Botulism

Cause

Botulism is a disease caused by the botulinum toxin, which is produced by the bacterium Clostridium botulinum.

Clostridium botulinum spores are common in the soil, and also in the gut of healthy normal cattle and other animals in tropical environments (which includes most of Queensland), where they are not a problem. Spores are the dormant form of the organism. Only the actively growing or 'vegetative' Clostridium botulinum bacteria produce botulinum toxin and it is the toxin that produces the disease. Clostridium botulinum spores will only germinate and grow under conditions where oxygen is totally excluded.

Botulism outbreaks can occur in intensively fed beef and dairy cattle when:

  • the feed is contaminated with botulinum toxin that has been produced by the botulism bacteria growing in rotting animal or vegetable material in the stored feed. Contamination is in the form of the actual toxin
  • actively growing botulism bacteria, eaten in feed contaminated with rotting animal or vegetable matter, continue growth in the rumen and gut of cattle, producing toxin. This source of disease is called Toxicoinfectious botulism. Sufficient toxin is produced for the animal to develop botulism. In this way, a relatively small source of the botulism organism, like a dead snake in contaminated feed, can infect large numbers of cattle fed a mixed ration
  • chicken litter, used as fertiliser on pastures and not incorporated into the soil properly, has been accessed by cattle and consumed.

Distribution in Queensland

Botulism is commonly seen in the phosphorus deficient areas of northern Australia, where it is often associated with cattle eating bones and carrion to satisfy a craving for phosphorus and/or protein.

Reports of botulism have become more frequent in parts of Queensland that are not phosphorus deficient. Most of these outbreaks have been in intensively fed beef and dairy cattle. An increase in intensive feeding practices has occurred in the dairy and beef industries in Queensland, so the increasing risk of botulism outbreak has simply mirrored the increase in these feeding practices.

Affected animals

cattle

Symptoms

Botulinum toxin is often reported as being one of the most potent toxins known to humans, with only a small quantity required to produce disease.

The toxin binds strongly to nerve endings, preventing nerve impulses proceeding to muscles. This typically leads to the type of paralysis where animals go floppy or flaccid because they cannot move their muscles.

Clinical symptoms of botulism vary dramatically depending on the dose of toxin to which cattle are exposed and any pre-existing immunity that may be present. Signs will vary from sudden deaths (animals collapse and die in a couple of hours) to a slowly progressive paralysis where animals may take days to die.

In the latter case, the first signs are cattle off their feed and water. They then develop a wobbly gait (staggers) and eventually go down. During the staggers stage, some cattle become aggressive. Not all cattle that develop botulism symptoms will die. Some mildly affected cattle will recover. Generally speaking, once cattle go down, their likelihood of recovery is poor.

Cattle affected by botulism do not develop a fever. They show no response to treatment for other common causes of 'downer cow syndrome' such as three-day sickness or milk fever. Cattle may progress to the stage where they have difficulty breathing and typically lie on their brisket with their hind legs stretched out behind them. Tongue paralysis may or may not be a feature of the disease (cattle cannot pull their tongue back in when it is pulled out of their mouth).

At post-mortem, there are no obvious signs other than those associated with being down. Evidence of carrion and bones in the gut is uncommon in intensively fed animals, as most do not have access to these sources of botulinum toxin, unlike animals from phosphorus--deficient areas.

Diagnosis of botulism is based on signs suggestive of the disease and the absence of other possible causes. Isolation of Clostidium botulinum organisms from the intestinal contents of an animal fed a prepared ration is highly suggestive but not conclusive evidence of the disease.

Tests for botulinum toxin can be used to identify the toxin in gut contents, blood from affected cattle and samples of suspect feed and water. The success rate of testing for toxin is variable and, in many cases, botulism diagnosis is made on clinical signs and by excluding all other possible causes.

Another test looks for antibodies to botulinum toxin in cattle that survive. This test may support the diagnosis if the vaccination status of the animals is known prior to the outbreak and the animals have not subsequently been vaccinated.

Impacts

The risk of botulinum toxin entering milk supplies from dairy cows suffering from botulism is considered insignificant. Cattle affected by botulism generally cannot be milked. In addition, the toxin is extremely unlikely to pass from blood into the cow's milk.

However, immediately notify milk factories if you suspect potentially affected cattle have contributed to your bulk milk supply.

If surviving beef or dairy cattle are fit, healthy and normal, there is no known risk of botulinum toxin remaining in the carcass and subsequently poisoning people.

Reports of botulism associated with other sources of toxin in prepared feed fed in feedlots or self-feeders in paddocks continue in Queensland, other states and overseas. Some of the worst botulism outbreaks have involved dairy cows being fed total mixed rations based on silage. In some cases, producers have lost two-thirds of their dairy herd over a 2-week period.

How it is spread

The usual source of toxin in intensively fed cattle is feedstuff contaminated with rotting animal or vegetable material.

Previous sources of rotting material in stored feed have included:

  • dead snakes and possums in grain augers
  • snakes and other animals in hay and silage (snakes and other animals are killed by mowers or hide in windrows of hay and are then incorporated deep inside large round bales of hay or piles of silage)
  • mice, when mouse plagues result in large numbers of mice dying in stored feedstuffs and grain augers, especially at the end of a plague
  • water, when animals that die in dams, tanks or troughs are left to decompose.

Once incorporated deep in piles of silage or hay bales, or buried in large amounts of grain (in a silo or grain auger), where oxygen is not present, botulism bacteria can grow and produce toxin. The toxin produced can subsequently leach into the surrounding feed.

Once produced, the toxin is quite stable and may remain in contaminated feed or water for some time. The time it takes to break down will depend on environmental conditions.

Risk period

Improperly made silage that rots instead of fermenting is an ideal breeding ground for botulism if spores are present. Other possible sources are Brewer's grains and other wet by-product feeds, like citrus pulp and cannery waste that have been allowed to rot.

Monitoring and action

If you suspect botulism, contact a veterinary practitioner immediately for help with diagnosis and treatment. The disease can result in rapid, significant losses.

A private veterinary practitioner should examine the affected animals and conduct post-mortem examinations to rule out other possible diseases, such as a mass poisoning. Notify a government veterinarian or stock inspector to rule out exotic diseases if losses are significant.

If you suspect that botulism is a cause of mortalities in intensively fed animals, carefully examine the current ration and water supplies for possible sources of the toxin. If you find an obvious source, remove it and any feed or water it may have contaminated. Wherever possible, obtain any new ration components from a different source and replace the current ration immediately. Avoid any major changes in ration composition to avoid digestive upsets. It is also essential to start a vaccination program immediately.

A vaccination program will be necessary to prevent ongoing losses after an outbreak in pastured cattle. Also pay attention to phosphorous and protein nutrition. This may assist in the longer term by addressing nutritional deficiencies that predispose cattle to the bone chewing that exposes them to the botulinum toxin.

Control

Prevention

In high-risk situations, vaccination against botulism is the only effective way to prevent botulism from occurring.

It is illegal to feed poultry litter or any other animal matter to cattle in Australia. Animals may graze pasture that has been fertilised with chicken faeces or litter if it is ploughed into the soil or given time to be incorporated into the soil before the animals are put to pasture.

Vaccination and antibiotics

All cattle in the herd should be immediately vaccinated with a bivalent botulinum vaccine; traditional formalised 'two shot' vaccine is the cheapest. The choice of which vaccine to use may depend on what is available at the time in sufficient quantities to vaccinate all your cattle.

Where cattle have been vaccinated early in the course of the outbreak, re-vaccination after 2 weeks has generally led to a cessation of new cases by significantly boosting the immune response. This recommendation for 2 weeks between first and second vaccinations differs from label recommendations on most botulinum vaccines. It is done in an attempt to accelerate the immune response to botulism and prevent further cases occurring. Field experience suggests it is an effective way of halting botulism outbreaks. If the revaccination interval is reduced, you should consult with the product manufacturer on whether they recommend a third shot of vaccine for long-term protection. This will depend on the type of vaccine used.

The use of oral antibiotics to prevent toxicoinfectious botulism from occurring has never been scientifically validated. The use of sufficient oral doses of antibiotics to kill Clostridium botulinum organisms may lead to significant digestive upsets in treated cattle.

Quality feedstuffs

Ensuring that feedstuffs are not contaminated with botulinum toxin is part of the assurance systems now implemented in all commercial feedlots and most dairies.

Vermin control during the preparation and storage of animal feedstuffs is very important.

Addressing nutritional deficiencies

In extensively grazed areas, the correction of any dietary deficiencies will help prevent bone chewing by cattle. However, it will not be a sufficient strategy on its own, so it should always be accompanied by an appropriate vaccination program. Addressing nutritional deficiencies will often result in some production improvements as well.

Avoiding problems with chicken litter on pastures

Chicken litter has been demonstrated many times to be a potent source of botulinum toxin. Where chicken litter is used as a fertiliser on pastures, it should be used only in situations where it can be incorporated into the soil immediately after being spread such as when pastures are being renovated. Cattle should not be allowed access to piles of stored chicken litter because they will eat it.

There is a chicken litter feeding ban under Queensland law; producers who knowingly feed their animals chicken litter or fail to take every reasonable measure to deny their animals access to the litter, may be prosecuted.

Vaccination the only long-term prevention strategy

Beef and dairy producers in Queensland who feed their cattle a prepared ration, especially those based on silage or by-products such as brewer's grains, should vaccinate their cattle against botulism.

Large areas of Queensland are phosphorous deficient and botulism is a common disease of grazing livestock in these areas. In addition, some areas are seasonally phosphorus deficient, such as in wet seasons in coastal north Queensland where lush growth results in a relative phosphorus deficiency in pastures. Vaccination is the most reliable means of preventing botulism losses in these areas.

Whenever possible, cattle should be vaccinated well before any suspected period of risk. Vaccinating well before the ration is introduced is recommended.

Various effective botulism vaccines are available. Some newer vaccines require only a single shot (rather than the traditional 2 shots a month apart). Both the 1-shot and 2-shot vaccines produce a similar end result, and the decision on which type of vaccine to use depends largely on product cost and convenience.

All vaccines require boosters to maintain protective levels of immunity. Consult package information or the vaccine manufacturer for advice on the timing of booster vaccinations.