39 free range wild horses (males and females) were brought from Danube Delta and kept altogether in a 3500 m2 square shaped paddock. Because of different reasons including infectious diseases issues and high aggression of the stallions in a compact group, we have decided that the sterilization of the adult males will be the best option to take.
Physiological and behavioral considerations:
Despite their normal peer preference, when they tend to form groups of 4-5 individuals, once a disturbance in their daily habit takes place, the horses stick together, are extremely cautious of the surroundings and gallop as a heard.
In comparison with the domesticated horses, in these horses we noticed a tendency for masking the underlying diseases and a higher capacity to cope with pain.
Field chemical immobilization/anesthesia consideration:
Excluding any type of direct drug administration due to their avoidance of human natural contact, we had to consider other remote drug delivery systems. Taking into account the fact that the CO2 dart gun or pistol presents a higher trauma index, and that these horses are now feeling more or less comfortable with people in a 5-10 m range distance, we have decided that the blowpipe drug delivery ) would be the best option for this particular situation. An overall assumption on the stallions weight would be around 300kg (+/-50kg) per animal.
As a technical fact we had to consider that the best ballistics for a 11mm blowpipe is the 3ml thin dart syringe. We managed to build those dart syringes from 3ml lure lock normal syringes.
The second important technical fact is that we had to choose the combination of drugs that would offer the highest concentration per ml and would anesthetize a horse in the shortest period of time possible with minimum side effects thus diminishing the stress.
The third fact is the availability of secure drugs with high concentration on Romania’s market. The literature (Walzer, Zoo Animal & Wildlife Immobilization and Anesthesia, 2007) suggests for the chemical immobilization of non-domesticated equids (Przewalski’s horse, wild ass, zebra) ethorphine in different combinations of tiletamine/zolazepam, acepromazine, detomidine or buthorphanol. Unfortunately ethorphine (Immobilon) is not available in Romania, consequently we had to address the situation resuming to the drugs marketed, in this case: tiletamine/zolazepam (Zoletil 50), detomidine (Domosedan), buthorphanol (Butomidor) ketamine (Vetased), acepromazine (Calmivet) and xylasine (Narcoxyl 2).
The highest concentration per ml is Domosedan, followed by Butomidor and then Zoletil, they all have smooth and fast induction effect, a safe toxicity level, they do not show any incompatibility between themselves and they all work on different receptors. Because of these reasons we have decided to use them according to the following protocol:
Zoletil is presented as a whitish powder and its solvent. We have removed the solvent and added one whole vile of 5ml of Domosedan and another 2 ml of Butomidor. The resulted mixture would be 7ml of 125mg tiletamine/125mg zolazepam (TZ), 50 mg detomidine and 20 mg buthorphanol. The drugs’ prospects recommend as followed: 3-8mg/kg TZ, 40-80 detomidine ug/kg and 10 mg butorphanol.
Theoretically the whole mixture should easily cover the sedation of two adult 300 kg horses, but only if given intravenous which in our particular case is impossible. The pharmacokinetics of the same drug given intravenous (i.v.) or intramuscular (i.m.) is different. Intra-muscular injections take longer for the effect to install (around 15 min ) in comparison with the i.v. ones (max 30 sec), and very important, it requires higher amount of drugs for i.m. injections to have similar effect with the i.v. administration.
Immobilization
The dart syringes (3ml maximum capacity) were filled each one with the above-mentioned mixture and delivered with the blowpipe whenever the animal was relaxed and close enough for a good aim in the hind legs.
In one case the delivery of only one dart was enough for the animal to lay down in recumbency in less than 10 minutes. For others (2 horses) it took up to four (4) fully discharged darts for the effect to take place and the horse to lay down on the ground (in those particular cases the full effect took place in 30-40 min). The average for all the horses would be 2-3 fully discharged 3ml dart syringes and a time of 15-20 minutes for the animal to be chemically immobilized on the ground and ready for the sterilization procedure.
Factors that can influence the immobilization procedure
Using a remote drug delivery system such as the blowpipe and the tranquilization darts raises some technical difficulties that can affect the overall process.
Aiming correctly at an animal is always a tricky procedure. Ideally the horse has to be close enough (around 5 m in range), not moving and relaxed. Many times the animals move or run away from humans right when the blowpipe is being raised. If the dart is blown while the animal is moving, the dart can bounce or deflect. There is also the possibility to miss the target.
Another problem can arise with the dart itself. During the handling it can loose pressure, the pistons can get stuck, it can get blocked in the blowpipe, etc. All these factors are hard to limit and have to be considered a souse of overall loss (drugs, darts).
To estimate this loss we have concluded that for one horse we use maximum 5 darts, with 25% chances for the dart not to hit the target or not to fully discharge.
Weather, tiredness, exterior pressure, low quality equipment can have a direct effect on this rate of loss.
Sterilization
Once the animal lays down due to the tranquilization, someone grabs the head of the animal and puts one’s weight on the neck and over the maxilla. A blanket will cover the head to protect and minimize any other stimuli.
If the animal had not been sedated enough, an intravenous injection of 5-10 ml of ketamine 10% has been administrated.
A rope tied to the pastern of the above hind leg was pulled anteriorly to give space for the procedure. A local disinfection with alcohol and betadine was performed and an intra testicular injection with 10 ml procaine 2% (local anesthetic) was injected in each testicle.
Two incisions were made over both testicles, parallel with the median line of the scrotum. Once the testis were separated from their covers, Raimer’s forceps were placed on each spermatic cord. The forceps were kept on place for 30 minutes to ensure a proper vasotripsy, then the testicles were removed. During this time the animal received antibiotic (1ml/25kg -penicillin/streptomicine –PenStrep), an analgesic/anti-inflammatory agent (20 ml metamizole-Novasul) and an anti-tetanus serum (1500 UAI). A local fast scarring agent (Germostop/Terramycin) and an insect repellent were pulverized on and around the incisions.
After the operation the horse was kept another 15-20 minutes with his head on the ground and then the blanket was removed. In most of the cases the horses got up immediately after the blanket was taken off or waited another couple of minutes.
The whole intervention, from the time the horse laid down until he raised on his feet took around 1 hour.
Post op considerations
In all male horse sterilization there is a normal swelling of the incision area that follows the intervention and lasts a few days. Because of the impossibility to asses the operation area without another tranquilization the horses were kept under close observation. We made the horses canter regularly in the paddock for the inflammation to decrease.
After 5-7 days the swelling disappeared and the horses appear to be in a good clinical shape.
Conclusions:
Field castration of wild horses implies knowledge on the behavior of animals and special logistics and know-how of the chemical immobilization methods. This knowledge reduced as much as possible the stress produced by the intervention. Because it cannot be compared with a normal domesticated horse anesthesia, where the drugs are injected directly intravenously, wild horse anesthesia is trickier, takes longer, is more expensive and presents some risks. After the operation the healing process takes up to one week without any other intervention.
by Ovidiu Rosu
