Ultrasonography and digital radiography findings in sheep with clinical disease associated with small ruminant lentivirus infection

Abstract

Digital radiography and ultrasonographic images were used in this case series to evaluate 4 ewes from a single flock for chronic weight loss and ill-thrift. On examination, all displayed tachypnea, dyspnea, coughing, and normothermia with abnormal thoracic auscultations. Three of the 4 animals were diagnosed with chronic respiratory disease associated with Maedi-visna (MV) infection confirmed via serologic testing. Diagnostic thoracic imaging identified characteristics consistent with pathological lesions associated with interstitial pneumonia in the 3 MV affected animals; these findings were absent in the animal that tested negative for MV.

Key clinical message:

Diagnostic imaging may be useful to clinicians looking to obtain further visualization of lung pathologies and as a reliable means of detecting thoracic lesions indicative of interstitial pneumonia on-farm. These results can be used to aid the practitioner in determining appropriate further diagnostic testing and treatment strategies while awaiting confirmatory test results for diagnosis of MV.

Maedi-visna (MV) is an underdiagnosed, chronic viral disease of sheep that causes devastating economic losses worldwide (1). Maedi-visna, also known as ovine progressive pneumonia (OPP), is caused by a non-oncogenic retrovirus belonging to the genus Lentivirus and is included in the small ruminant lentivirus (SRLV) group. It is characterized by chronic, progressive, debilitating pneumonia, as well as wasting, indurative mastitis, arthritis, and occasionally, neurologic signs in adult animals (2–8). A presumptive diagnosis of clinical disease associated with MV is frequently made based on progressive weight loss and chronic pneumonia in adult sheep, but the diagnosis is often overlooked if mildly affected or subclinical sheep are culled for other reasons (5). Several serologic and virologic diagnostic tests have been used to identify infected sheep for flock control programs; however, these tests must be performed at a reference laboratory, and it can take 1 to 2 wk to receive results (9–19). Therefore, additional methods to assist in reaching a presumptive diagnosis of clinical respiratory disease associated with MV on-farm are necessary to aid the practitioner in determining appropriate further diagnostic testing and treatment strategies while awaiting additional test results.

Even though radiography and ultrasound have been available for use in tertiary care settings for decades, there is a paucity of published reports on their specific use for respiratory disease in sheep (20,21). In addition, with recent improvements in portability, many food animal practitioners in private practice have access to portable digital radiography, which may allow for immediate on-farm viewing of radiographic images. The use of ultrasonography for purposes other than pregnancy diagnosis in food animal practice has also been rapidly increasing, and many researchers have begun to assess the use of thoracic ultrasonography for diagnosis of lung pathology in cattle and small ruminants (21–27). Thus, the increased availability of several on-farm diagnostic imaging modalities that can directly evaluate lung pathologies (21,24) may offer the ability to assist the practitioner more rapidly and accurately in identifying interstitial pneumonia and identifying MV as the most likely clinical diagnosis in those cases. In this case series, we report the successful use of both digital radiography and ultrasonography to aid in determining appropriate further diagnostic testing and treatment strategies while awaiting confirmatory test results for definitive diagnosis of MV.

Case descriptions

Four crossbred, black-faced ewes ranging in age from 4 to 5 y were brought to the Food Animal and Camelid Hospital (FACH) of Iowa State University College of Veterinary Medicine for evaluation. These animals originated from a flock of approximately 200 meat-breed ewes raised primarily for breeding sales as well as exhibition and thus were considered of high genetic value. The flock had recently experienced increased incidence of chronic weight loss, ill-thrift, and had a history of increased death loss. Elevated liver copper levels had been identified in some animals, indicative of potential chronic copper toxicosis affecting the herd, which prompted the referral.

On clinical inspection, Ewe #1 was quiet, alert, and responsive with adequate body condition and a body condition score (BCS) of 2.5/5. A spontaneous dry cough, tachypnea [44 bpm; reference interval (RI): 16 to 34 bpm] with increased (inspiratory and expiratory) respiratory effort, abnormal lung sounds bilaterally including wheezes, and mucopurulent nasal discharge were present. Body temperature was normal at 38.3°C (RI: 38.3 to 39.9°C). Heart rate was normal (64 bpm; RI: 60 to 80 bpm) with normal cardiac auscultation.

Ewe #2 was moderately dull compared to her flock mates and had a decreased BCS (2/5). A frequent spontaneous cough, tachypnea (54 bpm) with increased (inspiratory and expiratory) respiratory effort, abnormal lung sounds bilaterally including crackles and wheezes, and serous nasal discharge were present. Body temperature was normal at 39.6°C. Heart rate was elevated (100 bpm), but cardiac auscultation was normal.

Ewe #3 was moderately dull compared to her flock mates with poor body condition (BCS 1.5/5). A frequent spontaneous cough, tachypnea (54 bpm) with increased (inspiratory and expiratory) respiratory effort, abnormal lung sounds bilaterally including crackles and wheezes, and serous nasal discharge were present. Body temperature was normal at 39.5°C. Heart rate was elevated (120 bpm); however, cardiac auscultation was normal.

Ewe #4 was bright, alert, and responsive with normal body condition (BCS 3/5). A frequent spontaneous cough, tachypnea (44 bpm) with increased (inspiratory and expiratory) respiratory effort, inspiratory stertor, abnormal lung sounds bilaterally suspicious of referred upper airway sounds, and copious serous nasal discharge were present. Decreased airflow was apparent through the left nostril. Body temperature was normal at 38.8°C. Heart rate was elevated (100 bpm); however, cardiac auscultation was normal.

Clinical abnormalities expressed in all ewes were referable to the respiratory system, but other causes of poor body condition were also considered. Primary diagnostic hypotheses for signs of lower respiratory disease in sheep include acute bronchopneumonia (viral or bacterial), chronic bronchopneumonia (bacterial or parasitic), acute interstitial pneumonia (toxic or parasitic), chronic interstitial pneumonia (viral), and cardiac disease (left-sided heart failure resulting in pulmonary edema). These lower respiratory diseases can manifest by infection from Mannheimia haemolytica, Pasteurella multocida, maedi-visna virus (MV), ovine pulmonary adenocarcinoma (OPA), verminous pneumonia (Muellerius capillaris or Dictyocaulus viviparus lung worms), or pulmonary and thoracic lymph node abscesses caused by Corynebacterium pseudotuberculosis (caseous lymphadenitis — CL). The authors also considered causes of upper respiratory disease due to either nasal abscessation, enzootic nasal adenocarcinoma, or Oestrus ovis infection. To further assess the nature of the respiratory disease present, a complete blood (cell) count (CBC), serum biochemistry, thoracic radiography, thoracic ultrasound, and serology for MV virus were completed. In addition, a McMaster’s fecal egg count, urinalysis, serology, and fecal polymerase chain reaction (PCR) for Johne’s disease, as well as liver biopsy for a trace mineral panel were performed to further assess other causes for chronic weight loss and ill-thrift. The results of these ancillary tests are provided (Tables 1–8). External lymph node enlargement was not noted. However, CL also remained as a potential rule-out; further testing for this disease in all animals was not pursued.

Table 1.

Hematology report.

Parameter	Normal reference ranges	Animal identification
		Ewe #1	Ewe #2	Ewe #3	Ewe #4
PCV (%)	27.0 to 46.0%	30.1	26.7	28.7	25.0 (L)
Red blood cells/μL	9.00 to 15.00 × 106/μL	10.89	9.55	12.49	9.00
Hemoglobin (g/dL)	9.0 to 15.0 g/dL	10.7	9.8	10.9	9.2
MCV (fl)	28.0 to 40.0 fl	27.7 (L)	27.9 (L)	23.0 (L)	27.8 (L)
MCH (pg)	8.0 to 12.0 pg	9.8	10.2	8.7	10.3
MCHC (g/dL)	31.0 to 34.0 g/dL	35.6 (H)	36.6 (H)	37.9 (H)	37.0 (H)
MPV (fl)	4.4 to 8.1 fl	4.4	5.1	5.1	5.0
RBC morphology		Slight anisocytosis	Slight anisocytosis	Slight anisocytosis	Slight anisocytosis and rare polychromasia
Nucleated RBC		0.00	0.00	0.00	0.00
White blood cells/μL	4.0 to 12.0 × 103/μL	9.31	4.57	5.97	5.48
Band Neutrophils/μL	0.0 to 0.12 × 103/μL	0.00	0.00	0.00	0.00
Seg. Neutrophils/μL	0.7 to 6.0 × 103/μL	3.07	2.42	5.01	3.23
Lymphocytes/μL	2.0 to 9.0 × 103/μL	5.59	1.97 (L)	0.90 (L)	2.08
Monocytes/μL	0.0 to 0.75 × 103/μL	0.19	0.18	0.06	0.11
Eosinophils/μL	0.0 to 1.0 × 103/μL	0.47	0.00	0.00	0.05
Basophils/μL	0.0 to 0.3 × 103/μL	0.00	0.00	0.00	0.00
RDW	3.2 to 6.0%	17.4 (H)	16.7 (H)	20.0 (H)	17.9 (H)
Leukocyte morphology		Hyper-segmented neutrophil and granular lymphocyte	N/A	Reactive lymph	Reactive lymph
Platelets/μL	250 to 800 × 103/μL	520	366	779	522
Platelet clumps		Present	None	Present	None
Fibrinogen mg/dL	100 to 500 mg/dL	1000 (H)	300	500	500
Plasma protein	6 to 8.3 g/dL	8.0	7.1	7.0	7.6

PCV — Packed cell volume; MCV — Mean corpuscular volume; MCH — Mean corpuscular hemoglobin; MCHC — Mean corpuscular hemoglobin concentration; MPV — Mean platelet volume; RBC — Red blood cells; H — High; L — Low; N/A — Not available. Bolded numbers indicate a high or low value.

Table 2.

Biochemistry report.

Parameter	Units	Normal reference ranges	Animal identification
			Ewe #1	Ewe #2	Ewe #3	Ewe #4
Creatinine	mg/dL	1.0 to 1.9 mg/dL	0.8 (L)	0.6 (L)	0.6 (L)	0.7 (L)
ALP	IU/L	60 to 400 IU/L	68	55 (L)	42 (L)	46 (L)
AST	IU/L	55 to 150 IU/L	160 (H)	435 (H)	372 (H)	112
GGT	IU/L	40 to 100 IU/L	137 (H)	149 (H)	149 (H)	113 (H)
SDH		6.0 to 28.0	63.8 (H)	268 (H)	17.0	19.9
Creatine Kinase	IU/L	30 to 150 IU/L	231 (H)	194 (H)	183 (H)	103
Urea (BUN)	mg/dL	14 to 25 mg/dL	21	16	14	16
Bicarbonate	mEq/L		35	29	31	29
Glucose	mg/dL	50 to 100 mg/dL	64	78	59	67
Sodium (Na)	mEq/L	136 to 150 mEq/L	151 (H)	151 (H)	148	152 (H)
Potassium (K)	mEq/L	3.9 to 5.5 mEq/L	4.3	4.7	4.4	4.4
Chloride (Cl)	mEq/L	94 to 109 mEq/L	103	110 (H)	109	108
Calcium (Ca)	mg/dL	10.5 to 13.0 mg/dL	9.2 (L)	10.8	9.0 (L)	9.5 (L)
Phosphorus (P)	mg/dL	5.0 to 7.3 mg/dL	4.3 (L)	3.5 (L)	5.5	8.5 (H)
Magnesium (Mg)	mg/dL	1.82 to 3.65 mg/dL	2.59	1.86	1.64 (L)	2.40
Total protein	g/dL	6.0 to 8.0 gm/dL	8.2 (H)	7.3	7.0	7.6
Albumin	g/dL	2.4 to 3.5 gm/dL	3.2	3.0	2.3 (L)	3.0
Total Bilirubin	mg/dL	0.10 to 0.30 mg/dL	0.48 (H)	0.42 (H)	0.70 (H)	0.50 (H)
Bile acids — fasting	μmol/L		12.3	24.7	18.4	57.8
Anion gap			18	17	16	19
Lipemic indice			20.0	20.0	20.0	20.0
Hemolytic indice			15.0	15.0	31.0	23.0
Icteric indice			2.0	3.0	2.0	2.0

ALP — Alkaline phosphatase; AST — Aspartate aminotransferase; GGT — Gamma-glutamyl transferase; SDH — Sorbitol dehydrogenase; L — Low; H — High. Bolded numbers indicate a high or low value.

Table 3.

OPP serology report.

Parameter	Specimen	Animal identification
		Ewe #1	Ewe #2	Ewe #3	Ewe #4
OPP AGID*	Serum	Positive	Positive	Positive	Negative

^*OPP AGID reagents were manufactured and distributed by Veterinary Diagnostic Technology, Wheat Ridge, Colorado, USA.

Table 4.

Urinalysis report.

Parameter	Animal identification
	Ewe #1	Ewe #2	Ewe #3	Ewe #4
Source (e.g., cath/void/cysto)	Void	N/A	N/A	Void
Color	Yellow	N/A	N/A	Yellow
Appearance/Transparency	Hazy	N/A	N/A	Clear
Specific gravity (refractometer)	1.028	N/A	N/A	1.021
pH	8.5	N/A	N/A	8.0
Protein	Trace	N/A	N/A	Neg.
Glucose (urine)	Neg.	N/A	N/A	Neg.
Ketone	Neg.	N/A	N/A	Neg.
Bilirubin	Trace	N/A	N/A	Neg.
Blood	Neg.	N/A	N/A	Neg.
Casts	0	N/A	N/A	0
Leucocytes/HPF	Rare	N/A	N/A	Rare
Epithelial cells/HPF	0–1	N/A	N/A	0
Erythrocytes/HPF	0	N/A	N/A	0
Crystals	0	N/A	N/A	0
Bacteria	0	N/A	N/A	0
Lipids	Rare	N/A	N/A	0

N/A — Not available.

Table 5.

Fecal McMaster screen.

Parameter	Units	Animal identification
		Ewe #1	Ewe #2	Ewe #3	Ewe #4
Fecal egg count	Eggs per gram	No parasites identified	Eimeria = 100	Eimeria = 400	Eimeria = 100; Strongyles = 500; Strongyloides = 1400

Table 6.

Johnes serology report.

Parameter	Animal identification
	Ewe #1		Ewe #2		Ewe #3		Ewe #4
Johnes ELISA —	S/P	Result	S/P	Result	S/P	Result	S/P	Result
Serum/Plasma*	0.247	Negative	0.025	Negative	0.050	Negative	0.106	Negative

^*IDEXX MAP kit (IDEXX, USA), S/P < 0.45 = negative, S/P 0.45 to 0.55 = suspect; S/P > 0.55 = positive.

Table 7.

Johnes fecal PCR report.

Parameter	Animal identification
	Ewe #1	Ewe #2	Ewe #3	Ewe #4
M. para TB (Johne’s) — Life Tech*	Pooled sample — Negative: CT > 40

^*PCR = VetMAX GOLD MAP detection kit PCR used for pooled samples (positive = ≤ 35.0 Ct).

Table 8.

Tissue trace mineral analysis report: (liver biopsy).

Parameter (ppm)	Ovine normal reference ranges	Animal identification
		Ewe #1	Ewe #2	Ewe #3	Ewe #4
Cadmium		0.281	0.135	0.081	0.280
Calcium		417	778	92	292
Chromium		1.083	2.611	0.062	0.403
Cobalt	0.04 to 0.36	0.432 (H)	0.550 (H)	0.062	0.272
Copper	100 to 400	1478 (H)	1265 (H)	**	82
Iron	120 to 1200	492	609	115 (L)	246
Magnesium		615	744	161	557
Manganese	8 to 17.6	10.7	9.8	3.0 (L)	6.4 (L)
Molybdenum		4.74	4.87	1.62	5.88
Phosphorus		11 075	11 067	2898	10 367
Potassium		9818	11 007	2147	9452
Selenium	0.75 to 2.25	5.04 (H)	4.13 (H)	1.15	1.77
Sodium		3019	8735	1828	3579
Zinc	30 to 75	142 (H)	137 (H)	100 (H)	138 (H)

Liver biopsy results are on a dry weight basis. Reviewed by a veterinary toxicologist.

^**Ewe #3 was submitted for post-mortem evaluation and had further tissue trace mineral analysis by a different assay. Tissues evaluated included Kidney [Cu: 4 ppm (RR: 4 to 10 ppm)] and Liver [Cu: 360 ppm (H) (RR: 25 to 100)].

H — High; L — Low. Bolded numbers indicate a high or low value.

Digital thoracic radiography was performed on all 4 animals (Figure 1 A–D). Right lateral standing thoracic radiographic images (2 to 3 views per animal) were obtained using a Philips X-ray (Philips Medical Systems, Amsterdam, Netherlands) and Varix Imaging PaxScan 4336Wv4 Flat Panel detector (Sound, Carlsbad, California, USA) with exposure factors of 85kVp and 6 mAs. An additional standing dorsoventral (DV) view was obtained for Ewe #3, to determine left or right hemi-thorax location of lesions. Initial assessment was performed animal-side by the managing veterinarians. Images were evaluated by a Board-certified radiologist who generated an official report for the visit. Images were saved in a picture archiving and communication system (PACS) for later review. The radiograph of Ewe #1 (Figure 1 A) demonstrated evidence of a mixed moderate unstructured interstitial to mild alveolar lung pattern in the hilar and central regions of the lungs, suggestive of viral or mixed etiology. The radiograph of Ewe #2 (Figure 1 B) demonstrated evidence of severe diffuse unstructured interstitial pneumonia with additional ventral moderate alveolar lung patterns. Ewe #3 (Figure 1 C) demonstrated severe pulmonary changes, with a dorsal, unstructured interstitial pattern that coalesced ventrally to an alveolar pattern with a central air-bronchogram. In addition, several cavitary lesions, some filled with only gas, indicating the likely presence of emphysematous bullae, and others with a gas-fluid interface, indicating the likely presence of abscesses, were visible in the caudal central region. Ewe #4 (Figure 1 D) had moderate gas and fluid filling of the cranial thoracic esophagus with an otherwise radiographically normal thorax. The esophageal findings were attributed to eructation and/or aerophagia likely secondary to an obstructive upper respiratory lesion. Subsequent radiographs of the head revealed a soft tissue opaque mass in the left nasal cavity, extending into the left maxillary sinus and nasopharynx. Histology results of an endoscopy-guided biopsy of this mass were consistent with a nasal adenocarcinoma (Figure 2).

Figure 1.

Thoracic radiographs of the 4 ewes. Ewes #1, #2, and #3 (A–C) with abnormal thoracic radiographs and serologically positive for MV. A normal thorax from the ewe of the same flock serologically negative for MV (D) for comparison. The lungs of Ewe #1 (A) exhibit a caudodorsally distributed, unstructured interstitial pattern (black box). The lungs of Ewe #2 (B) were more severely affected than Ewe #1; a pattern of alveolar change in a ventral predominance but with extension dorsally into the caudal lobes. The lungs of Ewe #3 (C) had caudal central, severe patchy alveolar pattern transitioning ventrally to consolidation with air-bronchograms (white arrow) indicating patent bronchi. A cavitary lesion with gas and fluid (black arrow) is likely an abscess; other gas-only cavitary lesions are likely emphysematous bullae.

Figure 2.

Endoscopy report/images. Increased upper respiratory effort, inspiratory stertor, abnormal lung sounds bilaterally suspicious of referred upper airway sounds, and copious serous nasal discharge along with skull radiographs showing evidence of an increased soft tissue density prompted an endoscopic evaluation of the upper nasal pharyngeal region of Ewe #4. Images of the endoscopy are seen above. A tru-cut biopsy was performed and histopathologic evaluation of the submitted tissues indicated tightly packed nests and acini of neoplastic cells within a thin fibrous stroma as well as mitotic figures present, which is most consistent with a nasal adenocarcinoma. This ewe was diagnosed with a nasal adenocarcinoma.

Ultrasound images of the thorax were obtained stall-side using the SonoSite Edge II Veterinary Ultrasound System (Fujifilm SonoSite, Bothell, Washington, USA) and a (transrectal) linear array probe (L52X, SonoSite; 10-5 MHz transducer) at a depth of 5 to 9 cm depending on lesion size and depth. Scans were performed in a systematic manner, starting caudodorsally and continuing cranioventrally within the intercostal spaces (11th to 4th) on both the left and right sides of each animal. Wool was clipped over the lung fields to facilitate transducer contact. Initial assessment was performed by the managing veterinarians. Images were saved as both still image and video loops, loaded into PACS, and retrieved later for review by a Board-certified radiologist. On thoracic ultrasound of Ewes #1, #2 and #3, the normal hyperechoic pleural interface with corresponding A-line reverberation artifact was seen in the more craniodorsal aspects of the lungs (Figure 3 A). These regions corresponded to areas of less severe pulmonary parenchymal changes on the thoracic radiographs. Moving ventrally along the intercostal spaces, the pleural interface became focally thinned in multiple regions. In several locations of each of the 3 affected animals, the pleural interface was completely effaced by irregularly margined regions of hyperechogenicity due to confluent B-lines, referred to as “white lung” (Figure 3 B), which effaced the normal A-line reverberation artifact. In the ventral lung fields, the pulmonary parenchyma was markedly abnormal in Ewes #1, #2, and #3 (in order of least to most pronounced lesions) where tubular anechoic regions were seen coursing through parenchyma which caused the lung to have an echotexture similar to that of the liver, consistent with “pulmonary hepatization” (Figure 3 C). In contrast, for Ewe #4, the pleural interface was well-defined and uniform in thickness. The normal A-line reverberation artifact was present in all lung lobes. A few scattered, very small regions of pleural thickening with secondary perpendicularly oriented hyperechoic striations (i.e., B-lines) were seen extending into the deeper parenchyma of the lung.

Figure 3.

Thoracic ultrasound images of sheep affected with MV. Normal lung areas (A) had horizontally oriented A-lines (black arrows) running in parallel to the normal pleural-lung interface (white arrow). Multiple B-lines in the abnormal lung region (B) are so numerous that they are confluent and obliterate the A-lines making the lung appear white. Abnormal lung region (C) is hepatized in which normal lung is replaced with hypoechoic regions containing tubular centrally anechoic features consistent with fluid bronchograms or airways filled with fluid (white chevrons). Starred area in B represents a rib shadow.

The CBC results were generally unremarkable, and no evidence of a chronic inflammatory response was noted except for Ewe #1 which had elevated fibrinogen; WBC counts and WBC cell types were mostly within normal reference ranges except for a mild lymphopenia for Ewes #2 and #3 (Table 1). Several abnormalities were noted on the biochemistry reports; however, these were primarily due to concurrent elevation of liver copper which was diagnosed in Ewes #1, #2, and #3 (Table 2). Serological AGID results for MV, which were received 2 d later [with on-site access to an American Association of Veterinary Laboratory Diagnosticians (AAVLD) accredited, reference veterinary diagnostic laboratory], revealed that Ewes #1, #2, and #3 were positive for MV, whereas Ewe #4 was negative (Table 3). All other ancillary test results are listed in Tables 4–8.

To definitively confirm the presence of MV-associated disease in the flock, Ewe #3 was humanely euthanized and a full necropsy was performed. Gross necropsy examination revealed that the ventral regions of hepatized lung visualized on ultrasound corresponded to areas of consolidation likely from a previous bacterial pneumonia; culture revealed the presence of P. multocida in this area. Additional regions of severe bronchopneumonia with fibrous pleural adhesions, abscessation and pleuritis, as well as diffuse interstitial pneumonia in the caudal lung lobes (Figure 4) were also identified. One gas and fluid-filled cavitary lesion in the right lung parenchyma was confirmed to be an abscess from which C. pseudotuberculosis was cultured (Figure 4). In addition, both the mediastinal and tracheobronchial lymph nodes were markedly enlarged as well as edematous and hemorrhagic. Histopathological examination of the caudal lung lobes confirmed gross findings and revealed a severe, chronic, multifocal, lymphoplasmacytic interstitial pneumonia with type II pneumocyte hypertrophy and hyperplasia, consistent with MV, and not affected by the concurrent bronchopneumonia.

Figure 4.

Gross lung necropsy images from ewe #3. Caudal left lung lobes (A) were diffusely heavy, rubbery, and non-collapsing with rib impressions observed on the surface (white chevrons). The anteroventral lung lobes were firm in texture, consistent with consolidation (white asterisks) and displayed adhesions to each other and to the pericardium. In addition to similar lesions as seen on the left lung, the right lung lobes (B) displayed several distinct abscesses (black arrows). The gross lesions of interstitial pneumonia in the caudal lung lobes were compatible with typical lesions observed with MV.

Discussion

This case series demonstrates the usefulness of diagnostic imaging for the evaluation of clinical respiratory disease associated with MV. Both ultrasonographic and radiographic changes associated with interstitial pneumonia were visible in the clinically affected animals. Maedi-visna had not been previously diagnosed within this flock. However, the use of clinical history combined with physical examination findings and diagnostic imaging results, all of which could be obtained on-farm, allowed for rapid identification of chronic interstitial pneumonia with varying degrees of consolidation, leading to a strong suspicion that MV was a likely causative agent. These results were supported through serologic testing and histopathological examination of the lungs of an affected animal. Based on the results of these cases, the authors propose that the inclusion of diagnostic imaging such as thoracic ultrasound or radiography for on-farm evaluation of sheep with chronic weight loss and signs referable to the respiratory tract may be helpful to identify patients with abnormal interstitial pulmonary patterns. This information can be used to encourage further confirmatory testing of both individual animals and the rest of the flock for implementation of MV control programs.

The sonographic and radiographic abnormalities detected in Ewes #1, #2, and #3 are not pathognomonic for a single disease process and require integration of the clinical examination and laboratory test results to prioritize the most likely etiology. In addition, it is unclear at this time if the absence of these lesions can be used to rule out clinical disease caused by MV. The combination of “white lung” on thoracic ultrasound and unstructured interstitial pattern on radiographs can be seen with processes including but not limited to pulmonary edema (cardiogenic versus non-cardiogenic), viral infection, ovine pulmonary adenomatosis, pulmonary hemorrhage, or toxin induced pneumonitis. The ventral sonographic hepatization and radiographic alveolar pattern is most often associated with bacterial pneumonia. In a group of several adult sheep from one genetically valuable flock exhibiting respiratory signs with chronic weight loss and ill-thrift, infectious pneumonia of mixed viral/bacterial etiology is the most likely reason for the abnormal findings; with MV being a primary differential diagnosis.

Maedi-visna is a major concern for the sheep industry throughout the world; particularly because once infected, infection is lifelong (5–7). This chronic debilitating disease has a wide seroprevalence (range: 30 to 67%) in cull ewes across the United States, which increases with advancing age (3). Due to the insidious nature of MV infections, this disease often goes undiagnosed and is under-reported as few deaths are completely investigated; or other disease entities are presumed for the observed chronic wasting leading to early culling and/or death in the flock. Results from a National Animal Health Monitoring System study in 2011 indicated that 46.5% of sheep producers in the United States did not know the MV status of their flock (1). Much of the economic loss accompanying MV infections are commonly due to subclinical infections. These include loss of milk production due to udder damage, poor reproductive performance, respiratory disease, and chronic wasting (28). Consequently, the ability to identify clinically affected animals rapidly on-farm is important for the veterinary practitioner.

Currently, there are no rapid pen-side tests available to perform on-farm to definitively diagnose MV infection. Serologic tests including agar gel immunodiffusion (AGID), ELISA, and Western blot assays are all available but require submission to a reference laboratory and results may take several days to weeks to receive (5,7,8). In addition, some infected sheep never seroconvert, whereas others with advanced disease become transiently seronegative (6). These factors complicate interpretation of serological test results and suggest that alternative diagnostic methods may be important in assisting with clinical diagnosis.

Although animals positive for MV on serologic testing are considered to be actively infected with the virus, antemortem assignment of clinical disease due to MV positive status can be challenging on-farm using physical examinations exclusively. Physical assessment alone cannot reliably determine the extent, severity, and/or type of lung disease in every clinical scenario, and it can lead to ineffective selection of therapy or misinter-pretation of the respiratory condition (23). As demonstrated in cattle, there are also limitations to interpretation between lesion distribution of respiratory tract pathologies and auscultation of adventitious sounds (29). Hematologic changes in sheep with MV are non-specific (7,30), leaving the presumptive diagnosis of MV to be made solely on the basis of clinical signs of progressive weight loss and chronic clinical signs referable to the respiratory tract (5,6). Thus, the ability to use on-farm diagnostic imaging to improve the accuracy with which presumptive clinical diagnosis of interstitial pneumonia consistent with MV can be made while awaiting confirmatory testing is an important step in improving the decision-making process for treatment of these animals on initial examination. Maedi-visna infection may predispose affected sheep to secondary bacterial pneumonias due to immunosuppression as exhibited by multiple ewes in this case series. Antimicrobials can be used to control secondary bacterial pneumonia in MV-affected animals; however, most will succumb to the disease within a year of exhibiting clinical signs (3,31). With the increasing emergence of antimicrobial resistance in ruminant respiratory pathogens (32), use of tools such as digital radiography and ultrasonography to rapidly identify animals suspected to have chronic interstitial respiratory disease due to MV, and thus unlikely to respond favorably to antimicrobial therapy, will continue to become more important. Avoidance of unnecessary and/or ineffective antimicrobial therapy while awaiting serologic test results also allows for immediate removal from the flock once MV positive status is confirmed.

Thoracic ultrasound is a non-invasive modality employed to evaluate the pleura-lung interface, pleural space, and mediastinum, to facilitate establishing a more definitive diagnosis (21,22,24). Several recent studies detail the use of ultrasonography in respiratory disease in sheep, including its use for the diagnosis of ovine pulmonary adenocarcinoma, another chronic virus-associated condition (8,21,24–26,33). Although the ultrasonographic changes associated with interstitial pneumonia were subtle in this case series, they were repeatable across the affected animals.

The increased availability of portable digital radiography allows practitioners to immediately evaluate images on-farm, making this classic diagnostic tool potentially more accessible. Although the thoracic radiographs in this study were not obtained using a portable unit, acceptable images can be obtained with a portable unit. Respiratory motion due to tachypnea may decrease image quality on thoracic radiographs. In addition, human exposure to ionizing radiation during radiograph acquisition could be eliminated by using ultrasound. These factors may represent a continued limitation to on-farm thoracic radiography for patients with respiratory disease.

All the MV-positive animals represented in this case series were clinically affected animals; thus, it remains unclear as to the extent of lesions that may be visible via ultrasound or radiographs in animals sub-clinically affected with MV. Diagnostic imaging, often with thoracic ultrasound, is used in dairy cattle operations for identification of subclinical bronchopneumonia (22). One previous study in sheep investigated the usefulness of thoracic radiographs for identifying subclinical respiratory disease in experimentally infected lambs 3 mo after inoculation with MV, and failed to identify lesions associated with MV in these animals (20); however, MV is considered a disease of adult sheep (≥ 3 y old) and 3 mo would be considered a very short period of time after viral exposure. Further studies would be required to validate the efficacy of a test and cull program based on portable radiography and ultrasonography as screening tools for potentially MV afflicted animals. If proven effective, these tools could be used to increase suspicion/awareness in clinical cases, demonstrate lesions to owners/producers, and encourage further diagnostic testing of the flock and help producers in their control/culling efforts to reduce the transmission of MV infections within their flocks (1).

In summary, the ewes from this flock had clinical disease that was confirmed to be associated with MV, and recommendations were made to the flock owner and veterinarian to pursue whole-flock serologic testing for MV. Based on the positive results from diagnostic tests, our findings suggest that both radiography and ultrasonography can be useful aids in the diagnosis of active interstitial pneumonia caused by MV. These imaging modalities can be used in a private practice or an on-farm setting and can be easily implemented for further best-practices in identifying and culling sheep with signs of clinical disease as early as possible. CVJ