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Fertility Control

A common reason some species are overabundant is that they are adaptable to multiple or changing environments, thus their populations increase in spite of a rapidly changing landscape.

A growing interest in nonlethal methods for population control of nuisance or damaging species of wildlife has stimulated research in reducing fertility of these overabundant wildlife species. Fertility may be reduced by interfering with the fertilization of the egg (contraception) or interfering with the implantation or development of the fertilized egg (contragestion).

To have success in inducing infertility in a particular species, researchers must have an understanding of the reproductive behavior and physiology of that species and then select the most suitable infertility agent. Some recent infertility work has centered around development of a compound that reduces hatchability of bird eggs. Other infertility research has focused on immunocontraceptive vaccines, which control fertility by stimulating the production of antibodies against essential reproductive proteins or hormones. These antibodies interfere with the normal activity of the reproductive system. Two common targets for immunocontraception are a hormone, gonadotropin releasing hormone (GnRH), and a protein, zona pellucida (ZP).

Nicarbazin-- Induced Infertility as a Management Tool in Birds

Interfering with egg laying or the hatchability of the egg appears to be the best approach to reducing the reproductive capacity in birds. Expanding populations of resident Canada geese that remain in suburban and urban areas year-round often result in increased conflicts with humans. Non-lethal and humane means are needed for managing the size of Canada goose flocks residing near or on airports, golf courses, industrial parks, government sites, and city parks. A side effect of nicarbazin, a veterinary drug used to control coccidiosis in chickens, is decreased egg production and hatching. Studies on nicarbazin, and its eventual bait development, led to the OvoControl-G® (Innolytics LLC) product, which resulted in reduction in hatchability of 51% at treated sites compared to control sites in the field. Previous studies showed that nicarbazin is practically nontoxic and is environmentally friendly; timing and management of baiting will minimize non-target hazards. OvoControl-G® 2500 ppm nicarbazin baits recommended for incorporation into a comprehensive management plan as a reproductive inhibitor for use in controlling resident Canada goose flock sizes (Bynum 2005).

Gonadotropin-Releasing Hormone Vaccine

A GnRH immunocontraceptive vaccine can shut down the reproductive activity of both sexes by causing the development of antibodies that block GnRH. This, in turn, prevents the release of other essential reproductive hormones. The reduction or absence of these hormones leads to withering of reproductive organs and this induces infertility in both sexes. GnRH contraceptive vaccines have been evaluated for use in pets, cattle, horses, sheep and swine for more than 10 years, but little research has been done in wildlife species. In studies with Norway rats it was found that both males and females immunized with a GnRH vaccine were 100% infertile. A single injection of GnRH vaccine (GonaCon) can have a contraceptive effect for multiple years. USDA/APHIS, in conjunction with the U. S. Food and Dug Administration (and, in the future, the U.S. Environmental Protection Agency) is seeking registration of GonaCon for use in cervids (hooved animals).

Zona Pellucida

Zona pellucida (ZP) immunocontraceptives were the first to receive widespread publicity as contraceptives in deer and feral horses. The zona pellucida is a protein on the outer surface of the egg. A vaccine that produces antibodies to the zona pellucida causes infertility either by blocking the sperm from penetrating the ZP layer and getting to the egg, or by interference with the maturation of the egg. The ZP vaccine in use today comes from the pig ovary and is called Porcine Zona Pellucida (PZP). PZP vaccine (ZonaCon) has been used to produce immunosterilization in dogs, baboons, horses, burros, coyotes, and white-tailed deer. In a white-tailed deer study at Pennsylvania State University, an 89% reduction in fawning during the 2 years of active immunization was achieved. A 76% reduction in fawning was observed over the entire 7-year study. Similar to GonaCon, a single injection of ZonaCon will result in multiple years of infertility.

Status of Oral Immunocontraception

Orally delivered fertility control as a technology may soon be available in limited field situations. It is the most practical fertility control method for controlling free-ranging animal populations.

An oral contraceptive under study is 20,25-diazacholesterol, an extremely potent inhibitor of cholesterol synthesis in laboratory animals. This substance is delivered in baits and is active in both birds and mammals. Properly timed administration prior to the breeding season will result in temporary infertility. USDA/APHIS intends to submit a registration application to USEPA for use of diazacon on monk parakeets, prairie dogs and California ground squirrels.

References

BYNUM, K. S., C. A. YODER, J. D. EISEMANN, J. J. JOHNSTON, AND L. A. MILLER. 2005. Development of nicarbazin as a reproductive inhibitor for resident Canada geese. Proceedings of the Wildlife Damage Management Conference 11:179-189.

KILLIAN, G., L. MILLER, J. RHYAN, AND H. DOTEN. 2006. Immunocontraception of Florida feral swine with a single-dose GnRH vaccine. American Journal of Reproductive Immunology 55:378-384.

MILLER, L. A. 2002. Reproductive control methods. Pages 701-704 in D. Pimental, editor. Encyclopedia of pest management. Marcel Dekker, New York, USA. http://www.aphis.usda.gov/ws/nwrc/is/02pubs/mill022.pdf

MILLER, L. A., B. E. JOHNS AND D. J. ELIAS. 1998. Immunocontraception as a wildlife management tool: some perspectives. Wildlife Society Bulletin 26: 237-243.