As the most common iceberg disease in the UK sheep industry, increased awareness of ovine Johnes disease at a farm level is key to managing the disease. Katie Fallon reports.
Ovine Johnes disease (OJD) is recognised as one of the five important iceberg diseases affecting the UK sheep industry. However, the impact of the disease has received a relatively small amount of attention in comparison to its equivalent in cattle.
Dr Peers Davies, reader in livestock health and welfare at the University of Liverpool, has been heavily involved in research surrounding the disease and has been actively involved in raising both veterinary and farmer awareness of OJD.
While there is still little awareness surrounding the disease, studies have shown awareness has been increasing over recent years.
A survey conducted in 2014 found only 2 per cent of sheep farmers surveyed viewed OJD as a potential threat to their flocks.
Whereas a study completed in 2021 found 20 per cent of farmers viewed the disease as a threat, showing a gradual increase in knowledge.
A separate study from 2017-2018 found out of 62 flocks, 64 per cent were infected with the disease.
Dr Davies says: This makes it by the far the most common iceberg disease and more than double the number of flocks infected than the next most common which is Border disease. The other iceberg diseases are all in single figures in terms of flock prevalence.
So this begs the question, how have we got a disease which is so common, but so few people know about it.
He explains it boils down to one of the key biological facets of the disease, which differentiates it from cattle.
The symptoms of Johnes in cattle include weight loss and scour. However, a very small number of sheep with OJD will scour.
Dr Davies says most sheep will lose weight just like a cow, but they will not scour, which is why it is assumed something else is causing the problem, resulting in the sheep being culled without any diagnosis.
He says: Johnes is a disease which saps energy and causes loss of body condition, resulting in the animal being in a negative state of energy balance.
The first thing any animal does when experiencing a negative energy balance is sacrifice its reproductive system. Therefore, one of the first symptoms is ewes scanning empty, however, it is already common for empty ewes to be culled after scanning, without a cause being identified, resulting in the disease going undetected.
A further study carried out by the University of Liverpool, in collaboration with Hybu Cig Cymru in Wales, sampled pregnant and empty ewes from the same flocks to look at the proportions of Johnes-infected animals between the two groups.
Dr Davies says: We found was a substantially higher proportion of infected ewes in the empty groups, meaning it is disproportionately related to ewes being empty.
Dr Davies says an inevitable consequence of these findings is that flocks with OJD are culling a higher proportion of empty ewes and running at replacement rates of up to 30 per cent. This is compared with uninfected flocks, which are typically running at about 20-24 per cent.
He says: It means you are forced to run a younger flock, which is less productive as the optimum productive years for a sheep are really three, four and, if well managed, five, therefore you are sapping the productivity of the flock.
We need to be squeezing another year out of our breeding flock for it to be efficient; otherwise it is an unacceptable cost.
Managing the disease
To effectively manage OJD within the flock, Dr Peers Davies highlights two key areas as rough benchmarks which should be monitored.
He says: If you are at the stage where your rate of empty ewes is at 4 per cent or higher, or your replacement rate is higher than 20 per cent on average, the possibility of Johnes should be looked into.
Dr Davies advises manual body condition scoring (BCS) of ewes at tupping time. He recommends going through the adult flock and screening anything which is thinner than a BCS of two.
He says: If it is a full-mouthed ewe which is thinner than two it needs to be tested, but if it is a broken mouthed ewe which is thinner than a two, I would put it down to old age.
In terms of testing, Dr Davies says farmers and vets need to identify if a flock has the disease or not, rather than diagnosing individual ewes. This is because the disease needs to be managed as a whole flock rather than in individual animals, as is the case in dairy herds. As a result, the testing used in sheep is different to what is commonly used to control Johnes in cattle.
There are two testing options available: blood testing and faecal testing. Dr Davies says faecal tests look for the bacteria, whereas blood tests look for the antibodies of Johnes in the sample.
He says blood testing is the cheaper option, but is unsensitive, whereas the faecal tests are more likely to detect the disease. A faecal test costs about 36 for a pool of five ewes, whereas a blood test is 5/ewe.
Vaccination
In terms of managing the disease, Dr Davies highlights vaccination as an effective control method which has been used for decades in Australia and Spain.
He says: Even though the disease is incredibly common and incredibly unrecognised, it is one of the most straightforward of the iceberg diseases to detect and manage.
Kate Ingram, technical ruminant adviser for Virbac, says vaccination will not eradicate the disease, but should be viewed as a management tool to be used in conjunction with other management strategies.
She says other control strategies to use alongside the vaccine are aimed at minimising the exposure of lambs to the faeces of shedding ewes, which are the cause of the infection.
These include lambing high risk ewes away from lower risk, younger ewes to reduce faecal transmissions and reducing the potential of cross contamination from cattle by not spreading slurry on pastures to be grazed by lambing ewes or young lambs.
Ms Ingram says: Strategies used in the dairy industry, such as snatch calving, are just not practical on sheep farms, so vaccination remains one of the most effective ways of controlling the disease.
While the vaccination does not guarantee the animal will not become infected later in life, studies have shown it can delay the onset of clinical disease and the onset of shedding by up to 12 months.
Ms Ingram says the vaccine prolongs the productive life of the animal, while reducing mortality rates and the amount of bacteria shed, which is important for minimising the risk to the next generation.
She says: The aim is to get an extra crop of lambs out of every ewe, which is where the cost benefit of the vaccine comes into play, as well as being valuable in protecting future generations.
The vaccine is a long-term strategy for sheep farmers, who would be vaccinating their replacement animals every year.
The vaccine is licensed for use from four weeks old up to six months old, with a single dose required per animal across its lifetime.
Sheep are only required to be vaccinated once, at a young age, as this is the age at which they are most susceptible to infection. The benefit of vaccinating beyond six months old is unknown.
Although an effective control, the vaccine has not been widely adopted in the UK due it being a relatively new vaccine under UK license. However, Ms Ingram says its use has increased over recent years.
She says: There is an increasing industry wide focus on iceberg diseases, with it being a priority in the Governments Animal Health and Welfare Pathway. There are more conversations happening in farmer meetings and veterinary practices.
