The Population Factor in Wildlife Diseases and their Transmission to Man
Despite the fact that we are now in the 21st century, infectious disease remains a major issue for mankind and his domestic animals. The World Health Organisation estimates an annual human death toll of 10-15 million per year worldwide as a result of infectious disease.
Over the past few decades we have faced completely new and emerging diseases or infections, some of which we knew about but which have re-emerged as a consequence of environmental or other changes — the potentially lethal AIDS and the Marburg and Ebola viruses being some of them.
Epidemiological investigation has frequently implicated wildlife as the major source for man. This is clearly a consequence of the increasing interaction between wildlife and human activity. Many of these new diseases are now appearing outside their originally known range and are caused, either directly or indirectly, by man’s activities. These include changes in land use and deforestation resulting in disruption of wildlife habitats, changes in animal populations with increasing contact between wildlife and man or his domestic livestock, and climate change resulting in changes in distribution of animals and insect disease vectors. Such events are directly related to human population numbers — more people requiring more resources, either for themselves or for their livestock.
Some of the exotic-sounding viral infections that have surfaced in the last few decades have arisen either from primates via mosquito vectors or, increasingly, from a variety of bat species. Chikungunya fever and O’nyong’nyong, for example — characterised by encephalitis, joint pain and haemorrhage — carry a high rate of mortality and originate in wild animals in tropical forests and spread to man. Chikungunya is present in primates in Africa and South East Asia, causing occasional large outbreaks in man affecting hundreds of thousands, with hundreds of deaths every year. The related O’nyong’nyong virus hails from Uganda and surrounding areas, whilst various forms of equine encephalitis originate in birds but affect horses, which then can transmit to man via mosquitoes.
The well-known and related Marburg and Ebola viruses cause very high mortality (<80%) and are endemic to sub- Saharan Africa where transmission is the result of contact with body fluids. Because high mortality also occurs in wild primates it is thought that bats may be the primary reservoir, since these are frequently affected without disease — a common sign in a reservoir species. Interestingly, related viruses from Macaques in the Philippines produce disease in monkeys but seem to be non-pathogenic for man.
Rabies affects all species and is rightly feared as a result. Related viruses are frequently isolated from bats from several regions of the world. Australian bat lyssavirus is harboured by flying foxes in South East Asia and northern Australia and has resulted in a small number of human deaths from bite wounds. Currently, concern exists over European bat lyssavirus.
These few examples are of viruses that have RNA rather than DNA as their genetic blueprint. Such viruses generally have a higher rate of evolution than DNA viruses — which is a major cause for concern for these, as it is for avian influenza and other flu viruses.
These infections obviously are capable of jumping the species barrier and infecting more than one host species. Interestingly, it is multi-host infectious agents (pathogens) that predominate amongst emergent animal and human diseases. Ninety percent and 100% of the emerging diseases in cattle and domestic carnivores respectively are caused by such multi-host pathogens. Pathogens that infect wildlife hosts have a higher relative risk for emergence than species specific pathogens. For example, 77.4% of the 800 zoonotic diseases are caused by pathogens that affect wildlife, whilst 90.4% of the 125 emerging zoonotic diseases affect wildlife.
Many factors influence changes in wildlife disease incidence, including economic and climatic factors, which are directly related to population, and microbiological effects which are not.
Closer interaction between man and his livestock and wild animal reservoirs has led to increased incidence of infections affecting man and livestock. Forest clearances have resulted in an increase in contact between humans and forest-dwelling animals. Movement of animals or animal products can result in transmission of agents such as African and Classical Swine Fevers in adding on to Foot and Mouth Disease.
Changing climate, e.g periods of drought or flooding, affects disease incidence by alterations in land use or altered livestock rearing practices, and movement or changes in distribution of animal reservoirs or insect vectors. The latter may be able to survive in increasingly northerly latitudes, for example. Local increases in mosquito numbers have increased the risk of spread or introduction of disease such as Bluetongue (BTV) and the other haemorrhagic orbiviruses, including African Horse Sickness virus, Chikungunya virus and West Nile virus2. The recent expansion of BTV into Europe since 1998 provides a remarkable example of such changes with six new serotypes invading the region. It is likely that the expansion of the disease distribution was only partially due to the spread of established vector species into new areas. The higher temperature during the summer in northern Europe has also allowed previously unrecognised insect vector species to support virus replication and consequently to transmit the disease. Similar changes may have happened in the Americas.
Pathogen evolution may occur in response to changes of which man is not aware. Since evolution takes place at a greater rate in RNA than in DNA viruses, the risk of the development of H5N1 influenza virus adapted to man is of particular current concern with the associated risk of potential pandemic capabilities. Similarly the evolution of novel types of lyssavirus in bats suggests that evolution may be occurring in many hosts which are currently poorly monitored.
Other endemic diseases may also change in incidence for largely unknown reasons. Thus the cause of the increase of bovine tuberculosis in wild badgers and boar in the UK and Spain respectively is largely unknown. Similarly, the exact distribution of pathogens such as Francisella tularensis (tularaemia — a disease related to plague) and lymphocytic choriomeningitis virus is uncertain and factors affecting the reservoir populations, including rodents, are poorly understood.
Microbiological factors are largely beyond our capacity to influence — although it must be said that human activity has reduced the population size and therefore the gene pool size of many wild species with whom we have increasing contact. Reduced gene pool size itself can affect the ability of animals to respond to infection. There is variation in susceptibility in all species as a result of a combination of genes, the sum total of which is the phenotype. However, in many cases individual genes can have marked effects and may account for more than 50% of the measurable resistance to individual infections. Thus, reduced population sizes can skew the distribution of resistance phenotypes to a pattern where the predominant phenotype is highly susceptible with effects both on disease and on transmission to man and livestock.
There is much that remains to be learnt in terms of the sources of infection, how these affect individual wild animal species and the complex interactions between species in their natural environment. Many of these relatively delicate balances are being increasingly disturbed by man through environmental destruction with associated increased stress and pollution, increasing susceptibility to infection in wild species. However, the other factors, especially those affecting climate and our increasing contact with wild animal species are also largely the result of our numbers and encroachment on their territory.
Population-related influences are likely to increase in the short term to the detriment of both biodiversity and to man’s health and that of his livestock. Sticking plasters and fire brigade act on through vaccines, better diagnosis and surveillance are important in protecting human and livestock health and even that of wild species themselves but, at the end of the day, if we are not all to live in a global allotment with no wild space at all then the underlying factors needs to be addressed urgently by policy-makers with some radical action. As with all these issues, management of resources is important and currently vital, but there is little acknowledgement that it is not so much what we do but how many people there are doing it.
Thank you to the Jackdaw, an Optimum Population Trust Publication for this article from the www.optimumpopulation.org February 2010 Edition
This article was written by Paul Barrow, who is a Professor of Veterinary Infectious Diseases at the University of Nottingham.
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1) Visit www.who.int/whr/en/index.html
2) Visit the Defra website at www.defra.gov.uk and WHO website at www.euro.who.int