By Theresa L. Ollivett, DVM, PhD, DACVIM, University of Wisconsin-Madison School of Veterinary Medicine
BRD is economically challenging, subclinical disease exists, and producer based diagnoses lack in sensitivity. Therefore, monitoring BRD in young cattle should be considered a priority for maintaining proper drug use, animal well-being, and profitability. Incorporating thoracic ultrasound (TUS) at regular intervals will not only provide an understanding of the epidemiology of BRD in client herds but more importantly will help identify problems before they become catastrophic.
The first step in monitoring BRD at the herd-level with TUS is to acquire a baseline of the calves at risk (on a small farm) or a random subset of calves at risk. Eight to 12 calves could be a minimum number, although sometimes difficult to obtain in small herds, based on what has been recommended to assess herd metabolic health. A larger sample might be required when assessing an ongoing problem. Based on this information, along with clinical scoring, the distribution of the BRD subtypes, age of onset, duration of disease can be determined.
The competency of animal care personnel can be assessed by calculating detection rates and the proportion of calves scoring greater than “3,” on a 6-point ultrasound lung-lesion scoring system, at first treatment. Clinical detection rates are calculated by comparing the number of calves with pneumonia to daily treatment records, and should be as high as 85%. A high proportion of animals with lobar pneumonia affecting more than one lobe at first treatment suggests delayed detection and warrants additional training and protocol review with the employees.
Our studies have shown that excessive severe subclinical pneumonia can be present in herds with poor detection, inadequate treatment regimen, inconsistent nutrition, and significant environmental challenges despite new facilities and acceptable stocking densities. TUS data will help measure the impact of management changes that are not reflected by changes in clinical signs or treatment records.
Ultrasound Equipment, Calf Preparation, Restraint, Technique
Transrectal probes permit the best access to the axillary region and cranial thorax of the young dairy calf. They are widely used by bovine veterinarians making them most suitable for practical field-based TUS in young cattle. You can choose between using a machine with an attached screen, a wireless screen, or goggles, based on personal preferences.
Operator positioning, restraint, and transducing agents are all necessary considerations when scanning. During the examination, it is up to the operator to decide whether to stand or squat down next to the calf. Short individuals and those with low back pain typically prefer squatting; whereas those with knee pain or the gift of height often choose to stand. When standing, it is easiest to scan each side of the calf by reaching over the dorsum to the opposite side.
In most situations, restraint should be minimal, rarely requiring a halter, headlock, or chute, particularly in young dairy animals. Increasing the level of restraint often only manages to increase handling time, therefore reducing the practicality of the procedure. Most often, the young dairy calf can be restrained by placing the hindquarters in the corner and hand under the chin or in front of the chest. In headlocks, calves often lean backwards reducing access to the first few intercostal spaces (ICS) beneath the forelimb.
On the farm, 70% isopropyl alcohol is the transducing agent of choice and the hair is not clipped or shaved from the chest. A solution of 90% or greater isopropyl alcohol functions wells but is excessively drying to the US probe as well as the skin of both the operator and the calf, and should be avoided.
Coupling gel and even vegetable oil will work; however, both products create a substantial mess compared to alcohol. Significantly less alcohol is required if a household spritzer/spray bottle is used.
Technique is critical. Within reason, the TUS technique can be modified based on the goals of the exam. Individual sick or “poor-doing” animals are more likely than the average calf to harbor lesions, specifically lung abscesses, in the caudal lung lobe. In these cases, the caudal lung lobe should always be assessed as well as the more cranial lung lobes. This requires scanning the right lung from the 10th ICS cranial to the 1st ICS and the left lung from the 10th ICS cranial to the 2nd ICS.
When screening a group of calves for pneumonia, a different approach can be taken compared to that used for an individual sick animal. In this situation, TUS is used to screen specifically for bronchopneumonia. Bronchopneumonia reliably localizes to three specific lung lobes particularly during the early phase of disease. The cranial aspect of the right cranial lung lobe is most commonly affected, followed by the right middle lung lobe, and the caudal aspect of the left cranial lung lobe.
The caudal aspect of the right cranial lung lobe, the cranial aspect of the left cranial lobes, and caudal lung lobe are rarely consolidated without consolidation of the previously mentioned lobes. It is important to scan both sides of the thorax as since consolidation may occur unilaterally in up to one of three dairy calves.
When a systematic clinical score, such as the Wisconsin Respiratory Score is also incorporated, calves can be categorized by BRD subtypes including upper respiratory tract infections, clinical pneumonia, and subclinical pneumonia. In this context, upper respiratory infection is defined as a positive respiratory score and a normal TUS; clinical pneumonia is defined by a positive respiratory score and abnormal TUS; and subclinical pneumonia is defined by a normal respiratory score and an abnormal TUS. The distributions of BRD subtypes will vary from farm to farm.
The key to accurate TUS is being systematic. A systematic approach depends on an understanding of the external thoracic anatomy of the calf, the internal anatomy of the lung, and appropriate ultrasonographic landmarks. The external anatomy of the calf refers to the specific ICS where the probe is placed. The internal anatomy refers to the specific lung lobes that are being evaluated. Lastly, the ventral image landmarks provide unique identifiers for each lung to ensure that the high-risk locations for pneumonia are examined. Once comfortable with the technique and scoring system, an accurate ultrasonographic diagnosis can be made within 20 to 30 seconds.
In general, the recommended TUS examination extends from the caudal thorax to the cranial thorax by moving the probe along the grain of the hair in a dorsal to ventral fashion within each ICS. The probe should move parallel to the rib within the ICS. It is a common mistake to move the probe perpendicular to the ground. Instead, the probe should be moved slightly caudally staying within one ICS to avoid imaging the rib. Very slight adjustments can move the ultrasound beam onto or off the rib surface and/or enhance visualization of a lung lesion. These small movements include moving the tip or end of the probe side to side or rotating the footprint, or the the portion of the probe in contact with the body wall, so it is facing more cranial or caudal within the ICS. If the rib obscures the image of the lung, simply stop moving, readjust the angle of the probe until the lung is present, and then continue ventrally within the ICS.
A six-point scoring system is suggested for scoring lung lesions and has served as a practical means to document and monitor lung lesions on commercial dairy farms.
In order to properly score, the operator must be able to recognize the difference between aerated lung, aerated lung with diffuse pleural roughening (also called comet-tail artifacts), lobular lung lesions (also called lobular consolidations or lobular pneumonia), and lobar lung lesions (also called lobar consolidations or lobar pneumonia).
In the context of this ultrasound scoring system, lobular and lobar lesions simply reflect the extent of which the lung lobe is consolidated on the ultrasound image. Lobular lesions are relatively small discreet areas of consolidation within an otherwise aerated lung lobe. In other words, the hyperechoic pleural interface with reverberation artifact of normal lung can be seen both dorsal and ventral to the lobular lesion when the probe is placed vertically within the rib space. Lobar lesions indicate full thickness consolidation of the lung lobe that extends proximally from the tip of the lobe. In the ultrasound image, the hypoechoic parenchyma of the entire distal lung lobe is visible, and aerated lung cannot be seen ventral to the lesion.
In general, ultrasound scores 0 to 1 are considered normal and ultrasound scores equal to or greater than 3 are consistent with bacterial bronchopneumonia. Abnormalities such as pneumothorax, pleural fluid, abscesses, and necrosis are not inherently included in the scoring system. Instead, a comment is included within the record regarding the abnormality (e.g. US score 4 plus 4 cm abscess in right caudal lung lobe at the level of the 8th intercostal space).
The portable rectal ultrasound machines already in use by bovine veterinarians for reproductive examinations are a fast, accurate, and practical means of diagnosing the lung lesions associated with BRD in young cattle. When combined with respiratory scoring, systematic TUS allows for the differentiation of BRD into specific practical subtypes including upper respiratory tract disease, clinical pneumonia, and subclinical pneumonia; all of which can be performance limiting.
In individuals, TUS can be used to identify poor prognostic indicators such as caudal lung lobe consolidation, lung abscessation, and lung necrosis, and can aid culling and purchasing decisions. At the herd level, TUS can be used to identify specific populations at risk for developing the subtypes of BRD, monitor the prevalence and severity of BRD over time, and evaluate the impacts of management changes such as ventilation, vaccination, changes in treatment protocols or personnel. In conclusion, TUS can add to the services provided by bovine veterinarians, increasing their value and impact on animal health.
Information for this article was drawn, in part, from “On-Farm Use of Ultrasonography for Bovine Respiratory Disease,” Theresa L. Ollivett and Sébastien Buczinski, Vet Clinics of North America, 2016.