19. HCG injection
Gonadotropin-releasing hormones are substances in the blood that regulate the sexual glands (gonads): these are the testicles in males and the ovaries in females. The injection of a single gonadotropin-releasing hormone is referred to in the horse world as ‘the hormone injection’.
A hormone called HCG (Human Chorion-Gonadotropine) is produced in the placenta of pregnant women. It can be collected from their urine and then it can be used to make an injectable hormone preparation. The effect of HCG is like a gonadotropin-releasing hormone: it influences the sexual glands. If injected into male animals, it will stimulate the testicles to produce testosterone. An HCG injection in female animals stimulates the follicles to burst open and release the egg cell.
Olinde is a good breeding mare and is twelve years old. She gave birth to a foal nine days ago. She is taken from her box for observation: whether her behaviour indicates that she is willing to mate when she is close to a stallion. To that end, she is taken to another stable where a pony stallion is stalled. He is not the intended sire of the next foal: the teaser stallion may only make advances towards the mare; he may not service her. Standing in the corridor in front of the stallion’s box, the mare meets him through the railings. They sniff at one another and she shows an interest. When the young stallion becomes excited and gets an erection, Olinde turns her backside to the box and begins to urinate small amounts. The urine is cloudy and has a specific scent. The mare is in heat.
With my arm in her rectum, I examine her uterus with my fingers through the intestinal wall. It feels slack. There is a large, soft ovarian follicle on the left ovary with a diameter of around four centimetres. Our practice does not yet dispose of a scanner with which to measure the actual size. I then clean the opening to the vagina and insert a hollow, steel tube. Using a small light, I observe that the pink inner is shining with mucous. The cervix is lying limp on the bottom of the vagina. Olinde is optimally in heat. She can be serviced today. The studhorse owner is called. He will arrive later in the day with a lorry carrying his studhorses. The mare is to be covered naturally. Artificial insemination is still yet to become common in horse breeding. It is already customary in the cattle breeding sector.
checking the ovulation by ‘feeling up’ (rectal examination)
I check the ovulation two days later by rectal examination. The left ovary is back to its normal size: the follicle is gone and so the egg cell has been released. One of the sperm cells has meanwhile most probably penetrated the egg cell. The fertilised egg is now on its way through the fallopian tube to the uterus. That trip takes about five days in a mare. The ovary will start to produce pregnancy hormone and when the mare is observed a second time three weeks later, she will have no interest in the stallion. She will ‘reject him’ when he makes advances. But Olinde is observed to be most friendly towards the stallion the second time around; she urinates repeatedly and is visibly willing to mate. ‘She has come back’ the horse breeders say. This time, it is the right ovary that has a large follicle and she is ‘re-serviced’ that very same day.
Despite good advice and proper guidance, not every servicing leaves a mare with young. Good advice and proper guidance has no effect on the sperm quality of the stallion, for example. And the sperm quality differs greatly: gestation after a single cover differs per stallion in a range of ten to ninety percent. And the performance of a single stallion can also vary in terms of gestation results because the number of sperm is halved with each cover that takes place on the same day. And before AI became customary in the horse breeding sector, top stallions would sometimes service more than ten mares per day. And so the last mare on any given day received a lot less than the full load. At my request, Olinde is the first to be serviced that morning.
Two days later, there is still a follicle on the right ovary. It has grown somewhat in size. The mare is reserviced for that reason. But when I check again two days later whether the ovulation has occurred, the follicle is still present: its diameter now exceeds six centimetres. And Olinde has been in heat for five days now. Which is why she is given a hormone injection (HCG-injection); it will force the ovulation to take place. And she is covered for the third time in this cycle. She is no longer in heat after that. She is examined six weeks later to check the progress of the gestation. Feeling the uterus through the rectum, I find the uterus to contain twins.
It is not uncommon for mares to have two embryos, but twins are rarely born. One of the two usually dies prematurely. In this case, the two amniotic sacs differ somewhat in size: the smallest has probably died and will disappear. Embryos that die prematurely are not rejected through abortion, but rather they disappear due to resorption. This is not noticeable.
After ten months of gestation, Olinde’s udder is beginning to fill and the delivery follows a few days later. That is one month too early. Still, the stallion foal appears to be full-grown. But a few minutes later, a second foal arrives. It is a very small mare. Healthy-looking and lively, but clearly not full-grown.
Foals that are born prematurely have little chance of surviving, even in a single birth. In this case, the smallest of the two will probably not even survive two days. Intensive care is therefore essential and even then, the prognosis is doubtful. A few hours after the delivery, both foals are given a ‘foal injection’ (antibiotics) to prevent early infections. The mother is milked by hand in a small saucepan every other two hours, day and night. A rubber teat bottle is used to administer the colostrum to the smaller of the two foals. The stallion foal is strong enough to drink from its mother. The small mare becomes less lively in the course of a few days and lays down often. Blankets and warm water bottles are to no avail. Finally, she is fed by means of a stomach tube and she is livened up with drips. But all the efforts are in vain. She dies after a week.
Assessing the results
Olinde must have been with young after the first cover: the stallion foal was full-grown, after all. Still, a second follicle also ripened three weeks later, rendering her in heat once again despite the fact that she was already with young. That is exceptional. Perhaps a second ovulation might not have occurred had she not been given a HCG injection. And she would not have been given that injection had it been possible to a gestation diagnosis before the twentieth day after the first servicing. A gestation diagnosis that early on, however, requires a scanner (echography). And it would still be a few years before it became available for use in the horse veterinary practice.
© Leo Rogier Verberne