– Dr. Michelle Arnold, UK Veterinary Diagnostic Laboratory
Botulism is a disease caused by one of the most potent toxins known to man. This toxin is produced by Clostridium botulinum, a Gram-positive bacterium from the Clostridia family. This bacterium survives in the environment as a “spore” and contaminates plant material during harvest. For the bacteria to multiply and produce toxin, an anaerobic (“without oxygen”) environment must be maintained. Under certain conditions, round bale silage (or “baleage”) can provide the correct place for botulism toxin to form. In the absence of oxygen (as is found in wrapped hay) and a pH greater than 4.5 (poor fermentation), the spores enter a vegetative state, multiply and produce toxin. This toxin, once consumed and absorbed into the blood stream, blocks transmission of nerve impulses to the adjacent muscles. Two forms of the toxin, Types B and C, occur most frequently in KY cattle. Type B is associated with improperly fermented forage while Type C occurs from the accidental feeding of dead birds, dogs, cats or poultry litter contaminated with dead birds in the rations of cattle.
Round bale silage or “baleage” is an increasingly popular alternative to baling dry hay that allows shorter hay curing time and saves valuable nutrients in the face of approaching adverse weather conditions. Baleage is simply forage of a relatively high moisture content that is baled with a round baler and then stored in a sealed container, usually a long plastic tube or individually wrapped in plastic, to keep oxygen out. Both grasses and legumes can be preserved by this method if proper techniques are followed. Forage cut at the correct stage of maturity, allowed to wilt to a 40-60% moisture range, then tightly baled and quickly wrapped in 6 or more layers of UV-resistant plastic will undergo fermentation (“ensiling”), a process that should drop the pH of the feed below 5.0 (ideally below 4.5) where spoilage organisms (including Clostridials) do not grow well. Problems arise when there is a lack of adequate fermentation to reach this low pH, which occurs most often with small grains (rye, oats, wheat, barley) but can occur with any type forage. If fermentation is restricted, it is critically important to maintain the integrity of the wrap to keep an anaerobic environment in the sealed bale and preserve the silage. If wrapping is delayed or there is damage to the plastic covering, spoilage may result which supports the growth of Clostridial organisms. On the other hand, very wet, non-wilted, and/or overly mature forages wrapped for baleage have less soluble sugars available for completion of fermentation and are also at an elevated risk for botulism toxin formation. Bacteria from the Clostridia family thrive in wet environments where forage moistures are in the higher 67-70% range; greater than 70% moisture is very high risk for Clostridial growth and spoilage.
Both types of toxin produce the same characteristic clinical picture in cattle including:
- Typically, multiple cattle will be affected with symptoms at the same time; some cases may present as sudden deaths. Otherwise, animals first appear dull, depressed, lethargic and eventually become thin and dehydrated due to the inability to eat and drink;
- Progressive muscle weakness leading to recumbency (downers) depending on the amount of toxin ingested; clinical signs may be first observed from about 24 hours up to 17 days after exposure to the toxin;
Figure 1: Decreased Tongue and Jaw Tone are characteristic findings in botulism cases. If the tongue is grasped and pulled out the side of the mouth, the tongue may hang from the side of the mouth or is pulled in very slowly as the disease progresses. Without tongue control, a cow will have other associated signs such as a dirty nose, difficulty chewing and swallowing, drooling, and plunging the nose deep in a watering trough to drink (Photo: http://www.nadis.org.uk/bulletins/clostridialdisease-in-cattle.aspx)
- Decreased Tongue Tone (Figure 1) – Tongue weakness is characteristic of botulism. Without tongue control, a cow will have other associated signs such as a dirty nose, difficulty chewing and swallowing, drooling, and may plunge the nose deep in a watering trough to drink. Although they may appear to chew hay or grass, there is an inability to swallow so feed and forage may be seen to fall from the mouth or may be found within the mouth (Figure 2);
- Jaw Laxity and Decreased Muscle Tone – In affected cattle, back and forth movement of the lower jaw may be very loose; the upper eyelid and tail tone are often noticeably limp;
- Constipation/Raising the tail while straining. Sometimes see colic (abdominal pain) and a “hunched up” appearance;
- Most cattle that go down due to botulism toxin will die due to paralysis of muscles of the diaphragm, dehydration, or complications from being a “downer”. Cattle with a more gradual progression of signs and that maintain the ability to eat and drink may recover although it can take 30 days or longer to return to normal function.
Figure 2: Hay dropped from the mouth of a bull affected by botulism toxin
Treatment consists of supportive care including administering fluids for dehydration and propping cows up on the sternum (breastbone) to prevent them from lying down flat on their sides. A vaccine (toxoid) for Clostridium botulinum type B (Bot Tox B, Neogen Corporation) is approved for horses and can be used in an extra-label fashion in cattle if a valid veterinary-client-patient relationship exists. This vaccine will not reverse clinical signs already present but may help to prevent new cases. Dead animals must be disposed of properly as the meat is not safe for human consumption.
Diagnosis is difficult and is usually based on history and clinical signs. Rumen contents recovered at necropsy are the best sample for identification of the toxin. A sample of the suspected baleage should also be submitted for pH and moisture testing. Baleage testing for quality and a fermentation profile are highly recommended. Other possible causes of muscle weakness and downer cows include low blood levels of calcium, potassium or magnesium, ionophore toxicity (monensin, lasalocid), organophosphate or carbamate insecticides, heavy metals such as lead, and infectious causes such as listeriosis or rabies. Calves may exhibit extreme muscle weakness due to a lack of selenium. A thorough physical examination by a veterinarian will help rule out these other possible diseases.
Prevention is based on ensuring proper harvest and preservation of wrapped forages and maintaining proper feedout rates to reduce the risk of growth of organisms dangerous to cattle. Correct moisture content is of primary importance; there is a field method to assess moisture that will yield a general idea of moisture content but there are far more accurate methods available. Cut forage at the proper stage of maturity so it contains adequate levels of fermentable carbohydrates for good ensiling. See Quality Hay Production (AGR-62) for specific cutting recommendations for various forage crops http://www2.ca.uky.edu/agc/pubs/agr/agr62/agr62.pdf. Also, achieving the highest bale density possible, especially with high internal core densities, removes the maximum amount of oxygen with few air pockets. Wrapping the bales quickly after baling with a good quality plastic, preferably with an ultraviolet inhibitor and 6-8mm thickness, and using multiple (4-6) layers will extend the storage time. Bale weight can be a safety and equipment issue. Details of proper techniques can be found in the UK Extension Fact Sheet AGR-173 entitled “Baling Forage Crops for Silage” at your local extension office or on the web at http://www2.ca.uky.edu/agc/pubs/agr/agr173/agr173.pdf. Another excellent resource is the UK Forage Figure 2: Hay dropped from the mouth of a bull affected by botulism toxin website for more information: http://www.uky.edu/Ag/Forage/ForagePublications.htm#Silage/Balage0 and look for Baleage: Frequently Asked Questions. If holes appear during storage, these should be covered immediately with the proper repair tape. Store the wrapped bales on a north facing slope if available because prolonged exposure to the summer sun may cause the upper side and the south face of the bale to dry out, with the moisture condensing on the bottom or north face of the bale.
In summary, it is advisable to test the pH and moisture content of your baleage at the very least to insure adequate fermentation before offering it to cattle. Samples can be submitted to a forage laboratory such as Dairy One for quality and a fermentation profile requested. This type of forage analysis will include a pH and volatile fatty acid profile and will give a very good idea of the quality of feed produced. This is a common practice for corn silage and one should consider this with fermented forages of all types to avoid health risks. It is important to remember that thousands of round bales are wrapped annually with only a few cases of botulism occurring; the risk of disease is low if one applies the proper management techniques from time of harvest through feeding.