Disease Risk Assessment for Reintroductions—Are We Relying on the Wrong Data?
The importance of disease risk assessments and disease surveillance for reintroduction programs is universally accepted and comprehensive tools are now available to guide the process. However, the traditional approach of developing a list of diseases of concern, testing release candidates for those diseases, and making release decisions based on the test results suffers from several fundamental problems. First, most diagnostic tests are designed to detect an infectious agent (or the host response to an agent) in an animal showing appropriate clinical signs. Most diagnostic tests that are validated for the host species perform well in this situation. However, test performance declines significantly when such tests are used to screen apparently healthy animals (e.g., reintroduction candidates) for the same agent, particularly if the agent has a low prevalence in the population. This results in frequent false positives that are misinterpreted as true positives. The problem is compounded when multiple tests are performed in parallel, as typically happens with surveillance testing for reintroductions or translocations. In addition, there are few diagnostic or screening tests that have been validated for use in nondomestic species. This leads to reliance on potentially misleading test results for critical decision-making in reintroduction programs. A second problem occurs when surveillance is only conducted on the source population. In order to adequately evaluate the risk posed by the presence of an agent in the source population, one needs to know whether the agent is also present in the destination population. However, it is seldom feasible to sample sufficient numbers of animals in the field to answer this question, and the same interpretive problems with surveillance tests described above would apply. Finally, emerging disease outbreaks in domestic animals and wildlife over the past few decades have revealed that agents of highest concern may not even have been identified yet, making surveillance testing impossible.
Based on our experience with reintroduction and translocation programs for several species, we propose a different approach to disease surveillance and risk assessment. First, if all avenues of potential disease exposure are evaluated for the source and destination populations, a qualitative risk category can be assigned to each population. Movement of animals between similar risk categories reduces opportunities for inadvertent disease transmission, and reduces reliance on unreliable surveillance tests. While this is conceptually easier to apply to field translocation programs, it can also be used for reintroductions from zoos, but it may reveal that animals destined for reintroduction require greater biosecurity than zoos have traditionally provided.
Second, we believe that the most important disease surveillance information available to reintroduction program managers is long-term necropsy data from the population, not results of screening tests performed on individual, apparently healthy, candidates for reintroduction. Comprehensive postmortem evaluations suffer from few of the interpretive problems described above, and have the advantage of providing surveillance for a wide range of agents of concern (both anticipated and unanticipated), in a broader cross-section of the population, over a broader window of time. The use of qualitative risk categories and comprehensive necropsy data, combined with judicious use of carefully selected surveillance and confirmation tests, has the greatest potential to reduce disease risk for reintroduction programs. We propose that zoos engaged in reintroduction efforts place more emphasis on population-level biosecurity and comprehensive postmortem investigations to mitigate disease risk.