Introduction/Causative Agent:
Polioencephalomalacia (PEM) is a central nervous system (primarily brain) disorder caused by a lack of thiamine (vitamin B1) in body tissues. This deficiency is most commonly associated with a rapid dietary change to high levels of concentrates. A high concentrate/fiber ratio alters the bacteria in the rumen, producing enzymes that break down (metabolize) thiamine before it can be utilized by essential body organs. Other causes of polioencephalomalacia may include dietary deficiencies in young calves, poisoning by eating plants that contain thiamine-metabolizing enzymes (bracken fern, rock fern, nardoo, and Kochia scoparia), ingestion of moldy feed, and the intake of high amounts of sulfur-containing products such as molasses.Clinical Signs: Thiamine is essential in producing glucose for energy. When a lack of thiamine exists, the biochemical processes necessary to produce energy for brain cells cannot be completed. Hence, the nerve cells within the brain begin to swell and deteriorate, resulting in neurological defects such as poor weight gain, isolation from the herd, going off feed (anorexia), depression, and staggering. If it remains untreated, the disease may progress to include abnormal eye placement with the eyes positioned upward. This condition has been termed "star gazing." Progression of the disease may lead to head-pressing, abnormal posture with the head and neck extended over the back toward the tail, excitement, facial muscle twitching, repetitive chewing, teeth grinding, and blindness.
When the cause of PEM is related to a rapid change in dietary concentrates, decreased rumen contractions with a distended, fluid-filled rumen may also be present. This may also be accompanied with a watery, foul-smelling diarrhea. If the cause is related to the intake of high amounts of sulfur-containing feeds, a rotten egg, sulfur-like smell may be detected on the breath. In the latter stages of the disease, the animal may be unable to stand and exhibit seizures. Death usually occurs within 3-4 days after clinical signs begin if no treatment is undertaken.
Disease Transmission: Polioencephalomalacia is not contagious between animals. However, when the causative agent is feed-related, more than one animal within the herd may develop the disease.
Diagnosis: The diagnosis of this disease may be obtained by measuring rumen or fecal thiaminase (thiamine-metabolizing enzymes). However, the more practical diagnosis is obtained from past history of feeding and clinical signs, in addition to the response to thiamine therapy. If the animal(s) is dead, specialized microscopic evaluation of the brain by a pathologist can provide a precise diagnosis of thiamine deficiency.
Treatment: Polioencephalomalacia is a medical emergency and should be treated immediately following the observation of clinical signs. Affected animals should be isolated and treated with thiamine hydrochloride at the dose of 45-70 milligrams (mg) per 10 pounds of body weight (or 450-700 mg per 100 pounds of body weight). The first dose should be administered intravenously (IV). It is recommended that this initial IV dose be diluted with 5% dextrose or 0.9% saline solution and administered slowly. Following this initial IV dose, the same amount of thiamine hydrochloride should be given intramuscularly (IM), or subcutaneously (SQ) every few hours for the first day of treatment. The same treatment dosage should be continued, IM or SQ, 2-3 times daily for at least 3 days. This treatment is effective in restoring only blood and tissue thiamine concentrations. Therefore, it is also recommended that oral administration of B-complex vitamins, supplemented with thiamine, be administered to replenish thiamine levels in the rumen. If the affected animal is a calf, a single dose of 5 grams (g) of thiamine propyldisulfide should be given orally. In severe cases, medication to control seizures and reduce pressure within the brain may also be required. Phenobarbital, pentobarbital, and valium (diazepam) are medications used to control seizures. They should be administered under the direction of a veterinarian. To reduce pressure within the brain, furosemide or mannitol may be beneficial. The recommended dose of furosemide is 4.5 milligrams (mg) per 10 pounds of body weight (45 mg per 100 pounds) given IV, IM, or SQ twice daily. Mannitol may be given at 1-9 grams (g) per 10 pounds of body weight (10-90 g per 100 pounds). Mannitol must be administered IV as a 20% solution. Due to the intense monitoring required when administering furosemide or mannitol, direct administration and monitoring by a veterinarian is highly recommended.
Recovery: The rate of recovery is directly related to the promptness of diagnosis and treatment. If clinical signs are not severe, return to normal function should be restored within 2-3 weeks. However, if clinical signs have advanced to blindness and seizures, long-term supportive care including supplementing the diet with thiamine and feeds high in fiber, while decreasing the amount of dietary concentrates may be required. Recovery to normal vision and function may take up to 8 weeks.
Prevention: To aid in preventing polioencephalomalacia, gradual introduction of feeding high concentrate rations (at least 2 weeks) is recommended. Supplementing high concentrate diets with Brewer’s yeast or cobalt may also be beneficial. Where possible, frequent pasture rotation to avoid the need for concentrate supplementation can control outbreaks of the disease.