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Johne’s disease can be present and actively spreading among sheep, even if there are few clinical signs of infection, an animal health expert has said.
Speaking at a National Sheep Association (NSA) webinar, Dr Craig Watkins from Moredun Research Institute said that a long incubation period and lack of accurate diagnostic testing means Johne’s often goes undetected in flocks.
The incurable disease is more commonly associated with cattle, but it can affect all ruminant livestock, causing rapid weight loss and death. Johne’s is a bacterial infection that affects the intestine and limits an animal’s ability to absorb nutrients.
Symptoms
Watkins said that although some infected animals never display symptoms, it generally takes two to four years from infection until symptoms appear in sheep. It is also often incorrectly diagnosed as other health issues in sheep, such as fluke infestation.
“For every one or two animals that are at the clinical level [displaying symptoms], there could be between 15 and 25 other infected animals. That is why we call Johne’s an iceberg disease,” Watkins said.
Transmission
Infected animals that are not displaying symptoms can still spread the disease and young animals with developing immune systems are most susceptible to picking it up.
Ingestion of the bacteria from infected faeces is the most common form of transmission, but lambs can also get it by drinking colostrum from infected ewes. Other routes include between the ewe and foetus before birth and through semen from infected rams.
Another key issue with controlling Johne’s is that the bacteria can survive (but not replicate) in the environment for prolonged periods.
“In water it is nine months, slurry it’s 11 months and in soil it is up to 47 months, although that really deepens on the type of soil. It survives longest in wet soils with a low pH,” Watkins said.
Test
“There is no single test that will reliably detect all stages of the disease and sub-clinical carriers. This means that a negative result doesn’t necessarily mean no infection,” Watkins added.
Available diagnostics include an antibody test, which only picks up the disease at an advanced stage and a PCR test that can be used on blood or milk.
Growing a culture of bacteria taken from faeces is a reliable test, but it only works at the latter stages of infection and typically takes between eight and 16 weeks to grow in a laboratory.
“For a farmer or vet, the turnaround time is too long to be useful,” Watkins said.
His team at Moredun is aiming to develop a test that will reliably detect Johne’s at all stages and work is ongoing on a new test for cattle.
NSA members were encouraged to consider using existing tests on cull ewes or poor performers to see if pre-clinical cases of Johne’s are present in their flocks. All Johne’s-positive ewes and their offspring should be culled as early as possible, Watkins recommended.
Prevention
Most steps to avoiding Johne’s are also good practice to prevent other health problems. This includes maintaining a clean environment for young lambs, washing drinkers regularly and fencing off stagnant water.
Other steps include maintaining a closed flock, or only buying replacements from a flock that has been Johne’s testing for at least three years.
There is a vaccine for the disease in sheep, which reduces shedding of bacteria through faeces, but does not prevent the infection in the first place.
However, the vaccine means the antibody blood test can no longer be used for diagnostics. The vaccine is not recommended for use in cattle, as it interferes with TB skin test.
“There are some real limitations to the vaccine, but it is relatively cheap and is a single shot,” Watkins said.
Study shows OPA scanning is effective
A more commonly known iceberg disease in sheep is ovine pulmonary adenocarcinoma (OPA).
The disease is also referred to as Jaagsiekte sheep retrovirus and it causes infectious tumours which lead to respiratory issues and eventually death.
There is no diagnostic test, treatment or vaccine for OPA.
The only method of identifying the disease is by ultra-scanning sheep to find tumours in the lungs.
Speaking at the NSA webinar, Dr Christina Cousens from Moredun Research Institute said that scanning will typically only pick up tumours that are at least 1cm in size.
“It helps reduce transmissions, and culling infected ewes out before they are sick has an economic benefit to the farmer, as well as animal welfare benefits,” she said.
A recent study into the effectiveness of OPA scanning involved a post-mortem examination of 276 sheep that were deemed OPA positive at scanning and 174 that were negative.
Scanning was found to have a sensitivity of 96%, meaning very few sheep with tumours bigger than 1cm were missed. It had a specificity of around 80%, so a positive diagnosis is usually correct, although other lung diseases can be mistaken for OPA.
Cousens pointed out that accuracy will depend on the experience of the vet doing the scanning, but early culling of sheep with other lung problems is usually beneficial.
She presented the results of another study carried out on 14 different flocks, which found regular scanning and culling was effective at reducing OPA incidence.
“The whole flock should be scanned for it to be effective. Only scanning sheep that are for sale does not get to the root of the problem,” Cousens said.
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