Avoiding disease outbreaks when reintroducing endangered wildlife

Cute, little guppies swimming in a freshwater aquarium. Image credit, Marrabbio2

Scientists have tested a method for reducing parasitic infections in wildlife reintroductions. Using guppies (Poecilia reticulata) as a model species, they found that pre-exposing individuals to parasites and then curing them prior to reintroduction reduces infection after being released.

Disease outbreaks can compromise the success of wildlife reintroduction programs aimed at bolstering diminished populations. Individuals raised in captivity with limited exposure to pathogens such as parasites may be particularly susceptible to disease. This could pose risks to both the introduced individuals as well as the wild population.

So to help us figure out best practices for minimizing this problem, biologist Patricia Faria and fellow researchers ran a creative experiment using guppies as a proxy population. They wanted to find out whether pre-exposing guppies to infection and releasing them gradually versus all-at-once would reduce infection and mortality from the fish's main menace, the parasite (Gyrodactylus turnbulli). Guppies underwent one of the following treatments:

1) Pre-exposing guppies to the parasite and then curing them of infection prior to releasing them gradually;

2) Pre-exposing guppies to the parasite and then curing them of infection prior to releasing them en-masse;

3) Curing guppies without any pre-exposure and releasing them gradually; and

4) Curing guppies without any pre-exposure and releasing them en-masse.

For each of the four treatment groups above they ran four replicate introductions of 5 individuals into populations that had a low level of parasitic load similar to what might be found in the wild.

The study found that by the end of the experiment the parasite load was significantly lower for the reintroductions in which the guppies had been pre-exposed. In addition, the parasites only went extinct in the fish population when the introduced guppies had received pre-exposure.

They found weak evidence that adding fish gradually has benefits over reintroducing them all at once. Fish survival was highest for the guppies that were pre-exposed and released gradually (70%) and lowest for individuals that were not exposed and were released all at once (45%). However the differences were not statistically significant.

Pre-exposure also offers the advantage of allowing us to figure out beforehand which individuals are susceptible to infection (i.e. those that acquire a high parasitic load and do not shed the pathogens) versus those that are resistant. We can then remove the susceptible individuals prior to reintroduction and just release those that are resistant.

Of course pre-exposure of guppies in an experiment may not translate as well to a real-world setting. In the absence of a well-tested vaccine, wildlife biologists may be reluctant to pre-expose individuals to parasites when reintroducing endangered species that have small populations and are difficult to breed in captivity.  In such cases the researchers recommend other methods for testing susceptibility to disease such as measuring antibody levels, haematology parameters and stress responses.

--Reviewed by Rob Goldstein

Faria, P., van Oosterhout, C., & Cable, J. (2009). Optimal release strategies for captive-bred animals in reintroduction programs: Experimental infections using the guppy as a model organism Biological Conservation DOI: 10.1016/j.biocon.2009.06.002