Vaccination stops tumor growth in rhinoceros

The treatment was successfully conducted in southern white rhinoceros (Ceratotherium simum simum) and greater one-horned rhinoceros (Rhinoceros unicornis). Injecting the serum 'Improvac' influences the release of sexual hormones, which causes the female oestrous cycle to cease and thereby reduces hormone-dependent tumors.

Female greater one-horned rhinos in human care often have tumors in their reproductive tract, an important reason for problems in reproduction. These tumours have also been discovered in free-ranging Sumatran rhinos (Dicerorhinus sumatrensis) and are mainly innocuous tumors in the vagina or uterus (leiomyoma). If the tumors proliferate too much, fertility is reduced or the animal becomes completely infertile. The tumors also hurt the female during mating and lead to problems in conception, to miscarriage or stillbirth.

Tumor formation and growth are influenced by sexual hormones until the menopause, when the production of sexual hormones ceases. Therefore, tumor risk increases with age. Since resecting large tumors is not possible because of the thick skin of rhinoceros, a different procedure was tested in the recent study.

Three greater one-horned rhinos and four southern white rhinos (Ceratotherium simum simum) in zoos from several countries were treated with the vaccine 'Improvac', a hormone related to the body’s own gonadotropin-releasing hormone (GnRH). GnRH regulates the release of sexual hormones from the hypophysis and hence ovarian activity and ovulation. After inoculation, antibodies are produced which block the body’s own GnRH. As a result, the hypophysis stops releasing sexual hormones and the oestrous cycle as well as the female's fertility stop. With the sexual hormones put 'on the back burner', hormone-dependent tumors should stop growing or even reduce in size.

Although the ovaries of the rhinos were active before the vaccination, the animals were infertile because of intense tumor growth. Already three months after the first inoculation, ultrasound examination showed a reduction in the size of the tumor to half of the original size. After one year, no more newly emerging proliferations were detected.

The tumor risk depends on whether the animals have already had offspring or not. Gestation at an early age, for example, prevents reproductive organs from tumor formation and existing tumors stop growing if an animal becomes pregnant. Therefore, early reproduction of rhinos should be promoted to avoid the formation of small tumors in young females. “If they are not able to have offspring because of external circumstances, for instance in the absence of a suitable mate, a vaccination could help prevent tumor formation“, explains Robert Hermes from the Leibniz IZW, expert for reproductive medicine and rhinoceros. “The sexual cycle is ’put on ice‘ and can be reactivated for pregnancy later on without loss of the female's fertility or the risk of tumors.“

Further examinations will be required to determine whether the effect of the vaccination is a completely reversible process. In this study, the inoculation was conducted in free-ranging animals for the first time. The same treatment has already been successfully tested in horse mares, which became fertile again with a normal sexual cycle some time after the inoculation. “If the oestrous cycle of female rhinoceros can be interrupted by a simple inoculation and, later on, fertility can recover again, the vaccination is a huge veterinary advance. This is a great benefit, particularly with regard to the future health care management of rhinos in human care“, says Hermes.

These results have been published in the scientific Open Access journal PLOS ONE.

Text: Leibniz-IZW
Photos: Ltshears, CC BY-SA 3.0 (leadphoto: greater one-horned rhinoceros); Pixabay