Meningitis, Encephalitis and Myelitis

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Overview

Meningitis is an inflammation of the meninges. These form a tough, protective layer of fibrous tissue that covers the brain and spinal cord and sits just beneath the bones of the skull and vertebrae. Meningitis can occur on its own, in which case it results in pain without neurological deficits. Encephalitis is an inflammation of the brain and as such it is usually associated with neurological deficits. As this inflammation usually affects most of the brain, the resultant neurological deficits often reflect involvement of multiple areas of the brain. These deficits can be of varying severity and severe encephalitis can easily be fatal. Myelitis is an inflammation of the spinal cord. It can occur primarily on its own but it is usually combined with both meningitis and encephalitis. Causes of these three related disorders can be either infectious and non-infectious. Infectious causes can be due to viral, bacterial or fungal disorders. Viral causes include rabies and distemper in dogs and rabies and feline infectious peritonitis (FIP) in cats. Bacterial causes include some of the tick-borne organisms like Lyme disease as well as Q fever. Protozoa like toxoplasmosis and a very similar disease of dogs called neosporosis occur occasionally. The most common fungal disorder in British Columbia is cryptococcosis, which has recently become a particular problem in parts of Vancouver Island. Non-infectious causes include the breed-specific disorders of pug dogs, Maltese terriers and Yorkshire terriers along with the disorder known as GME or granumolatous meningoencephalitis.

Figure 14: CT scans from a dog that presented almost comatose and demented due to an acute encephalitis. A: This scan was made before administration of intravenous contrast and the brain appears to be normal. B: The dog was given an intravenous contrast agent and the scan was repeated. Several areas appear much lighter than in the first scan (arrows). This is because the contrast agent has leaked out of blood vessels and into inflamed areas of the brain causing them to enhance with contrast. The dog did not respond to treatment and a post mortem confirmed that it was suffering from granulomatous meningoencephalitis (GME).

The prognosis for animals with these inflammatory disorders of the nervous system varies greatly depending on the specific cause. Many animals will either make a spontaneous recovery or will respond well to antibiotics and often return to normal. Other animals, such as those suffering from distemper, FIP, cryptococcosis, GME or the breed-specific disorders have a somewhat more guarded prognosis. The main complication is progression of neurological disease despite treatment.

Further Resources

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