Reduce The Chances Of Listeriosis And Botulism By Using Best Management Practices When Making Silage

With Dr. Grant Burcham’s permission, the paper he authored and was part of the proceedings shared with attendees of the recently held “Heart of America Grazing Conference” follows. Dr. Burcham is a diagnostician at the Heeke Animal Disease Diagnostic Laboratory that is found at the Southern Indiana Purdue Agricultural Center. Dr. Burcham has identified listeriosis and botulism as causes of death in multiple necropsies performed. I thought it was prudent to share this information at the beginning of the growing season. As forages are harvested and ensiled, follow guidelines so fermentation can achieve a pH of at least 4.5. Forage too mature, packaged at too high or too low a moisture content, loosely packed, put in a structure or wrapped bale that doesn’t exclude air, and with soil contamination can create an unforgiving issue. Please read on!

bale of hay

Livestock Disorders Associated with Preserved Forages:
Listeriosis and Botulism
Grant N. Burcham, DVM, PhD, Dipl ACVP
Veterinary Diagnostician, Heeke Animal Disease Diagnostic Laboratory
Purdue University College of Veterinary Medicine
gburcham@purdue.edu

Preservation of forages via fermentation results in high-quality feed that can be stored and fed to livestock many months after cutting and processing the forage. Ensiled forages typically retain more nutrient content than dry hay, giving producers a high-quality feed option throughout non-growing months. Fermented forages come in three main types: silage, haylage, and baleage. Silage has a higher moisture content than haylage or baleage. Haylage is typically chopped like silage and can be stored in a silo or bag. Baleage is baled and then sealed in an airtight wrap.

The preservation of forages occurs over three steps. First, bacteria and plant cells will process sugars in the presence of any remaining oxygen. This first step is known as the “aerobic phase” and lasts about a day. Next, fermentation occurs. Bacterial species that can process sugars without oxygen (anaerobic bacteria) will do so, and produce acid as a byproduct of metabolism. Acid decreases the pH, preserving the forage and preventing breakdown of nutrients. Low pH also prevents growth of undesirable bacteria and molds. At this point, the material reaches an equilibrium state, known as the “stable phase,” and can be maintained under anaerobic conditions for months. If/when aerobic conditions are reintroduced, molds and other bacteria can grow and cause spoilage. Once opened, ensiled forages should be used as quickly as possible to minimize chances of spoilage. If problems occur during this process, bacteria that are harmful to livestock can grow and pose a threat to animal health. This presentation will cover two more common bacterial causes of disease associated with feeding preserved forages: listeriosis and botulism.

Listeriosis

Listeriosis is a condition caused by the bacteria Listeria monocytogenes. Listeria is a psychrophilic, gram positive species of bacteria that is found throughout the environment. Listeria thrives in silage or other preserved forages that do not reach the appropriate pH level during the fermentation phase; pH <4.5 inhibits growth of this potentially harmful bacteria. The disease resulting from Listeria infection, listeriosis, usually manifests as one of three different syndromes: encephalitis, abortion, or sepsis. Encephalitis, or inflammation of the brain, manifests as asymmetric facial paralysis, ataxia, and/or circling to one side. Circling is such a prominent feature of disease that this manifestation of listeriosis has been called “circling disease” by producers. Dullness, anorexia, and head pressing can also be observed. A second manifestation of listeriosis in adult livestock is abortion or stillbirths. Fetal loss due to listeriosis usually occurs in the last trimester for cattle, and fetuses are usually autolyzed. Retention of the placenta is common, and cows may experience fever or anorexia in association with abortion. Interestingly, outbreaks of encephalitis and abortion do not usually occur in the same herd; producers only see one syndrome at a time. The reason for this phenomenon is not completely understood, although, it may be due to the route of entry of the bacteria, as encephalitis occurs secondary to abrasions in the oral mucosa, while abortion likely results from maternal sepsis. Various strains of the bacteria likely play a role in the different types of infection as well.

Another manifestation of listeriosis that can be overlooked by producers and veterinarians alike is sepsis in young ruminants. This syndrome occurs in animals that are just a few days of age to those that are a few months of age. Neonatal infections likely stem from in utero infections; those infections in older animals could presumably come from the environment or feed material. Clinical signs of neonatal infection are usually vague, including listlessness and inappetence. Cattle producers and veterinarians sometimes refer to cases with this type of presentation as “weak calf syndrome.” In the spring of 2022, the Heeke Animal Disease Diagnostic Laboratory (ADDL) saw multiple cases of septicemic listerisosis in cattle. Affected calves ranged from 4 days of age to 48 days of age. In all cases, the history included calves “not thriving” or doing poorly. Affected producers were feeding fermented forage materials, including one farm feeding orchardgrass baleage. In one case, cows were started on corn silage the last week of February and the cows started calving the first week of March. By mid-March, 2-week-old calves were “not thriving” and had increased mortality. Listeria was cultured from major organs of dead calves, and pathologic changes associated with bacterial sepsis were present. Adult animals never exhibited clinical signs. Diagnosis of listeriosis in animals requires a post-mortem examination and culture of affected organs. Testing suspect forages is not considered helpful. Treatment of affected animals involves antibiotic therapy, usually with penicillin; however, treatment is not often successful. There is no available vaccine for listeriosis.

Botulism

Botulism is a disease caused by toxins produced by the bacteria Clostridium botulinum. This bacteria is considered an anaerobe, meaning that the bacteria favor low to no oxygen environments—the exact environment created during the fermentation process. To counter growth of C. botulinum, producers need to ensure that the pH of the product is <4.5, at which botulism bacteria cannot grow. Different livestock species have varying levels of susceptibility to botulism intoxication. Horses have a low threshold for botulism intoxication and are the species usually covered in discussions about botulism. Cattle have an intermediate to low susceptibility to botulism; however, ruminants are usually those species fed ensiled forages, meaning that, overall, there may be more opportunities for botulism intoxication in cattle as compared to horses.

Because cattle have a higher threshold for botulism intoxication, clinical signs may be prolonged and, seemingly, mild. Cattle may even spontaneously recover from intoxication, which also creates difficulty in diagnosing the condition. While horses may exhibit weakness leading to complete paralysis in a matter of 2-3 days, cattle may be affected for several days to weeks. Typical clinical signs include muscle tremors, symmetrical weakness, motor paralysis, and recumbency. Cattle with botulism may seem ataxic (wobbly) or weak, but otherwise normal. Affected animals typically progress to sternal recumbency. Histories from botulism cases submitted to the Heeke ADDL over the last few years have included producers that say calves “just won’t get up.” Affected calves are aware of their surroundings and will eat and drink, but, remain down until they recover or the disease progresses. If the disease progresses, animals will become laterally recumbent and, eventually, die due to paralysis of muscles used for breathing.

Botulism diagnostics include testing gastrointestinal contents of affected animals or feed materials using a test known as the mouse bioassay. In this test, the material to be sampled is purified and injected into mice that have been vaccinated against botulism and those that have not. This test has poor sensitivity, meaning that a negative test may not be helpful, especially in the face of compatible clinical signs. Diagnostic testing based on detection of the botulinum toxin within a sample is forthcoming and will be the next generation of botulism testing. Treatment for botulism is expensive and involves administration of anti-toxin. Prevention via vaccination is available for horses, but not other livestock species.

Management Recommendations

Both of the diseases discussed here are caused by infection with bacteria that are widespread throughout the environment, in soil and vegetation. Thus, the causative agents are commonly present to some degree in large amounts of forage, especially ensiled forage. There are two specific recommendations for decreasing the risk of these diseases to livestock:

  1. Ensure that ensiled forage hits the target pH of <4.5 to inhibit growth of harmful bacteria. Testing ensiled forage for pH levels can be accomplished at private laboratories.
  1. Heavy soil contamination of forage for any reason can be a risk factor for both of these diseases. Having forages tested for ash content can give producers an idea about how much soil contamination is present in a particular product and, therefore, associated risk level.

Feeding ensiled forages has both benefits and risks. Minimizing risks associated with bacterial infections involves proper fermentation technique, testing the finished product for pH level and ash content, and expeditious use of ensiled forage once exposed to air.

References

Anderson, ML. Chapter 2: Disorders of Cattle. In: Kirkbride’s Diagnosis of Abortion and Neonatal Loss in Animals. 4th ed. Edited by Bradley L. Njaa. Wiley-Blackwell. 2012

Maxie MG, Youssef S. Nervous System. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 5th ed. Saunders Elsevier. 2007

Radostits OM, Gay CC, Hinchcliff KW, Constable PD. Veterinary Medicine: A textbook of the diseases of cattle, horses, sheep, pigs, and goats. 10th ed. Elsevier. 2007

Romero JJ, Castillo MS, Burns JC. Forage Conservation Techniques: Silage and Haylage Production. NC Cooperative Extension Resources. Available at:

NC State Extension - Forage Conservation

NC State Extension – Forage Conservation

 

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