Respiratory disease is common in sheep, and is a significant constraint on production across many varied management systems (Lacasta et al, 2019). Thoracic radiography is impractical and costly, and the reliability of auscultation in diagnosing pulmonary pathology in people, cattle and sheep, is questionable (Wipf et al, 1999; Scott, 2017; Pardon et al, 2019), respectively. However, thoracic ultrasonography has been shown to be an effective technique in the control of ovine pulmonary adenocarcinoma (OPA), in addition to identifying and monitoring other thoracic pathologies at the point of care (Scott et al, 2013; Scott, 2017; Scott and Cousens, 2018). In this article, the authors share their opinions and advice on building experience with thoracic ultrasonography in sheep.
Scanner selection and set-up
It is possible to obtain diagnostic quality images using linear and curvilinear rectal-style transducers; however, the shape of the probe is difficult to hold and maintain contact with the convex surface of the chest. Better quality images are achieved, and with greater ease, by using a micro-convex array transducer. It is difficult to achieve diagnostic images using a sector scanner such as those used for ovine pregnancy diagnosis. The authors commonly use transducers with 6.5 MHz frequency, using the pre-programmed settings for the canine abdomen, with the gain increased to 100. The depth is initially set at 6 cm, although this may need to be increased to completely visualise large lesions.
Handling, restraint and positioning
As with many farm animal veterinary jobs, the efficiency of the day's work is maximised by a good handling system that allows large groups to be collected before being moved quickly into and through a race system. For ultrasonography, this should ideally be covered to protect the equipment and operators from rain, and to reduce the glare of sunlight on the viewing screen. These considerations are less important if using a rectal-style scanner with viewing goggles.
For lung scanning, the sheep's ipsilateral foreleg is flexed at the elbow and drawn forward to image the cranioventral lung field and base of the heart. This area is fleeceless and can be scanned without clipping. Some clipping of the fleece would be required to image the dorsal lung field. However, this is not commonly performed by the authors in a pragmatic compromise between sensitivity and costs, and avoidance of Muellerius spp. lesions.
Handling systems with conveyor belts or clamps allow for the easy, calm restraint of sheep with just one operator; however, in order to gain adequate access to the axilla, the front third of the animal protrudes from the clamp or conveyor. This then requires an assistant to support much of the animal's bodyweight when the leg is drawn forward, which rapidly becomes tiring. In the authors' experience, commercially available handling crates are generally more of an encumbrance than a help, as framework gets in the way of the sonographer and it is difficult to extend the ipsilateral leg to expose the axilla.
The authors have no experience using turnover crates but they may present a significant welfare concern if used in animals with OPA as the surfactant gathered in the ventral lung moves the unaffected dorsal lung. Where fixed restraint systems are used the operator must move from side to side to visualise both sides of the chest. This is fine if using viewing goggles, but can be more challenging if using a screen, although one of the authors has had some success on a concrete yard by strapping the ultrasound unit and screen to a wheeled office chair.
As a result of these difficulties, the authors prefer manual restraint of the sheep against a hurdle, with one handler at the chin and another at the rump. The scanner can then be placed on a table behind the hurdle out of harm's way. Sheep are turned 180° by the handlers in order to view the opposite side; this set-up is illustrated in Figure 1. This system works very effectively, although it does require two handlers in addition to staff moving animals through the race. Figure 2 (and Video 1 online) shows a custom modified handling system mounted on a turntable, which may be considered as an alternative.
Performing the scan
The transducer head is firmly held at right angles against the skin overlying the intercostal muscles of the 4–7th intercostal spaces. The starting point is approximately 2–5 cm above the point of the elbow when the foreleg is held horizontally. This point orients the operator to the junction between the lung and the base of the heart. The probe head is moved 2–5 cm dorsally to image lung only then cranially then caudally over adjacent intercostal spaces at this level. The probe head is then lowered by about 5 cm and moved into adjacent intercostal spaces to further examine the ventral margins of the lung lobes especially caudally.
When the diaphragm is reached, the image appears to swipe from left to right between thoracic images and abdominal images — this occurs as the diaphragm contracts and relaxes with respiration. The liver is commonly seen on the right-hand side, with the reticulum/rumen seen on the left. Images obtained cranial to the heart should be interpreted with care so as not to confuse normal mediastinal tissue with pathological lesions.
Identifying lesions
Normal pleural surface and reverberation artefacts
The ultrasound waves are reflected at the visceral pleura (lung surface) if the lung immediately deep to it is aerated. This causes the visceral pleura to appear as a continuous bright white (hyperechoic) line moving in time with respiration. Any lesions deep to aerated lung will not be identified by ultrasonography.
Equally-spaced reverberation artefacts are often seen in sheep in poor body condition, and an artefactual reflection of the curve of the transducer head can sometimes also be seen below the pleural line. This artefact is also common when the transducer is overlying a rib, rather than the intercostal space. Normal pleural surfaces and reverberation artefact are shown in Figure 3.
Ovine pulmonary adenocarcinoma and B-lines
Lesions associated with OPA result in the loss of the hyperechoic line at the visceral pleura are usually sharply-demarcated, and may appear similar in echogenicity to liver (‘hepatoid’). These are often wedge shaped and are commonly found in the ventral lobes, displacing the myocardium away from the body wall. In advanced cases, the bronchial tree may be seen at certain angles. These lesions are described in greater detail in the companion article (Hinde et al, in press), as well as Scott et al (2018). B-lines (also described as ‘comet tails’ or ‘sun beams’) are often seen at the pleura dorsal to the larger lesion, potentially due to alveolar/interlobular oedema, and are often the first signs seen by the authors when scanning animals with OPA. Examples of these lesions are shown in Figure 4.
Pleural effusions and pleurisy
Pleural effusions are seen as an anechoic/hypoechoic area separating the parietal and visceral pleura. They accumulate ventrally under the influence of gravity and may be unilateral owing to the complete mediastinum. In severe cases, the collapsed ventral portion of a lung lobe can be seen floating in this effusion. Pleurisy may have a similar ultrasonographic appearance but the fibrin content gives the fluid a more hyperechoic, heterogenous appearance and large fibrin clots may be present. Examples of images obtained from these conditions are shown in Figure 5.
Pleural abscesses
These are commonly found cranioventrally, presumably as a sequela to a previous fibrinous pleurisy. They are rounded in shape, often with a thick hyperechoic capsule and anechoic contents with hyperechoic foci caused by microscopic gas bubbles. These abscesses can be very large (up to 16–20 cm diameter), occupying most of one half of the thoracic cavity in some cases. Adjusting the field depth aids their identification, as has been performed in Figure 6.
Pulmonary consolidation and abscesses
The bright hyperechoic line of the visceral pleura is interrupted, with deeper areas of mixed echogenicity representing areas of consolidation, and even fibrosis. These may contain encapsulated abscesses. Examples of these lesions are shown in Figure 7; however, these lesions can be extremely difficult to distinguish from those caused by OPA, especially OPA with secondary infection and abscessation. Necrotic centres to tumours may also confuse this picture. Medical management and isolation, followed by repeat ultrasonography may be considered; although owing to the increased labour required to treat animals in isolation and the guarded prognosis for full recovery, clients may often choose to cull such animals. The difficulty in distinguishing these lesions from those caused by OPA emphasises the need to confirm a proportion of diagnoses by necropsy and histopathology.
Recording lesions
It is essential to record lesions as this allows one to monitor the progression of borderline cases and assess responses to treatment of pleurisy, abscesses and pneumonic consolidation. Reviewing ultrasound findings to compare with necropsy findings is an important learning experience; such reflection is only possible with high quality ultrasound recordings. Recording lesions is also advisable as they may be useful in cases of legal dispute.
Still frame images can be transferred from many portable rectal scanners by cable or, on newer models, wirelessly. However, it can be difficult to capture a lesion in a single frame, and the authors prefer to use short video clips. Many scanners have the ability to record short clips which may be transferred wirelessly or via USB memory stick, but it can be difficult to capture these clips while simultaneously scanning. It is easier to use a laptop connected to the scanner via S-video or VGA cable adaptors and record brief MP4 files using video capture software. When a lesion of interest is identified the operator can stop scanning, quickly set the software to start recording, and then scan the lesion again. The authors have had positive experiences with Elgato software and connectors, although many other brands are available. Telemedicine allows rapid expert interpretation of uncertain diagnoses.
Building numbers and speed
Scott (2017) demonstrated that in an infected flock, the costs of scanning for OPA at a rate of 50–80 animals per hour charged at £70 per, were met by the value obtained by selling preclinical sheep fat before they deteriorated alone, without considering benefits to production. This rate of scanning may seem intimidating at first, but the authors have found that such rates can be quite quickly achieved, or even exceeded (up to 160 sheep per hour), with experience and good handling systems.
Initial experience can be gained with smaller flocks or by scanning groups of bought-in animals while visiting the farm to collect blood samples for quarantine serology. The authors have also gained significant experience using small groups of cull ewes in very poor body condition, which have then been necropsied. This approach is commonly undertaken at the University of Edinburgh's Farm Animal Hospital, and at CPD events across the country. However, this approach can also be used in the field, with on-farm post mortem examinations giving opportunity to correlate findings with gross pathology, to sample for histopathology and to investigate for other causes of weight loss. This confirmatory process is vital for building experience in interpreting images, improving self-confidence, and gaining the confidence of the client.
Conclusions
Thoracic ultrasonography is a useful technique for diagnosing and monitoring pulmonary and pleural pathology. Experience can be gained quickly in practice, particularly if used alongside post-mortem examination and histopathology to confirm diagnoses.
KEY POINTS
- Respiratory disease is a common condition in sheep, and thoracic ultrasonography is a useful diagnostic tool in commercial practice.
- A variety of affordable commercially-available ultrasound machines may be used to obtain diagnostic images.
- Good handling and restraint are key to building speed when scanning; however, these can be achieved with basic facilities on farm.
- Post-mortem examination, followed by histopathology, is an important tool for confirming diagnoses and building confidence in the thoracic ultrasonography.