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Several areas of active research are ongoing by the PZP Wildlife Contraceptive Group, and these areas of research reflect current shortcomings of the PZP vaccine. These current research efforts include the following:
A One-Inoculation Vaccine: Because of the need to inoculate animal twice the first year, and the difficulty of doing this with wild species, research is ongoing toward a “one-inoculation” vaccine. Such a vaccine would permit a single inoculation to cause one or more years of contraception. The approach under study incorporates the PZP into non-toxic, biodegradable material which can be formed into small pellets. The pellets can be designed to release the vaccine at predetermined times after inoculation (at one, three, nine or more months) much the way time-release cold pills work. Initial trials are encouraging and continued trials are ongoing (see Eldridge et al. 1989; Turner et al. 2001, 2002).
New Adjuvants: The good news is that PZP works in many mammalian species, because the ZP molecule has been conserved over millions of years of evolution. This means the molecule is similar (but not identical) among the many species of mammals. The bad news is that this similarity across species means that PZP is not very good at causing antibodies to be formed. Thus, it must be given with a general immunostimulent, known as an adjuvant. This adjuvant, when given with a specific vaccine, causes the body to make greater concentrations of antibodies against the vaccine. Currently the adjuvant of Choice is a product known as Freund’s MODIFIED adjuvant. It cannot cause false tuberculosis-positive tests after use, as some earlier adjuvants did. The research team is constantly examining new adjuvants. (see Harrenstein et al. 2004; Lyda et al. 2005; USDA 1997).
Improved Marker Darts: Available marker darts have a fair degree of reliability but their ranges are limited. The need here is for smaller marker darts that can be accurate at greater ranges.
A Genetically Engineered or a Synthetic ZP Vaccine: The ability to produce large quantities of the PZP vaccine at low cost will permit the increased use of this vaccine in wildlife. This is a long-term effort requiring resources well beyond those of our own research group. While this is a desirable goal, its attainment is thought to be rather far off (see Kaul et al. 1996).
Other areas of wildlife contraceptive research are being carried out by other groups. For a variety of reasons, we see these directions as less fruitful and are not pursuing these goals. The list below illustrates some of these efforts, but is not all-inclusive.
Oral Contraceptives: It is intuitive that the ability to deliver contraceptives to wildlife in baits would be easier and more cost effective. However, for safety and ethical reasons, both the public and regulatory agencies are likely to demand that any oral contraceptives must be species-specific (that is, they must work in only one species for which it was intended). This will be very difficult with the PZP vaccine. Other groups are pursuing this goal (see Eldridge et al. 1989).
Viral-Vectored Contraceptives: Researchers working with the Australian government are seeking to engineer the gene for PZP and similar contraceptive molecules into non-pathogenic viruses. The viruses could then be transmitted from animal to animal in wild populations. While there are some attractive features to this approach, the safety, environmental, and ethical issues associated with this technology will raise some strong objections from the public and regulatory agencies in the U.S., and this approach will probably never be used here (see Robinson et al. 1997).
Abortifacients: At least two research groups are seeking to administer compounds which will cause abortion in the recipient animals. This has already been shown to be feasible in deer, but we are not pursuing this approach because of the stresses that impinge upon the wildlife.
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