The big sleep: How do you anesthetize a hippopotamus?

A new procedure is now described by the group of Chris Walzer at the Research Institute of Wildlife Ecology of the University of Veterinary Medicine, Vienna and published in the current issue of the Journal of the American Veterinary Medical Association.

All zoo animals -- and sometimes also wild animals -- occasionally need veterinary treatment and anesthesia is clearly required in many cases. For most animals the procedures are well established but for a variety of reasons it has proven difficulty to anaesthetize hippopotamuses. The thick skin and the dense subcutaneous tissue make it difficult to introduce sufficient amounts of anesthetics and opioid-based anesthetics often cause breathing irregularities and occasionally even death. In addition, the level of anesthesia is only rarely sufficient to enable surgery to be undertaken: few vets wish to be around when a drugged hippopotamus starts to wake up.

Together with Thierry Petit from the Zoo de la Palmyre, France, and collaborators in Germany and Israel, Gabrielle Stalder and Chris Walzer from the Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna (Vetmeduni Vienna) have now developed a new anesthetic protocol based on the use of two non-opiate drugs, medetomidine and ketamine. The procedure has been tested on a total of ten captive hippopotamuses, all of which were successfully anesthetized to an extent that enabled surgery -- although due to the difficulty to estimate their exact weight some animals needed additional doses of anesthetic before they could be safely handled. Crucially, all animals recovered rapidly and completely from the procedure and showed no lasting after-effects.

Diving during anesthesia? This does not mean that the anesthesia always passed without incident: five of the ten animals stopped breathing for periods of up to nearly ten minutes. But in each case the hippopotamus spontaneously recommenced breathing without the need for any intervention. The VetMed scientists interpret the temporary suspension of breathing as a dive response: their aquatic lifestyle means that hippopotamuses are able to hold their breath for relatively long periods, so it is likely that the animals also "dived" during the period of unconsciousness.

The researchers thus had a unique opportunity to learn what happens when hippopotamuses stop breathing. The level of oxygen in the blood naturally decreases but this is not associated with an increase in heart rate nor, surprisingly, with increased levels of lactate. As Walzer says, "all diving mammals have evolved a strategy to cope with the shortage of oxygen while they are underwater. The reaction of hippopotamuses to anesthesia suggests that they do not switch to anaerobic metabolism when they dive but possibly have other mechanisms to help them use the oxygen in their blood more efficiently. The hooded seal is known to have very high levels of myoglobin in its muscles: maybe the hippopotamus has a similar trick to help it survive?"