Prospects for managing African elephant population growth by immunocontraception: a review

Immunocontraception has been proposed as a tool for managing African elephant populations threatening to. 'outgrow' a wildlife reserve. To date, however, the only immunocontraceptive technique tested on elephant cows is porcine zona pellucida (pZP) vaccination, in which solubilized pZP is injected together with an adjuvant to induce formation of circulating antizona pellucida antibodies, which block fertilization. A review of the literature on the use of pZP vaccination in free-ranging mammals reveals that the contraceptive efficacy ranges between 22% and 100% (15 trials, 2 in elephants). A pZP vaccine can be delivered by dart, but at present more than one inoculation is needed to ensure contraceptive antibody titres. Initial studies in elephants suggest that pZP vaccination is safe, even in pregnant animals, does not pass through the food chain and is reversible, at least in the short term. However, little is known about possible long-term side effects. Elephants are social animals that live in matriarchal herds, and inhibiting individual fertility and herd growth may have unforeseen longer-term consequences on behaviour and social structure. There is also a fear that immunization may favour weaker animals by preferentially sterilizing individuals capable of mounting a vigorous immune response, or that animals may become resistant to vaccination. In short, while pZP vaccination appears to be a promising tool for controlling elephant population growth, questions about the long-term side effects need to be answered before use on a large scale can be recommended.