimbabwe. E-mail: tonym.MONDA@gmail.com
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