Tackling a terrible toxin
Fun fact — botulinum toxin is one of the most potent toxins in the world.
A cow only needs to ingest 10 micrograms of toxin to result in death from botulism, and those 10 micrograms could be within one to three grams of consumed affected matter.
Considering your average cow consumes 11-15kg of dry matter a day, this is an astonishingly small amount.
Botulinum toxin can also cause death in humans, birds and wildlife in the same manner.
The word ‘botulism’ comes from the Latin ‘botulus’ meaning sausage, because food poisoning in humans in the late 1800s was attributed to tainted sausage.
Botulinum toxin is produced by the bacteria Clostridium botulinum, part of the same bacteria family that causes tetanus, enterotoxaemia and pulpy kidney.
The bacteria are readily found in soil, mostly as spores, and can last a long time there.
The problem arises when the right conditions of low air and moisture occur, allowing the bacteria to multiply and produce toxin.
A rotting carcase or sometimes even rotting plant material meets the requirements well and can be the source of toxins that cause a disease event.
In Australian cattle production systems, there are two main situations that can expose cattle to botulism.
In the northern extensive grazing systems there can be a higher risk of phosphorous deficiency, causing cattle to develop the behaviour of pica, which makes them chew at bones and carcases.
This increases the risk of exposure to botulinum toxin produced by the C. botulinum bacteria found in these carcases.
In dairy systems, the risk comes from ensiling carcases of animals (for example, mice, rats, cats, wallaby, fox) and then exposing cattle to the toxin when feeding out that silage later.
Once ingested the toxin is absorbed through the intestinal wall of the cow into the circulation and out to the peripheral nerves. Once there it binds to receptors between the nerve and muscle cells, where it disrupts messages from the nerves to the muscles, meaning the muscles of the limbs and head can no longer be controlled.
The toxin does not disrupt the central nervous system. So, while there is paralysis of the legs, head and particularly the tongue, the awareness of the animal remains intact.
In humans, administration of antitoxin can bind any toxin still circulating in the blood but will have little effect on toxin that is already bound at the nerve endings. Much of the treatment for botulism is related to providing supportive care.
In cattle, antitoxin is not available and supportive care becomes difficult to provide. However, vaccination against C. botulinum is possible, with long- and short-term vaccines available for use either in the face of increased risk or in an outbreak situation.
Botulism can occur in sporadic cases as well as in outbreak scenarios.
A colleague of mine recently dealt with an outbreak of botulism in Tasmania where 129 adult milking cows died from a herd of 750.
On this farm, animals were fed pasture, potato waste and pit silage where a mixer wagon was used to feed out on a feedpad. Initially 15 affected cows were found recumbent without many other signs, but then developed to having difficulty breathing with their tongues hanging out and death.
Suspected causes included botulism but also acute bovine liver disease, nitrate poisoning, metabolic disease and acute toxicity.
Laboratory testing ruled out other causes, and on discovery of fragments of wallaby carcase on the feedpad, botulism was highly suspected.
At this stage testing for the botulinum toxin was sent for the wallaby fragments and postmortem samples from the cows, however, the result at that stage was negative.
A vaccination program was started within three days of the initial signs of the outbreak on this farm.
Eventually an additional carcase sample sent two months post-outbreak proved a positive result for botulinum toxin, finally confirming what was suspected.
The swift action in identification of signs, implementation of a vaccine program and attempts to reduce contamination of the ration bought about control of the botulism outbreak on this farm.
So, if you are considering signs of botulism in your cattle, act swiftly and call in your veterinarian to start an investigation and put in place an action plan.
As there is no cure for botulism, vaccination of at-risk animals is highly recommended.
Dr Rosie Waite works for Scottsdale Veterinary Service in north-east Tasmania and enjoys the opportunity to talk cows and drive around the rolling hills. Rosie has spent time working along the northern coast of Tasmania at Smithton and Ulverstone, and has completed her Master’s in Dairy Science where her focus was the use of young dairy bulls.