by Linda M. Wilmot, D.V.M., M.S.

Control of wildlife populations may be necessary for several reasons, including control of predators or prevention of disease dissemination. Furthermore, the rapid urbanization of heretofore untouched wilderness land or rolling farmland and the resultant loss of wildlife habitat has forced many species of animals to assimilate to a new niche or live in smaller, more confined areas of their natural environment. Uncontrolled proliferation of a species beyond environmental carrying capacity, such as the feral horses on Assateague Island, which live within a confined environment, could lead to increased incidence of disease and starvation, threatening the health of the individual horses and the group. These scenarios do not present a positive outlook for long-term survival of populations of wild animals.

Some species, such as the gray squirrel, have become common place in backyards and suburban parks, with relatively little interference into "man's" world. On the other hand, animals such as white-tailed deer have also become a common sight in the urban and suburban areas of the northeastern United States. Vehicular--deer collisions, animals running through plate glass windows, and destruction of private home landscapes have also become a frequent occurrence. The animals have no where to go, and they are often left living on an island of forest surrounded by developed areas, with heavy traffic and dense human population.

How are these situations dealt with so that man and animals can live in harmony or that a beloved natural treasure does not procreate itself into ill-health and death? The need for contraception among wildlife has been acknowledged by wildlife researchers for over 30 years. Until recently, traditional methods of hunting, trapping, translocating, poisoning, or controlled confinement have been used to maintain population levels. However, with approaching suburbia, and the growing humane movement within the United States, these methods may no longer be viewed as acceptable, either from a logistical or a moral standpoint.

It is noted that the size of populations such as those discussed above is dependent upon several factors: mortality (death), natality (birth), emigration, and immigration. Generally, emigration and immigration have minimal impact on these island (be they surrounded by concrete or water) populations. Mortality and natality are largely affected by environmental forces such as abundance of food, weather, and natural disasters.

Research into population control has usually been directed towards modifying the natality rate. The effort has become a global one, as evidenced by the Conference on Fertility Control in Wildlife, held in Melbourne, Australia, in 1990. Such conferences or workshops are common now, and provide a forum for discussion of the current research into the various techniques at controlling a population birth rate. Early anti-fertility method research evaluated for use in wildlife species included administration of sex steroids in a bait or implant. Such administration interferes with ovulation of ova from the female, implantation of the fertilized egg, or impairment of spermatogenesis in males. Steroid administration has been evaluated in several species, including skunks. This form of birth control is less than optimal because of the persistence of steroids in the environment and concern over the potential for persistence of these hormones within the food chain. Other chemicals which prevent ovulation, such as certain fungicides and anticholesterol agents, have also been suggested as a means of population control.

Vaccination, with generation of immune responses to reproductive hormones, glycoprotein antigens of the pituitary-gonadal axis, antigens specific for early pregnancy, or gamete-specific antigens of the oocyte or spermatozoon, provide a possible mechanism of fertility control among wildlife species. Immunization of female animals with a preparation of porcine zonae pellucidae (PZP), which is a glycoprotein obtained from pig ovaries, has received recent publicity as applied to the feral horse population on Assateague Island. The experimental product is injected into animals, and the animal then forms antibodies and subsequent disruption of ovarian function. Thus far, research indicates that this form of population management is reversible, although the long-term effects are still being determined. Studies involving the use of homogenized cat ovaries injected into rabbits and then back into cats have proven unsuccessful in response to the need to control certain African lion populations. Antibodies against the gonadotropic hormones, e.g., luteinizing hormone, have been of limited success thus far in control of canine reproduction, and widespread application to wildlife species is, as yet, not feasible. The use of monoclonal antibody technology may assist in this methodology.

Other novel methods of birth control have been proposed including infection of house mice with Capillaria hepatica, a liver parasite. Theoretically, infection with this nematode leads to impairment of fertility. It should be noted that a population must be of a certain size in order for the infection to persist. Finally, the future looms like a science fiction novel. On the horizon are investigations into virus-vectored immunocontraception and plant products which interfere with reproduction.

It appears that with each new proposed method, the problem of wildlife population control becomes more complex. Questions regarding the effect of the contraceptive on the environment, non-target species, social behavior within the population to be contracepted, long-term effects upon the contracepted population, and how the methodology will be applied in conjunction with traditional methods of population control must be addressed. Furthermore, the issue of reversibility must also be addressed. The issue of wildlife contraception is one, however, that must be aggressively pursued as we find wildlife coming in ever closer contact with us.

Discussions between FDA, the Environmental Protection Agency (EPA), and the Department of Agriculture (USDA) have led to the determination that evaluation of products which interfere with conception in animals and are identified as drugs come under the jurisdiction of the FDA's Center for Veterinary Medicine. The Center acknowledges that the EPA and USDA have expertise in evaluation of certain areas, such as bait and vaccine development, and consults with them as needed. The Center is aware of the problem of population control among certain animal species and encourages the development of products to assist in their management.