Ovine pulmonary adenocarcinoma diagnosis and control in practice

Jaagsiekte sheep retrovirus (JSRV) is an oncogenic retrovirus that causes ovine pulmonary adenocarcinoma (OPA). This is a progressive, fatal, bronchiolo-alveolar carcinoma of sheep (Griffiths et al, 2010), caused by virally-induced transformation of secretory epithelial cells of the distal respiratory tract (Ortin et al, 2019). JSRV can be transmitted from sheep with OPA tumours mainly by the respiratory route (Ortin et al, 2019) and in colostrum and milk, transplacental transmission has not been ruled out. There are insufficient data to determine the relative importance of these routes, and they should all be considered when designing plans to control the disease (Ortin et al, 2019).

OPA is generally considered a chronic wasting disease with progressive respiratory distress, and is invariably fatal. Initial clinical signs of the disease include weight loss, a gentle cough and tachypnoea. This progresses to marked elevations in respiratory effort while the animal is at rest; however, the animal's appetite remains good and unless there is a secondary bacterial infection, the sheep remains afebrile (Scott et al, 2013). Over-production of pulmonary epithelial lining fluid (lung fluid) is a common sign of OPA. Fluid gathers within the respiratory tract and first appears as a scant serous nasal discharge, and during the advanced stages of clinical disease may flow freely from both nostrils when the head is lowered during feeding. This quantity may exceed 50 ml if the hindquarters are raised when the head is simultaneously lowered (colloquially referred to as the ‘wheelbarrow test’, although a negative test does not rule out the disease). OPA lesions may predispose to secondary bacterial pneumonia causing sudden death despite antibiotic treatment.

The incubation period from JSRV infection to the start of tumour development is not known. Clinical disease is most commonly observed in 3–5-year-old sheep, although it is occasionally seen at less than 8 months old. Tumour growth varies considerably, but Scott et al (2018a) reported 2–5 mm hypoechoic circles at the viscera pleural, progressing to hypoechoic areas occupying the whole screen and extending to 60 mm deep within 95 to 290 days.

Diagnosis

Currently there are no serological tests available commercially because of the lack of detectable immune response to JSRV in infected animals. In the molecular blood tests that are available, sensitivity is low (Ortin, 2019); the sensitivity of a single blood test in field samples is only 0.11 (i.e. it identifies only 11% of animals that ought to give a positive test result (Lewis et al, 2011)).

In clinically-affected animals where the disease is suspected, the wheel barrow test may be performed where the animal is tipped with its back legs in the air and fluid runs from the nose and is collected in a container. Although a positive wheelbarrow test is pathognomonic for OPA, 38% of 282 OPA cases produced no fluid even in the advanced stages of disease (Cousens et al, 2009). There are also serious animal welfare concerns unless the sheep is killed immediately aft er a positive result.

Thoracic auscultation does not always aid diagnosis (Cousens et al, 2009; Scott et al, 2010) and post-mortem examination of the lungs supported by histology and immunocytochemistry remains the gold standard diagnostic test for OPA (Lee et al, 2019).

However, while post-mortem diagnosis with histological evaluation of any suspect tumours is valuable in determining disease presence in commercial flocks, it is clearly not a useful screening test to help decrease the incidence in flocks.

Cull ewe screens have been used successfully in some practices to identify ‘iceberg diseases’, especially OPA where 10 cull ewes are selected annually and blood samples are tested for caseous lymphadenitis (CLA), border disease, maedi visna (MV); pooled faecal samples for Johne's disease; and ultrasound examination of the lungs is performed for OPA. This allows flocks that have no previously-diagnosed problems to check animals annually and detect any problems on farm. Attendance at local abattoirs, if at all possible, to check on pluck for the presence of iceberg diseases, is a useful tool in clinical practice.

Thoracic ultrasonography is currently the best available technique for the ante-mortem diagnosis of OPA, and has a high level of sensitivity and specificity (Cousens et al, 2015, 2019). However, although 37% of flocks in Scotland tested positive for JSRV (Griffiths et al, 2009) only 60–90 OPA diagnoses are officially recorded from the entire UK national flock (c.14 million adult sheep) each year. However, anecdotally, the number of practitioners offering the technique to their clients has increased recently, with good attendances at OPA ultrasonography CPD courses across the country.

Ultrasound examination for ovine pulmonary adenocarcinoma

A full description of the technique for thoracic ultrasonography is provided in the companion article in this issue, as well as by Scott et al (2018).

Ultrasonographic findings

It is imperative to examine both sides of the chest because OPA lesions may only affect one lung. OPA lesions not involving the visceral pleural membrane cannot be identified during ultrasound examination.

The first suspicion of OPA is a slight increase in width and intensity of the hyperechoic line representing the normal lung surface with widespread ‘comet tails’ or B lines (Figure 1), which are likely caused by mild interlobular oedema.

When this sonographic appearance is seen it is essential to carefully scan both lungs for early signs of OPA, such as sharplydemarcated hypoechoic 5–10 mm deep areas in the ventral lung lobes displacing the heart from the chest wall (Figure 2).

A sharply-demarcated area of consolidation representing an OPA tumour extends for 3 cm into the lung parenchyma (Figure 3) and occupies the whole lung field revealing a bronchial pattern (seeFigure 4). A bronchial pattern is commonly seen in advanced OPA — patience is important to demonstrate this appearance (seeFigure 4 and Video 1 [Sheep-thor-ultra-video 1.mp4]).

The ‘hepatoid’ echogenic appearance of OPA tumour masses is highlighted in Figure 5 and Video 3 [Sheep-thor-ultra-video 3.mov] where both organs are imaged together with OPA lung pathology dorsally (to the left) and liver ventrally separated by the diaphragm, which appears as a broad convex hyperechoic line in the centre of the recording.

An examination rate up to 120–150 sheep per hour yields an immediate cost benefit in flocks with disease prevalence above 1.5%. Twenty two of 42 commercial flocks (median size 700) had an OPA prevalence rate greater than 2% at first scanning with seven of these flocks greater than 4% OPA prevalence (Scott et al, 2019).

Authors' practical tips

The authors have a number of practical tips for diagnosis in practice:

• Do not interpret liver as OPA on the right hand side, check position of probe and ensure no biliary vessels present on the image
• Use the position of the heart base to orient yourself, particularly if scanning in an unusual position
• Ensure very good contact (pre-wetted skin and ultrasound gel) between the ultrasonography scanner probe and the sheep's skin in order to get the best image
• Avoiding scanning around the time the farmers have been organophosphate dipping — this may be a popular time with farmers as they are handling the sheep, but it can be very detrimental to the operator's health
• Try to time OPA screenings close to a time when the farmer will be able to make management decisions. Some flocks scan at prehousing for lambing as this is when they will have most of the sheep available (not to sell when late in lamb but the sheep can be lambed separately and kept away from non infected sheep); lesion identification at this stage may reduce transmission during housing. Weaning time is another natural point in the husbandry/production cycle when culling decisions can be made
• It is essential to confirm ultrasound diagnoses of OPA either on farm, at the slaughterhouse or knackery. Histopathology confirmation is essential — even board-certified pathologists mistake other lesions for OPA and vice-versa (Lee et al, 2017)
• Good communication is essential. Make sure that the farmer knows exactly what you are trying to achieve. Serial ultrasonography has reduced flock prevalence in almost all flocks and allowed the sale of affected animals at a good cull price. While prevalence has been reduced by up to 80–90% after 4 years in some flocks, elimination of OPA has yet to be achieved
• After identification of other lung pathologies, scanning every 1–2 weeks allows antibiotic treatment of bacterial infections causing lung consolidation and/or fibrinous pleurisy to be monitored
• Recorded lesions may be referred to colleagues for second opinions if the diagnosis is unclear (e.g. using telemedicine)
• Where doubts exist concerning the accuracy of diagnosis of small lesions these sheep should be isolated and re-scanned 4–6 weeks later; however, few farms have robust biocontainment facilities.

Control of the disease in flocks

There are no vaccines available for OPA, so control methods have to be based on other strategies. Efforts to eliminate OPA from sheep flocks in the UK have targeted sheep in poor condition showing exercise intolerance and an increased respiratory rate, but experience from general veterinary practice indicates that such generic culling has proved largely unsuccessful in that disease persists in many flocks.

Control methods based on colostrum and milk management have been demonstrated to be effective, but cost prohibitive for most commercial flocks (Ortin et al, 2019). This involves creating a new flock with lambs separated from ewes at birth with feeding of heat-treated colostrum and milk following the principles of MV control (Voight et al, 2007).

During discussions with other practitioners, some have suggested advising clients to simply maintain a young flock in order to control clinical disease, but the peak age prevalence is reported to be 3–4 years. In addition, this strategy will increase replacement costs and unless animals are sold direct to slaughter, the draft ewes may go on to introduce clinical disease into new flocks.

So far the best strategies for controlling the disease have been quarantining, cleaning, disinfection of contaminated areas and removal of affected animals and their offspring from the flock when the disease is detected (Voight et al, 2007). The Icelandic eradication programme indicated possible routes of reinfection that need to be identified in a control programme are infected ova or sperm cells, intrauterine transmission, perinatal infection in the contaminated maternal environment, persistence of the virus in the cleaned and disinfected sheep shed, or the reintroduction of the disease by rams, personnel or other vectors (Voight et al, 2007). Consideration of other potential risk factors for transmission (e.g. stocking densities, trough feeding, lick buckets) is also important. In Iceland when eradicating the disease they used the presence of OPA in a flock as a ‘flock test’, in that the whole flock was killed not simply the infected individual. This principle has been successfully used for bTb control in Australia.

Early results from biannual ultrasound screening of 42 flocks show a reduction in OPA prevalence after 1 year (Cousens et al, 2009; Scott et al, 2018b) with these reductions repeated in years 2, 3 and 4 (unpublished data). With appropriate handling facilities, an experienced veterinary operator can examine both sides of the chest of up to 150 sheep per hour, making such screening affordable even for commercial value (non-pedigree) sheep. In addition to controlling risk factors for transmission, ultrasonography therefore appears to be the most effective approach for controlling OPA in commercial flocks. The case studies below illustrate how this technique has been employed successfully by a mixed-animal practitioner (DH) in commercial flocks in North Yorkshire.

Case studies

The following farms are clients of a mixed practice in North Yorkshire and the scanning is performed by a mixed practitioner with an advanced practitioner certificate in sheep health and production.

Farm 1

Farm 1 was a 1500 ewe flock (Figure 6), which had a high cull rate due to ‘pneumonia’ as diagnosed by the farmer. When a flock health plan was implemented on the farm, it was discussed how this rate was too high (30%) and it was decided to investigate these deaths/culls further. Post-mortem evaluation revealed hard lesions in the lungs of one sheep that had died. This was sent for histological diagnosis and OPA was confirmed.

It was decided that because of the hill grazing nature and some sheep lambing outdoors that lamb capture and milk/colostrum management was not possible. As a result of possible damage to reputation as a breeding unit, selling out the flock at a young age was also decided as not practical, so it was determined that ultrasound diagnosis would be performed to control the disease (Figure 7). This is performed twice yearly in the flock and 500 animals are scanned at a time at an average rate of 100 per hour, as described in Table 1.

It has been discussed how the disease is still prevalent, but this farm has shared grazing with a neighbour who will not invest in control, so elimination (eradication is the term used for national programmes) may not be possible on this farm.

Farm 2

Farm 2 was a 650 ewe ‘open’ flock (Figure 8), where thin ewes had been a concern for some time; 10 sheep were presented for the cull ewe screen, and sheep had OPA lesions detected on ultrasound (Table 2). Interestingly in the case of farm 2, 50% of all the ewes detected as having OPA lesions present were purchased from the same farm, so farm 2 made the decision not to buy ewes from this flock again.

Case discussion

In both case studies, ultrasound evaluation of the lungs has been important in reducing the annual incidence of clinical disease on farm as well as control. The limitations of scanning was communicated to the farmers, and they both feel that the scanning is beneficial as affected animals can be walked off the farm and a cull ewe price obtained for them, as well as reducing spread within the flock, and minimising the economic impact of the disease. Both farms are continuing to scan, farm 1 every 6 months and farm 2 every 12 months. In both cases, when ultrasound evaluation was started, post-mortem examination of some animals was also performed to confirm the presence of the disease and that lesions were where they were expected to be.

Conclusion

OPA is manageable on farm although eradicating is currently very difficult. The management of the disease however, needs careful discussion with the client before undertaking.

KEY POINTS

• Ultrasound evaluation is a useful tool in the diagnosis and control of ovine pulmonary adenocarcinoma (OPA) but requires open communication with the farmer.
• If ultrasound is being performed then post-mortem/abattoir evaluation of the lungs with subsequent histology is essential.
• Management of lambs removed from ewes at lambing is a control method for OPA.
• Quarantine, cleaning and disinfection, as well as removal of affected animals, is the ultimate control plan.