Management of terrestrial vertebrate pests - Invasive Species Management - page 182

second the importation into Australia and New Zealand of a new virus, rabbit
haemorrhagic disease, which naturally evolved pathogenicity from an existing
calicivirus found in rabbits (Cooke and Fenner 2002). Both diseases caused large
initial reductions in rabbit abundance at most places, but the effi cacy of both has
waned, as the myxoma virus has evolved competing attenuated strains (Williams
et al . 1995) and for unknown reasons for the calicivirus (Parkes et al . 2008b).
Both of these viruses only lethally infect the target species, and to date few other
candidate viruses with such specifi city have been identifi ed as potential candidates
to control terrestrial vertebrates—the exception being another lagomorph calici-
virus, European Brown Hare Syndrome virus (Frölich et al . 2003), which could
be used (if required) to control hares ( Lepus europaeus ) in countries where they are
an exotic problem. Other pathogens like rinderpest, anthrax, and canine distem-
per are not biocontrol candidates because the costs to desirable vertebrates would
hugely outweigh any benefi ts.
12.3.3.5 Fertility control
Controlling the fertility of invasive species has been suggested as a more humane
method of controlling their abundance (Fagerstone et al . 2002). However, most
models show the benefi ts, in terms of population regulation, are less than when the
same number of animals is killed, although a combination of lethal and fertility
control is usually best (costs to achieve both being ignored; Cameron et al . 1999).
A major restriction on the use of fertility control agents has been the lack of reliable
one-shot oral delivery (i.e. in a bait) systems, requiring that animals be somehow
injected or dosed individually, often more than once in their lifetime, which in
turn usually requires their capture. This has proved possible for populations of feral
horses in the USA (Turner et al . 1996), but is too impractical for most wild animals
and often far more expensive than simply killing them.
To overcome these problems, a number of research programmes have aimed
to develop genetically engineered, non-pathogenic agents that express proteins
that act as immunological blocks to the target vertebrate's fertility (Tyndale-Biscoe
1994). Ideally these agents spread by themselves, but they can also be delivered
alive or dead in baits. h ere are even eff orts to combine vaccines that, for example,
reduce possum fertility and protect them against tuberculosis infection (D. Collins,
pers. comm.). h us far, attempts to make this biotechnology work in the fi eld
against mice, rabbits, and foxes in Australia have failed (T. Peacock, pers. comm.).
Mice failed because the engineered cytomegalovirus would not transmit in the
fi eld; rabbits failed because the proportion of females sterilized by the engineered
benign Myxoma virus was too low to overcome compensatory fecundity in the
non-sterile females (Twigg et al . 2000); and foxes failed because no candidate vec-
tor was found. However, research continues using both living vectors (engineered
nematodes) and non-living, bait-delivered systems in New Zealand for use against
possums (Cowan 2000), and there are models supporting development of a com-
bined lethal and fertility control solution (e.g. Ramsey 2005).