Hidden diseases pose major risk

The presentation by UK health specialist Fiona Lovatt at the recent Teagasc sheep conference left those attending in no doubt as to the importance of facing up to a new wave of hidden diseases. She said that so-called iceberg diseases such as OPA, CLA, Ovine Johne’s and Maedi Visna were becoming more commonly diagnosed in the UK and represent a similar threat to Irish producers.

They are referred to as iceberg diseases because of the high number of undiagnosed cases and small number of clinical cases, failing to represent the severity of the problem.

The diseases are all hard to diagnose, partly due to a slow, deceptive onset and absence of simple and accurate testing procedures, but can have significant production-limiting consequences in a flock.

The disease Ovine Pulmonary Adenocarcinoma (OPA) caused by Jaagsiekte sheep retrovirus (JSR) is characteristic of this description. JSR has been on the rise in pockets of the country in recent years, but the true prevalence is not known.

The difficulty is finding a readily available accurate method of diagnosis, with Fiona explaining that extensive research has been undertaken in the Moredun Research Centre in Scotland, but has yet to yield positive results in identifying a testing mechanism.

A recent large-scale study of the lungs of over 1,900 sheep identified 1.6% prevalence, with cases in Donegal, Kerry, Kilkenny, Offaly, Tipperary, Waterford and Wicklow.

The study estimates that the prevalence in Ireland is likely to be in line with the UK, where there is a high prevalence in geographical regions.

The symptoms are an increased respiratory rate, with sheep possessing a soft cough in the early stages of the disease. This can progress into increased respiratory problems, with sheep coming under pressure when flocked and exhibiting a frothy fluid discharge from their nostrils when their head is down.

This stems from the disease presenting as cancer in the lungs, with the incubation time generally three years, but ranging anywhere from two months to 11 years depending on the severity of the infection. A secondary bacterial pneumonia can also develop and is more common in affected flocks.

Fiona says the disease can be spread through a number of avenues, with the virus capable of lasting weeks in the environment.

It can be spread by respiratory aerosol (disease passing through the air) and contaminated troughs and water, while it can also spread to progeny via milk or colostrum.

There is no immune response, so blood serology is not an avenue of identification.

Fiona says that PCR is possible in the respiratory and peripheral blood lymphocytes, but this is not commercially available and even if it was, it is not always accurate, as it has low sensitivity and virus levels can fluctuate.

Thoracic ultrasound can also be used to identify infected animals, but again this method has shortfalls, with lesions of greater than 2cm required on the surface of the lung.

The limited avenues of identification make the disease hard to control. Suspect cases should be explored, with infected ewes and their offspring culled. Where present in a flock, housing single-age animals can help to limit its spread.

Taking newborn lambs immediately from ewes and artificially rearing has been shown to be an option and could be used for high-value animals.

Management is also an important consideration, with stocking density and feeding management tailored to reducing close contact between animals.

Another similar disease, ovine Johne’s disease, which is caused by Mycobacterium avium paratuberculosis, is also likely to be hugely underdiagnosed in Ireland and the UK.

In contrast to cattle, sheep with Johne’s do not present with characteristic symptoms of scouring and significant weight loss; the only real symptoms are sheep becoming very thin and emaciated for no apparent reason. This generally leads to higher condition-related culling in a flock.

It is also hard to identify, with blood tests available not very accurate and some are prone to returning false positives. PCR on faecal samples works OK, while the best avenue of identification is post-mortem analysis.

Fiona says the cost of tests is also another issue contributing to low flock diagnostic testing and is a barrier in eradicating the disease in a flock.

She advised farmers of the importance of at least investigating suspect cases and delivered a useful alternative management option to explore suspicious animals.

This included quarantining sheep that had lost excessive weight and giving preferential treatment. If animals failed to recover in such a scenario, then there is a greater risk of some underlying health concern which merits investigation. The disease is often identified in this manner in rams, with their higher value encouraging examination, whereas ewes are often sold without further testing.

Limiting the spread of disease in a flock through early culling of infected animals and removal of offspring is a key part of controlling disease. While expensive, central to this is regular diagnostic testing, with farmers advised to carry out blood testing or faecal sampling two to four times a year.

In terms of management, limiting lambing in wetter areas (disease can survive for nine months in water, 11 months in slurry and 48 months in soil) and practicing high standards of hygiene, along with keeping young stock away from adults, will help.

A vaccine, Gudair (Virbac), is available in the UK and has a recommendation of vaccinating all animals less than one year old and all purchased replacements.

However, take note that vaccinated animals will still shed Johne’s in faeces and therefore will present as positive for the disease, which will prevent live export.

The disease Caseous lymphadenitis, or what is more commonly known as CLA, targets lymph nodes and is best characterised by abscessation of lymph nodes.

Fiona says the most prevalent symptom is discrete swellings around the head and neck (below the ear), which can burst to discharge a thick pus. The discharge is highly contagious and can last in the environment for up to eight months in soil and 24 hours in dip.

The greatest risk of the disease spreading is by infected animals coming into close contact with healthy animals, with shearing and dipping highlighted as two particular risks.

Shearing presents a particular risk, as equipment can become contaminated with bacteria if abscesses burst or are lanced by the shears.

Infection spreads through the lymph nodes in the chest and internal organs and it is this action that causes the greatest damage. The enlargement of abscesses in the airways can also restrict breathing.

It is again difficult to diagnose and Fiona’s advice is to handle any suspect cases with care and seek veterinary assistance so that tests can be carried out.

Abscesses should not be unnecessarily punctured, with suspect animals quarantined. Control options include isolating bought-in sheep and implementing a quarantine period.

The disease is more common in older animals, with low prevalence in sheep up to a year old, while there is a lower spread in pre-weaned lambs. As such, younger animals should be checked first when carrying out surveillance to lower the risk of spreading during handling.

Fiona says there is a limited volume of vaccine imported into the UK annually. A vaccine has been developed by Moredun, but it is not yet commercially available.

Border disease was highlighted as a notable concern. It is similar to Bovine Viral Diarrhoea (BVD) in cattle, with adult sheep most at risk when pregnant.

If infected during pregnancy, it can lead to abortion or persistently infected animals being born, which will act as a reservoir for shedding virus.

Dealing with the disease is complicated by the fact that lambs can be born normal, weak or as the characteristic hairy shakers that are most representative of the disease.

Fiona advised farmers to be cautious of the disease and added that the prevalence should hopefully reduce greatly as a result of the BVD eradication programme.

Industry challenge

The iceberg diseases were highlighted as presenting a major challenge to the sheep sector, with the potential for rapid transmission through the national flock.

One of the most limiting factors in preventing disease spread is low awareness, both at farm and industry level. Fiona advised all sheep farmers to take greater precautions when purchasing sheep and to implement a quarantine procedure.

She said there is a need for the industry to promote accreditation, monitoring and consideration of the health status of flocks, while also being aware of issues raised by diagnostic challenges.

Maedi Visna, a disease not thought to be present in Ireland, is increasing in prevalence in the UK. It causes respiratory issues and progressive nervous issues, reflected in the animal dragging its hind leg.

Fiona said it is important that Irish producers are aware of the disease and that importation requirements are strictly adhered with to prevent the disease from gaining entry.