Nervous System The nervous system is composed of

Nervous System

§ The nervous system is composed of billions of neurons with long, interconnecting processes that form complex integrated electrochemical circuits. It is through these neuronal circuits that animals experience sensations and respond appropriately.

Basic Sensory and Motor Functions: § The peripheral nervous system (PNS) is formed by neurons of the cranial and spinal nerves. The central nervous system (CNS) is formed by neurons of the spinal cord, brain stem, cerebellum, and cerebrum.

§ Evidence of disease in other body systems may be associated with inflammatory, metabolic, toxic, or metastatic neoplastic disorders of the nervous system. External signs of traumatic or toxic exposure may support these mechanisms of disease.

The neurologic examination consists of evaluation of the following: § 1) the head. § 2) the gait. § 3) the neck and thoracic limbs. § 4) the trunk, pelvic limbs, anus, and tail. Initially, an attempt should be made to relate all deficits to a focal anatomic lesion.

Mechanisms of Disease: § Disease processes affecting the nervous system may be congenital or familial, infectious or inflammatory, toxic, metabolic, nutritional, traumatic, vascular, degenerative, neoplastic, or idiopathic. § Infections of the nervous system are due to specific viruses, fungi, protozoa, bacteria, rickettsia, and algae

Rabies

§ Rabies is an acute viral encephalomyelitis that principally § § § affects carnivores, although it can affect any mammal including food animals. Rabies is most often transmitted by the bite of infected wild animals including skunks, raccoons and foxes. Bats are the important species in which carriers are known to occur Rabies is endemic in Texas. A raccoon with rabies. Courtesy of Dr. Thomas Lane

Etiology and Epidemiology: § Rabies is caused by lyssaviruses in the Rhabdovirus family. Lyssaviruses are usually confined to 1 major reservoir species in a given geographic area, although spillover to other species is common.

Transmission and Pathogenesis § Transmission is almost always by introduction of virus-laden saliva into the tissues, usually by the bite of a rabid animal. Although much less likely, it is possible for virus from saliva, salivary glands, or brain to cause infection by entering the body through other fresh wounds or through intact mucous membranes

Clinical signs § The most reliable signs, regardless of species, are acute behavioral changes and unexplained progressive paralysis. Behavioral changes may include sudden anorexia, signs of apprehension or nervousness, irritability, and hyperexcitability (including priapism). The animal may seek solitude. Ataxia, altered phonation, and changes in temperament are apparent

Clinical signs cont. § Due to the vagueness of signs and lack of consideration of rabies as a potential diagnosis, cattle handlers, veterinarians and other people in contact are often exposed to the virus. During the active course of the disease, the virus is excreted at high levels in saliva. § Cattle with furious rabies can be dangerous, attacking and pursuing humans and other animals. Lactation ceases abruptly in dairy cattle. The usual placid expression is replaced by one of alertness. The eyes and ears follow sounds and movement.

Diagnostic § Rabies testing should be done by a qualified laboratory, designated by the local or state health department. § Fluorescent antibody testing of the brain, the presence of Neigri bodies on histopathology.

Control § Vaccines approved for use in cattle, sheep and horses are available and should be seriously considered for high-risk exposures or high-value animals. Due to costs of vaccine and relative low incidence rates, vaccination is not routinely used in most cattle herds.

Polioencephalomalacia (PEM) § is an important neurologic disease of ruminants that is seen worldwide. Cattle, sheep, goats, deer, and camelids are affected. § Historically, PEM has been associated with altered thiamine status, but more recently an association with high sulfur intake has been observed.

Etiology, Pathogenesis, and Epidemiology § The disease is seen sporadically or as a herd outbreak. In general, younger animals are more frequently affected than adults. Animals on high concentrate diets are at higher risk, but pastured animals also develop PEM. . § Thiamine inadequacy can be caused by such factors as acute dietary deficiency of thiamine or ingestion of plant thiaminases or thiamine analogs.

Clinical Findings: § PEM may be acute or subacute. Animals with the acute form manifest blindness, recumbency, tonic-clonic seizures, and coma. Those with a longer duration of acute signs have poorer responses to therapy and higher mortality. Animals with the subacute form initially separate from the group, stop eating, and display twitches of the ears and face.

Lesions § Acutely affected animals may have brain swelling with gyral flattening and coning of the cerebellum due to herniation into the foramen magnum. Slight yellowish discoloration of the affected cortical tissue may be present.

Lesions cont. § Polioencephalomalacia. Transverse section of the dorsal parietal cortex of a feedlot steer with PEM viewed under ambient illumination. Affected segments of the cortical gray matter have a yellowish discoloration and irregular contours. Courtesy of Daniel H. Gould

Treatment § The treatment of choice regardless of cause is thiamine. Therapy must be started early in the disease course for benefits to be achieved. If brain lesions are particularly severe or treatment is delayed, full clinical recovery may not be possible. The dosage of thiamine is 10 -20, mg/kg, IM or SC, tid. Initial treatment may be administered IV.

Listeriosis § Listeriosis, a disease of the central nervous system, is caused by the bacterium Listeria moncytogenes. § This bacterium can live almost anywhere-in soil, manure piles, and grass. § Listeriosis is common in cattle, sheep and goats and can occur in pigs, dogs, and cats, some wild animals, and humans.

Etiology § § § Listeria moncytogenes. Gram-positive Ovoid to rod-shaped Facultative anaerobe Acid but not gas producing from glucose Can multiply at temperatures between ~0 and 45°C

Reservoirs L. monocytogenes. § – decaying vegetation § – soils § – animal and human feces § – sewage § – silage § – water It has been isolated from a wide range of retail foods.

Pathogenesis: § Listeria organisms that are ingested or inhaled tend to cause septicemia, abortion, and latent infection. Those that gain entry to tissues have a predilection to localize in the intestinal wall, medulla oblongata, and placenta or to cause encephalitis via minute wounds in bucal mucosa.

Clinical Findings: § Encephalitis is the most readily recognized form of listeriosis in ruminants. It affects all ages and both sexes, sometimes as an epidemic in feedlot cattle or sheep. The course in sheep and goats is rapid, and death may occur 2448 hr after onset of signs; however, the recovery rate can be up to 30% with prompt, aggressive therapy

Lesions: § In listeric encephalitis, there are few gross lesions except for some congestion of meninges. Microscopic lesions are confined primarily to the pons, medulla oblongata, and anterior spinal cord. Lesions of listerial encephalitis (microabscessation) in the midbrain of a cow. Courtesy of the Department of Pathobiology, University of Guelph

Diagnostic § Listeriosis is confirmed only by isolation and identification of L monocytogenes. Specimens of choice are brain from animals with CNS involvement and aborted placenta and fetus

Treatment and Control: § L monocytogenes is susceptible to penicillin (the drug of choice), ceftiofur, erythromycin, and trimethoprim/sulfonamide. High doses are required because of the difficulty in achieving minimum bactericidal concentrations in the brain. § Penicillin G should be given at 44, 000 U/kg body wt, IM, daily for 1 -2 wk; the first injection should be accompanied by the same dose given IV

Zoonotic Risk § Whether animals serve as a reservoir of infection for humans may be questioned, because Listeria organisms have been isolated from feces of a significant number of apparently healthy people as well as other animals. § L monocytogenes can be isolated from milk of mastitic, aborting, and apparently healthy cows. § Listeria also have been isolated from milk of sheep, goats, and women.