Perhaps while they are young. The blood-brain barrier is more hindrance than helpful for neurological disorders caused by thathological organisms, as it also applies to the immune system. (White cells don't enter nerual tissue. The brain has to combat pathogens in different ways.)
A 1000 year old elf, suffering the ravages of multiple infections would be quite scenile, on a good day. Stark raving mad as a hatter with shakes on a bad one.
CJD is essentially the same disease as mad cow, except caused either by inheritance or by ingesting human (or human like) flesh of people afflicted with it. It causes spongiform encephalitus, which is incurable, and progressive. Elven cannibalism would make them highly vulnerable to this disease.
Trichinosis is spread by blood sucking parasites, like ticks and mosquitos, as well as eating poorly prepared animal flesh that is contaminated with it. It causes a persistent, endemic infection of muscle and neural tissues with small cysts containing the parasite, which manifests as shakes, pains and aches, loss of strength and stamina, and loss of coordination in the host. (Just the sorts of things predators look for in prey animals.) As such, it tends to bounce between herbivorous animals via insect bites, and high level predators via ingestion. Wild pork, bear, wolf, and weasles are primary vectors for human contagion.
Equine encephalitis is a virus that is spread though insect bites and through contaminated water supplies. It causes menningitis like swelling of the brain and brainstem, causing coma, convulsions, hallucenations, and a whole host of terrible sympthoms.
Rocky mountain spotted fever is a bacterium spread predominantly by tick bites, which causes a characteristic rash, high fever, unendurable pain, hallucenations, and batshit insanity, followed by death.
Elves may have strong constitutions, but repeated exposure to these pathogenic agents would be very debilitating over time, even for a race with perfect natural memory.