When illness strikes and begins to kill off animals in your collection with no apparent pattern, it is a heartbreaking and deeply frustrating experience. When you feel powerless to predict which animal will be targeted next, you become suspicious of all the dangerous possibilities lurking behind every corner.
Is that seemingly innocuous vine, that languidly winds itself through the fence, poisonous? Could it be a plant that is not present, its very absence illustrating its preference by one or many in the herd, its deadly toxins heightening its mouth watering qualities? Is the paint chipping of that railing lead-based? A nutrient deficiency? Genetic abnormality? Pesticides? Insecticides? Parasites? Malicious intent? These difficult to diagnose illnesses can affect only one animal, one stage of life, disappear for years at a time, ride on the seasons, only strike in certain pens…during the equinox when I use the red shovel with my left hand and forget to turn around three times before entering the enclosure!
The mystery that I wanted to present here was causing years of misery to the keepers involved. I will tell you right away that we may not have solved the mystery. Regardless, our thought processes involved may help to solve someone else’s mystery so here it is:
The Problem: Reindeer experiencing neurological problems that make them appear drunk – symptoms include lack of co-ordination, stumbling and falling over, swaying and in two cases cannot stand up from lying posture and required euthanizing.
Emus suffered from neurological problem. Necropsy found Baylisascaris procyonis parasite in brain tissue.
Reindeer spend majority of time in Pen #1.
Reindeer shifted to Pen #2 because they had eaten everything green in Pen #1 and Pen #2 needed mowing!
Following shift to Pen #2 bull developed neurological issues within a few days.
Bull was moved into isolation pen and given ivermectin (assumed it was B. Procyonis). After a few days bull recovered.
Cow in Pen #2 exhibiting same symptoms. She was moved to isolation pen and also recovered after a few days.
Two more reindeer develop symptoms. They are isolated and ivermectin is administered but they do not recover. Symptoms progress until they can no longer stand and must be euthanized. No results on necropsy.
All reindeer are moved back to Pen #1.
Since the original cases, reindeer experienced problems in all seasons except winter.
Up until this year, all cases occurred after reindeer were shifted to Pen #2.
First case in Pen #1 occurred this year.
The Thought Process:
Baylisascaris procyonis – B. procyonis was suspected due to its confirmed presence on site. B. procyonis is a roundworm which infects a high percentage of raccoons in N. America. Eggs are passed from the raccoon in its feces and infects other species by being involuntarily ingested in fecal contaminated plant matter. Transmission may also occur directly through ingestion of infected tissue. The larvae will then hatch in the intestine of the intermediate host, penetrate the gut wall and migrate through the tissues. Migration to the brain tissue causes the neural symptoms described. Although, this has been documented in a number of zoo species(from birds to possibly lemurs) it does not seem to affect ungulates neurologically. Therefore, it was ruled out in this case.
Toxic Plants – When it was determined that illness was localized to Pen #2, all animals were removed and all vegetation in and around the pen was examined by horticulture staff. Dog strangling vine (Vincetoxicum rossicum or Cunanchum rossicum) and Common buckthorn (Rhamns cathartica) or Glossy buckthorn (Rhamnus frangula) were identified as possible suspects. These plants are also present in other areas of the zoo but no species, other than emu and reindeer, have shown neurological symptoms that could be associated to plant toxicity. Although, the Canadian Poisonous Plant database lists buckthorn as having a potential laxative effect, this did not even refer to the species of buckthorn found on site. All plants identified were ruled out as causes of the neurological symptoms experienced. This does not exclude the possibility of plant toxicity but it was considered less likely.
Diet – There are a number of nutrient deficiencies that can have neurological effects, such as vitamin E, selenium or thiamine deficiency. Contaminated hay was also being considered. However, as all reindeer had been fed the same diet for a number of years regardless of illness or pen, diet was eliminated in the list of possible culprits. Certain medications can also cause interactions that may require a higher level of certain nutrients. The reindeer are medicated monthly with Panacur and every few months with ivermectin but no pattern could be detected between medical treatment and the onset of symptoms.
Pesticides, herbicides, insecticides – No chemicals of any type are used in or around the pens.
Parasite infection – Although, B. procyonis does not produce neurological symptoms in reindeer, a vet friend of mine identified for me another that does affect ungulates in a rather fascinating manner. Parelaphostrongylus tenuis is a parasite that begins its life cycle in white-tailed deer. The female P. tenuis lays its eggs in the deer’s blood vessels and the larvae are transported to the lungs. The white-tailed deer are unaffected by the nematode. Once in the lungs, the deer coughs up the larvae and swallows them which leads them to travel through the digestive system of the deer and end up in the deer’s feces. The journey doesn’t end there! The larvae then infect snails as they slide over the feces. The snails or slugs are then inadvertently ingested by animals grazing on the plants they sit upon. The larvae travels from the stomach, through the abdomen until it reaches the spinal cord. It then shimmies up the spinal cord up to the brain. In white-tailed deer, the cycle continues in the blood vessels of the brain but in other ungulates eggs are not laid and the larvae wreak havoc on the delicate neural tissues. Eureka! It was found that white fallow deer were housed in Pen #2 years earlier and that there is a healthy (in abundance not lack of parasite) white-tailed deer population that comes up to the perimeter fence. Those snails that survive the winter dormancy will pass the parasite along in the following spring hence the seasonal patterns. This diagnosis has not been confirmed but it is being considered and steps are being taken to prevent infection.
Toxic plants – When toxic plants are suspected all suspected species of plant can be manually(not chemically) removed from inside and around the perimeter of exhibits. Animals will go to great lengths to eat plants that appear out of reach so make sure that there is a large enough vegetation-free zone around the exhibit to prevent unwanted grazing. Observing animals and recording the species of plants that are being consumed may help identify problem plants.
Diet – Always use diets that are formulated based on the nutrient requirements for the species of animal to which it is being fed. Although, animals may survive on a poor diet. A properly balanced diet is a good investment towards the health and well-being of your collection.
P. tenuis and B. procyonis – As these zoonotic diseases are found in the native population of snail and raccoon it is impossible to eradicate it completely from the environment. The only means of protection appears to be preventing interaction between the hosts and medicating daily to prevent infection. A stone border around paddocks can reduce the incidence of snails carrying P. tenuis into enclosures to protect those species that can be affected. B. procyonis is even more difficult to stave off. Even birds within enclosed aviaries are not safe as raccoons may defecate on the roof and dried feces will carry the parasite into the exhibit. There isn’t an effective method to treat infected individuals but it can be prevented with daily oral doses of pyrantel pamoate (Nemex®, Strongid®). This can be added to your dry feed formulation for ease of administration in large collections.
This mystery may not have had an Agatha Christie conclusion that tied up all the loose ends but few animal mysteries do. At least, all the suspects have been detained in the study until we can determine if it was indeed Colonel Mustard in the library with a lead pipe. However, Professor Plum is not beyond suspicion. Or was it Miss Scarlett with a candlestick?
Do you have an animal mystery? Tell us about it!
References and Links:
Dr. Chris Dutton and Dr. Deanna (Russell) Daneshment, personal correspondence. Thanks!
Anderson, RC. Neurologic disease in reindeer (Rangifer tarandus tarandus) introduced into Ontario. Canadian Journal of Zoology, 1971, 49(2): 159-166
Gavin PJ, Kazacos KR, Tan TQ, Brinkman WB, Byrd SE, Davis AT, Mets MB, Shulman ST. Neural larva migrans caused by the raccoon roundworm Baylisascaris procyonis. Pediatr Infect Dis J. [2002 Oct;21(10):971-5.
Nichols DK, Montali RJ, Phillips LG, Alvarado TP, Bush M, Collins L. Parelaphostrongylus tenuis in captive reindeer and sable antelope. J Am Vet Med Assoc [1986, 188(6):619-621]
Russell, DJ. Avian neural larva migrans due to Baylisascaris procyonis: natural and experimental infections in psittacine birds. University of Guelph. Dept. of Pathobiology. 2006
Sorvillo F, Ash LA, Berlin OGW, Yatabe J, Degiorgio C, and Morse SA. Baylisascaris procyonis: An Emerging Helminthic Zoonosis. Emerg Infect Dis. 2002 Apr.8(4)