This article is a reprint from the Science and
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 Freunds 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
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.