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The development of non-surgical contraceptives for cats may facilitate population control of the species. The purpose of this study was to investigate the utility of GnRH for immunocontraception of male cats. Male cats (n ¼ 12) were divided into groups of three and were immunized once with 0 (sham), 50, 200, or 400 mg synthetic GnRH coupled to keyhole limpet hemocyanin and combined with a mycobacterial adjuvant to enhance immunogenicity. GnRH antibody titer, serum testosterone concentration, and scrotal size were determined monthly. At 6 months, semen was collected by electroejaculation and testes were examined histologically. GnRH antibodies were detected in all cats receiving GnRH vaccine by 1 month post-treatment and persisted throughout the study. No dose effect of GnRH was observed; titers were not different among cats treated with 50, 200, or 400 mg GnRH (P ¼ 0:5). Six of nine treated cats were classified as responders based on high GnRH antibody titers (>32,000). By 3 months post-treatment, responder cats had undetectable testosterone concentrations and testicular atrophy. Non-responder cats had GnRH titers of 4000–32,000 and testosterone concentrations intermediate between responder and sham-treated cats. At 6 months, total sperm counts were similar for sham-treated cats (3:1 _ 1:8 _ 106 sperm) and non-responder cats (3:4 _ 1:6 _ 106 sperm; P ¼ 0:7). Only one of the six responder cats produced sperm, none of which were motile. Combined testicular weights of responder cats (1:3 _ 0:1 g) were lower than sham-treated controls (5:3 _ 1:3 g; P ¼ 0:02) and non-responder cats (2:9 _ 0:3 g; P ¼ 0:02). Histologic evaluation of the testes revealed that in responder cats, the interstitial cells that were present were pale and shrunken compared to the plump, polyhedral eosinophilic cells in sham-treated cats.