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The immune response and fertility of koalas (Phascolarctos cinereus) immunised with porcine zonae pellucidae or recombinant brushtail possum ZP3 protein

Kitchener, AL, Kay, DJ, Walters, B, Menkhorst, P, McCartney, CA, Buist, JA, Mate, KE & Rodger, JC 2009, Journal of Reproductive Immunology, vol. 82, pp. 40-47.

Zona pellucida, a membrane that surrounds mammalian ova, was used to immunise koalas against pregnancy. Three treatments were applied: zona pellucida from pig (PZP), zona pellucida protein from brushtail possum (recBP-ZP3) and a control. In 29 koalas for which fertility after immunisation was evaluated, both PZP and recBP-ZP3 were found to raise antibodies in the koala, but only PZP was effective as contraception.

  Fifty female koalas were captured, sedated and immunised at two sites under the skin near the groin and twice into the muscle of the thigh. The koalas were tagged for subsequent radio tracking to enable recapture. One group of koalas received a control treatment, the second received PZP immunisation and the third received immunisation of a cloned protein from possum found in zona pellucida (recBP-ZP3). The koalas received a second immunisation three to five months after the first. Of the 50 koalas, 43 were recaptured three or more times and their fertility assessed. Of these, 14 were labelled as non-breeding as they were never observed to breed before or after treatment. The remaining 29 bred a total of 31 times: 89% of the controls bred and 90% of koalas immunised with recBP-ZP3 bred. In contrast, only 30% of koalas immunised with PZP had young, which was significantly less than the number of young produced by koalas in the recBP-ZP3 treated and control groups. Number of antibodies produced against the antigen, also called titre, was high in PZP treated and recBP-ZP3 treated koalas. The titre continued to increase for 3 months and then plateaued after 4 months. Thirty-three months after the first immunisation, antibodies against recBP-ZP3 and PZP were found in the ovaries, uterus and vagina. Ovarian tissue of treated and non-treated animals appeared normal. When recBP-ZP3 protein was fragmented and allowed to react with serum from immunised koalas, recBP-ZP3 treated koalas reacted to many fragments while PZP treated koalas reacted only to a few fragments.

  Fertility control using PZP immunisation has been effective in brushtail possums, tammar wallabies and now koalas. Interestingly, while recBP-ZP3 treatment has worked for possum and grey kangaroo fertility control, it did not work as a contraceptive for koalas despite raising an antibody response. This suggests that koalas have a different immune response to recBP-ZP3 than possums and grey kangaroos. Importantly, the authors of this study determined that the prevalence of chlamydial infection in the studied population (Snake Island, Victoria) was higher than previously realised. As chlamydia is known to negatively affect koala fertility, the authors warn that results must be interpreted with caution as it is hard to determine whether koalas were infertile as a result of immunisation or from an acquired chlamydial infection.

  In the southern extent of the koala’s range where overcrowding can lead to leaf and tree destruction and ultimately koala starvation, fertility methods such as immunisation with pig zona pellucida fragment may be effective. Additional research will be required to confirm the study’s findings of PZP effectiveness in controlling fertility in chlamydia-free koalas.


Summarised by Alexandra Selivanova


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