PREDICTING THE EFFECT OF A NOVEL VERTEBRATE BIOCONTROL AGENT - A MODEL FOR VIRAL-VECTORED IMMUNOCONTRACEPTION OF NEW-ZEALAND POSSUMS

1. A model is described for predicting the outcome of biological control of New Zealand possums, using viral-vectored immunocontraception based on a sexually transmitted herpes-type vector. 2. The model shows that success is possible in ecological terms, and identifies the probable circumstances under which it is achievable. These provide targets which a genetically modified virus must meet, aiding in the quest for suitable vectors and appropriate genetic modifications to them. 3. In particular, the female reproductive system rather than the male's should be targeted, contact rate (i.e. number of potentially infectious contacts) per possum carrying the virus (i.e. infected, not infectious) must be in the order of one or more per year and at least 75% of females carrying the virus must be sterile at mating. Achieving this incidence is critical, with only slightly lower values exerting a disproportionately lower effect on possum densities. 4. Spatial aggregation of the viral vector and the existence of a recovered and immune class of possums would both reduce substantially the impact of the control agent, but the presence of even limited vector-induced mortality would dramatically enhance it. 5. Immunocontraception is likely to confer a selective advantage on the engineered virus by allowing multiple matings for an affected female. This will raise the contact rate and prevalence, and allow the vector to compete successfully with any existing wild strains. 6. Given an appropriate mechanism of action on the reproductive system, viral-vectored immunocontraception of this kind would, if successful, offer a uniquely acceptable control for a vertebrate pest such as the possum, being humane, species-specific, cost-effective and environmentally benign. 7. Even if immunocontraception caused only limited suppression of an otherwise uncontrolled population, it could contribute to successful integrated control by greatly reducing the need for conventional poisoning operations. Such integrated control also reduces possum densities more rapidly than would occur with immuno-contraception alone.