Several  Protozoa may cause nervous symptoms e.g., Babesia canis,   Encephalitozoon cuniculi ,   Toxoplasma gondii ,   Neospora caninum and   Trypanosoma spp.

Infections with some strains of B. canis often terminate with signs of cerebral damage such as paddling of limbs, ataxia, mania and coma. This is the result of brain damage caused by obstruction of the brain capillaries by parasitized red blood cells. There is usually no evidence of neuronal degeneration but there is dilatation of the perivascular spaces and interstitial  oedema.

 Encephalitozoonosis (  Nosematosis) is caused by the obligate intracellular microsporidian Encephalitozoon cuniculi. The disease has been described in rodents, lagomorphs, primates and several species of carnivores. Asymptomatic infection usually occurs in rodents and lagomorphs. In carnivores the neurological signs include repeated turning and circling movements, especially after disturbance, dysmetria, dysergia,  blindness, and a terminal semi-comatose state. Lesions described are  encephalitis and segmental  vasculitis. The course of the illness is usually 5–12 days.

The neuropathology associated with canine and feline toxoplasmosis has been described in detail. In these species, toxoplasmosis is characterized by focal  necrosis and vascular damage in acute infections, and by glial  nodules, repair, and scar formation in  chronic infection. Cerebral calcifications, common to chronic toxoplasmosis in children, appear to be rare in animals. In dogs extensive areas of necrosis, gliosis and demyelination are found. Clinical nervous signs include depression, trembling, opisthotonus, head tilt, incoordination, blindness and paraplegia. In puppies, it may resemble distemper, clinical toxoplasmosis occurring sometimes together with this disease. Skeletal muscle atrophy due to damage of lower motor neurons has been associated with a case of clinical canine toxoplasmosis.

Incoordination and spinal paralysis have been reported in dogs infected with T. brucei brucei.

 Neosporosis( N. caninum) is mainly reported in young dogs. Puppies show a hind limb paresis that develops into a progressive paralysis. Neurologic signs are dependent on the site that is parasitized. The hind limbs are more severely affected than the front limbs, and often in rigid hyperextension. The cause of this hyperextension is not known, but is most likely due to a combination of upper motor neuron paralysis and myositis which results in rapidly progressive fibrous contracture of the muscles that may cause fixation of joints.

Cerebral coenurosis, due to Coenurus serialis and   Cysticercus cellulosae has been described in cats and dogs showing neurological disorder.

Various nematode species may invade the central nervous system.   Angiostrongylus cantonensis is a metastrongylid lungworm of the rat. In unnatural hosts, such as dogs the parasite develops in the spinal cord and to a lesser extent the brain, and usually dies without reaching the lungs. Infection leads to an eosinophilic meningo-encephalitis and a periradiculoneuritis. Clinical signs are slight paresis of the hind legs, uncertain straddle gait and  hypersensitivity of the skin.

Cerebrospinal nematodosis caused by   Ancylostoma caninum has been reported in a dog. A 12-week-old cocker spaniel had signs of imbalance, torticollis and pain on flexion of its  neck that eventually progressed to tetraparesis and death. A young adult female A. caninum was found in the haemorrhagic cervical spinal cord.

Adult heartworms   Dirofilaria immitis usually inhabit the right side of the heart or pulmonary arteries of carnivores. Occasionally adult worms have been observed in the brain where they invade the lateral ventricle, or in the meningeal arteries with subsequent occlusion. The clinical course is characterized by intermittent convulsion, blindness, ataxia, behavioral changes and circling.  Microfilaria of D. immitis have also been reported within the meningeal arteries, deep arteries and capillaries of the brain and extravascularly within the brain.

Larvae of   Toxocara canis have been recovered from the brains of experimentally infected dogs, but with little clinical illness. A severe granulomatous inflammation of the hypothalamus and adjacent neurohypophysis caused by T. canis larvae have been reported in a dog suffering from diabetes insipidus.

The larvae of Cuterebra species (  Diptera) normally mature in subcutaneous tissue of Rodentia and Lagomorpha, but occasional infection in dogs and cats may occur. In these abnormal hosts the larvae have been observed in the brain, causing neurologic clinical signs.