Fertility of brushtail possums (Trichosurus vulpecula) immunised against sperm

The brushtail possum (Trichosurus vulpecula) is a major pest species in New Zealand. Research to develop a possum-specific method of immunologically-based fertility control (immunocontraception) has begun. This study examined the effect of immunization against sperm on the fertility of possums. Possums were assigned to 16 mating groups (two females and one male per group) before the start of the breeding season. One female in each group and eight males were injected subcutaneously with 5 x 10(7) sperm in complete Freund's adjuvant. Animals were given booster injections containing 5 x 10(7) sperm in incomplete Freund's adjuvant 4 and 12 weeks later. Control animals received adjuvants emulsified with phosphate buffered saline. Serum and sperm samples were collected monthly and the number of offspring recorded. Reproductive tract samples were collected from eight immunized and eight control possums of each sex at the end of the breeding season. Samples were screened for sperm antibody binding by an indirect possum sperm ELISA. Increased sperm antibody binding was detected in sera of 22 (15 females and seven males) of the 24 animals immunized against sperm. Immunization significantly reduced the proportion of females producing offspring (2/16 immunized versus 12/16 control) but had no effect on the fertility of males. Immunization increased sperm antibody binding activity in vaginal secretions of female and in prostatic tissue of male possums. Results indicate that there are antigens in possum sperm with immunocontraceptive potential for possum control.