List,
A challenge study is when two groups of animals are exposed to natural
disease and then the incidence of disease in the two groups are compared.
The two groups usually consist of unvaccinated control animals and vaccinated
animals. They could also consist of animals vaccinated one year vs. animals
vaccinated five years.

Remember that in order to do the testing to prove that your horse is safe, it
is possible that other horses had to die.

I recently talked with a veterinarian trying to get a rabies vaccine approved
for goats. They did an approved challenge study that was a complete bust,
because none of the unvaccinated animals came down with rabies, and so there
is no way to compare whether the vaccine protected the other group, or if the
live virus was damaged somehow. We do know for a fact that goats get rabies,
so it wasn't that.

Belinda S. Thompson DVM
Pine City Veterinary Clinic
Pine City, NY
belindavet@...