Effect of GnRH immunisation on hormonal levels, sexual behaviour, semen quality and testicular morphology in mature stallions

Abstract

Summary The aim of this study was to investigate the effect of gonadotrophin‐releasing hormone (GnRH) immunisation on mature stallions that had been used for breeding. Four Standardbred stallions were used in the study: 3 experimental animals and 1 control animal. Semen was collected regularly, i.e. twice/week, during the 4 months prior to the experimental period. The stallions were immunised against GnRH with a GnRH‐BSA conjugate. Equimune was used as the adjuvant. The stallions were immunised on 5 occasions, 4 at 2 week intervals, and the fifth 4 weeks after the fourth. Blood samples were taken once a week for analysis of GnRH antibody titre and every third week for testosterone and oestrone sulphate analyses. Semen was collected once a week, and libido and sexual behaviour were observed. Ejaculate volume, sperm concentration, total number of sperm in the ejaculate, sperm motility and sperm morphology were evaluated. Testicular size was measured once a week. At the end of the study, the stallions were castrated, and a histological examination of the testes performed. All immunised stallions produced antibodies against GnRH, and plasma testosterone concentration decreased. However, the effect of immunisation varied between stallions. In 2 of the stallions, high levels of antibodies were found, while in the third, the level was moderate. Four weeks after the first immunisation, a decrease in libido was observed. Two months after the first immunisation, marked changes in semen quality were observed in the 2 stallions with high antibody titres. Fourteen weeks after the first immunisation, the total number of sperm/ejaculate had decreased from >8.6×109 to<2.7×109, sperm motility from >59 to<10% and the frequency of morphological normal spermatozoa had decreased from >60 to<14%. The dominating abnormalities were abnormal head shapes, proximal cytoplasmic droplets and detached heads. In the third stallion, only slight changes in semen quality were found. No changes were observed in the control stallion. Decreases in testicular size were noted in all of the experimental stallions. Pronounced histological alterations in the testes were observed in 2 of the stallions. It is concluded that the vaccine was effective in stimulating production of GnRH antibodies and in suppressing testicular function and androgen secretion. However, there was an individual variation in the responses among the stallions and, further, libido was not totally suppressed.

Effects of GnRH immunization in sexually mature pony stallions

“…Testis size was reduced to the same extent as reported in other GnRH immunization studies in stallions (Dowsett et al, 1991(Dowsett et al, , 1993(Dowsett et al, , 1996Malmgren et al, 2001). However in the studies of Dowsett, relatively young stallions were involved of which the testes most likely were still developing.…”

“…Titers were only slightly increased in two stallions, while two other stallions responded with substantial antibody titers. Such a large variation in antibody responses after GnRH immunization is shown in the majority of the GnRHimmunization studies in which sexually mature subjects are involved (Chappel et al, 1980;Lincoln et al, 1982;Schanbacher et al, 1983;Grizzle et al, 1987;Simms et al, 2000;Malmgren et al, 2001). Several factors may affect the immunogenicity of the applied vaccines, such as the type and amount of antigen, the adjuvant, vaccination schedule and the species involved.…”

“…The oil-based vaccine was not well tolerated since severe local reactions appeared at the injection site. Only recently, a GnRH vaccine was tested in sexually mature stallions (Malmgren et al, 2001). It appeared to be effective in two out of three treated stallions, though five injections were required and adverse reactions at the site of injection were observed, despite the fact that a mild adjuvant (Equimune and aluminium hydroxide) was used.…”

“…Unfortunately, this vaccine has not been applied in sexually mature stallions, which most likely will be the most important treatment group. In a recent study, three adult stallions were treated with a GnRH vaccine with a commerciallly available 'horse adjuvant' Equimune (Malmgren et al, 2001). However, this vaccine failed to induce high antibody titers in one of the three stallion, despite the fact that four booster injections were given.…”

Vaccination against GnRH may suppress aggressive behaviour and musth in African elephant (Loxodonta africana) bulls - a pilot study

“…Most studies with GnRH vaccine in other species failed to produce an adequate response in 100 % of treated animals. It seems that a certain number of non-responders can be expected 23,32,49,51,52 . The reasons for individual variations are not completely understood.…”