2.6 Climate and animal disease
The effect of climate on an animal disease varies considerably depending on the disease, and on the management and husbandry systems used. Here we briefly describe various types of interaction between climate and disease through a number of examples, using work from a number of recent studies in Australia.
For some diseases, climate factors may determine the pattern of the disease through direct effects on the distribution of the agent. Screw-worm fly (SWF) is a case in which the disease is the pest itself. Although SWF is not found in Australia, it is considered a serious threat to Australia's livestock industries, and studies have been undertaken to determine the areas at risk if it were introduced, and optimal control strategies.
Climate may modulate the persistence and expression of a disease. Anthrax, which can affect a wide variety of domestic and wild animals, and humans, is an example of this type of interaction.
More complex interactions between climate and disease can occur if there are multiple hosts and vectors involved. Arboviral diseases such as bluetongue and Akabane in animals, and Japanese encephalitis and Ross River virus in man, fall into this category.
For other diseases, spread may usually be determined primarily by local conditions, but under certain circumstances, climatic factors can play a significant role. Foot-and-mouth disease (FMD) is an example of such a disease. Although it is spread mainly by close contact between susceptible animals and infected animals, suitable weather conditions can in some situations lead to long distance spread of airborne virus. FMD is considered one of the most serious diseases of livestock and because of its potential to cause major disruptions to Australia's trade in livestock and livestock products has been the subject of intensive study.
Because of the complexity in the interaction between climate and disease, multi-disciplinary approaches involving skills in epidemiology, modelling, meteorology and geographic information systems are often required to understand and study the processes involved. Some recent examples of modelling studies undertaken in Australia are described below.
Screw worm fly
Screw-worm flies (SWF) are obligate parasites of warm-blooded animals, including humans. Adult SWF lay their eggs on the edges of wounds and the larvae burrow into the underlying tissues to feed on blood. Parasitism of animal tissues by SWF larvae (myiasis) causes serious production losses. Although female SWF may range for up to 25-50 km in search of wounded animals in which to lay eggs, they do not actively migrate. SWF prefer moist well-shaded areas and are unlikely to survive in completely open country, particularly if subjected to intense heat and low humidity. The optimal temperature range for the fly is 20-30°C. Flies will not move at temperatures below 10°C, and in the range 10-16°C they are very sluggish and probably will not mate (DPIE 1996).
Simple climate-matching models, such as Climex or Bioclim, can indicate both sporadic and potential endemic areas in which the pest could survive over winter (Sutherst and Maywald 1985). Mayer et al. (1994) reported on a more comprehensive assessment of potential SWF abundance and spread using a hybrid simulation technique incorporating deterministic simulation models and a geographic information system (GIS). This work has formed the basis of a bioeconomic decision-making tool developed by the Qld Department of Primary Industry to assist with the management of a SWF incursion.
Anthrax
Anthrax is caused by a spore-forming bacterium. It is endemic in many countries of the world, particularly in tropical and sub-tropical areas, but the disease is usually only seen in well-defined endemic areas where environmental conditions appear particularly favourable for the survival of the spores. Anthrax spores are very resistant to inactivation and can persist in soil for many years, particularly in warm climates and in soils with a neutral to alkaline pH and that contain organic matter. Environmental factors that contribute to the risk of disease include soil type and weather conditions. To some extent, the soil type might be considered a 'static' variable. Outbreaks tend to be associated with periods of hot, dry and humid weather - conditions that enable anthrax spores to germinate and generate higher numbers of spores in favourable niches that could infect susceptible animals. Drought is another factor - stock ingest more soil while grazing, and this can increase the chance that spores are ingested.
A review of the 1997 anthrax outbreak in Victoria highlighted the importance of weather conditions as a predisposing factor for the outbreak (Galvin 1997).
Bluetongue
Bluetongue is an insect-spread disease of ruminants characterised by inflammation, congestion, swelling and haemorrhages. The disease is variable in severity. Sheep are generally the worst affected, with cattle having milder disease. For Australia, where bluetongue infection without clinical disease is recognised, it is an important issue in the export of cattle and sheep. Hence, there has been considerable interest in describing the relationship between presence of the virus and climatic factors. Statistical models, using a variety of techniques have been developed (e.g. Wright et al. 1993, Ward 1994, Ward and Thurmond 1995).
More recently, Ward and Carpenter (1996a, 1996b) have used simulation modelling to investigate infection of Australian cattle herds with bluetongue viruses.
In 1997, the Australian Quarantine and Inspection Service (AQIS), commissioned a study into the feasibility of developing a forecasting system for bluetongue transmission based on existing data, with a view to better management of the arboviral risks in the live cattle export trade. Following a positive report by the consultants, AQIS sought tenders for the development of a computerised area forecasting system that would permit regionalisation for bluetongue in Australia that is dynamic, taking into account the variability of climatic conditions which control the distribution of bluetongue virus vectors. Ausvet Animal Health Services subsequently assembled a multidisciplinary team that includes experts from the private sector, from the NSW Department of Agriculture, Commonwealth Bureau of Resource Sciences, NT Department of Primary Industries and fisheries, Qld Department of Primary Industry and WA Department of Agriculture to undertake this project which is due to be completed in 2000.
Japanese encephalitis
Japanese encephalitis (JE) is a mosquito-borne disease that occurs over much of Asia. Humans, horses and pigs are at risk of clinical disease, with pigs being very important amplifying hosts. For other species - cattle, sheep, goats and some wild species -infection is rarely clinically apparent. Water birds (herons and egrets) are the main reservoir and amplifying hosts for the virus. Their migration also has the potential to spread the disease considerable distances. A number of different mosquitoes can act as the vector to spread the virus, both between birds and also to and between other hosts (DPIE, 1996).
In 1995, JE reached the northern tip of Australia. Mosquitoes capable of spreading the disease exist throughout Australia, although it is not known how efficient the species of mosquito in Australia will be as vectors. Assessing the risk of the disease spreading to different parts of Australia, and the seasonal changes in potential levels of disease is complex. Such assessment needs to take into account the interactions between populations of the various hosts and vectors. To a large extent, the environmental component is concerned with estimating the population dynamics of the vector and requires information about temperature, humidity, rainfall, and surface water. The Bureau of Resource Sciences (BRS) has undertaken an assessment of potential spread of JE in Australia using a GIS approach.
BRS is also working with the National Centre for Epidemiology and Population Health, ANU using modelling and GIS to study another important arboviral disease in Australia, Ross River fever.
Foot-and-mouth disease
FMD is one of the most contagious of animal diseases affecting cloven-hoofed animals. It spreads rapidly through a farm and to adjoining farms by close animal-to-animal contact. Animals are infected by ingesting or, especially for ruminants, by inhaling the virus. Australia is free of FMD and maintains strict quarantine controls on animals and products from infected countries. Contingency plans (DPIE, 1996) are in place to eradicate the disease should it be introduced.
The BRS has developed a sophisticated FMD simulation model. This has been used to assist disease planners by predicting the potential size, duration and impact of FMD outbreaks under Australian conditions (Garner 1993, Garner and Lack 1995a), and to evaluate various control strategies (Garner and Lack 1995b, Garner et al. 1997).
The major method of spread of FMD is by the movement of infected animals or contaminated products or equipment. However, under the right conditions - a concentrated source of the virus, high humidity, stable atmospheric conditions and presence of susceptible livestock downwind - long distance spread by wind can occur. Although it is thought that most wind-borne spread over land travels less than 10km, there is good evidence that spread of FMD has occasionally occurred over distances of 60km over land, and 250km over water (Hugh-Jones and Wright 1970; Gloster et al. 1982). Studies in New Zealand indicate that meteorological conditions favourable to wind-borne spread occur there. Sanson et al. (1991) describe a computerised disease recording and information system (EpiMAN) developed at Massey University for use in an exotic disease emergency. The system incorporates a database management system, a GIS, a simulation model for FMD, a virus plume model, and expert system elements. The system is being adapted and tested for use in Europe.
Recognising the importance for animal health authorities to have an indication of the potential for windborne spread of FMD under Australian conditions, in 1994 the Meat Research Corporation commissioned a study on this issue. Weather records and livestock distribution data were used to identify areas at risk. Aerosol virus production and extent of spread that could be expected from typical Australian livestock enterprises was modelled. The study involved the use of viral production models, plume models, deposition models and geographic information systems to integrate and analyse the available data (Garner and Cannon 1995).
Current projects being undertaken in the area:
Foot-and-mouth disease modelling studies. Dr M.G. Garner, Bureau of Resource Sciences. Ph: (02) 6272 5369; Fax (02) 6272 4533; graeme.garner@brs.gov.au
Northern Cattle Export Enhancement Project. Dr F.C. Baldock, AusVet Animal Health Services, 12 Thalia Court, Corinda, Qld 4075. Ph: (07) 3225 1712; Fax: (07) 3278 195; ausvet@eis.net.au
Japanese encephalitis: risk and spread modelling. Dr R. Smart, Bureau of Resource Sciences, PO Box E11, Kingston, ACT 2604. Ph: (02) 6272 4707; Fax: (02) 6272 4687; robert.smart@brs.gov.au
Climate factors in the surveillance of arboviral diseases. Dr C.S. Guest, National Centre for Epidemiology and Population Health, Australian National University, ACT 0200. Ph: (02) 6249 3503; Fax: (02) 6249 0740; Charles.Guest@nceph.anu.edu.au
References
1. DPIE; AUSVETPLAN Disease strategy (cost sharing) (1996). Section 4, Foot-and-mouth disease; Section 8, Screw-worm fly; Section 12, Japanese encephalitis; available from http://www.brs.gov.au/aphb/aha
2. Galvin, J. (1997). An unusual outbreak of anthrax in Victoria. Animal Health Surveillance Quarterly 2(1), 4-6.
3. Garner, M.G. (1993). Modelling foot-and-mouth disease in Australia. In Proceedings of the National Symposium on Foot and Mouth Disease, Canberra, 8-10 September 1992, pp 157-175.
4. Garner, M.G. and Cannon, R.M. (1995). Potential for wind-borne spread of foot-and-mouth disease virus in Australia. A report prepared for the Australian Meat Research Corporation, Bureau of Resource Sciences, Canberra, 88 pp.
5. Garner, M.G. and Lack, M.B. (1995a). Modelling the potential impact of exotic diseases on regional Australia. Australian Veterinary Journal 72, 81-87.
6. Garner M.G. and Lack, M.B. (1995b). An evaluation of alternate control strategies for foot-and-mouth disease in Australia - a regional approach. Preventive Veterinary Medicine 23, 9-32.
7. Garner, M.G., Allen, R.T. and Short, C. (1997). Foot-and-mouth disease vaccination: a discussion paper on its use to control outbreaks in Australia. Bureau of Resource Sciences, Canberra, 98 pp.
8. Gloster, J., Sellers, R.F. and Donaldson, A.I. (1982). Long distance transport of foot-and-mouth disease over the sea. Veterinary Record 110, 47-52.
9. Hugh-Jones, M.E. and Wright, P.B. (1970). Studies on the 1967-8 foot-and-mouth disease epidemic: the relation of weather to the spread of disease. Journal of Hygiene, Cambridge 68, 253-271.
10. Mayer, D.G., Atzeni, M.G., Butler, D.G., Anaman, K.A., Glanville, R.J., Stuart, M.A., Walthall, J.C. and Douglas, I.C. (1994). Biological simulation of a screwworm fly invasion of Australia. Project Report Series Q094005, Department of Primary Industries, Brisbane, 56 pp.
11. Sanson, R.L., Liberona, H. and Morris, R.S. (1991). The use of a geographic information system in the mangement of a foot-and-mouth disease epidemic. Preventive Veterinary Medicine 11, 309-313.
12. Sutherst, R.W. and Maywald, G.F. (1985). A computerised system for matching climates in ecology. Agricultural Ecosystems and Environment 13, 281-299.
13. Ward, M.P. (1994). Climatic factors associated with the prevalence of bluetongue virus infection of cattle herds in Queensland, Australia.Veterinary Record 134, 407-410.
14. Ward, M.P. and Carpenter, T.E. (1996a). Simulation modelling of the effect of climatic factors on bluetongue virus infection in Australian cattle herds. I. Model formulation, verification and validation. Preventive Veterinary Medicine 27, 1-12.
15. Ward, M.P. and Carpenter, T.E. (1996b). Simulation modelling of the effect of climatic factors on bluetongue virus infection in Australian cattle herds. II. Model experimentation. Preventive Veterinary Medicine 27, 13-22.
16. Ward, M.P. and Thurmond, M.C. (1995). Climatic factors associated with risk of seroconversion of cattle to bluetongue virus in Queensland. Preventive Veterinary Medicine 24, 129-136.
17. Wright, J.C., Getz, R.R., Powe, T.A., Nusbaum, K.E., Stringfellow, D.A., Mullen, G.R. and Lauerman, L.H. (1993). Model based on weather variables to predict seroconversion to bluetongue virus in Alabama cattle. Preventive Veterinary Medicine 16, 271-278.
