Johne’s disease (pronounced “Yo-knees”)
Something that all farmers have no doubt heard mentioned by a vet at some stage or seen on their farms.
Suggested to be prevalent on around 60% of farms in recent publications but I would suggest it could be up to around 90% of Dairy farms. It would be considered relatively endemic!
Youngstock (calves, lambs, fawns) are most susceptible to new infection and especially those animals under 6 months of age. The bacteria are primarily spread via faeces but can also be present in milk and can pass via the placenta. Often infection occurs at the time of birth or when they are just newly born. Repeated exposure increases the chance of more severe disease presentation later in life, but as the disease is very slow to get going (long incubation period) it cannot be detected at this early stage.
Adult animals are the main source/reservoir of infection, and they begin to shed the bacteria in their faeces before they show clinical signs. Once clinical signs (weight loss, bottle jaw, diarrhoea) start to appear (3-7 years of age) they are shedding the bacteria in huge numbers. At this stage, diagnostic tests are reasonably good at giving a diagnosis. Some animals can shed the bacteria in huge numbers whilst appearing clinically normal (“Super shedders”).
The bacteria can survive for several months in effluent, water, or on pastures so once shed they pose a risk to any younger animals grazing those areas.
We don’t have any treatments that are effective against Johne’s disease and once it has been picked up and it starts to invade the gut lining and lymph nodes it will replicate and slowly and insidiously cause increasingly severe damage.
So how can we manage this? Unfortunately, we can’t completely eradicate it, but we can stop it from causing us problems by keeping on top of it, it doesn’t need to cause us financial losses in terms of cows needing to be destroyed on farm!
In dairy systems, testing of our adult herds at the time of routine milk herd testing is a great way to manage the risk associated with Johne’s. If we can pick up that a cow has Johne’s by doing a herd screen on the milk test, then we can either monitor her or cull her depending on her circumstances and what levels of infection we find within the herd. This will reduce the risk of her becoming clinical and shedding high numbers or giving birth to a calf that will also turn out to be infected.
Making sure our youngstock have minimal exposure to pastures grazed by adult cattle recently and not using colostrum/milk from Johne’s positive cows to feed calves will also reduce the risk of spread. Moving youngstock off the grazing platform as early as possible will help to reduce risk also.
When testing our herds using the milk test (considerably more cost-effective than individually blood testing the whole herd); it would be sensible to test at the second herd test for the year as this will mean any positive cows are hopefully still going to be in good condition and can be sent for slaughter to get the cull value back. If we wait till late in the season there is a potential for some false positives to occur but the infected cows at that stage may be lighter than ideal and therefore unable to go to the works, at this point they will not put weight on, so they become a dead loss.
With sheep/beef and deer farms the options available consist of diagnostic blood testing and faecal testing (in some situations). For Deer Farmers, there is good info on the DINZ website.
To summarise, Johne’s can be managed but talking it through with your vet is the best way to put a plan in place and testing is part of that strategy.