Johnes disease is quite a hot topic in Cornwall at the moment with some herds really starting to feel the effects. Whether you have it in your herd or not hopefully the following information will be useful.
Johnes is contracted by calves, 80% of infections in the first month of life. However due to the long incubation period, (average five years) signs of disease don’t show until later in life. Three years plus.
Many of these infected calves will be clinically normal carriers.
Infected cattle will generally increase the amounts of bacteria they shed as they get older and nearer to showing clinical signs.
The classic Johnes cow is thin, old and scouring. However there are often signs before this. Persistent lameness, recurrent mastitis, high cell counts.
Johnes infected animals tend to shed the bacteria intermittently especially early on which is why an animal can test positive to one test and negative to another four months later. An animal is more likely to test positive the older it gets.
NMR run a milk testing screen which is very useful for identifying which cows need which management options. This screen is best done on an individual cow level ideally three times a year. It categorises cows into reds, ambers and greens with options for all depending on numbers affected. The bulk milk test is only of limited value.
Blood testing is the gold standard but again repeated testing is the ideal.
Neither blood nor milk testing should be carried out within six weeks of a TB test.
Testing bought in animals
There is no point blood testing an animal younger than two to three years old. Remember they pick up the disease in the first few months of life but don’t start reliably shedding bacteria and therefore testing positive until they are older. If you are buying young stock ask to have the dam tested.
Control for beef herds is more limited than for dairy herds and essentially involves hygiene, hygiene and more hygiene. This is particularly important regarding young calves. Avoid faecal contamination of water and feed troughs and in particular calving areas. Avoid grazing cattle and sheep together. Johnes can survive up to a year in acid soil, alkaline soil tends to be less of a risk.
Don’t fertilise land with anyone elses slurry.
Otherwise beef herds with suspected or confirmed Johnes must think about blood testing and management based on the results of that. I will come to that at the end.
In dairy herds don’t pool colostrum, only give colostrum as a single feed from highly infected farms or even colostrum alternatives and then onto milk replacer. Hygiene around calving and keeping calves away from the dung of adult cattle.
Again the remaining management advice will be based on testing results. Dairy herds have the luxury of ‘snatch calving’ from known positive cows – ie remove the calf immediately.
For dairy and beef don’t keep the offspring of known positives for replacements.
Testing and control plans
For dairy herds the NMR scheme is a great way of full herd screening on a regular basis. Once the status of all the cows is known a management plan can be readily put in place.
For those not milk recording then a targeting blood test approach similar to what I will describe for beef farms is the way forward.
Choosing cows to blood test
This will depend to some degree on whether you have already had Johnes positive cases confirmed. There are very few herds that can confidently say they are Johnes free.
A starting point will be to select 10-20 older, thinner cows to test or even just to test cull cows. From here you will get a picture of levels and ages of infected animals. You can test the progeny of test positive animals and you may find an age range that is particularly affected and then choose to test all cattle within a certain age range.
The gold standard would be a whole herd test and this will often need repeating so for many this is not considered an option but certainly may be cost effective.
Management decisions can start to be made once you know the level and identity of cattle affected. Decisions based on which cattle to keep as replacements, which to cull, which to monitor closely for the onset of clinical signs and then cull at the first instance.
There is a vaccination available but it does have its problems. It has to be imported under veterinary license. It needs to be given ideally in the first few hours of life. Once you have started vaccinating these animals can no longer be tested for Johnes. The vaccine does create more lumps on a TB test and APHA would need to be informed but it is not impossible.
For more information please contact our vet Nicky Hallows BVMS MRCVS on 01280 813258.