 |
Future Research
- the PZP Wildlife Contraceptive Group
A One-Inoculation
Vaccine
Because of the need to inoculate animals twice the first year,
and the difficulty of doing this with wild species, research
is proceeding toward a "one-inoculation" vaccine.
Such a vaccine would permit a single darting to cause one
or more years of contraception. The approach under study incorporates
the PZP into a non-toxic, bio-degradable material which can
be formed into small pellets. The pellets can be designed
to release the vaccine at predetermined times after injection
(at one and three months currently) much the way time-release
cold pills work. Initial trials were encouraging and continued
trials are underway (see Eldridge et al. 1989).
New Adjuvants
The good news is that the PZP vaccine works in most mammalian
species because the ZP molecule has been conserved over millions
of years of evolution. That 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 immunostimulant, known as
an adjuvant. This adjuvant, when given with a specific vaccine,
causes the body to make greater concentrations of antibodies
against the vaccine. The best available adjuvant, known as
Freund's Complete Adjuvant (FCA) also causes false positive
tuberculosis tests after injection and this restricts it use
in many species. New adjuvants, which do not cause these false
TB+ tests must be identified and tested. Several adjuvants
are under study at the moment. Success in this area may lead
to more relaxed regulation of the vaccine by the FDA (see
USDA 1997).
Conjugated
PZP
It is also possible to conjugate PZP (attach it) to other
immunogenic molecules. Experiments are underway to conjugate
PZP to an immunogenic protein known as keyhole limpet hemocyanin,
or KLH. Initial tests with PZP-KLH conjugates suggest that
this form of the vaccine can lead to cellular immune responses
(destruction by white blood cells) rather than humoral responses
(interference by antibodies). This in turn may lead to irreversible
sterilents as well as more effective contraceptives. The ability
to cause sterilization rather than temporary contraception
can be a huge advantage with some species in some situations
(see Millar et al. 1989).
Improved
Marker Darts
Available marker darts have a fair degree of reliability but
the various dyes have fallen short of the mark. Deer in particular
have a tendency to lick the dye off from the injection site.
New and more permanent dyes must be found that will survive
attention by the target animal and will persist over at least
a three to four week period.
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).
|
|
