|
|
Part 1 covered general information about the parasite, its life cycle, and that very few deer in this area do not eventually become infected with it. This column will deal with how many first stage larvae are on deer pellets, infection of gastropods, and their ingestion by deer (or other host).
Estimating the number of larvae on deer pellets was quite time consuming. A sample of 12 to 15 pellets from each deer is suspended (by a fine mesh) in a small container of water for a day. The larvae drop off and fall to the bottom of the container. Most of the water is removed and the remainder (which contains the larvae) is put into a shallow glass container with a grid etched on the bottom. Then a dissecting microscope is used to count the number of larvae between the grid lines (to keep from not counting some or counting others twice). The total number of larvae in each sample is divided by the weight (in grams) of the pellet sample and the result is expressed as larvae per gram of pellet.
One might expect the greatest number of larvae to be passed by adult deer. However in our study of 1,480 deer, the opposite occurred. Fawns passed the greatest number of larvae per gram of pellet which decreased with increasing age of deer. The number of larvae per gram of pellet also varied through the year; the greatest numbers were passed in March through May and the least in summer and fall. The average number of larvae per gram of pellet for fawns was 121 during March, 85 for yearlings in April-May, and 50 for adults in March.
Another step in the study was to determine what percentage of gastropods became infected with the parasite during summer and fall. This was done by placing wetted cardboard sheets and the ground and collecting the gastropods on the bottom of them once each week. Two sites were involved; one was in a deer wintering area near the shore of Lake Superior and the other was on deer summer range a few miles farther inland.
Each gastropod was identified to species and then placed in a shallow glass Petri dish and digested in a solution of pepsin and hydrochloric acid and the contents were examined for parasite larvae under a stereoscopic microscope. Gastropods become infected with the larvae of a number of parasite species so each had to be identified as to which parasite species it was. This is quite time consuming as the larvae of many species are quite similar.
As a side note, the larvae of a snowshoe hare parasite was the most frequently encountered. Apparently hare also ingest many gastropods while feeding.
Of the over 12,000 gastropods collected, only 10 contained brain worm larvae (average of 3.2 larvae per infected gastropod). That means, on average, a deer (or other host) ingests about 1,200 gastropods before becoming infected with the brain worm in this area. Since most fawns become infected during their first year of life, they obviously ingest many gastropods while feeding. Infected gastropods were twice as numerous (per area sampled) in the wintering area than on the inland summer range site.
Therefore, deer living inland should be less likely to become infected with brain worm than those living year around along the shore. Most deer that summer inland migrate to the shore for the winter. Early migrants may face increased chance of becoming infected if they arrive while deer are still able to feed on ground level vegetation and ingest gastropods. Inland deer that linger on winter range after snow has melted also may ingest infected gastropods while feeding on newly exposed vegetation and litter. Although migrant deer deposit additional pellet groups (and therefore additional first stage larvae) in wintering areas; larvae deposited in snow suffer high mortality.
Moose living inland from Lake Superior also should be less likely to become infected with brain worm than those living near the shore. Twelve (43%) of 28 confirmed cases of brain worm infected moose I handled were within 1 km (0.6 mile) of Lake Superior.
Deer do not keep accumulating more brain worms each time they ingest an infected gastropod. Our studies found that old deer had the same average number of adult worms as did fawns. Sometime after a deer becomes infected, the immune system produces antibodies which kill newly ingested larvae. The adult worms are not killed, only the new larvae. Young animals have poorly developed (weak) immune systems which are less able to fight off infection by diseases and parasites than are the immune systems of older animals in good condition.
Dr. Lankester demonstrated this also happens in moose. Two moose calves were hand-raised for five months and then each was given three brain worm larvae. Within 6 weeks they exhibited front leg lameness and hind quarter weakness. By 3 months the signs had almost disappeared and at necropsy only one worm was found in each of them.
Four other moose calves were not infected until 9.5 months of age. One was given 10 larvae and another was given 15 larvae. Both then exhibited the same slight signs as above. At 95 days they were necropsied and 1 and 3 worms, respectively, were recovered. The other 2 calves were each given 5 larvae, neither showed lasting signs of infection, and no worms were recovered. Brain worm infection is not always fatal for moose, age of the animal and number of larvae ingested may be important factors.
Part 3 will be on additional brain worm questions that need answers, problems for other species, and other parasites that cause similar signs.
Leave a Reply