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 infected 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 can 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 dogs or cats but may also aberrantly wander into the CNS and localize on the cerebrum or cerebellum.
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 migrates through portions of the CNS and produces an often fatal neurologic disease.
These parasites are normally free-living within the animal's environment 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 situ within the CNS may provoke additional tissue damage.
Before implementing therapy for a pathogenic helminth or arthropod, other possible causes of neuropathology should be carefully considered. In particular, rabies (see Rabies 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.