A number of metazoan parasites (helminths and arthropods) can cause central nervous system diseases in animals and may be categorized as described as follows. Zoonotic conditions are also noted.
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 (tapeworm) is acquired by the canine definitive host when an affected dog ingests the infective larval stages of the worms. Coenurus cerebralis affects the brain and spinal cord of the ovine intermediate host, causing ataxia. Dogs (carnivores) can then more easily prey upon 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 preferred site, the dorsum of the back.
Erratic or Aberrant Parasites
These parasites typically have a predilection for non-neurologic sites in the definitive host; however, occasionally they 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 aberrantly be found in the CNS and localize in the cerebrum or cerebellum.
Incidental Parasites
These parasites are found in an atypical host. For example, Parelaphostrongylus tenuis is usually found in neurologic sites in the definitive host, white-tailed deer, and 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.
Facultative Parasites
These organisms are normally free-living within the animal's environment but, on occasion, can develop into parasites. For example, Halicephalobus deletrix, a saprophytic soil nematode that is found free-living in the environment, has been reported to produce disease when found in the CNS of horses.
Treatment and Control
Ivermectin and organophosphates kill larval bots and at least some nematodes; however, killing parasites in situ within the CNS may provoke additional tissue damage.
Before implementing treatment for a pathogenic helminth or arthropod, other possible causes of 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.
