An immuno-contraceptive vaccine causes an animal's immune system to produce antibodies that act against some essential event or structure in the reproductive process. The antibodies can act against sperm, eggs or reproductive hormones, which prevent either fertilization or the production of sperm and ova.

Proponents of the technique, which was first tested nearly 20 years ago, regard it as more humane than the conventional methods of controlling wildlife populations, such as shooting, trapping, poisoning or viral diseases. It has drawn support from some politicians and animal-welfare agencies.

An expert in mammal reproduction, Professor Cooper, of the UNSW School of Biological, Earth and Environmental Sciences, questions immuno-contraception on three grounds.

"Firstly, immuno-contraceptives are ineffective against substantial minorities of animals, probably for genetic reasons," Professor Cooper says. "If so, the genes responsible for this lack of response will be passed on to offspring. Within a few generations most of the population will be unresponsive to the immunocontraceptive, so its effectiveness as a form of birth control is likely to be short-lived."

Professor Cooper's research reveals that than in 27 out of 32 immunocontraceptive control trials around the world, 10 per cent of animals remained fertile because they were unresponsive to self-antigens.

His second concern is that the technique is effectively a form of genetic engineering: "Immunocontraception alters the gene pool of the targeted species because those animals who are unresponsive are genetically unusual individuals," he says.

"The job of the immunological system is to combat disease-causing organisms. But there is a danger that the selected minority might be more susceptible to diseases, or that they would be better at carrying disease-causing agents which could affect other animals with which they come in contact.

"Unless we are certain about these effects, altering the genetic capacity of a wild population to mount immune responses should not be permitted.

"The third concern is that animals are suffering side-effects from multiple injections and adjuvant substances (catalysts that modify the action of other agents) designed to boost the effectiveness of immuno-contraceptives.

"The most commonly used adjuvant, Freund's adjuvant, also induces a range of undesirable side-effects and its use is being challenged on animal-welfare grounds."

About immuno-contraception

Several immunocontraceptive vaccines are under development, including vaccines against brain reproductive hormones such as gonadotropin-releasing hormone (GnRH); pituitary hormones such as luteinizing hormone (LH), follicle-stimulating hormone (FSH); vaccines against steroid reproductive hormones such as oestrogen and progesterone; and vaccines against the sperm and the egg, which in turn prevent fertilization.

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Materials provided by University of New South Wales. Note: Content may be edited for style and length.