Emerging Infectious Diseases of Wildlife Sarah Cleaveland
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Emerging infectious diseases of wildlife Sarah Cleaveland has shown that pathogens Many pathogens that can infect wildlife are are not fussy more than twice as likely to about their host cause emerging diseases (in species. As Sarah both humans and live- stock) as those that do Cleaveland not affect wildlife. This describes, this includes newly recognized means that wildlife pathogens, such as HIV-1, hosts can play a HIV-2, BSE, Hendra virus, key role in the Nipah virus and the SARS emergence of coronavirus, as well as human and those that are spreading domestic animal (re-emerging), such as diseases. rabies and West Nile virus. Although we currently lack denominator data on wildlife diseases (and hence cannot quantify risk factors for emergence), pathogens that infect several host taxa have also been responsible for most emerging wild- life epidemics, including several that have threat- ened endangered species, such as rabies, which decimated populations LEFT: of the Ethiopian wolf Rapid decomposition and (one of the world’s rarest scavenging of wildlife carcasses creates considerable difficulties for carnivores) and canine the collection of appropriate distemper, which has diagnostic samples. The caused major declines development of robust and in Channel Island foxes practical field techniques is and Lake Baikal seals. required for effective wildlife disease surveillance. Generalism appears to be a PHOTOS SARAH CLEAVELAND common characteristic of (VULTURE) AND SARAH DURANT We often think of human, veterinary and emerging pathogens, whether in human or animal (LEOPARD) Gwildlife medicine as separate disciplines, but populations. this distinction makes little sense when we Several factors may underlie disease emergence, but in consider the epidemiology of infectious diseases. Most practice, most emerging human diseases are associated pathogens that cause human and animal diseases are with ecological changes, such as changes in land-use, ecological generalists – they can infect and be trans- environment, climate, human demography or animal mitted by more than one host species – and for many, the and human movement patterns. Many of these factors spectrum of hosts includes wildlife species. This is true can lead to increased human–wildlife contact and for almost half (44 %) of all pathogens that cause human thereby facilitate transmission of potentially pathogenic diseases and 70 % of the pathogens that cause the most agents that are maintained in wildlife hosts. For internationally important veterinary diseases (e.g. example, deforestation and encroachment into new rinderpest, foot-and-mouth and avian influenza). habitats have been linked with the emergence of California encephalitis virus, Ross River virus, Ebola and G Multi-host pathogens and emerging Marburg viruses (all of which co-infect wildlife). infectious diseases Although we know very little about the dynamics of Human factors in emerging wildlife infectious agents in most wildlife populations, wildlife diseases hosts appear to play a key role in the emergence of For emerging diseases of wildlife, similar ecological and human and domestic animal diseases. A recent analysis anthropogenic factors come into play. For example, the MICROBIOLOGY TODAY VOL 30/NOV03 155 RIGHT AND BELOW: appearance of canine Recent wildlife epidemics distemper as a devastating of rabies and canine distemper in wild carnivores in the Serengeti, new disease in Serengeti Tanzania, have been associated lions has been associated with a rapid expansion of domestic with a rapid expansion of dog populations. domestic dog populations PHOTOS SARAH CLEAVELAND living adjacent to the park; a marked increase in mycoplasmal conjunctivitis among passerine birds in north America has been attributed to habitat changes are limited. Many strategies, such as culling and creation and artificial feeding; and the emergence of chytridio- of barriers, invariably result in harm to wild animals. mycosis (a fungal skin disease), which has resulted in Conventional approaches to disease control in animals, major population declines in amphibians in Australia such as vaccination or treatment have limitations in and the Americas, has been linked to climate change and wildlife populations. Specific vaccines and treatments movements of captive amphibians. are often unavailable or untested for wildlife, and While medical attention may focus on zoonotic risks delivery in field settings is beset by logistic, financial and from wildlife infections, disease risks from humans ethical considerations. The success of wildlife oral rabies are a growing concern for conservationists. Disease vaccination campaigns in Europe and North America transmission from both local people and tourists has long points a way to the future and has stimulated research been recognized as a threat for wild primates – outbreaks into other non-lethal approaches, such as sterilization. of measles in mountain gorillas, polio and pneumonia in A third consequence of wildlife involvement in human chimps and, more recently, Ebola in great apes have all diseases is the potential threat to the wildlife tourism been associated with human contact and proximity. industry. The economic damage caused by a decline in However, human diseases are also emerging as threats to visitors to countries suffering from SARS and Ebola virus non-primate vertebrates, and recent outbreaks of clearly highlights this problem. Equally clear is the Mycobacterium tuberculosis (presumed to originate from important lesson learnt from the SARS epidemic about humans) have led to high mortality in meerkats and the need for open exchange and dissemination of banded mongooses in southern Africa. With HIV/AIDs epidemiological data. Balancing these needs presents a causing widespread immunosuppression in the human dilemma for wildlife managers, particularly in those population and enhancing the potential for pathogen countries dependent upon wildlife tourism for economic transmission, the risk of future wildlife outbreaks from development. Additional dilemmas will invariably arise humans must surely be set to increase. as molecular tools increasingly allow detection of patho- gens in an expanding range of wildlife hosts. A major Implications of wildlife infections in the challenge for the future will be the epidemiological control of human emerging diseases interpretation of these results and appropriate evaluation The link between wildlife and human health poses and management of potential disease risks. several problems. First, the lack of knowledge of In summary, the links between infectious diseases of infection dynamics in wild animal populations limits wildlife and public health have far-ranging impacts and the development of effective strategies for disease implications that pose considerable challenges to control. Even in the public health sector, disease medical scientists, veterinarians and wildlife managers. surveillance is often not a high priority. In almost all To date, there has been little integration between these wildlife populations, surveillance is rudimentary or non- sectors – wildlife ecologists tend to show little interest in existent – and this is particularly true in the developing human health and public health scientists often have world where many emerging diseases have originated. little knowledge of wildlife issues. But this interface Detection of pathogens in free-living wildlife is notori- provides exciting opportunities to develop innovative ously difficult, hampered by the enormous practical and collaborative approaches that will mitigate problems of finding, collecting and storing appropriate emerging disease risks for humans and minimize adverse samples under field conditions, as well as the lack impacts on wildlife. of species-specific diagnostic tests. Even where con- siderable investment has been made (as with bovine Dr Sarah Cleaveland is a Lecturer in Tropical tuberculosis and badgers in the UK), the complexities of Animal Health at the Centre for Tropical Veterinary infection dynamics in multi-host systems make it Medicine, University of Edinburgh, Easter Bush, difficult to identify optimum methods of control. Roslin, Midlothian, EH25 9RG, UK. Second, even where wildlife hosts and/or reservoirs Tel. 0131 650 6404; Fax 0131 651 3903 have been identified, the options for control in wildlife email [email protected] 156 MICROBIOLOGY TODAY VOL 30/NOV03.