Botulism in farmed ruminants
Published 1 November 2018
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1. Introduction: Botulism
Botulism is a paralysing disease which kills most affected animals. It is caused by a highly lethal toxin which is produced by Clostridium botulinum, a bacterium found in soil, which multiplies in rotting vegetation or carcases. Since 2003 there has been a marked increase in the number of confirmed outbreaks of botulism in cattle and sheep in the UK.
2. Sources of the bacterium, Clostridium botulinum
In UK ruminants the disease is often caused by animals coming into contact with the litter of broiler chickens especially if it contains any carcase material. There are also more sporadic but very large outbreaks due to feeding forage that has been contaminated with carcase material, including carcases that have been ensiled during grass cutting. If there are outbreaks of diseases in wildlife on a farm, for example myxomatosis in rabbits, the risk of botulism in livestock may be increased because dead wildlife carcases may be present in the grass fields that are to be cut for silage. There is also a risk if game shoots leave carcases behind on farms. The increased popularity of feeding livestock a wide array of materials considered unfit for the human food chain in a bid to reduce costs and waste, also poses a risk if not suitably stored.
Chicken litter is usually brought onto farmland as a source of fertiliser. Animals have become affected through direct access to litter when it has been heaped or spread in the field where they are grazing, and indirectly from litter on a neighbour’s premises, possibly as the result of carcase movement by scavenging birds and animals. Windy weather is also thought to be implicated in some outbreaks by spreading contaminated dust. Occasional cases have also occurred in animals fed silage made from fields fertilised with broiler litter.
Figure 1. Broiler manure on field awaiting spreading
Figure 2. Chicken carcase remains found in broiler manure (warning: image of a chicken carcase)
3. Clinical signs in affected animals
The clinical signs in affected animals vary from sudden deaths, to animals becoming stiff, wobbly, and going down before dying. Some animals have slowly become weaker developing flaccid paralysis and are unable to eat or drink, necessitating euthanasia on welfare grounds. Very few animals survive once they develop the disease. The onset of clinical signs is variable and can be within 24 hours of exposure but can be delayed by up to two to three weeks, with the majority affected within the first seven days.
Figure 3. Holstein-Friesian heifer in sternal recumbency (warning: image of a diseased cow)
Figure 4. Botulism affected ewe in sternal recumbency (warning: image of a diseased sheep)
4. The effect of the disease on farms
The main costs to the farmer are the deaths of animals and the purchase of replacements. The loss of livestock may range from a few animals to large numbers. There are also costs of veterinary visits, supportive treatments that may be given to affected animals, and the cost of post mortem examinations. There may also be the cost of vaccination if the farmer decides to vaccinate. In addition to economic losses there will inevitably be an emotional impact and associated stress.
5. Food safety
With botulism outbreaks no clinically affected animals or their produce should enter the food chain. This means that meat and milk from clinically affected animals may not be used for human consumption. If there are any recovering animals these should be restricted for 18 days following withdrawal from the suspected source and cessation of clinical signs.
6. Diagnosis
At the start of an outbreak it is recommended to seek specialist advice from a Veterinary Investigation Officer (VIO) at a Veterinary Investigation Centre (VIC). The diagnosis of botulism is based on the history, clinical examination, and ruling out other conditions that may result in ataxia and recumbent animals. Although there are no specific findings on post mortem examination (PME) of affected animals, PME is useful because other differential diagnoses can be ruled out. With suspect botulism outbreaks affecting several animals PME examination at a VIC is recommended. Samples for possible toxicological examination can be collected. For cattle, sheep, and goat carcases submitted for PME to VICs where botulism is suspected or cannot be ruled out approximately 2 x 50 ml of small intestine contents, 50 grams faeces, a blood (serum) sample and 50 grams of liver are collected from each animal. When further laboratory testing is required to enable a diagnosis to be reached, APHA subcontract this work to a laboratory specialising in this analysis. All other samples are stored at - 20° C. When samples are analysed, the testing looks for the presence of botulinum toxins. To date, confirmed disease in ruminants in the UK has been caused only by the C and D forms of the botulinum toxin.
Where the outbreak is thought to be feed-related (other than contamination of the feed by poultry carcases) a sample of feed (approximately 5 kg) should be collected and stored at - 20°C. This may be tested if specific criteria are met.
Positive results are reported as toxin type C, D, or C/D (where C/D is the chimeric botulinum toxin). If no toxin is found on direct examination, samples are tested by PCR for the bacterium. The PCR is not usually carried out if toxin is found on direct examination, but with some outbreaks tests for the identification of botulinum toxins and detection of the bacterium may be performed.
7. Risk to humans
Although botulism is occasionally identified in people, the risk to humans from affected ruminants is considered to be very low, because the C and D forms of the botulinum toxin very rarely affect humans.
8. Examples of some botulism outbreaks investigated over recent years
Botulism outbreaks investigated by the Animal and Plant Health Agency (APHA) over the last few years include cases associated with broiler litter, suspected random carcase material on pasture land, and those associated with carcases contaminating feed, in particular silage. A few botulism outbreaks are commented on below.
8.1 Cases associated with broiler litter
In 2006, on eight premises in Devon, a total of 52 of 378 (around 14%) cattle at risk died of botulism. This was through direct and indirect contact with broiler litter from a single source farm.
In 2008, whilst grazing fields in Herefordshire, 150 of 500 ewes died. This was through direct contact with broiler litter.
In 2015 a severe outbreak of botulism was diagnosed in a group of approximately 600 one-year-old lambs which were destined for slaughter into the food chain. Around three- and-half weeks before the outbreak the lambs were put in one field which had a large amount of poultry litter stacked in it. Most of this broiler litter was fenced off from the sheep however two trailer loads were not. On the first day of the outbreak 70 lambs were found dead and 30 others showed clinical signs of weakness and flaccid paralysis. The group were immediately moved out of the field but new cases continued to occur over the following two weeks. In total 230 lambs died.
8.2 Cases associated with carcase material contaminating feed
There can be high numbers of deaths associated with feeding contaminated feeds. There is frequently only one carcase / remnant of carcase confined to one area of feed, however the use of feed mixers / wagons can disseminate C. botulinum and botulism toxins.
In 2015 botulism was diagnosed in a beef suckler herd consisting of 38 animals. In total 8 heifers were affected and 5 heifers died. The animals were housed in a straw yard when the outbreak occurred. The suspected source was a goose carcase within the silage being fed. PME was carried out on one heifer. No other causes of death were found. C. botulinum type C/D toxin was detected in intestinal content. Following the initial cases the group were turned out to grass and no further cases occurred.
Figure 6. Depressed ewe in sternal recumbency (warning: image of a diseased sheep)
In 2017 botulism was diagnosed in a group of 39 housed high yielding dairy cows which were fed on a silage-based total mixed ration. On the first day of the outbreak there were four cases with the four affected cattle showing weakness and flaccid paralysis. They all died within six hours. Over the next four days there were further losses including animals with severe signs that were euthanased on welfare grounds. A total of 38 high yielders died. Two cows were submitted for PME. Botulinum toxin testing confirmed the presence of toxin type C/D in intestinal content, and the presence of C. botulinum type DC bacteria was confirmed by PCR.
9. The prevention of botulism
Information for farmers:
- Maintain a tidy farm. Clear up any dead carcases promptly. Take measures to prevent introduced carcase material from entering feed.
- Ensure that feed (silage, other forages, concentrates) that are fed to livestock are of good quality with no spoilage or contamination.
- Have a rodent control programme in place.
- Be alert to potential diseases in wildlife on farms and thus dead carcases.
Broiler litter is a useful source of nitrogenous fertiliser but there is the potential for botulism occurring in animals which come into contact with the fertiliser:
- Broiler litter containing visible carcase material must not be used as fertiliser (Animal By-Products Order).
- Heap litter in fields well away from livestock.
- Store litter securely well away from livestock and prevent access to birds and other scavengers.
- Ideally, only use the litter on arable land.
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It is illegal to use poultry litter as bedding material for other livestock.
- Cattle and sheep can be vaccinated against botulism – Farmers should seek veterinary advice first.
10. What to do if botulism is suspected
Information for farmers and veterinary surgeons:
- Farmers should seek advice from their veterinary surgeon.
- Livestock should be immediately prevented from having access to the suspect source.
- Farmers should immediately prevent animals from accessing broiler litter by moving them to fresh pasture or at least fence off any heaped manure.
- A diagnosis of botulism may require the examination of affected animals. For further information a local APHA Veterinary Investigation Centre should be contacted by the farm’s veterinary surgeon. VIOs are happy to assist with the investigation of suspect botulism cases. As there are potential implications for the safety of the food chain the APHA prefers to be contacted early in the outbreak so that risk assessments can be carried out and so that essential advice can be given. All investigations performed by VIOs are done through the farmer’s private veterinary surgeon.
- In botulism cases that are due to broiler litter - dispose of contaminated manure / broiler litter by deep ploughing it into arable land as soon as possible.
- In cases associated with contaminated feed – do not use the problem feed. A sample may be sent for testing. Advice should be sought on the safe disposal of contaminated feed.
- Vaccination may be considered. There is not a licensed vaccine that is available in the UK, however through the private veterinary surgeon an overseas vaccine can be obtained with a Special Import Certificate via the Veterinary Medicines Directorate. Farmers should seek further advice from their veterinary surgeon.
11. Further information
For more information, farmers should contact their private veterinary surgeon. Private veterinary surgeons should contact their local APHA Veterinary Investigation Centre.
APHA Veterinary Investigation Centres and surveillance pathology partners
Risk assessment links:
Animal By-Products links:
https://www.gov.uk/government/collections/guidance-for-the-animal-by-product-industry
https://www.gov.uk/guidance/how-to-use-store-or-move-manure-guano-and-digestive-