With the changing of the seasons comes new challenges for dairy producers. As day length, temperature and environmental pests and pathogens change throughout the year, we see swings in intakes, production and animal health.

While some of these shifts can be positive, many are negative. We try to mitigate the effects through management practices to achieve as much consistency as possible.

As the leaves begin to change color and the air becomes crisp, it is time to start thinking of the approaching winter and the challenges that come with it. One such challenge is winter dysentery.

Winter dysentery is a highly contagious virus that results in severe diarrhea. A large number of animals tend to become infected within a herd, and although most will recover spontaneously within a few days, severely depressed milk production of up to 50 percent or more, reduced intakes and mild respiratory signs are typically seen.

While there is still a question as to the cause, bovine coronavirus (BCV or winter dysentery), which likes cool temperatures and low light levels, is commonly found in association with the onset of the disease and is likely a causative agent.

Due to the rapid onset and spread of winter dysentery, it is important for producers to properly diagnose sick cows, understand risk factors and implement management practices that can help limit the impact of the virus on the farm.

Diagnosing winter dysentery

Cows are susceptible to many diseases that cause acute diarrhea. The correct diagnosis is essential to ensure cows receive the appropriate treatment.

Cows with winter dysentery will have liquid stool that is a consistent dark green to black color. Blood may be present in the stool of some animals. Prior to or during the onset of the disease, cows may exhibit a nasal discharge or a cough.

Cows will often appear lethargic and show signs of depression, anorexia and dehydration.

One of the more telltale signs of winter dysentery is how quickly the disease spreads. Cows usually become symptomatic within two to three days, and the disease spreads rapidly between cows. While a winter dysentery outbreak can easily affect 50 to 100 percent of the herd, death loss is low. The whole herd tends to be clear within one to two weeks.

To confirm the diagnosis of winter dysentery, producers can rule out other causative agents, such as salmonella (negative fecal culture), coccidiosis (negative fecal floatation) and bovine viral diarrhea (negative mucosal lesions).

Risk factors contributing to winter dysentery

To understand the risk factors of winter dysentery, producers need to recognize how the virus is spread and which cows are most susceptible. BCV is contracted through fecal-oral transmission, where infected cows shed the virus in their feces, and the virus is then contracted through contact with the mouth or nose of an uninfected cow.

Because winter weather often leads to cows being housed in close confinement with poor ventilation, BCV spreads easily amongst these herds.

When intakes fluctuate due to abrupt weather changes or extreme ration changes, cows can experience a shift in the rumen microbial population that allows harmful pathogens to colonize the rumen and the lower gut (small and large intestines).

These pathogenic bacteria reduce digestion and absorption of nutrients in the small intestine as well as cause damage to the lining of the lower gut. The presence of other viruses, such as bovine viral diarrhea virus (BVDV), depresses the cow’s immune system, making her more at risk for contracting BCV.

It’s important to note that cows previously exposed to winter dysentery often form immunity and will not be susceptible to the virus for three to five years. That leaves first-calf heifers and cows new to the herd as the most susceptible animals on a dairy that has previously had an outbreak.

Since some degree of immunity to winter dysentery appears to develop, recurrences within the same herd are realized at one- to five-year intervals.

Management practices that can help limit winter dysentery

There is no USDA-licensed BCV vaccine for winter dysentery. Prevention and mitigation management practices should be a producer’s focus.

During winter housing conditions, emphasis should be put toward maintaining a clean environment.

• Cleaning and scraping bedding frequently will help limit the growth of BCV in the environment.
  
  
• Maintaining clean water sources will help eliminate the infection of new animals.
  
  
• Machinery or tools used for manure management should not be used on feed.
  
  
• Ensuring cows always have ample fresh feed will help maintain a healthy, stable rumen environment.

There are no antibiotics effective at reducing the length or severity of winter dysentery. When an outbreak does occur, the best plan of action is to reduce the spread through the herd and treat the clinical symptoms of infected cows.

Isolating infected cows will limit the extent of herd contamination. Staff should be instructed to increase sanitation practices to limit cross-contamination between pens. Special attention should be placed on staff that work with both adult cows and calves, or on two facilities.

Even with the best management practices, the risk of a winter dysentery outbreak remains. Feeding an effective probiotic throughout the year provides cows with extra digestive and immune support when faced with challenges such as winter dysentery. BCV likes to invade the gut tissue of the small and large intestine.

It causes significant damage to the tissue responsible for transferring nutrients from the gut to inside the body, causing severe diarrhea and reduced production efficiencies.

Probiotics are “good” bacteria that work in the gastrointestinal tract to improve overall gut health and feed efficiency. Many strains of probiotic bacteria have been selected for their ability to compete with, and reduce the load of, harmful bacteria in the gut.

Select probiotic bacteria strains communicate with the animal’s immune system to increase antibodies and immune defense cells. Probiotics aid in physically protecting and repairing the GI tract. This results in gut tissue that absorbs nutrients more efficiently while blocking harmful pathogens and toxins from entering the body.

Making it through the winter

While winter dysentery does not require antibiotic treatment, and the majority of cows recover on their own within a few days, the lasting effect on milk production can be significant. Due to the extensive damage BCV causes to the gut tissue, nutrient digestion and absorption is hampered for an extended period.

To help prevent long-term milk production losses, producers should focus on management practices that minimize the growth of the virus and limit the spread of the virus if it does occur, as well as feeding to maintain a healthy gut that leads to an overall healthy animal.  

Jill Havlin, Ph.D., P.A.S., is a technical services manager – Pacific Northwest, Chr. Hansen. Havlin provides dairy nutrition technical support to the sales team, customers and consultants, including insight on cow comfort, cow movement, bunk management and feed management. Havlin has her doctorate in animal biology from the University of California – Davis. Email Jill Havlin. 

PHOTO: Clean water sources are important in preventing the spread of winter dysentery. Photo by Mike Dixon.