Trenton Schoeb, University of Florida School of Veterinary Medicine
December 1998

During the Spring and Summer of 1998 we have examined four alligators from Lake Griffin that were identified by Game and Fish Commission personnel as behaving abnormally and possibly ill. The first three animals were profoundly depressed and unresponsive. For example, one did not attempt to escape even when the boat bumped into it, and was thought to be dead at first. They were humanely sacrificed and subjected to a standard diagnostic necropsy, but we found nothing to explain their clinical condition. In particular, their livers were normal, indicating that hepatotoxic cyanobacterial toxins probably were not responsible.

We conferred with several individuals around the world with experience with crocodilians, to no avail, but Dr. Elliott Jacobson suggested that we try electromyography to try to determine whether or not the animals had a condition affecting their nervous system. Electromyography is a procedure in which needle electrodes are inserted over a nerve such as the sciatic nerve, and another electrode into the muscle of the foot. (The procedure is conducted under anesthesia, and the electrodes are small needles, like hypodermic needles, so there is little stress associated with the procedure.) The electrodes are connected to a computerized device that sends tiny electrical signals to the nerve, and, from the distance between electrode and the time required for the muscle to respond, calculates the nerve conduction velocity. Similarly, the conduction velocity along the spinal cord can be assessed, and by conducting a repetitive stimulus response test, the function of the neuromuscular junction, the junction between the motor nerves and muscle, can be evaluated. The results thus provide information that helps one localize a neuromuscular problem, which in turn helps narrow the list of possible causes.

Game and Fish Commission personnel captured a fourth abnormal alligator and a clinically normal one from Lake Woodruff National Wildlife Refuge to use as a control. The animals were housed in a modified dog run at the Animal Resources 34th Street facility. The animal from Lake Griffin was more responsive than the previous three, but nonetheless had observable abnormalities in locomotion, notably dragging the dorsal surfaces of its feet. This did not abate over several days of observation, so we don’t think it was due to muscular injury associated with capture and restraint. Anesthesia was administered by Dr. Terry Heaton-Jones, and electromyography and electroencephalography were conducted by Dr. Roger Clemmons. The abnormal animal had a reduced nerve conduction velocity compared to the normal one. In Dr. Clemmons’s laboratory, values below 50 m/sec are considered abnormal. The value for the control alligator was well above that value; whereas that for the abnormal one was well below, at 41 m/sec. (Both were examined after several days of acclimation and at similar temperatures.) Both of these animals were humanely sacrificed and subjected to detailed neurologic necropsy, which includes removal of the spinal cord and collection of numerous nerve and muscle samples for special histologic stains and for electron microscopic examination. Processing of these samples will be completed when we are able to obtain funds to do so. We also have tissue samples from the above animals for toxicologic analysis when funding becomes available and when we have obtained additional information to help determine which analyses that are needed.

These very limited preliminary data suggest that future efforts should be devoted to (1) determining whether or not there actually is a neurologic disease problem in Lake Griffin alligators; (2) if so, identifying the underlying changes in structure and function in the nervous system; and (3) collecting samples for later analyses for specific causes such as toxicants affecting the nervous system. We plan to examine up to six clinically healthy alligators from the Lake Woodruff to collect baseline electromyography and other electrophysiologic data. These data are needed because there is very little of this type of information available for crocodilians, almost none for alligators, and, in particular, none for our laboratory, other than that from the above two animals. Without these results, it will be difficult or impossible to evaluate the results from the Lake Griffin animals, because we would not know enough about the range and variation of results to expect from normal animals. We will examine additional abnormal alligators from Lake Griffin. These animals will be subjected to clinical examination and detailed neurologic necropsy, including light and electron microscopic examination of nervous and muscle tissue. Samples of nervous tissue, stomach contents, and lake water also will be analyzed for neurotoxic cyanobacterial toxins. Other analyses may be done, based on the clinical and necropsy findings.

We also hope to be able to examine some dead alligators during the winter. The cooler water temperatures may help slow decomposition enough that it may be possible to locate some animals that are not too decomposed for examination to be useful.