What is disease surveillance?

Since Covid, we all understand that testing for diseases is not just important for the health of individuals, but also for the greater, or public health. This is equally true for the animal and wild animal world. In fact, the health of humans, animals and the ecosystems that contain us all, are intricately connected. We refer to this as the One Health concept. Research has found that over 40% of all newly emerging diseases in humans have a wildlife origin, although only a small proportion of them have high risks and impacts for public health. In addition, over the last decades, several infectious diseases, such as chytrid fungus in amphibians or white nose syndrome in bats, have caused significant declines and even extinctions of wildlife species. It is therefore clear that a good knowledge of what diseases occur in wildlife is important.

When we systematically investigate animals within populations for the presence of diseases and health outcomes, we call this health or disease surveillance. There are two different types of disease surveillance: active surveillance and passive surveillance.

If we conduct active surveillance, we specifically go out to test animals for particular diseases. A current example is surveillance for avian influenza (bird flu) in water birds. There are different types of avian influenza: low pathogenic (doesn't usually cause significant disease) and highly pathogenic (causes significant disease and/or death). Some strains of highly pathogenic avian influenza have resulted in mass deaths in wild birds and mammals overseas, and can also infect humans. We don't currently have highly pathogenic avian influenza in Australia, but we want to know as early as possible if it arrives here with migrating birds, so we can be as prepared as possible and avoid it from spreading if at all possible. Therefore, as part of Australia's avian influenza strategy, the government, through Wildlife Health Australia, funds regular testing of both live and dead wild birds in high risk areas.

With passive surveillance, we usually focus on examining animals who have died from natural causes or have been killed, in order to get a better understanding of the range of diseases and health issues that occur in those populations. Typically, we conduct a full necropsy (post-mortem examination) of the animal, and collect a range of samples that allows us to look at any disease-related changes in their organs and tissues using microscopsy (histopathology), as well as collecting samples to test for the presence of specific diseases (e.g. serology, molecular testing).

By investigating why wild animals have died, we can achieve a number of important things.

We can build up a better baseline understanding of which diseases occur in the population, how often they occur, whether there are particular risk factors for the disease (e.g. age or location), and whether they are related to any significant changes in the body that would impact on the animal's health and quality of life.
We can detect new diseases in the population by matching observed changes in the body to likely causes. These new diseases could be ones that occur elsewhere (in other Australian populations or overseas), or they could be completely new. For example, Tasmanian Devil Facial Tumour was completely new when it was first detected.

Both of these things are essential to succesfully managing wildlife populations, preventing or mitigating disease outbreaks and protecting domestic animal and humans health.