Botulism: a potential disease problem in intensively fed beef and dairy cattle
Lee Taylor, Senior Veterinary Officer, Biosecurity, Biloela
On this page:
Key points
Background
Symptoms
Diagnosis
Sources of the botulinum toxin
Toxin in the feed or water
Toxicoinfectious botulism
Dealing with outbreaks of botulism
Risk to humans
Vaccinations
Prevention
Further information
Key points
1. Botulism outbreaks in intensively fed beef and dairy cattle can occur 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.
2. Specific advice on how to deal with outbreaks of botulism is provided.
3. Advice on vaccination programs is provided. In high risk situations, vaccination against botulism is the only effective way to prevent botulism from occurring.
Background
Botulism is a disease caused by the botulinum toxin, produced by the bacterium Clostridium botulinum. 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. In these areas, where the risk of losses due to botulism is very high, vaccination against types C & D botulism has been widely adopted and is now standard industry practice.
However, 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. There has been an increase in intensive feeding practices in the dairy and beef industries in Queensland and the increasing risk of outbreaks of botulism has simply mirrored the increase in these feeding practices.
Some of the most publicised reports of significant mortality due to botulism in feedlots in Australia and overseas involved feeding cattle with poultry litter as a source of non-protein nitrogen. Dead birds in the litter were the source of toxin. This practice has now been outlawed in Queensland.
However, reports of botulism associated with other sources of toxin in prepared feed fed in feedlots or in self feeders in paddocks continue in Queensland, other states of Australia and overseas. Some of the most spectacular outbreaks of botulism 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 two-week period.
Recently, outbreaks of botulism have been reported in dairy herds where poultry litter has been used to fertilise pastures. Cattle have eaten litter piled ready to be spread on pastures or litter that has been spread on pasture but not incorporated into the soil. Legislation now prohibits feeding animal matter including chicken faeces and chicken litter to livestock and livestock must be denied access to this material. Stock are allowed to graze pasture fertilised with chicken faeces or litter provided it is ploughed into the soil or otherwise given time to be incorporated into the soil before the stock are put on the pasture.
Botulism symptoms
Botulinum toxin is often reported as being one of the most potent toxins known to mankind, as only a small quantity is required to produce disease.
The toxin binds strongly to nerve endings, preventing nerve impulses proceeding to muscles. This leads to the type of paralysis typically seen with botulism 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. Then they develop a wobbly gait (staggers) and eventually go down. During the staggers stage, some cattle become aggressive because they feel helpless. 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" like 3-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
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. There are tests for botulinum toxin that 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.
Sources of botulinum toxin: where does botulism come from?
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.
Toxin in the feed or water
The usual sources of toxin in intensively fed cattle are feedstuffs contaminated with rotting animal or vegetable material.
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 in grain augers, especially at the end of a plague.
- Water may also be a source of toxin if animals that die in dams, tanks or troughs are left to decompose.
Once incorporated deep in piles of silage, hay bales, or buried in large amounts of grain (in a silo or grain auger), where oxygen is not present, the botulism bacteria can grow and produce toxin. The toxin produced can subsequently leach into the surrounding feed. Improperly made silage that rots instead of ferments is an ideal breeding ground for botulism if spores are present. Brewer's grains and other wet by-product feeds like citrus pulp and cannery waste that have been allowed to rot are also possible sources.
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.
It has been argued that large scale feeding practices would mean any small source of botulinum toxin would be greatly diluted in the preparation of tonnes of feed. However, the converse could be that mixing and distributing a prepared ration could potentially expose large numbers of animals to botulism.
Toxicoinfectious botulism
This form of botulism occurs when cattle eat actively growing (vegetative) botulinum bacteria in contaminated feed. These actively growing Clostridium botulinum bacteria continue to grow in the rumen and gut of cattle and produce toxin, leading to the onset of botulism days after contaminated feed has been removed from the diet. Scientific opinions still differ on the likelihood of this form of botulism being significant or even if it occurs at all. However, in several of the most spectacular outbreaks of botulism recorded in Queensland in intensively fed cattle, the pattern of losses has suggested it has occurred.
Dealing with outbreaks of botulism
If botulism is suspected as a cause of mortalities in intensively fed animals, the current ration and water supplies should be carefully examined for possible sources of the toxin. If an obvious source is found it should be removed along with any feed or water it may have contaminated. Wherever possible, new ration components should be obtained from a different source and the current ration immediately replaced, avoiding any major changes in ration composition to avoid digestive upsets.
A private veterinary practitioner should examine the clinically affected animals and conduct post mortem examinations to rule out other possible diseases, such as a mass poisoning. If losses are significant, a government veterinarian or stock inspector should also be notified so that exotic diseases can be ruled out.
Risk to humans
In the case of dairy cows affected by botulism, the risk of botulinum toxin entering milk supplies from cattle suffering from botulism is considered by experts to be 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, milk factories should be immediately notified if you suspect your cattle have botulism and potentially affected cattle have contributed to your bulk milk supply.
In the case of beef or dairy cattle that survive outbreaks of botulism and are fit, healthy and normal, there is no known risk of botulinum toxin remaining in the carcase and subsequently poisoning people.
Vaccinations
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 two weeks has generally led to a cessation of new cases by significantly boosting the immune response. This recommendation for two 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.
Oral antibiotics
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.
Prevention
Quality feedstuffs
Ensuring that feedstuffs are not contaminated with botulinum toxin is part of good quality assurance systems now implemented in all commercial feedlots and most dairies. However, despite all efforts, contamination is still possible.
Avoid 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. 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. The reasons for this are explained in a DPI Note called "Chicken litter and chicken faeces - It's illegal to let livestock eat it". Another DPI Note entitled "Chicken litter as a fertiliser on dairy pastures - Managing health risks to dairy cattle" explains in greater detail the precautions that should be taken if chicken litter is used as pasture fertiliser. These are available by contacting the Business Information Centre (see below for details) or on the DPI&F website at www.dpi.qld.gov.au/dairy/12503.html.
Vaccination the only long-term prevention strategy
The only effective long-term prevention strategy for botulism is vaccination with bivalent botulinum vaccines.
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.
Whenever possible, cattle should be vaccinated well before any suspected period of risk. Vaccinating well before the ration is introduced is recommended.
There is a range of different botulism vaccines on the market that are highly effective. Some newer vaccines only require a single shot where the traditional vaccine requires two shots a month apart. Both the one shot and two 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 be given to maintain protective levels of immunity. Consult package information or the vaccine manufacturer for advice on the timing of booster vaccinations.
Further information
- The DPI&F web site (www.dpi.qld.gov.au) provides easy access to DPI&F information, products and services.
- The DPI&F Business Information Centre (Queensland residents: phone 13 25 23; non-Queensland residents phone 07 3404 6999) provides generalist information and specialist referral services for the cost to Queensland residents of a local call. The DPI&F Business Information Centre is open Monday to Friday 8 a.m. to 6 p.m. (excluding public holidays). E-mail address: callweb@dpi.qld.gov.au.
This DPI Note is also published on the DPI's PrimeNotes CD-ROM
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: APH0162
Last updated 31 May 2004
