A number of metazoan parasites (helminths and arthropods) are associated with pathology in the CNS and may be categorized as described below.

Immature (Larval) Stages of Parasites of Carnivorous Animals

These developmental stages may induce behavioral changes in the intermediate host that are likely to enhance transmission to the definitive host by means of predation. For example, Taenia multiceps multiceps is acquired by the canine definitive host when the dog ingests the infective larval stages of the tapeworm Coenurus cerebralis in the brain and spinal cord of the ovine intermediate host. In sheep, C cerebralis causes ataxia, which allows the dog (a carnivore) to more easily prey upon the sheep.

Immature Stages of Parasites Exhibiting a Neurotropic Affinity

These developmental stages require conditions provided by the host's CNS for their growth and development. For example, Hypoderma bovis in cattle must migrate through the spinal cord and adjacent tissues to reach its predilection site, the dorsum of the back.

Erratic or Aberrant Parasites

These parasites are normally found in non-neurologic, predilection sites within the definitive host but, on occasion, may wander erratically into some portion of the CNS. For example, larvae of Cuterebra spp are normally found in subcutaneous sites in the dog or cat but may also aberrantly wander into the CNS and localize in the cerebrum or cerebellum.

Incidental Parasites

These parasites are found in a different host than that in which they normally are found. For example, Parelaphostrongylus tenuis normally is found in neurologic sites within the definitive host, white-tailed deer, in which the parasite is nonpathogenic. However, in an incidental host, such as moose, elk, or llama, the parasite produces an often fatal neurologic disease.

Facultative Parasites

These parasites are normally free-living but, on occasion, can develop into a parasitic existence. For example, Halicephalobus deletrix, a saprophytic soil nematode that is found free-living in nature, has been reported to produce pathology in the CNS of horses.

Successful chemotherapeutic treatment for cerebrospinal nematodiasis has been reported with diethylcarbamazine at 100 mg/kg (45 mg/lb). Ivermectin and organophosphates kill larval bots and at least some nematodes, but killing parasites in the CNS may provoke additional tissue damage.

Before implementing therapy for a pathogenic helminth or arthropod, other possible etiologies for neuropathology should be carefully considered. In particular, rabies should always be included in the differential diagnoses. The animal's age, vaccination status, exposure status, and history are factors that should be considered when rendering a diagnosis.

Last full review/revision July 2011 by Charles M. Hendrix, DVM, PhD