Controlling Johne’s disease is a challenge for farmers and veterinarians. It is also a challenge for researchers. There are a few reasons for that. A big one is that the disease is chronic. It can take years between the time an animal is infected and the time it shows evidence of infection. Only a few of the cattle get sick compared to the number that are actually infected. The ones that don’t show clinical Johne’s disease are called subclinically infected and they do not do as well as uninfected cows. And thirdly, it is not easy to prove that an animal is infected (or not infected.)
Many countries have Johne’s disease control programs in place. These provide an opportunity to study Johne’s disease because control programs require that cattle be routinely tested for evidence of infection. One recent study tried to answer the question of whether or not there was a risk that a calf would end up infected with the bacteria that caused Johne’s Disease even if her dam appeared not to be infected when the calf was born.
During the trial, all cows were tested every three months using their milk. Cows were classified according to their Johne’s status at the time the calf was born and over the 12 months immediately after the calf was born and then for the rest of its life. The Johne’s infection status of the calves was followed starting at the beginning of their first lactation using the same milk test and they were then tested every three months.
In addition to the Johne’s infection status of the dam, information about the birth of the calf was collected because early life experiences are believed to be an important risk factor of Johne’s infection. Information on cow density in the fresh pen and cleanliness of the dam and other cows in the pen and cleanliness in the calving area was recorded along with the amount of time the calf spent in the calving area. Information on calving difficulty was recorded. Also recorded was the timing of feeding and the amount, quality and source of colostrum.
The researchers were able to include 440 calves in six herds born in 2012-2013. Those calves started their first lactations beginning in June, 2014. They were then followed by testing every three months during each lactation until March, 2017 to see if they showed evidence that they were infected with the Johne’s disease bacterium.
Surprisingly, none of the factors related to birth were more important than whether their dam tested positive for Johne’s disease. It also didn’t seem to matter when the dam tested positive, even if it was at the time that the calf was born.
If the dam was positive when the calf was born, that calf was 2.6 times more likely to also become positive with Johne’s compared to a calf from a negative-tested dam. But if the dam was negative when the calf was born but then tested positive some time in the 12 months after the calf was born, the calf was 3.6 times more likely to become positive for Johne’s. If the dam was negative when the calf was born and for the 12 months after but became positive later on, the calf was 2.8 times more likely to become positive for Johne’s disease.
It is easy to understand that a calf would be more at risk of becoming infected if her dam was positive when she was born. But it was surprising that the risk was just about the same even if the cow did not become positive until a year or more later. That means that all heifer calves born to cows that end up being positive for Johne’s disease should be considered at high risk of being infected, not just the calves born after the cow becomes positive for Johne’s.
It is important not to discount how important cleanliness and colostrum management are to reduce the risk of Johne’s disease to newborn calves based on the results of this study. The study herds were already in Johne’s control. These farms might have effectively controlled much of the risk from Johne’s disease immediately after birth. But it is still important to pay close attention to implementing a plan to limit risk.
