Viruses for rabbit control

The myxoma virus was imported into Australia in 1936, and extensively studied before being released into the environment in 1950. The virulence of the myxoma virus is rated on a scale of one to five, with one being most virulent. The original myxoma virus strain released into the environment was rated one, and provided a spectacular 99% rabbit mortality when first released into the environment in the early 1950s. Without rabbits grazing on the landscape, the amount of forage available for sheep production soared and farmers were very happy with the financial windfall.

But the spectacular success did not last. There is an ecological interaction over time involving the pathogenicity or virulence of a microbe and the genetic suseptibility or immunity of the host population. A microbe that is 100% successful in killing its host would go extinct along with its host. So, ecological theory favors the evolution of a less virulent microbe that does not kill the entire host population. Indeed, the evolution of reduced myxoma virus virulence and increased rabbit immunity became a classic epidemio-logical case.

Over time the virus attenuated, and the virulence of field strains of myxoma virus declined from one to three. At the same time, the rabbit population developed greater immunity to myxoma virus. Releasing the highly virulent myxoma virus strain rated one into the environment no longer produces the 99% rabbit kill seen in the 1950s. At the end of the twentieth century, myxoma virus was producing a more sustainable rabbit kill of between 40% and 90%. This reduced rabbit kill is still very important to integrated control programs. Indeed, in the absence of myxoma virus rabbit populations can still soar to very high levels.

In theory, integrated pest management can incorporate multiple techniques, each providing a percentage of pest population suppression. In practice, studies in both central and south Australia demonstrate the advantage of combining multiple techniques into an integrated pest management program for rabbits. When myxoma virus is used alone, as is typically the case, rabbit populations rebound to high levels in subsequent years. However, when rabbit warrens are ripped or plowed after myxoma virus has already reduced the rabbit population, the area can remain almost devoid of rabbits for many years.

Bolstered by the half century of continuing success with myxoma virus, it has only been natural to look for additional rabbit pathogens to introduce into the environment. In 1984 Chinese scientists identified an acute infectious rabbit disease called Rabbit Calicivirus Disease (RCD). RCD was subsequently identified in Europe, Asia, Africa, and Mexico. Australia began studying RCD on wild and laboratory rabbits and non-target species in 1991. Several months after a 1995 field trial on South Australia's Wardang Island, RCD was detected on the mainland. In 1996, RCD was officially recognized as

Brown rats converge at a garbage dump. Rats can spread diseases that affect livestock and people. In addition, they eat and/or contaminate feed and their gnawing destroys buildings. (Photo by Jane Burton. Bruce Coleman, Inc. Reproduced by permission.)

a biological control agent under the Commonwealth Biological Control Act.

In some parts of South Australia RCD has killed over 90% of the rabbits, and longer term rabbit populations are down 17%. RCD has been spreading at the rate of 250 mi (400 km) per month. An insect vector traveling in the wind is believed responsible for this rapid spread. However, humans and implements in contact with carrier rabbits may also spread the disease. More time is still needed to see how well the combination of myxoma virus and RCD work against rabbit populations.

Less lethal fertility control agents, a humane solution favored by animal welfare groups, are also under development for rabbits, foxes, and mice. One idea is to engineer a virus like the myxoma virus with an antigen causing animals to produce antibodies that reduce fertility. An estimated 60% to 80% of female rabbits need to be stopped from breeding in order to reduce rabbit populations. Immunocontraception will likely be tested on wild rabbits sometime before 2010.

Even if not wildly successful by themselves, new techniques like immunocontraception will likely play a role in integrated pest management systems targeting rabbits and other pest mammals. It is clear from over a century of rabbit control efforts that no single pest control technique by itself will work everywhere. Even myxoma virus, which looked so promising with its initial 99% rabbit control, is no longer viewed as a stand-alone solution. The integrated pest management paradigm of combining multiple control techniques and ecological principles is the wave of the future for combating invasive pests.

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