Immunocontraception of Free-Ranging African Elephants in Kruger National Park, South Africa
In order to seek a more publicly acceptable alternative to the management of African elephants by culling, a test was conducted to determine if a porcine zona pellucida (PZP) vaccination could effectively contracept females of this species. Initially, ovaries were recovered from culled animals and slices were incubated with immunogold-labeled rabbit antibodies against PZP. Significant staining of the elephant zona suggested that PZP would be an effective contraceptive vaccine. In a second experiment, three captive female zoo elephants were inoculated with the PZP vaccine (400 µg PZP + 300 mg RIBI). These non-breeding animals were tractable and blood samples were recovered and assayed for anti-PZP antibodies. Antibody titers (1:500 dilution) peaked (0.75–1.3 OD) at 1–2 months following the initial inoculation, declined to 0.1–0.34 at 6 months–1 year, and peaked again following a third inoculation (0.8–2.3). These data indicated that African elephants would mount a significant antibody response to the PZP vaccine and together with the histochemical study, suggested the vaccine would be a successful immunocontraceptive in this species.
In October 1996, 100 elephants in Kruger National Park (KNP) were captured by etorphine immobilization from a helicopter. Forty-one were determined to be non-pregnant on the basis of transrectal ultrasound examination conducted by Thomas Hildebrandt and colleagues (Institute for Zoo and Wildlife Research, Berlin). On the strength of the ultrasound determinations, it was found that the helicopter pilot and park veterinarian could identify non-pregnant females with 85–90% accuracy, on the basis of the size of calves with them. Twenty-one non-pregnant adult females received an initial IM dose of 600 µg of PZP vaccine emulsified in RIBI adjuvant, and another 20 received only saline and adjuvant. The treated females were fitted with radio collars and the control animals with colored collars without radios. Approximately 1 month later the experimental group received a 600-µg PZP-RIBI booster remotely, by darts fired from the helicopter. Eight months after the initial treatment, the treated animals received a second 600-µg PZP-RIBI booster remotely, via darts.
In October 1997, all 20 control cows and 19 of the 21 PZP-treated cows were located, immobilized, and examined for pregnancies by ultrasound. All cows still had their calves at their sides, indicating no adverse behavior reactions to the vaccine. Eighteen of 20 control elephants were pregnant (90%). Three of the 19 treated females were pregnant, and the size of the fetus indicated that they were in the early stages of pregnancy at the time of initial PZP treatment and ultrasound examination. Of the remaining 16 treated animals, only six were pregnant (37.5%) and the difference in pregnancy rates between control and treated animals was significant. Antibody titers among the treated elephants, across the 1 year, were similar to those in the three zoo elephants, with an initial rise after the first booster inoculation, a steady decline during the next six months, and a significant elevation after the second booster inoculation.
These results indicate that the PZP vaccine can contracept African elephants. On the basis of both the zoo and free-roaming elephant antibody titer data, a second trial has been carried out with six additional non-pregnant females, in which the PZP vaccine was given at day 0, day 14, and day 35–42. These animals have been fitted with GPS collars as well. The hypothesis is that the change in the timing of booster inoculations will raise contraceptive efficacy to near 100%. Half of the previously treated and non-pregnant females were given another booster inoculation to test longer-term contraceptive effects and the other half were left untreated, to test reversibility of contraceptive effects.
Data retrieved thus far indicates that 1) PZP vaccination will inhibit fertility in adult female African elephants, 2) inoculations can be given remotely, 3) family group integrity is not affected by PZP vaccination, 4) reversibility of contraceptive action is probable, based on the declining antibody titers, and 5) no abscesses formed at the injection sites. Further studies may provide sufficient data to provide managers of African elephants with a non-lethal and publicly acceptable management tool.