Chemical Restraint and Surgical Removal of a Perineal Mass from a Nile Hippopotamus (Hippopotamus amphibius)
An approximately 36-year-old, overweight female Nile hippopotamus (Hippopotamus amphibius) was evaluated on several occasions for an enlarging perineal swelling on the right side of the base of the tail. Historically, the animal had been bitten in this area by a pen mate 5 years earlier. The swelling was monitored and became progressively larger over the 5 years prior to diagnostic evaluation. Several evaluations were made over a period of 1 year in an effort to identify the nature of the mass located in this area.
With the help of behavioral conditioning,1 ultrasound evaluations and biopsies of the mass were collected prior to surgery. The histopathologic diagnosis was reactive tissue with possible foreign body etiology. There was concern that this mass (approximately 25–30 cm diameter) would eventually impinge on the rectum enough to affect the animal’s ability to defecate normally. The animal began to exhibit intermittent reluctance to come out of the water or eat. Chemical restraint and mass removal were planned due to the increasing size of the mass and recent signs of illness.
Injections were given in a specially designed restraint chute that the animal had been conditioned to over the past 6–7 months. Detomidine hydrochloride (Pfizer Animal Health, Pfizer, Inc., West Chester, PA 15380 USA) 100 mg was administered by hand intramuscularly (IM) into the right lateral cervical region. After tranquilization was achieved, 300 mg butorphanol tartrate (Fort Dodge Animal Health, Fort Dodge, IA 50501 USA) was given IM by hand caudal to the first injection site. Chemical dosing was based on an estimated body weight of 2270 kg. Tranquilization adequate to work with the animal was reached at 22 min after the first injection. Marked respiratory depression and suspected hypoxia occurred shortly after induction but were alleviated once the animal was intubated. Endotracheal intubation was accomplished using a 36.0 mm endotracheal tube and digital manipulation of the endotracheal tube through the glottis. Oxygenation was maintained with 100% oxygen delivered directly into the endotracheal tube with a high flow demand valve. Monitoring of vital parameters was very limited due to the difficulty in acquiring electrocardiographic or pulse oximetric readings. Doxapram hydrochloride (Fort Dodge Animal Health, Fort Dodge, IA 50501 USA) 200 mg intravenously (IV) was given at 68 minutes postinduction due to a sluggish respiratory pattern.
The mass was surgically removed and weighed 15 pounds. It was described histologically as dense mature granulation tissue attributed to foreign body reaction although no foreign body was identified. No neoplastic cells were located on several sections of this mass. It is suspected the reaction was initiated by material becoming imbedded into the tissue subsequent to a bite wound this animal received 5 years previously.
The large wound remaining was packed with gel foam and sterile gauze to reduce hemorrhage. The wound was closed with 2 vicryl in a simple interrupted pattern leaving 2 inches open at the bottom of the suture line for drainage and removal of gauze later.
The entire anesthetic period lasted approximately 2 hours. Reversal of the anesthesia was achieved with yohimbine hydrochloride (Wildlife Laboratories Inc., 1401 Duff Drive, Suite 600, Fort Collins, CO 80524 USA) 50 mg given intravenously (IV) and 30 mg given IM 125 min after induction and an additional 50 mg IM at 142 min after induction. Naltrexone hydrochloride (Wildlife Laboratories Inc., 1401 Duff Drive, Suite 600, Fort Collins, CO 80524 USA) given 300 mg IV was administered at 126 min post induction and 250 mg was given IM at 146 minutes postinduction. Intravenous administration of reversal agents was achieved opportunistically by use of a distended superficial dermal vessel overlying the medial aspect of the mass near the base of the tail.
This hippopotamus was held out of water immediately post-operatively to avoid the risk of drowning. Despite her ability to walk to a grassy, outdoor holding pen upon recovery, and 24-hour monitoring with encouragement to stand, drink and eat during the night, the animal expired the next day due to pulmonary edema and atelectasis resulting in hypoxia. Actual body weight prior to necropsy was 2246 kg. Obesity, advanced age, an overdistended stomach (noted on postmortem exam), prolonged surgical and post-operative recumbency all likely contributed to the pulmonary compromise and, ultimately, to the death of this animal.
The large size and weight of the Nile hippopotamus as well as their physical shape make this species prone to respiratory compromise when recumbent. Recumbency can lead to ventilation:perfusion mismatching, possibly resulting in respiratory distress, the most common complication of chemical restraint in this species.3 Recommendations for preanesthetic preparation of hippopotamus include a fasting period of 24–48 hours and restricting water intake for 12–24 hours.2 Additional risk factors involved in this particular case included obesity and the inability to withhold water from her prior to the anesthetic procedure. Based on the recent episodes of inappetence and lethargy, it was suspected that this animal may have had underlying health problems unrelated to this mass. No specific additional pathologic processes were noted on necropsy. However, acute cardiac failure cannot be ruled out. It is possible that a small lesion could have been missed during tissue sampling of such a large heart. Small lesions can lead to fatal arrhythmias, acute cardiac failure, and pulmonary edema. The effects of such lesions can be exacerbated by hypoxemia.
Several hippopotamus have been anesthetized for limited time periods using etorphine and reversed with diprenorphine or naltrexone.2,3 In this citation, etorphine had variable results. Supplemental oxygen seemed to be key in the most successful anesthetic procedures. The animal in this report was provided supplemental oxygen by nasal insufflation during tracheal intubation and then directly by the endotracheal tube with delivery accomplished using an oxygen demand valve. Even though sternal recumbency occurred, the head remained propped on the front gate of the chute helping to keep the neck extended and the airway open. A large animal respirator would have provided more effective lung expansion and oxygenation of this animal but was not available for this procedure.
By using the detomidine and butorphanol combination, our hope was to avoid the adverse effects of narcotics and the associated complications of deep anesthesia as well as to shorten the recovery period. We cannot exclude the possibility that our doses of detomidine and butorphanol may have been too high for this individual as the hippo did appear to be in a deep plane of anesthesia with pronounced respiratory depression during the procedure.
It is suspected that the prolonged recumbency necessary for the surgical procedure complicated the recovery of this animal significantly. Recumbency time seems to be a critical factor in the recovery of hippopotamus from anesthetic procedures. Recommendations for future procedures in these animals would include a maximum recumbency time of 1 hour whenever possible, withholding from water 24 hours prior to the procedure and oxygen delivery by positive pressure ventilation with a large animal ventilator. In animals of advanced age such as this one, consideration of a lower induction dose of detomidine and butorphanol may be prudent.
We would like to thank all of the staff of Busch Gardens who devoted so much of their time and effort to working with us and with this animal for this process. Special thanks to Dr. Keith Beheler-Amass, Dr. Avery Bennett and Dr. Geoffrey Pye for their assistance with the biopsy collections performed prior to this surgery.
1. Dumonceaux GA, Burton MS, Ball RL, Demuth A. Veterinary procedures facilitated by behavioral conditioning and desensitization in reticulated giraffe (Giraffa camelopardalis) and Nile hippopotamus (Hippopotamus amphibius). In: Proceedings from the American Association of Zoo Veterinarians and American Association of Wildlife Veterinarians Joint Conference. 1998:388–390.
2. Loomis MR, Ramsay ED. Anesthesia for captive Nile hippopotamus. In: Fowler ME Miller RE, eds. Zoo and Wild Animal Medicine, Current Therapy 4. Philadelphia, PA: W.B. Saunders Co.; 1999;638–639.
3. Ramsay EC, Loomis MR, Mehren KG, Boardman WSJ, Jensen J, Geiser D. Chemical restraint of the Nile hippopotamus (Hippopotamus amphibius) in captivity. J Zoo Wildl Med. 1998;29(1):45–49.