Johne’s disease in calves
The usual entry point of Johne’s disease onto a farm is through the purchase of infected animals, which are not usually exhibiting signs of the disease, irrespective of what test(s) were carried out on them before arrival, or in quarantine post arrival, particularly if the animal(s) were young at the time of importation. The disease can consequently spread silently in the herd to the point where the imported animal(s) and other infected animals show clinical signs of the disease, which could be years later.
Other routes of entry include:
- imported colostrum,
- imported slurry,
- shared livestock trailers,
- use of short term conacre, (often for replacement heifers), which may previously have had (Johne’s contaminated) slurry spread on it from another farm.
Johne’s disease is not always easily recognised in a herd, often because of high culling rates, which are associated with other factors, such as high SCC, poor production, infertility, lameness etc. and also because no testing has been carried out to ascertain the level of prevalence in the herd.
Calf infection with Johne’s
The ingestion of faeces is considered to be the main entry point for the organism, with young calves thought to be more susceptible than older animals. Older cattle may become infected from the shedding of Johne’s by infectious animals in the herd, but the consensus is that the level of exposure must be very high for this to happen. Mature cows can ingest contaminated faeces, producing a short term immune response and subsequently shed the organism, but may not be truly infected in the long term.
A calf can get infected in the womb, which is largely determined by how advanced the disease is in the dam. If the disease is at an advanced stage in the dam, the estimate is ~ 1 in 3 calves get infected this way, if the infection is at an earlier stage in the dam, around 1 in 10 will get infected in utero.
Ingestion of the bacterium via colostrum/milk from an infected dam is a primary source of infection, as is infected bedding material in a calving pen, so hygiene in calving pens is extremely important. Replacement heifers that get infected in utero, via drinking infected colostrum/milk, or via the ingestion of faeces from their environment can subsequently pass the bacterium on to their herd mates through faecal shedding, (the one to many scenario) and by so doing, ensure that the disease persists in a herd and the cycle from dam to calf continues.
Summary and Recommendations
- If high risk animals are in-calf, (as determined by testing), they should be calved in a separate isolated calving pen, with the calf removed immediately post calving and fed colostrum from a known Johne’s negative cow. (Ideally all calves should be removed at calving to minimise faecal ingestion).
- Pooling of colostrum should not be practiced, as it has the potential to infect many calves from one infected dam.
- Do not import colostrum from other farms, Keep a supply of frozen colostrum from non-infected cows for emergency situations and for calves from possibly infected dams.
- Do not feed waste milk to calves and definitely not to replacement heifers. (They should be reared on their own mothers’ milk only initially and then on powdered milk). Pasteurisation of colostrum and milk does kill the organism, but may not eliminate it completely if the contamination load is high. Pasteurisation is a helpful tool to further reduce the risk in feeding low risk milk, but it should not be relied upon for high risk milk, (from test positive animals).
- Slurry/manure particularly from cows, should not be spread on fields where youngstock graze.