UK Vet Livestock - Diagnosis of respiratory disease in adult cattle using ultrasonography

Abstract

This article features video recordings to emphasise the vague presenting clinical signs of chronic respiratory disease affecting adult cattle, the common sonographic findings, and response to correct antibiotic therapy in selected cases. Identification and quantification of lung and pleural pathologies can be readily achieved within 1–2 minutes using a 5 MHz linear transducer (‘rectal probe’).

In veterinary textbooks wheezes are described as prolonged musical sounds that usually occur during inspiration, and occasionally throughout the breath cycle, resulting from vibration of airway walls caused by air turbulence in narrowed airways. Crackles are loud, explosive, short duration (typically 10–30 mseconds), non-musical, ‘rattling or bubbling’ sounds. Crackles are thought to be caused by air bubbling through, and causing vibrations of respiratory secretions within the larger intrathoracic airways, including those that are pooling within the dependent part of the rostral thoracic trachea.

A recent article in The Veterinary Record commented on the generally poor diagnostic performance of lung auscultation as a confirmatory test for pneumonia in cattle (Pardon et al, 2019), which could concern farmers and potentially lead to inappropriate use of antimicrobials. The authors further suggested that improvements in diagnostic accuracy could be made by using lung ultrasound examination as a reference test. These findings were not surprising because previous articles had shown the inability of chest auscultation to identify and quantify lung and pleural pathologies in sheep (Scott et al, 2010) and adult cattle (Scott, 2013). An accurate assessment of respiratory pathology is essential to establish the likely cause, select an appropriate antibiotic and offer an informed prognosis. The potential application of ultrasound examination in adult cattle is clearly demonstrated in Figures 1 and 2, and video recording [Video 1], taken before and after administration of the correct antibiotic.

Figure 1. Holstein cow at referral following a 3 day course of marbofloxacin and meloxicam. Figure 2. The same cow 6 weeks later after a change in antibiotic therapy to penicillin [Video 1]. Similar cases are shown in Videos 2 and 3.

Video 1

cow 6 weeks later after a change in antibiotic therapy to penicillin

Video 2

cow 6 weeks later after a change in antibiotic therapy to penicillin

Video 3

cow 6 weeks later after a change in antibiotic therapy to penicillin

During auscultation of the chest it is important to recognise that attenuation of heart and lung sounds results from lung consolidation and significant pleural effusion in cattle, and large (>4 cm diameter) pleural abscesses and >1–2 cm fibrinous pleurisy, which are common pathologies in sheep (Scott, 2017). In many cases of respiratory disease, reduced/absent lung sounds are clinically more significant than increased adventitious lung sounds. Auscultation can lead to an overestimate of lung pathology when there is muco-pus within the larger dorsal airways, which generates loud crackles audible over much of the lung field. Simultaneous ultrasonography and auscultation highlights the limitations of the latter in identifying and defining lung pathology, and such comparison can be readily undertaken in practice.

Clinical presentation of chronic respiratory disease

Ultrasonography is essential to differentiate between many of the common causes of reduced milk yield, poor appetite and weight loss in early lactation including liver abscesses, peritonitis, traumatic reticulitis, chronic pneumonia, endocarditis and pyelonephritis, as many of these conditions cause pain (Figures 3–5). The clinical signs associated with many infectious diseases may be further complicated by painful foot lesions causing lameness. Reduced feed intake during the early post-partum period may predispose to left displaced abomasum [Video 4].

Figures 3 and 4. Heifer and cow present with poor milk production and weight loss caused by recrudescence of chronic respiratory disease after calving. These animals appear dull and disinterested in their surroundings. A nasal discharge is not always present in chronic pneumonia. See Video 5.

Video 4

Video 5

Figures 5 and 6. Note the cows’ stance with the neck extended, head lowered, and the ears directed caudally suggestive of pain. These cows are dull and disinterested with poor milk production and low body condition scores suggestive of chronic disease but which one?

Ultrasound examination of the chest

A 5 MHz linear transducer, routinely used trans-rectally for early bovine pregnancy diagnosis, works perfectly well to examine the pleurae and lungs in both sheep and cattle. While field depth is restricted to 10 cm, and lesions such as pleural abscesses may extend beyond 16–20 cm from the chest wall in both species, this limitation does not adversely affect decisions relating to either diagnosis or prognosis. Where possible, video recordings should be captured as mp4 files using Elgato Video Capture software (www.elgato.com), which permits accurate monitoring of lesions and reflective learning should necropsy results become available later.

Where necessary, a 5–7 cm wide strip of hair is shaved from both sides of the thorax extending in a vertical plane from the point of the elbow to the caudal edge of the scapula (visible in Figures 4, 6–8). The skin is soaked with warm tap water and ultrasound gel liberally applied to the wet skin to ensure good contact; alcohol is not needed and can be irritant. The transducer head is firmly held in a vertical plane against the skin overlying the 5th to 7th intercostal spaces. The probe head is moved from the dorsal (normal) lung ventrally to below the point of the elbow. This examination routine shows normal lung then pathology that has a cranio-ventral distribution in almost all cases.

Figures 7 and 8. The neck is outstretched with the head held lowered and the ears directed caudally with a painful expression. The cows are emaciated; the abdomen is very drawn-up with an obvious sunken triangle behind the rib cage caused by poor rumen fill. The coat lacks its normal sheen and is covered with dried faeces (left). Both Holstein cows are suffering from recrudescence of chronic pneumonia following the stress of calving, although the disease process has not been detected early enough and/or treated correctly. See Video 6.

Video 6

The sonograms are presented with the probe head/chest wall at the top of the image, dorsal is to the left and ventral to the right of the image. Centimetre markers are displayed on the margin of the images and should be consulted to ascertain the depth of field presented. An air interface, created by aerated lung parenchyma reflects sound waves and appears as a bright white (hyperechoic) linear echo (Figure 9). The area visualised below the linear echo, including any reverberation artefacts, does not represent lung parenchyma. Comet-tail artefacts (Figure 10) represent a series of closely spaced discrete echoes indicating the focal accumulation of a small amount of highly reflective material, often gas bubbles (Babkine and Blond, 2009), and are of no clinical significance. It is much easier to interpret ultrasound findings in video recordings than images, and readers are encouraged to study the recordings and compare them with lung pathology shown at necropsy.

Figure 9. The surface of normal aerated lung (visceral or pulmonary pleura; white arrows) is characterised by the continuous hyperechoic (white) linear echo 1.5 cm (chest wall) from the top of the image. 5 MHz linear scanner. Figure 10. Unlike sheep, ‘comet tails’ (white arrows) are commonly observed in cattle arising from the lung surface and are of no clinical significance. 6.5 MHz microarray probe.

Ultrasonographic examination of the chest

While pleural effusion associated with right-sided heart failure and cardiac tumours cause much reduced lung and heart sound ventrally, there is obvious peripheral oedema that would be detected during the clinical examination. Significant accumulations of pleural exudate is rare in cattle and associated with per-acute disease (Scott, 2022).

Chronic suppurative pneumonia/bronchiectasis in cattle

As the probe head is advanced ventrally from normal lung tissue present in the dorsal lung field, the first ultrasonographic changes attributed to chronic bacterial pneumonia in cattle are hypoechoic ‘columns’ extended 2 to 6 cm from the visceral pleura bordered distally by bright hyperechoic lines representing lobular consolidation (Figure 11). Moving the probe ventrally, the columnar appearance of lung pathology in the dorsal lung field changes to a very large hypoechoic area representing lung consolidation which extends down to the ventral lung margin (Figures 14–20). The consolidation extends horizontally 8–10 cm from the visceral pleura to involve the full depth of lung. Discrete well-encapsulated 1–3 cm diameter abscesses can be identified within consolidated lung in some cases but are not often widely distributed. The significance of such abscesses to the clinical presentation is further limited by the abscess capsule. Active bacterial infection is much more often present within the airways with pus exuding when the lung is compressed at necropsy (Figures 21 and 22). Bronchiectasis is the term given to permanently-dilated small bronchi and bronchioles located in the ventral parts of the lungs (Figures 20 and 22), which contain a wide range of microorganisms, predominantly Trueperella pyogenes, causing chronic disease. While antibiotic treatment of encapsulated lung abscesses would have no clinical benefit, treatment of bronchiectasis often yields a very good response (Figures 1 and 2, 23 and 24) with much improved demeanour and appetite within 1–2 days. Dectection of halitosis on clinical examination, a productive cough and purulent nasal discharge indicate likely bronchiectasis.

Figure 11. A hypoechoic ‘column’ (arrows) extends 2 cm from the visceral pleura bordered distally by bright hyperechoic lines (5 MHz linear scanner). This area represents lobular consolidation (Figures 12 and 13). Figure 12. Necropsy reveals a lobular pattern of lung consolidation dorsally. The dark purple areas ventrally (dorsal margin delineated by white arrows) representing lung consolidation contrast with normal inflated (raised) pink lung lobules (black arrows). Figure 13. Cross section of junction between normal lung lobule (black arrows) and pathology revealing lobular consolidation (white arrows) with pus within sectioned airways.

Figures 14 and 15. Sonograms demonstrating the extent of lung pathology which is sharply demarcated from normal lung dorsally (black arrows). 5 MHz linear probe. Figure 16. Sharply-demarcated dorsal margin of lung consolidation (black arrows) typical of severe bronchiectasis (lung shown at necropsy in situ). Also note the absence of pleurisy.

Figures 17 and 18. There is sharp demarcation in these images between the normal hyperechoic (bright white) visceral pleura dorsally (to the left; black arrows) and lung consolidation ventrally (to the right) represented by the large hypoechoic area. 5 MHz linear scanner. Figure 19. Consolidation (‘hepatoid change’) at the ventral lung margin displacing the base of the heart from the chest wall. 5MHz linear scanner. There is excess pericardial fluid. See Video 7 and Video 8.

Video 7

Video 8

Figure 20. Sharply-demarcated bronchiectasis ventrally (dorsal margin marked by black arrows) demonstrated at necropsy. See also Figure 16. There is also some fibrous pleurisy in this case. Figures 21 and 22. Consolidated lung with pus expressed from the airways (black arrows) in sections of an affected ventral lung lobe. See Video 9.

Video 9

Figures 23 and 24. These images feature the same heifer before treatment (left) and 10 days later (right). Note the much improved demeanour and full abdomen (right). The coat is much cleaner (right) from self-grooming.

The dorsal margin of lung consolidation (Figures 12 and 16) can be measured vertically from the point of the olecrannon to quantify lung pathology. As a general guide, in a 600 kg cow the prognosis is considered poor when the sonographic changes representing chronic pneumonia extend more than 40 cm above the level of the olecranon when values for both sides of the chest are added together (Scott, 2013). Be aware that lung pathology defined ultrasonographically may change little during antibiotic treatment despite marked clinical improvement; antibiotics administered in bronchiectasis cases treats bacterial infection within the airways and lung consolidation/fibrosis may not resolve. Penicillin is selected as the antibiotic of choice because of its action against T. pyogenes (Galán-Relaño et al, 2020). Antibiotics should not be selected on the basis of the suspected role of Pasteurellae spp. A critically important antibiotic, such as marbofloxacin, should never be considered as an appropriate choice in chronic respiratory disease. The action of penicillin is time-dependent and an extended course of treatment is necessary.

Prevalence of chronic respiratory disease

There are no large scale studies reporting the prevalence of chronic respiratory diseases in UK herds. The extensive pathology observed at necropsy in some cases indicates that cows may fail to show clinical signs until disease is at an advanced stage, however, many cases can be successfully treated if correctly diagnosed and treated with an effective antibiotic. A survey of gross pathological conditions in cull dairy cows at slaughter in USA revealed 10.3 and 23.5% had severe and mild lung lesions, respectively (Rezac et al, 2014). Such lung pathology could be readily and accurately detected by trans-thoracic ultrasonography taking only 1–2 minutes. Chronic pneumonia was also one of the more common causes of death in a necropsy-based study of dairy cows in Colorado (McConnel et al, 2009).

Conclusions

Ultrasonography is essential to differentiate between many of the common causes of reduced milk yield, poor appetite and weight loss in early lactation cows including liver abscesses, peritonitis, traumatic reticulitis, chronic pneumonia, endocarditis and pyelonephritis. A 5 MHz linear transducer, routinely used trans-rectally for early bovine pregnancy diagnosis, works well for examination of the lungs and pleurae and trans-thoracic ultrasonography takes only a few minutes.

KEY POINTS

Chest auscultation fails to accurately detect and define lung pathology.
Attenuation of heart and lung sounds can also result from pleural effusion.
Ultrasonography is essential to differentiate between many of the common infectious causes of reduced milk yield, poor appetite and weight loss in adult cattle.
A 5 MHz linear transducer, routinely used trans-rectally for early bovine pregnancy diagnosis, works well to examine the pleurae and lungs.
Procaine penicillin is the drug of choice in cases of bronchiectasis caused by Trueperella pyogenes.