Porcine Zona Pellucida Immunocontraception in Captive Exotic Species: Species Differences, Adjuvant Protocols, and Technical Problems
Kimberly M. Frank, BS; Jay F. Kirkpatrick, PhD
Since it was first reported to the American Association of Zoo Veterinarians (AAZV) in 1992, the use of porcine zona pellucida (PZP) immunocontraception in zoos has increased to include 95 distinct species in 87 zoos. At the present time there are 30 species for which five or more animals are on protocol. Ungulates form the largest single group of taxa and have provided the most information regarding effectiveness and safety. As data have accumulated, it has become clear that species differences require differing treatment protocols, that adjuvant protocols must be adjusted to the individual species, and that a certain amount of precision must be adhered to during the treatment phase to avoid technical problems and contraceptive failures.
Most of the initial data regarding PZP contraception came from wild horses and white-tailed deer, where two initial inoculations and a single annual booster inoculation could cause and maintain contraceptive antibody titers. Both species, however, are highly seasonal with respect to reproduction. Over the past 10 yr it has become clear that not all captive mammals mount identical immune responses with regard to duration of contraceptive effects, and that less seasonal breeding patterns among zoo animals require more frequent booster inoculations. Currently 59 giraffe and 42 zebra (all species) are on protocol. Among 35 giraffe and 29 zebra for which complete data exist, contraceptive efficacy has been 94% and 96%, respectively, for animals given booster inoculations every 7–9 mo. Big horn sheep (Ovis canadensis) and Rocky Mountain goats (Oreamnos americanus) provide an even more dramatic example of species differences among ungulates. Three big horn sheep inoculated in 1996 with a series of three PZP injections over 6 wk remained infertile for 2 yr; one lambed in 1999 and again in 2000, and another lambed in 2000. Among eight mountain goats inoculated with a single series in 1996, only one lambed by 1999 (and again in 2000) and one other lambed in 2000. Thus in goats and sheep a single initial PZP series will maintain infertility for more than 2 yr and in most cases longer. A single California sea lion (Zalophus californianus) that was treated with a single series of PZP inoculations in 1995 and bred with a proven male did not produced a pup until 1998, and none since that time despite being bred by the male. Other ungulate species for which sufficient data exist to document PZP contraceptive efficacy include ibex (all species, 38), white-tailed deer (19), sambar deer (16), North American bison (15), kudu (15), sika deer (2 subspecies, 15), Himalayan tahr (13), wapiti (9), caribou (7), bongo (7), markhor (5), and axis deer (5). In every case we recommend an initial series of three inoculations, with a single booster inoculation every 7–9 mo, unless the animal is markedly seasonal.
In equids, where no reliable TB test exists, the adjuvant protocol of choice is an initial inoculation with Freund’s Complete Adjuvant (FCA) followed by one or two subsequent inoculations with Freund’s Incomplete Adjuvant (FIA) over 6 wk. In all other ungulates the protocol is 3 inoculations over 6 wk with only FIA, and no differences in efficacy between the two protocols have been seen. More recently, we have used a new protocol which utilizes an initial inoculation with Freund’s Modified Adjuvant (MFA) followed by subsequent inoculations with FIA. The FMA does not lead to false positive TB tests. Among 4 fallow deer (Dama dama) that received an initial inoculation with FMA followed by a single booster with FIA the antibody titers were no different from 5 animals given an initial inoculation with FMA and s single booster inoculation with FMA. Thus, it appears that two subsequent inoculations of FMA can be given without the occurrence of abscesses, unlike FCA, and that either FMA or FIA will suffice for booster inoculations.
Among 35 giraffe for which data have been recovered, there were 14 failures during the first year of treatment and one during the second year and all could be attributed to technical failures. Of these 14 failures, six animals were already pregnant at the time of first treatment, two animals did not have the three-inoculation series completed, one had an inadequate injection because of the dart needle’s length (too short), and one was bred before the inoculation series was completed. Presumably antibody titers had not reached contraceptive levels before the treated animal was reintroduced to the male. We recommend at least 2 wk following the third inoculation before females can be safely placed back with males. In another two animals boosters were given late, after the 9-mo recommendation. The one second-year failure was due to a booster inoculation that was not given at all. In addition, there were two clear contraceptive failures.
Among the 29 zebra for which data has been retrieved, there were four first-year failures, two second-year failures and one third-year failure. Three first-year failures were attributable to incorrect timing of the initial three inoculations. Another was already pregnant. Both of the second-year failures were due to boosters not given on time. The third-year failure one was a contraceptive failure, thus contraceptive success, barring these technical problems was 96% (one failure).
Based on 14 yr of data from horses, the PZP vaccine appears extremely safe over both the short and long-term treatment, safe to give to pregnant animals, and almost completely reversible through 5 consecutive yr of treatment and we hope to analyze similar safety data for captive exotic species during the next 2 yr. In summary, (1) the timing PZP booster inoculations must conform to the species differences in maintaining adequate antibody titers and contraceptive effects, (2) treated animals must be given adequate time to mount significant antibody responses before they are placed back with the male, and (3) the choice of adjuvant must conform to the species being treated.
Special thanks to the many zoo veterinarians and keepers who have been diligent in their efforts to provide us with timely and accurate data, a list too long to recite here, and to Robin Lyda, of the ZooMontana SCC for her careful preparation of PZP vaccine.