Johne’s Disease: A Silent Threat

 By Michael Cox for American Dairymen Magazine

 Livestock losses can be frustrating at the best of times, even when we have a clear cause of death such as milk fever or pneumonia. However, a disease such as Johne’s can often be an even more frustrating ‘silent killer’ and lie undetected in animals for years before showing clinical and rapidly fatal symptoms. For non-treatable diseases such as Johne’s, prevention and damage limitation can be the best course of action. Although a ‘silent killer’, losses from Johne’s can add up significantly over time, with an estimated 90% of herds carrying the disease.

Dormant period
Johne’s is caused by a bacterium ‘Mycobacterium avium’, which is similar to tuberculosis causing bacteria. The bacteria typically infects young calves and does not show clinical rapid weight loss and diarrhea symptoms until an animal is 2 years or older. Often times the disease can lie dormant for long periods and only become apparent after a stressor event for the herd, such as fast changing weather events at the beginning of a new season. Once clinical signs begin to appear, the outcome is almost always death or early culling. The bacteria shows clinical symptoms by targeting the intestinal wall which causes the wall to thicken and become impenetrable to water and nutrients. The cow will then lose weight rapidly due to poor nutrient absorption. Cows can also appear dehydrated and show diarrhea, as water pass-through in the intestines is compromised.
To best minimize outbreaks of Johne’s and limit the spreading of the disease within the herd we must try to limit the contact of ‘at-risk’ animals from infected animals which may be shedding the bacteria. Newborn calves are the most at risk group, and can easily pick up bacteria from faeces or by nursing on contaminated udders.

Prevention
A simple practice of ‘snatching calves’ i.e. removing calves as quickly as possible from the maternity pen after they are delivered can help to reduce exposure to these vulnerable animals.
In an ideal world, every calf would receive colostrum from it’s dam. But in reality, many dairies pool colostrum from several dams to feed to newborn calves. Although colostrum is a wonderful feed full of beneficial antibodies, unfortunately Johne’s disease can be passed through in the colostrum to newborn calves. For this reason, dairies with a significant Johne’s problem or for dairies wishing to take a pro-active role in minimizing the disease, should consider pasteurizing pooled colostrum to reduced disease spread. Feeding a powdered colostrum replacer is the gold standard in ensuring newborns are not at risk from infected colostrum, however powdered colostrums can be cost-prohibitive, particularly when there is a natural supply of colostrum on the dairy.

Practical Steps
Blood testing individual cows and/or full herds has limited benefits in identifying ‘clean’ and ‘carrier’ cows. As over 90% of herds have some level of subclinical Johne’s it would be very difficult to source cows from a clean herd. The ELISA blood test also has limitations, in that the cow must be shedding at the time of testing, as false-negative results can be an issue in low-stress herds.
Other considerations for minimizing the disease is to cull clinical cows as soon as symptoms appear, keep maternity pens and udders as clean and disinfected as possible, keep water troughs and feed-bunk areas clean and free of fecal debris and feed colostrum from repeat Johne’s negative cows if possible. Maintaining good parentage records can also help identify future offenders if an older cow shows clinical symptoms. Generally, 50% of mature clinical cows will have daughters that show clinical signs in later life. Culling on lineage can reduce outbreaks in future.
A vaccine is permitted for use in calves less than 30 days old. However, it’s use is quite rare, as it will only reduce clinical symptoms and not eliminate or protect against new infections. Vaccines may be an option for herds struggling severely with the disease, but the other practical steps mentioned above have a greater chance of limiting the disease spread within the herd.
As Johne’s disease can be such a tricky disease to try and tackle head-on, producers should seek support from their veterinarian and State Johne’s prevention program, where applicable. Taking measures to prevent the spread of the disease can show few clear benefits and it is easy for producers and the farm team to become complacent and slip into old habits. Committed dairies will see the benefits over time however, and as Johne’s prevention steps also align with excellent animal health and management standards, the benefits of a strong Johne’s prevention program could be huge.