In normal animals, electrical signals travel from the brain down the spinal cord and then follow the path of individual peripheral nerves. The nerves branch and supply the muscles of the body. When the electrical signal reaches the muscle, it results in muscle contraction and movement. Therefore, widespread diseases of the peripheral nervous system cause muscular weakness and in severe cases, paralysis.

Dogs with polyradiculoneuritis often begin showing signs of weakness one to two weeks following raccoon exposure, although the disease has been reported in animals with no history of exposure. Something in the raccoon's saliva is thought to initiate the damaging immune response. Weakness most often starts in the hind legs and the dog's bark may also be noticeably weaker than normal. Signs generally worsen over a period of about 10 days, although however complete paralysis can occur within 24 hours in rapid cases. In these cases, there is also a larger risk that respiratory muscles will become paralyzed and the dog will not be able to breathe.

Other diseases that cause similar clinical signs include:

Botulism. Botulism is caused by the toxin produced by a type of bacteria called Clostridia. Dogs are most likely to be exposed to the toxin following ingestion of dead animals. The bacteria also proliferate in spoiled foods. The overall symptoms of botulism are quite similar to polyradiculoneuritis but there are subtle neurologic differences that can be picked up by the veterinarian. Special tests can be run to identify the toxin in animals with suspected toxicity.

Tick paralysis. This is another disorder caused by a toxin. The toxin comes from certain species of ticks and produces paralysis. Typically, physical exam reveals an engorged tick and removal of the tick causes resolution of the paralysis.

Protozoal polyradiculoneuritis. Infections with protozoal organisms (Toxoplasma and Neospora) can cause neurologic dysfunction as well. This type of polyradiculoneuritis is most often seen in young dogs, and may involve other clinical signs in addition to the neurologic disease.

Myasthenia gravis. Myasthenia is another immune mediated disease that prevents normal transmission between nerves and their respective muscles. It most often causes episodes of weakness with periods of recovery in between. It rarely causes weakness as profound as what is seen with polyradiculoneuritis.