The Veterinary Journal 2000, 160, 87–89 doi: 10.1053/tvjl.2000.0501, available online at http://www.idealibrary.com on
Guest Editorial Global Climate Change and Vector Borne Diseases
Global warming, if it does occur as predicted, can be expected to have considerable effects on worldwide patterns of disease occurrence. In particular, it has been suggested that arthropod-borne diseases will have greatly expanded ranges. Wittmann and Baylis (2000) in this issue of the Journal outline their opinions on the effects climatic change may have on the distribution in the UK of two viruses of veterinary economic significance, which are transmitted by Culicoides species. Placing their concerns in wider context, both in terms of international animal disease control and the quantitative likelihood or risk that such events will occur, is a natural development of the debate. Predictions of the extent and effect of climate change involve many variables and are complex and fraught with uncertainty. This was recognized by the Inter-Governmental Panel on Climate Change (IPCC) in the preparation of its Special Report on the regional impacts of climate change (Watson et al., 1997). Globally, by 2100, the projected increase in annual surface temperature is between 1 and 312°C. It is projected that the global mean sea level will increase by 16 cm and that there will be spatial and temporal patterns of precipitation. Importantly, the Report makes it clear that local climatic effects due to such unexpected events as changes in the direction of flow of marine water streams (such as the Gulf Stream) cannot be predicted with confidence. Within the UK, similar temperature changes are expected and geographical and seasonal contrasts in rainfall will become more evident. Scotland and the north will become wetter, and more rain will fall in winter (Hulme & Jenkins, 1998). The IPCC Report predicts that the human health consequences of these changes in global terms will be relatively small compared with the total burden of ill health. Likely mechanisms by which climate change might affect human and 1090-0233/00/020087 + 03 $35.00/0
animal health include increases in heat stress mortalities, vector-borne diseases and urban pollution. Increased temperatures, urban population density and flooding might also increase problems together with salmonellosis, cholera and giardiasis. Increased sea surface temperature and level could also increase the risk of water-borne toxicities such as algal blooms or red tides of dinoflagellate algae (Patz et al., 1996), with a consequent increase in the risk of cholera due to associated zooplankton (Epstein, 1995). The timeliness of the paper by Wittmann and Baylis is reinforced by the recent outbreak of West Nile virus (WNV) in New York City in the Summer and Autumn of 1999. West Nile virus, a flavivirus that is transmitted by Culex, Aedes and Anopheles mosquitoes and is endemic among bird populations in Africa, the Middle East and Asia had never previously been identified in the Americas (Studdert, 2000). The portal of entry of the virus remains unknown. The virus is capable of causing signs ranging from pyrexia through to fatal myeloencephalitis and hepatitis in a range of hosts, and in this outbreak was responsible for the deaths of seven of 62 human cases and 13 of 24 affected horses. One hundred and ninety-four of 550 birds from nine different orders that were tested at the Center for Disease Control and Prevention were positive for WNV (OIE, 2000). Arthropod vector-borne diseases are particularly likely to be influenced by climatic change, mainly due to the influence of climate on the distribution, physiology and behaviour of the vectors. Although the complexity of the relationship between climate and vectorial capacity is made clear by Wittmann and Bayliss, it is generally expected that global warming will tend to increase the suitability of large areas of the world for colonization by mosquitoes and the ceratopogonidae and many other species. © 2000 Harcourt Publishers Ltd
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There is a considerable body of literature directed at predicting climate-induced changes in the risk of vector-borne diseases of humans. For example, Patz et al. (1998) predicted that by the year 2050 climatic changes would result in the aggregate epidemic potential of dengue fever in selected cities increasing by between 31 and 47%. Bryan et al. (1996) used a commercially available climate-matching model and predicted that by 2030 Anopheles farauti sensu stricto, an important vector for malaria in Australia, is likely to extend its range by 800 km south along the coast of Queensland into areas of significantly higher population density. However, it has also been argued that human public health activity is likely to be a much more important factor in deciding whether malaria will become established than climatic considerations alone (Walker, 1998). Other important considerations that impede useful prediction of disease distribution include the interaction of local effects, and the failure of linear relationships between climatic indices and vector suitability. For example, increased rainfall in normally dry areas might increase vector breeding, but only where it results in an increase in the availability of standing water. In wet areas, reduction in rainfall might lead to pooling of rivers that normally flow, with associated increases in vector breeding (Hales et al., 1997). Factors that may have confounding effects with climate on animal and human health include increasing poverty and urbanization, increasing international travel by humans and animals, trade liberalization, civil conflict and migration of refugees, water availability for stock and humans, increasing landscape fragmentation and heterogeneity, population growth and immune status, land-use patterns, resistance profiles of vectors and pathogens, and weakening public and veterinary health infrastructure. The effects of reaching global thresholds for these factors, non-linear dynamic effects, and unforeseen interactions among them mean that it is impossible to accurately predict outcomes (Hales et al., 1997). Clearly, the factors likely to influence disease patterns in association with global climate change differ substantially between developing and developed economies. However, reflecting on the complex web of factors influencing the distribution of any vector species, Sutherst (1998) concluded that ‘Despite all these influences, it is evident that climate is the dominant factor that most often sets the limits to the species’ distribution.’
Two further outbreaks of disease, both reported on ProMED-mail this year, are worthy of mention. In the first small outbreak Theileria annulata was responsible for the death of two cattle at a research institute in the UK, probably a result of accidental infection from deliberately infected research cattle (ProMED-mail, 13 and 19 June 2000). It appears that the infection has been contained and, in any case, the organism’s usual vector, the Hyalomma spp tick, is not known to exist in the UK. The second incident was the confirmation of anaplasmosis in a herd of bison in Saskatchewan, well outside the normally expected range of the normal vectors of Anaplasma spp (ProMED-mail, 15 and 20 June 2000). These incidents highlight deficiencies in our knowledge of the epidemiology of vectorborne diseases outside their normal range. Is it possible for Theileria annulata to be transmitted by other vectors, or can it cycle via mechanical transmission? To what extent can biting flies and iatrogenic mechanical transmission maintain anaplasmosis in a susceptible population? Unfortunately, the experimental work needed to address these questions often carries with it some risk of escape infections. In the UK, the potential for new vectors and pathogens to become established is a possibility and suitable micro and meso climatic conditions undoubtedly exist. Corridors of opportunity, whether import of infected animals and passage through quarantine or physical transmission of infected arthropods across geophysical barriers, must be identified. For the epidemiologist, the subject provides fertile grounds. Surveillance will be a lynchpin in maintaining an exotic vector and/or pathogen-free state. Using accepted terminology, it will inevitably be a combination of active and passive surveillance that will provide the best means of identifying exotic diseases; targeted methods are expensive and, although useful for detecting ‘expected’ occurrences, are ineffective for unexpected, rare events. Passive, clinical sampling methods, whilst subject to inherent sampling bias and unsuitable for establishing quantitative measures of incidence and prevalence (particularly of infection rather than disease), do benefit a priori from the importance of the sample having come from a sick animal. A good example was the detection of the West Nile Virus in New York described above, a pathogen that it is unlikely to have been the focus of a targeted active surveillance programme. The likelihood of outcome is central to these considerations.
In terms of international disease control, risk analysis (risk assessment, risk management and risk communication) is now well established as a tool appropriate for establishing import and export policy. The first step in this process is the identification of the hazard, a step, in relation to climate change, that Wittmann and Baylis have taken. We now have a responsibility to face up to the challenges that they have identified and establish the risks associated with these hazards. There can be little doubt that lack of data will be an issue in any such risk assessment and, whilst developing methods address uncertainty and variability, it is important that the models are seen as a starting point and are used to identify data gaps and direct future research. Only when the risks are established can the identification and implementation of strategies that will mitigate the risk be rationally formulated. N. N. JONSSON1 AND S. W. J. REID1,2,3 Farm Animal Medicine and Production and 2 Comparative Epidemiology and Informatics, Department of Veterinary Clinical Studies University of Glasgow Veterinary School, Bearsden Rd, Glasgow G61 1QH and 3Comparative Epidemiology and Informatics, Department of Statistics and Modelling Science, Livingstone Tower, University of Strathclyde, Glasgow G1 1XH, UK 1
REFERENCES BRYAN, J. H., FOLEY, D. H. & SUTHERST, R. W. (1996). Malaria transmission and climate change in Australia. Medical Journal of Australia 164, 345–347. EPSTEIN, P. R. (1995). Emerging diseases and ecosystem instability: new threats to public health. American Journal of Public Health 85, 168–172. HALES, S., WEINSTEIN, P. & WOODWARD, A. (1997). Public health impacts of global climate change. Reviews on Environmental Health 12, 191–199.
HULME, M. & JENKINS, G. (1998). Climate change scenarios for the United Kingdom, Summary Report, UK Climate Impacts Programme Technical Report No. 1. Norwich: Climatic Research Unit, University of East Anglia. PATZ, J. A., EPSTEIN, P. R., BURKE, T. A. & BALBUS, J. M. (1996). Global climate change and emerging infectious diseases. Journal of the American Medical Association 275, 217–223. O. I. E., (2000). West Nile fever in the United States of America: final report. Disease Information 13. http://www.oie.int/info/AIS_03.HTM. PATZ, J. A., MARTENS, J. M., FOCKS, D. A. & JETTERN, T. H. (1998). Dengue fever epidemic potential as projected by general circulation models of global climate change. Environmental Health Perspectives 106, 147–153. ProMED-mail. ProMED-mail, the Program for Monitoring Emerging Diseases, a program of the International Society for Infectious Diseases. http://www.promedmail.com:8080/promed /promed.home STUDDERT, M. J. (2000). West Nile virus finds a new ecological niche in Queens, New York. Australian Veterinary Journal 78, 400–401. SUTHERST, R. W. (1998). Implications of global change and climate variability for vector-borne diseases: generic approaches to impact assessments. International Journal for Parasitology 28, 935–945. WALKER, J. (1998). Malaria in a changing world: an Australian perspective. International Journal for Parasitology 28, 947–953. WATSON, R. T., ZINYOWERA, M. C., MOSS, R. H. & DOKKEN, D. J. (Eds) (1997). The Regional Impacts of Climate Change: An Assessment of Vulnerability, a Special Report of IPCC Working Group II Published for the Intergovernmental Panel on Climate Change. November 1997. ISBN: 92-9169-110-0. WITTMAN, E. J. & BAYLIS, M. (2000) Climate change: Effects on culicoides-transmitted viruses and implications for the UK. The Veterinary Journal 160, 107–117
(Accepted for publication 11 July 2000)