The most important predators are golden eagles, grizzly bears, wolverines and other scavengers. The golden eagles are the most important predator of the calves when they are born at the calving grounds. Adult caribou fall prey to wolves. Wolves harvest approximately 3-5 percent of the herd each year. Grizzly bears are found on both the summer and winter ranges of the Porcupine Caribou Herd and eat adult caribou as well as many kinds of plants, berries and other animals. Wolverines cannot kill healthy adult caribou, but they do prey on newborn calves, cows giving birth and other weak adults.
Mosquitoes are sometimes referred to as "micro-predators," because of the damage they cause to the caribou. A Russian study revealed that mosquitoes can take as much as 125 cc of blood daily from adult animals. Malnutrition results when feeding patterns are affected. Mosquitoes distract the animals from feeding and prevent the cows from nursing their young. Animals sometimes rush about in frustration and injure themselves or other caribou.
Over the years, the PCMB has seen many health "scares" concerning the Porcupine Caribou.
Following the explosion of the Chernobyl nuclear reactor in 1986, levels of radioactive cesium rose in the herd for some time. The PCMB responded by conducting an extensive public education campaign and arranged for the residents of the caribou communities to be tested for radiocesium by Health and Welfare Canada. The tests revealed extremely low levels of contamination and concluded that there was no health risk to people, even on a steady diet of caribou.
In the fall of 1992, the Canadian Wildlife Service released a report which stated that there could be health risks from cadmium contamination for people eating Porcupine Caribou under certain conditions. After a thorough examination of data and a public education program, the PCMB concluded that there was no realistic health risk to anyone eating caribou kidneys or livers. Several Old Crow residents were tested for cadmium toxicity, and there was none found. Tests from caribou samples for 1991-97 show that cadmium levels are not increasing in the Porcupine Caribou Herd.
The only other contaminant in the Porcupine Caribou that has been investigated is mercury. It was reported at the November 1995 meeting that although mercury levels in caribou for 1991-94 were slightly higher, the mercury was not in the toxic methyl mercury form and so is not considered a health hazard.
The health advisory issued by the Yukon Health and Social Services in 1997 recommends limiting consumption of Porcupine Caribou kidneys to 25 servings per person for the year. The consumption limit for Porcupine Caribou livers should be 12 servings per person for the year.
Caribou samples sent in by hunters and any samples taken during body condition monitoring are submitted for testing every year.
Parasites and diseases are a natural part of any wildlife population. However, if there is a change in the frequency of occurrence or if the environment changes, the effects of these parasites and diseases may become more important to the caribou.
The larvae of tapeworms (Echinococcus granulosis) live in the lungs of caribou in a fibrous cyst. When animals eat infected lungs of caribou, they can grow in the animal's gut. Those animals in turn can pass the worm's eggs in their feces. These tapeworms can pose a serious risk to human health. People become infected by ingesting food or water contaminated by infected wolf or dog feces. The eggs hatch and larvae form a cyst in humans that then have to be surgically removed.
Another type of flatworm larvae found in caribou is Taenia hydatigena, which produces cysts on the livers of caribou. These cysts probably don't affect the health of the caribou, and there is no known danger to people from this species of tapeworm.
A third kind of flatworm larvae found in caribou is the Taenia krabbei, which migrates to the heart and muscles to form yellowish-white cysts.
Thread lungworms (Dictyocaulus) are roundworms found in the lungs where their eggs are laid and hatch. The tiny immature worms migrate up the windpipe and are eventually swallowed and deposited on the vegetation in the feces. They grow and, if eaten by a caribou, travel through the bloodstream to reach the lungs where the cycle starts again. The impact of thread lungworms can cause pneumonia.
A protozoan called besnoitia can cause cause "cornmeal disease," so named because it causes roughening and pitting in the bones and tendons. Most infestations have been light, well tolerated and of no consequence to the animal.
Brucellosis is a bacterial disease that is transmissible to humans by caribou. In female caribou, the infection can cause abortion, retained afterbirth (which may cause infection) and giving birth to weak calves with a poor prognosis for survival. In humans, the disease is called undulant fever. Humans can be infected by milk, aborted fetuses or the body organs and marrow from infected caribou. The flu-like symptoms can be severe, and hospitalization might be required.
Warble flies (Oedemagnena tarandi) and nasal bot flies can cause the caribou to run about wildly and lose the body fat the need to gain during the summer months. Warble flies are known to affect the nutritional balance, cause allergic responses and suppress immune responses in caribou. Animals weakened by warble infection are more susceptible to other diseases, predation and environmental stress. Nasal bot flies can cause nasal discharge and coughing and may block breathing to some extent, but their effect on the overall body condition of the caribou is unclear.
- Further information on wildlife diseases and parasites, and what hunters should do if they shoot a sick animal, can be found on the NWT Wildlife & Fisheries website.
Caribou that are chased by snowmobiles and aircraft can suffer from stress syndrome. Sudden and violent exertion causes changes in the muscle as chemicals from the functioning of the muscles build up faster than the blood can remove them. Such changes in the muscle can cause the caribou to die hours, days or even weeks after the chase. As well, a panicked caribou can injure itself, or suffer frostbite in the lungs from panting in extreme cold.