Inflammation of the meninges, the membranous covering of the brain and spinal cord (meningitis), and inflammation of the brain (encephalitis) often are seen simultaneously (meningoencephalitis), although either can develop separately. Causes of meningitis, encephalitis, and meningoencephalitis include infection by bacteria, viruses, fungi, protozoa, or parasites. Viruses (for example rabies virus, herpesvirus, and more) and protozoa (for example, Sarcocystis neurona) are more frequent causes of meningitis and encephalitis than are bacteria.
Meningitis and encephalitis are less common than infections of other organs, because the nervous system has protective barriers. However, infections may occur when these protective barriers are injured or weakened. Infections can also spread to the central nervous system from the sinuses, guttural pouches, the inner ear, vertebrae, or spinal disks; these infections may result from bite wounds or other traumatic injuries near the head or spine. Brain abscesses also can arise from direct infections or from blood poisoning.
Bacterial meningoencephalitis often affects very young farm animals because of blood poisoning caused by Escherichia coli or a streptococcal infection. Actinobacillus equuli is an important cause of meningoencephalitis in foals. Meningoencephalitis caused by Mannheimia haemolytica has been reported in horses, donkeys, and mules. Actinomyces, Klebsiella, and Streptococcus species are sporadic causes of meningitis in horses.
The usual signs of meningitis are fever, neck pain and rigidity, and painful muscle spasms. Horses may have these signs without any sign of brain or spinal cord dysfunction. However, in meningoencephalitis, signs such as depression, blindness, partial paralysis of the face or the limbs, loss of balance or motor control, seizures, behavior changes, agitation, head tilt and circling behavior, difficulty eating, and loss of consciousness can develop, depending on the severity and location of the inflammation. The analysis of cerebrospinal fluid from a spinal tap is the most reliable and accurate means of identifying meningitis or encephalitis.
Cases resulting from rickettsial and bacterial infections can be treated with appropriate antibiotics, while fungal infections can be treated with specific antifungal drugs. The outlook for recovery depends on the cause, the severity of the infection, and whether or not the infection has resulted in irreversible damage to the nervous tissue. Supportive care may include pain relievers, anticonvulsant drugs, fluids, nutritional supplements, and physical therapy.