Editorial – Wildlife diseases: costs and benefits
Image - Don Robson.
When we talk about wildlife diseases, what tends to spring to mind are usually diseases such as rabies, avian influenza, West Nile virus, severe acute respiratory syndrome (SARS) or the latest, Middle East respiratory system (MERS) – diseases that originate in wild animals and now threaten people or New Zealand’s agricultural industries. The global trend for people to increase their contact with wildlife, their habitats, and with domestic animals means it is more and more likely that new diseases will be transmitted between people and wildlife.
But the importance of wildlife diseases goes far beyond the threat to people and their livelihoods, as this issue of Kararehe Kino demonstrates. New Zealand’s native biodiversity is also under significant threat from pathogens. First, diseases new to New Zealand are a potential threat. Dan Tompkins and his collaborators (see article on seabird surveillance) have recently evaluated the risk of new diseases of wildlife and people arriving through the natural movement to New Zealand of migratory birds from Australian waters. Richard Hall and Dan White (see article on viruses in bats) have similarly been undertaking the first intensive survey of New Zealand bats for viruses. This work complements the Ministry of Primary Industries national wildlife surveillance programme. Second, the spread of existing diseases within New Zealand is also a concern. Avian malaria has been in New Zealand for many years and infection has been linked to deaths of a wide range of native bird species. Chris Niebuhr (see article on avian malaria) has recently been investigating whether disease mortality is important at the population level, as impacts may worsen with the climate-change driven spread of the mosquito disease vectors and the disease.
Finding new diseases that may be potential threats has been greatly helped by new DNA-sequencing approaches (metagenomics). These approaches have also been applied recently to help find potential causes of new serious illnesses detected in native wildlife – Dan White (see article on disease in kākāpō) describes the hunt for the causative agent of a cloacal disease in kākāpō, and Wray Grimaldi (see article on feather loss in penguins) describes the hunt for the cause of feather loss in Adélie penguins. The difficulty when native wildlife is found with a new illness is that there is almost always very little information about naturally-occurring diseases, and so little information is available on which to base a diagnosis and decide on possible treatments.
The wildlife disease which receives the greatest attention and expenditure in New Zealand is bovine tuberculosis (TB), because of its potential economic impact. Possums are the main culprit infecting livestock, but other wildlife (deer, pigs , ferrets) are also involved through their interactions with possums and each other (alive and dead). As New Zealand moves towards TB eradication, understanding the combined role of all these wildlife species in maintaining TB will be critically important and has been investigated by Mandy Barron and her colleagues (see article on TB persistence). Part of this research requires good estimates of transmission of TB within species, which Carlos Rouco and his collaborators have recently provided with the first direct measure of TB transmission rate between possums (see article on estimating TB transmission rates). They concluded that a better understanding of the conditions leading to high rates of transmission would allow better targeting of TB control efforts.
Wildlife diseases are usually thought of as causing unwanted harm, but in some cases that harm can be exploited for good, such as for pest wildlife control. Biological control of mammal pests using diseases has been around for many years – i.e., the control of rabbits with myxomatosis and, latterly, with Rabbit Haemorrhagic Disease (RHD). Host-pathogen systems usually involve a continuous process of co-evolution, as hosts develop immunity and pathogens alter their pathogenicity. Recent research by Janine Duckworth and colleagues (see article on rabbit haemorrhagic disease) has clearly shown the evolutionary changes that have occurred in the RHD virus since its release in New Zealand, with virus strains in different parts of New Zealand now differing significantly in lethality to rabbits. Strain differences may in future allow better matching of strains to local conditions to enhance RHD efficacy. Biological control using wildlife disease may also exploit disease effects on the host more subtle than mortality. Infection of rodents with the parasite Toxoplasma gondii changes their behaviour and makes them more susceptible to trapping (and predation). Dan Tompkins and Clare Veltman (see article on rat control) used computer models to assess how seeding rat populations with the parasite could contribute to improving their control with traps – a novel idea certainly worth following up given the very high costs of ongoing suppression of rat populations!
If New Zealand follows global trends then wildlife disease issues are going to become increasingly important, both for people and their livelihoods and for native biodiversity. While new technologies for disease discovery will help with early detection and diagnosis, the lack of good baseline information about pathogens in New Zealand wildlife will continue to be a major constraint on effective response to disease outbreaks.