Winter is normally a good season for dairy cows in terms of environment temperature (no more heat stress) but with winter comes other challenges. Of the many issues (water bowls freezing, frozenmanure, challenges with ventilation, etc.) one that often shows up on farms is winter dysentery.


According to the Merck Veterinary Manual “Winter dysentery is an acute, highly contagious gastro intestinal disorder that affects housed adult dairy cattle, primarily during winter. Clinical features include explosive diarrhea (sometimes accompanied by dysentery), a profound drop in milk production, variable anorexia and depression, and mild respiratory signs such as coughing. The disease has a high morbidity but low mortality, and spontaneous recovery within a few days is typical.” The word “explosive” seems excessive but many farms can attest that it is real!

One of the main signs of winter dysentery (WD) is the sudden start of diarrhea in one or more animals that then starts spreading to other animals in the herd. In fact, this sometimes is a good way to confirm that the problem may be WD, when
animals eating different diets and in different pens start having diarrhea. The diarrhea is usually dark green to black in colour and at times, may contain blood.

It is rare that affected animals die, but the number of affected animals is high (20 to 50% will show clinical signs within a few days and close to 100% within a week, according to the Merck Veterinary Manual). If anything related to this disease can be considered positive, it is that some degree of immunity appears to develop, because recurrences, if seen in the same herd, are noted at one to five year intervals.


Research indicates that a precise cause of WD has not yet been confirmed. The most common accepted theory implicates a bovine coronavirus (similar to the one that causes diarrhea in calves). What is interesting is that it has not yet been possible to consistently reproduce WD through oral inoculation of adult cattle with the bovine coronavirus. The Merck Veterinary Manual indicates, “… concurrent risk factors, such as changes in diet, cold temperatures, closed confinement with high animal density, poor ventilation, and presence of other microorganisms may be required before bovine coronavirus causes clinical disease in adult cattle.”

In terms of transmission, we know this virus is transmitted via the fecal-oral route, through ingestion of feed or water contaminated with feces from clinical cases or clinically healthy carrier cows. Viral particles present in respiratory secretions of affected animals may further enhance transmission. Transmission of disease is promoted by close confinement. One of the problems with this disease is that it is highly contagious and easily introduced to barns by visitors, carrier animals, and fomites (objects or materials that are likely to carry infection, such as clothes and utensils).


Affected cows will normally have a sharp drop in milk production. This is due to a combination of dehydration and a reduction in feed intake. Provide a well-balanced, palatable and well mixed diet and push feed often to encourage intake. Since the animals are dehydrated, the presence of fresh, clean water is fundamental. Offering free choice salt may also help the animals cope with the loss of some salts through the feces. Although the disease in the herd typically subsides in one to two weeks, milk production may take weeks to months to return to normal due to the extensive damage that the virus causes to the gut tissue.

Since there is no vaccine for WD and it is a very contagious disease, biosecurity measures are especially important. The Merck Veterinary Manual recommends isolation of newly introduced cattle for two weeks and isolating any adult cow with diarrhea to decrease the likelihood of disease introduction into a herd. In an outbreak, access to the farm should be restricted, and all persons in contact with affected cattle should sanitize boots and clothing after working with the animals to reduce the risk of spreading the disease. WD can be spread easily from farm to farm.




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