46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
77
B. Zona pellucida-based immunocontraception
Introduction
Currently, the primary contraceptive in use in both elephant and equid populations is
the porcine zona pellucida (pZP) vaccine produced from the ovaries of pigs collected
at slaughter, and the antibodies induced are able to recognise and bind to ZP
proteins across a wide range of species. The pZP vaccine acts by stimulating an
immune response antigenically identical to zona pellucida (ZP) proteins surrounding
the released oocyte. The exact mechanism of action of ZP-based vaccines is not
completely understood, and may be the result of one or more possible routes of
action. Anti-pZP antibodies may affect sperm-oocyte binding, either by interfering
with receptors directly or through binding with epitopes nearby. The binding of
these antibodies is believed to prevent fertilization of the oocyte by spermatozoa,
and therefore conception.
A major limitation to the use of this vaccine, particularly in large population groups,
is the cost: primarily due to the current requirement for multiple vaccinetreatments. Elephant cows, for example, require two to three initial vaccinations
approximately five weeks apart, followed by annual boosters, generally via
helicopter-based darting. A synthetic or recombinant vaccine may prove more
economical, a potentially limitless amount may be manufactured and any risk
associated with an animal-derived vaccine eliminated when compared with the
current pZP vaccine. New vaccine formulations must aim to ensure vaccine safety
and with adequate efficacy over a longer time period, increasing the interval between
boosters to decrease management requirements and potentially lower
implementation costs. Recently, a recombinant vaccine based on the expression of
porcine ZP3 and ZP4 in E. coli was developed and has been tested in mice.
ZP-based vaccines have been extensively researched in mares since being first
reported in 1989. In addition to its contraceptive potential, pZP vaccination was
found to be reversible and appeared to produce no adverse effects on the oestrous
cycle or ovarian function in most mares. This has led to its application particularly
for management of feral horse populations in North America. In other species,
negative side effects, which include reduced numbers of developing follicles,
abnormal oestrous or menstrual cycles and permanent infertility, are reported.
Considerable interest in the use of immunocontraception in equines, as well as in
elephants, calls for further investigations into possible adverse effects of these
vaccines. In particular, the precarious state of elephant populations between labels of
“threatened” and “overabundant” makes these studies vital in ensuring the safety of
elephant populations in which immunocontraception is currently practiced or
proposed. The anatomy, size and demeanour of the mare allow trans-rectal
monitoring of ovarian events, together with serial blood collections, at a scale not
feasible in a species such as the elephant.
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