Porcine Zona Pellucida
What is PZP and How Does It Work?
A non-cellular membrane known
as the zona pellucida (ZP) surrounds all mammalian eggs. The
ZP consists of several glycoproteins (proteins with some carbohydrate
attached), one of which, ZP3, is thought to be the sperm receptor
(the molecule which permits attachment of the sperm to the
egg during the process of fertilization). The PZP vaccine
is derived from pig eggs. When this vaccine is injected into
the muscle of the target female animal, it stimulates her
immune system to produce antibodies against the vaccine. These
antibodies also attach to the sperm receptors on the ZP of
her own eggs and distort their shape, thereby blocking fertilization
(see Paterson and Aitkin 1990).
Thus far PZP has been a promising
form of contraception in wildlife because
1. it has prevented pregnancy an average of 90% of the time
in treated animals
2. it can be delivered remotely by small darts
3. the contraceptive effects are reversible
4. it is effective across many species
5. there are no debilitating health side-effects even after
6. it has almost no effects on social behaviors
7. the vaccine cannot pass through the food chain
8. it is safe to give to pregnant animals (see Kirkpatrick
et al. 1996b).
How is the Vaccine Delivered?
The PZP vaccine must be injected into the muscle of the target
animal. This can be done by hand if the animal is restrained,
or by dart, for remote delivery. There are many commercial
dart systems available but the thick viscosity of the vaccine
requires a large needle and a quick injection. Thus far, Pneu-Dart(r)
systems (Williamsport, PA) seem to work the best. The Pneu-Dart(r)
1.0 cc barbless darts can be fired from Pneu-Dart(r) capture
guns or from several other commercial dart guns (Pax-Arms(r)
or Dan-Inject(r), for instance). The darts are disposable,
and after hitting the animal in the rump or hip (the only
acceptable location for darting) they inject by means of a
small powder charge and pop out. Because of their bright colors
the darts are usually retrieved in the field. Undischarged
darts cannot be discharged by stepping on them or by any other
kinds of casual contact. Over a ten year period on Fire Island
National Seashore, and more than 1,500 dartings of deer, only
6 darts have not been recovered.
Normally, each animal is darted twice the first year, with
the first injection being given up to a year before a booster,
just preceding the breeding season (March for wild horses
or September for deer). Thereafter, a single annual booster
inoculation will maintain contraception. The second inoculation
of the first year requires
1. that you are able to recognize the individual animals,
2. that you do the first inoculation with a special "marker
dart" which leaves a dye mark on the animal at the same
time it injects the vaccine, or
3. that animals are treated opportunistically and randomly,
with the hope of eventually treating a large proportion of
the total population over the course of several years.
An alternative strategy is to give only a single inoculation
the first year, from which there will be little contraception,
and then a single annual inoculation thereafter, from which
there will be significant contraception (see McShea et al.
1997). Current research is aimed at the development of an
effective and longer acting one-inoculation form of the vaccine
(see Turner et al. 2002).