Bill May:

Several weeks ago, whilst on call at the weekend I saw two milking animals that were down and unable to stand. Both were relatively fresh calved and one of the two was a first calved heifer so milk fever, whilst possible, was an unlikely differential. Both animals had also been treated with calcium and anti-inflammatory/painkillers with no improvement and appeared to be getting weaker with time. The noticeable finding on a clinical examination was the muscle weakness and in particular the lack of muscle tone in the tongue, which made me remember previous cases of botulism seen many years ago.

In ruminants, botulism is effectively a poisoning with botulinum toxin, which is produced by the bacterium Clostridium botulinum and commonly found in decaying organic matter such as bird or animal carcases. Contamination of broiler litter with the carcases of chickens that have died from various causes during production, can render the litter dangerous for ruminants and poisoning incidents are often linked to broiler litter in some way. Even small fragments of carcases transferred onto pasture by scavenger animals, such as foxes, dogs or crows, may pose a risk to grazing ruminants.

Cattle and sheep of all ages are highly susceptible to botulism, which is characterised by progressive muscle weakness (paralysis). Affected animals may be weak, stagger about or go down and can look remarkably similar to milk fever cases in the early stages, especially if the affected cases are freshly calved. Cattle characteristically display flaccid paralysis and occasionally protrusion of the tongue out of the mouth. If grasped the tongue feels limp and is easy to hold, which is a very unusual sign in cattle where it is more usually like trying to grasp hold of an eel! In most cases the disease is fatal (both of these animals died) although some animals may recover. In many cases of botulism euthanasia is justified on welfare grounds as affected animals cannot stand and find feeding increasingly difficult. Cattle are extremely sensitive to the effects of the toxin meaning that ingestion of very small amounts can result in clinical disease or death. The progression and severity of the disease depends on the amount of ingested toxin. When a large amount of toxin has been ingested, the animal may be found dead without having shown any signs of disease. Conversely, if only a small amount of toxin is ingested the progression of the disease may take a more chronic course and clinical signs may be less severe.

Careful disposal of all animal or bird carcases and poultry litter is essential to minimise the risk of botulism to livestock. Poultry carcases should be promptly removed and disposed of by incineration or rendering as required by the Animal By-Products (Enforcement) Regulations (NI) 2015. Following removal of the broiler crop, all poultry house doors should be kept closed until the litter is removed. The litter should not be removed from the house until it can be loaded directly onto spreading equipment, covered vehicles or immediately stacked and covered. At no time should it be accessible to dogs, foxes, crows or other scavengers that may carry carcases onto adjacent pasture or into livestock housing. Washings from poultry houses and yards should be collected in tanks rather than be allowed to flow onto adjacent land.
Poultry litter should not be spread on agricultural land that is to be grazed, or from which silage or hay is to be harvested, in the same year. This is because fragments of carcases, containing botulinum toxins, may persist on pasture for a considerable time. If litter must be spread, it should be deep-ploughed into arable ground. If this is not an option and litter must be disposed of by spreading on pasture, ruminants should not have access to the treated fields for at least several months. However, there is no guarantee that the treated fields would then be safe for cattle and it is important to remember that fragments of carcases on pasture may be transported by scavenger animals and birds to neighbouring fields. Spreading litter on a windy day may also pose a risk of contaminating adjacent fields.

Any animal or bird carcases, or portions of carcasses, visible on pasture or in livestock houses, should be promptly removed. Even small fragments of such material may be dangerous to livestock and should be disposed of by incineration or rendering, as required by current legislation.

Botulism in dairy cows

A progressive muscle weakness and paralysis is common in cases of Botulism – cases can appear very similar in their presentation to milk fever cases in the early stages

Botulism in dairy cows

A flaccid or ‘limp’ tongue, which remains outside of the mouth when grasped, is a characteristic finding in cases of Botulism

One of the interesting things about botulism is the potency of the toxin produced. The potency of a poison or toxin is measured by the amount of that substance that is required to kill 50% of people that ingest it (LD50) and any LD50 less than 500mg/Kg is considered ‘highly toxic’. The LD50 for various commonly ingested toxins is shown to the right:

  • Alcohol: 7 grams per kilogram body weight (a pint of beer contains approximately 16 grams of alcohol)
  • Caffeine: 140 milligrams per kilogram body weight (a typical cup of coffee contains 50-100mg of caffeine)
  • Arsenic: 13 milligrams per kilogram body weight
  • Botulinum toxin: 2 nanograms per kilogram body weight (a nanogram is one-millionth of a milligram, or one billionth of a gram)
  • Polonium 210 (the radio-isotope used to kill Alexander Litvinenko): <1 nanogram per kilogram bodyweight

Given this toxicity, it is even more interesting that Botulinum toxins are used medically (in very low doses) in BOTOX to paralyse facial muscles and reduce wrinkles!