Polioencephalomalacia

What it is

Polioencephalomalacia (PEM), also called cerebrocortical necrosis, is a lesion with several possible causes: laminar necrosis of the grey matter of the cerebral cortex, with cerebral oedema. In cattle it is associated mainly with two mechanisms:

• Excess sulphur: rumen bacteria reduce sulphate from the diet and water to hydrogen sulphide, which accumulates in the rumen gas. It is eructated and inhaled, and absorbed; it interferes with the energy metabolism of neurons. Sources are high-sulphate water, by-products such as distillers' grains, some plants (crucifers) and sulphate-containing supplements.
• Disturbed thiamine metabolism: from thiaminases in plants such as bracken fern or horsetail, from thiaminases produced by rumen bacteria on high-concentrate diets, or from analogues such as amprolium at high doses. The adult ruminant does not need dietary thiamine because it is synthesised by rumen microorganisms.

In practice, sulphur-associated outbreaks are now the most recognised presentation, and in many of them blood thiamine levels are normal.

Who it affects

Young growing cattle, especially feedlot cattle, on diets rich in concentrate or in high-sulphur by-products, or drinking high-sulphate water. The risk is highest in the first weeks after a change to a high-sulphur diet and in hot weather, when animals drink more. Sheep and goats are also affected.

Anatomy involved

• Brain: necrosis affects the cerebral cortex, especially of the occipital and parietal lobes, which explains the cortical blindness with preserved pupillary reflexes. The gyri appear swollen, flattened and yellowish, and oedema can push the cerebellum into the foramen magnum. Necrotic areas fluoresce under ultraviolet light.
• Rumen: this is where hydrogen sulphide is produced from ingested sulphate and where bacteria synthesise thiamine or, on some diets, produce thiaminases. Sulphide accumulates in the dorsal gas cap of the rumen, and a low ruminal pH, as on high-concentrate diets, favours its release as gas.

Clinical signs

At first the animal separates from the group, stops eating and appears disoriented. Then come cortical blindness (no menace response but normal pupillary reflexes), head pressing, aimless walking, muscle tremors, strabismus, nystagmus, teeth grinding and the head drawn upwards (opisthotonus). In advanced stages the animal becomes recumbent with seizures and paddling, and may die.

Diagnosis

• Clinical: signs of cerebral cortical disease in an animal from a risk group. A rapid response to thiamine in early stages supports the diagnosis.
• Sulphur investigation: analysis of total sulphur in the ration and sulphate in the water; measurement of hydrogen sulphide in rumen gas or thiosulphate in the urine of unaffected penmates.
• Necropsy: swollen, yellowish cortex, fluorescence under ultraviolet light and histopathology with laminar necrosis.
• Differential diagnosis: lead poisoning, salt poisoning or water deprivation, listeriosis, thrombotic meningoencephalitis due to Histophilus somni, vitamin A deficiency and rabies.

Treatment and prevention

Treatment is based on parenteral thiamine, started as early as possible and repeated over several days, even when the cause is sulphur. Anti-inflammatory and other drugs to reduce cerebral oedema and supportive care are also used. The sulphur source must be removed and the group's ration reviewed. Animals treated early can recover, sometimes with persistent blindness or other deficits; those recumbent with seizures have a poor prognosis. Drug use in food animals is subject to national regulations and withdrawal periods.

Prevention consists of controlling total sulphur intake, adding up ration and water, testing drinking water, adapting gradually to diets rich in concentrate or by-products and ensuring enough fibre.

Key studies

The evidence on this disease is limited: there are no randomised clinical trials of treatment and no systematic reviews, and studies on sulphur are small experiments or outbreak studies.

• Apley (2015), review of the evidence on treatment. No randomised, masked, prospective clinical trials of any therapeutic intervention exist for naturally occurring bovine PEM; the use of thiamine and anti-inflammatories rests on pathophysiology, induced models, data from sheep and parallels with human diseases. It is a narrative review, cited because it is the reference on the subject in the absence of anything better.
• Cammack et al. (2010), 77-day randomised trial with 96 forage-fed steers: low-sulphur water, high-sulphur water or high-sulphur water plus dietary clinoptilolite. Steers on high-sulphate water had more PEM, with 12 cases confirmed by cortical lesions, and clinoptilolite did not protect them. It shows that water alone can cause the disease.
• McAllister et al. (1997), epidemiological study in a feedlot with 14 steers with acute PEM, 26 healthy steers and 6 years of records. Blood thiamine in affected steers was within reference ranges; ruminal sulphide was higher at 3 weeks after arrival, when incidence peaked, and most cases occurred between 15 and 30 days after starting a high-sulphur diet. Annual summer outbreaks began when a well with high-sulphate water came into use. It does not support the idea that PEM is always a thiamine deficiency. It is a single feedlot.
• Clarke et al. (2019), retrospective review of 159 necropsies of cattle with central nervous system disease at a diagnostic laboratory in Georgia (United States) between 2001 and 2017. PEM was the most common non-infectious diagnosis (17% of cases). It reflects cases submitted to a laboratory in a single region.