Animal Writes
From 27 February 2005 Issue

By Priscilla Cohn, Ph.D. - [email protected]

Human contraceptives were first tested on animals, and then later used to control fertility in animals. Starting in the 1960s, hormonal contraceptives were tested on a large number of animals including, among others, dogs, lions, deer, horses, rats, and birds. Delivery of these compounds was comparatively simple for zoo animals where much of the early research occurred, but difficult for free-ranging wildlife, because large and frequent doses were necessary, and also because baits were often avoided. In addition, social behavior was sometimes adversely affected. A major disadvantage concerned passage of these steroid hormones through the food chain so that other, non-targeted species, such as predators, or even humans, might ingest them. By 1987, data clearly revealed that hormonal contraceptives were causing health problems in, at least, some animals.

Since the late 1980s, much of the research has been directed toward immunocontraception, a method whereby an individual animal is injected with a substance that causes it to produce antibodies that interfere with a process necessary for reproduction. Many different kinds of immunocontraceptives are possible. Vaccinations could counteract the effect of hormones made by the brain, by the pituitary gland, and so forth. The vaccine that has been most widely tested and developed is PZP (porcine zona pellucida).

The zona pellucida is the non-cellular membrane that surrounds the mammalian egg. When the zona pellucida from a pig is inoculated into an animal of a different species, that animal produces antibodies that prevent sperm from penetrating it own zona pellucida, thus preventing fertilization, and reproduction.

There are a number of practical advantages, some of which translate into ethical advantages, in using PZP. It is, on average, approximately ninety per cent effective, and does not pass through the food chain. Since only a small amount of the vaccine is needed, it can be delivered remotely by a dart, thus avoiding the stress of capture, or anesthesia. PZP does not significantly affect social behaviour and, so far, has not been found to cause any serious health problems. It is safe for pregnant animals, is reversible, and is effective on a broad range of species. The vaccine itself is very inexpensive. Costs for an immunocontraceptive program vary largely according to personnel expenses.

In general, then, there are many advantages to PZP immunocontraception. It is humane for animals, since it causes neither death nor discomfort. It represents no danger to humans, since it does not involve guns or razor-tipped arrows. It is effective since there is no rebound reproduction, a phenomenon that occurs when large numbers of animals are suddenly removed. Bizarre and often troublesome animal behavior is avoided, since the young are not orphaned, and social structure is not affected. Genetic diversity is not lost since it is reversible. Lethal methods, such as hunting and culling, offer none of these advantages.

Immunocontraceptives in general, and PZP in particular, seems to be desirable if it is used as an alternative to culling or hunting to control wildlife populations, or if it is used to limit the reproduction of zoo animals so that so-called "surplus" animal are not born.

Hunters, of course, do not view immunocontraception as desirable for they fear that its use threatens their "life style." They are, however, not alone in their criticism of PZP. Some animal rightists have also raised ethical objections claiming that immunocontraception is simply one more example of human domination and manipulation of the natural world, one more way in which we control nature for our own ends, and with our own welfare as the primary criterion. This criticism is valid, but it would also apply to our habitual ways of treating animals, which include shooting, poisoning, gassing, habitat manipulation, and so on. Seen within this context, immunocontraception appears benign, even if it does not represent the ideal of non-interference with, and respect for, all non-human animals.

Another objection concerns the fact that PZP is a slaughterhouse product made from pig ovaries. It is claimed that purchasing these ovaries only increases slaughterhouse profits, and encourages more slaughter. This objection is also valid, but, again, it can be argued that the profit increase is very small. Furthermore, whether or not their ovaries were sold, pigs would still be slaughtered for their meat. Eliminating the use of PZP would not lessen the number of pigs killed. This objection would be nullified if the ongoing research that is directed at genetically engineering PZP were successful.

A third, although infrequently heard, objection to PZP is that the development of this vaccine inevitably leads to more animal experimentation. Most scientists believe it is a necessary part of the scientific method to kill and necropsy animals to order to prove that PZP is harmless. According to this view, the fact that PZP vaccinated horses lived longer, and were in better condition than those who were not treated, does not militate against this necessity.

A fourth objection is the ease with which PZP can be "misused." It is one thing, critics assert, to use PZP as an alternative to killing, and quite another to control the number of predators to protect their prey for the benefit of hunters or fishers. Actual instances of such misuses have already occurred. Denying that they had over-fished, Canadians wanted to contracept seals because they claimed that the seals were responsible for the lack of fish. Similarly PZP is being used in Canada to reduce the number of wolves to prevent them from preying upon caribou herds that hunters want to exploit.

Scientists continue their research, not only on PZP to make it effective for a longer period, but also on other kinds of immunocontraception. Scientists are also working to produce a sterilant for domestic cats and dogs that is as effective, but easier and less invasive than surgically spaying and neutering.

Immunocontraception: ethically desirable or the opening of Pandora's box? The jury is still out.

For more information on immunocontraception and PZP in particular, see 

This essay will appear in Andrew Linzey's Animal World Encyclopedia, Kingsley Media Publishing, forthcoming 2005.

[Professor Priscilla Cohn is an internationally known philosopher. She has a doctorate in philosophy from Bryn Mar College, and is professor emeritus of Abington College, Pennsylvania State University, USA. She has written on animals, environmental issues and ethical problems, as well as on contemporary philosophers and the history of philosophy, publishing both in English and Spanish. Among her books are Etica aplicada (Applied Ethics) with Jose Ferrater Mora, and, more recently, Ethics and Wildlife (Edwin Mellen Press, 1999). She has lectured on five continents, including Spain where Queen Sofia attended her summer school course on animal rights. She is the founder and director of PNC Inc., a non-profit animal rights foundation, which organized the first international conference on contraception in wildlife, and which also initiated and funded the first study on fertility control in deer using the immunocontraceptive agent PZP.]

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