How to Use Thoracic Ultrasound to Screen Foals for Rhodococcus equi at Affected Farms

1. Introduction

Rhodococcus equi, a gram-positive facultative intracellular pathogen, is an important cause of pneumonia in foals between 3 wk and 5 mo of age. Rhodococcus is a saprophytic inhabitant of soil and is widespread in the environment of horse-breeding farms [1]. R. equi is present in the environment of most horse farms; its associated clinical disease may be unrecognized or sporadic on some farms and enzootic and devastating on others with morbidity rates sometimes exceeding 40% [2,3]. Methods for control and prevention of R. equi are not well understood. Presently, there are no effective vaccines available. The only prophylactic strategy proven to be effective is the administration of R. equi hyperimmune plasma [4]. Antimicrobial treatments for this disease are expensive, prolonged, associated with adverse effects, and may not be successful [5]. The disease may also be associated with a decreased chance of racing as an adult; additionally, farms reputed to have the disease can suffer loss of clients [6]. Confirmation of R. equi pneumonia usually requires isolation of R. equi using transtracheal aspiration and radiographic evidence of pulmonary abscesses. However, thoracic radiographic examination is not a practical diagnostic test for the field practitioner. Because complete prevention of the disease is not possible, control of R. equi on enzootic farms depends on strategies to decrease the incidence of disease. Diagnostic thoracic ultrasonography has been shown to be an accurate alternative imaging modality for detection of pulmonary pathology attributed to R. equi pneumonia in foals when thoracic radiography is not available [7].

2. Material and Methods

Foals

The study population were foals from farms in central Kentucky with endemic R. equi pneumonia confirmed by isolation of R. equi from a transtracheal wash (TTW) or gram-positive intracellular coccobacilli observed in cytology of TTW fluid (morbidity > 15% during past 5 yr) [7,8]. Thoracic ultrasonography was started at 30 days of age and repeated at 2-wk intervals until the foal was weaned (16 - 20 wk). All the foals had been administered hyperimmunized R. equi plasma during the first 24 h of life and again at 25 days of age [a,b].

Scanning Techniques

Ultrasonographic examinations can be performed with a multifrequency 5.0- or 7.5-MHz linear transrectal transducer. The 7.5-MHz will be able to display a depth of 4 - 12 cm, which is ideal for the thoracic examination of a foal. Isopropyl alcohol is copiously applied to the hair coat to provide surface contact between the transducer and the foal. The alcohol helps to reduce the intervening trapped air [9]. The thorax was thoroughly scanned in a dorsal to ventral plane from the 16th to the 3rd intercostal space.

Sound waves are completely reflected at the normal aerated lung interface, allowing only the pleura surface to be evaluated [10,11]. Therefore, the normal visceral pleural edge of the lung appears as a straight hyperechoic line with characteristic equidistant reverberation air artifacts that indicate normal aeration of the pulmonary periphery [12]. The pleural edge of the lung is imaged gliding dorsally and ventrally when watching the patient breath during thoracic ultrasonography. Only when fluid or cellular accumulation in the lung occurs immediately beneath the visceral pleural surface will an acoustic window be created, allowing visualization of pulmonary pathology. The affected area of the lung is hypoechoic and/or lacks the normal air echo at the surface. It is critical to examine the lung carefully during exhalation and inhalation, because lesions can move beneath an adjacent rib or the inhaled air into the alveoli. Surrounding airways will cause reflection of the sound waves and thereby prevent the visualization of pulmonary disease.

Pulmonary abscesses are variable in size and are located anywhere in the lung. Abscesses are identified ultrasonographically in the lung by their cavitated appearance and the absence of normal pulmonary structures within the abscess. The center may appear hypoechoic, isoechoic, or septate, depending on the type of fluid present [9].

Pulmonary consolidation is hypoechoic and/or lacks the normal air echo at the surface. Ultrasonographic visualization of the consolidated lung occurs because of the replacement of alveolar air with fluid or cells, which produces an acoustic window [9].

Grading Pneumonia

Pulmonary lesions were assigned a grade according to the severity. The grading scale ranged from 0 (normal) to 10 (the entire lung surface is affected). A grading scale was implemented to aid in the documentation of lesions, to determine if treatment was successful, and to help with the communication and description of pneumonia. The foal's grade was determined not by the total number of lesions that were visualized but by the highest grade visualized. For example, a foal with multiple grade 1 lesions of the left hemithorax and one grade 3 lesion would be identified as a grade 3 in the left thorax.

• Grade 0: No evidence of pulmonary consolidation. Pleural irregularities appear as vertical hyperechoic lines and are described as reverberation artifacts (Fig. 1) [13].
• Grade 1: Lesions are <1 cm in diameter/depth (Fig. 2).
• Grade 2: Lesions are 1.0 - 2.0 cm in size.
• Grade 3: Lesions are 2.0 - 3.0 cm in size (Fig. 3).
• Grade 4: Lesions are 3.0 - 4.0 cm in size (Fig. 4).
• Grade 5: Lesions are 4.0 - 5.0 cm in size.
• Grade 6: Lesions are 5.0 - 6.0 cm in size (Fig. 5).
• Grade 7: Lesions are 6.0 - 7.0 cm in size.
• Grade 8: Lesions are 7.0 - 9.0 cm in size. If pleural effusion is present, the lesion is assigned this grade regardless if lesser grades of consolidation or abcessation are present.
• Grade 9: Lesions are 9.0 - 11.0 cm in size.
• Grade 10: The entire lung is affected.

Figure 1. Grade 0. Note the vertical hyperechoic lines that are reverberation artifacts in the ventral tip of the lung.

Figure 2. Grade 1 pulmonary abscess measuring 7.4 mm in depth.

Figure 3. Grade 3 pulmonary abscess measuring 2.0 cm.

Figure 4. Grade 4 pulmonary abscess measuring 3.3 cm in depth.Note the hypoechoic center.

Figure 5. Grade 6 cavitated pulmonary abscess located in the ventral tip of the lung.

Treatment

All foals with a graded pulmonary lesion were placed on antibiotic treatment that consisted of azithromycin [c] (10 mg/kg, q 24 h, PO for 7 days and then every other day) and rifampin [d] (5 mg/kg, q 12 h, PO). Treatments were continued until ultrasonographic resolution was determined.

3. Results and Discussion

Since 2001, >800 foals from various endemic farms have had thoracic ultrasonography performed as an aid in the early diagnosis of R. equi (Table 1, Table 2, Table 3, Table 4, Table 5 and Table 6). These farms have had a history of increased mortality and morbidity associated with this disease. Ultrasonography is generally considered to be an insensitive test for assessing the lung because of its inability to detect lesions located deep in the normal aerated lung. However, it has been suggested that the lung periphery is often affected, enabling these lesions to be identified ultrasonographically [9]. Thoracic ultrasonography is easy and quick, and the results are instantaneous. One of the difficulties associated with treatment is knowing when treatment should be discontinued. Foals often appear clinically better long before the abscesses/consolidation are gone, and discontinuing treatment too early results in increased relapses. Initially (Farm A), we found that as many as 50% of the foals were subclinically affected. With the continuation of the diagnostic ultrasonography and treatment over the next several years, the subclinical incidence dropped to 31%, and clinical disease dropped to 0%. Farms B and C had a higher rate of subclinically affected foals during their first year compared with Farm A. We expect the subclinical incidence to decline on these farms after several more years of thoracic screening. Infrequent complications (<6%) associated with azithromycin and rifampin included hyperthermia and tachypnea. In a study of 43 foals from two farms in Japan, the prevalence of fecal shedding was higher among foals from an R. equi-endemic farm (94%; 16/17) than among foals from a farm with no history of R. equi pneumonia (73%; 19/26) [14]. Fecal concentration of the bacterium was observed to increase in two foals that developed disease. The organism can be isolated from feces of foals <1 wk of age, and shedding increases with age [1,15,16]. Therefore, it is speculated that the decrease of subclinical infections may be associated with the treatment of these foals and the decreased fecal shedding of virulent R. equi. None of the resident foal population on these farms showed any evidence of disease, and only a few had elevated temperatures. Meanwhile, foals that were shipped onto the farms during the first 6 - 8 wk of life had an increased incidence of elevated temperatures compared with the resident foals. Only 5 of 87 non-resident foals (Farm A) were actually clinically apparent at the time of examination (elevated respiratory rate and coughing), and thoracic ultrasonography confirmed their disease status. The implementation of thoracic ultrasonography on our endemic farms seems to be highly sensitive, because no foals that were diagnosed as a grade 0 developed clinical disease. The farms that implemented thoracic ultrasonography had no mortalities and a marked reduction of clinical disease associated with R. equi. Thoracic ultrasonography is a practical, quick, accurate, and useful diagnostic modality when screening for R. equi. In addition, the pulmonary lesion grades should be routinely employed when monitoring treatments and communicating the description of the lesions.

Footnotes

1. Rhodococcus equi plasma (950 ml; United States Department of Agriculture approved), Lake Immunogenics Inc., Ontario, NY 14519.
2. Rhodococcus equi (700 ml) and Clostridium perfringens-blend plasma (400 ml; not United States Department of Agriculture approved), Mg Biologics, Ames, IA 50014.
3. Zithromax, Pfizer Inc., New York, NY 10017.
4. Rifadin, Merrell Pharmaceuticals Inc., Kansas City, MO 64137.