Why are moose susceptible to brain worm and deer are not?
The internal parasitic worm, Parelaphostrongylus tenius, is believed to have evolved with white-tailed deer over hundreds, if not thousands of years. The brain worm has evolved to live within a deer and the intermediate hosts of snails and slugs, without being a significant detriment to these animals. Moose thrive in the northern boreal forests where deer have not been able to survive because of deep snows in winter. Therefore, moose have had little exposure to the brain worm. Moose have not had time (in an evolutionary sense) to build its immune defenses to the brain worm, nor has the parasite had adequate time and exposure to moose to adapt to the slightly different metabolism of a moose.
