Pathology in Practice

Shelley J. Newman Newman Specialty VetPath, Hicksville, NY

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Richard W. Gerhold Department of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN

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Stephen A. Smith Department of Biomedical Sciences and Pathobiology, Virginia-Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA

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History

A 12-year-old 62-kg Finn-Shetland crossbreed ewe was euthanized (captive bolt followed by exsanguination from the carotid artery) because of the recent progression of a mammary mass. The ewe was from a small farm flock from which fleeces were harvested yearly. The ewe had not been bred or had any offspring for the past 5 years but was deemed by the owner to have been healthy, with a good appetite, activity level, and behavior. The flock was maintained on pasture (approx 2 acres with paster rotation) year-round, with the diet supplemented by corn and hay during the colder months of the year. Mineral supplement was provided free choice, and lambs were vaccinated with a multivalent clostridial vaccine. No regular deworming program was practiced, as annual fecal examinations did not demonstrate clinically important parasite egg numbers.

Clinical and Gross Findings

A complete field postmortem examination was performed. The left mammary gland was 27 × 14.5 × 10 cm and 1.35 kg without skin; the right mammary gland was 12 × 7 × 5 cm and 0.78 kg. The left mammary gland mass was expansile, was multinodular to multicystic, and, on cut surface, was composed of multiple nodules and cysts separated by white fibrous tissue within a capsule. The largest and most defined cysts within the mammary mass included 2 large fluctuant cysts (2 × 2 × 0.8 cm to 10 × 10 × 3 cm) and 4 small coalescing fluid-filled cysts. The largest cyst contained approximately 50 mL of brown fluid admixed with brown clotted material.

Throughout all lung lobes were multiple firm tan to brown nodules (1 to 3 mm in diameter). On cut section, these nodules were firm cystic structures that contained scant brown fluid. At the dorsomedial area of both caudal lung lobes, there were tan to white, slightly raised consolidated regions, which on cut section were often wedge-shaped, slightly bulging, and of firmer consistency (Figure 1). Additionally, multiple firm tan nodules were along the serosal surface of the cecum and small intestines, mainly jejunum, and multiple yellow gritty nodules were in and adjacent to the mesenteric lymph nodes. Throughout all liver lobes were multifocal raised tan nodules (range, 0.8 × 0.5 × 0.5 cm to 1.0 × 1.0 × 0.5 cm) that were solid on cut section and accompanied by multiple white depressed capsular scars.

Figure 1
Figure 1
Figure 1

Dorsal perspective (A) and cut surface (B) of the lungs of a 12-year-old 62-kg Finn-Shetland crossbreed ewe that was euthanized because of recent progression of a mammary mass. A—The dorsomedial aspects of the caudal lung lobes contain white to tan nodules, and there is a peripheral targeting of lobe margins. B—A raised, consolidated, white to tan pulmonary nodule is evident.

Citation: Journal of the American Veterinary Medical Association 260, 3; 10.2460/javma.20.12.0663

Histopathologic Findings

Histologic changes in the mammary gland supported the diagnosis of carcinoma developing within an adenoma. The left mammary gland mass was expansile but composed of lobular arrangements of tubular epithelium, with fewer solid lobules (< 5%), with slight peripheral invasion and a desmoplastic response. In the areas of malignant transformation, the mitotic count was 1 mitotic figure/10 hpf (2.37 mm2).

Histologic examination of lung tissues revealed multifocal airway and parenchymal eosinophilic granulomas. Foci of eosinophilic material, interpreted to be eosinophil degranulation product, were surrounded by intact eosinophils admixed with macrophages, lymphocytes, plasma cells, and fibroblasts. Recognizable inciting agents were rarely identified within these granulomas. Larval nematodes (approx 40 µm in diameter) were free in the parenchyma and surrounded by lymphocytes, eosinophils, and epithelioid macrophages arranged in circumferential sheets (Figure 2). These larval nematodes contained a thin cuticle layer and hypodermis, a thin musculature, a pseudocoelom with multiple accessory hypodermal chords and an intestinal tract lined by multinucleated epithelium, characteristic of metastrongyles and more specifically that of pulmonary lungworm larvae. Diffusely, the alveolar septae were thickened 2- to 3-fold by fibrous connective tissue.

Figure 2
Figure 2
Figure 2

Photomicrographs of tissue sections of lung (A) and mesenteric lymph node (B) from the ewe described in Figure 1. A—Multifocal eosinophilic and granulomatous inflammation is characterized by a core of neutrophils mixed with larger numbers of eosinophils, rimmed by reactive macrophages and multinucleated giant cells. Two nematode larvae (arrows) are evident in cross-section. H&E stain; bar = 200 µm. B—Cross and longitudinal sections of intralesional nematode larvae (arrows) are present, and eosinophilic and histiocytic lymphadenitis is evident. H&E stain; bar = 120 µm.

Citation: Journal of the American Veterinary Medical Association 260, 3; 10.2460/javma.20.12.0663

Enlarged mesenteric lymph nodes contained multifocal eosinophilic granulomas, and a few similar granulomas were within the surrounding mesentery. In rare instances, sections of nematode larvae were evident (Figure 2). In all sections of liver, there were numerous variably sized chronic eosinophil-rich granulomas with eosinophils surrounded by macrophages and multinucleated giant cells. These granulomas resembled those previously described in other tissues; however, no recognizable organisms were noted in these hepatic granulomas, which may have been due to section of cut or scarcity of the larvae. There was extensive fibrous connective tissue surrounding portal triads.

Additional Clinicopathologic and Histologic Findings

Ten 3-µm scrolls of paraffin-embedded lung sections containing nematodes were sectioned and submitted to the University of Tennessee Parasitology Laboratory for PCR assay with the use of extracted DNA1 and NC1 and NC2 primers (Integrated DNA Technologies Inc) as previously described.2 The resultant sequence had a 96.9% to a 99.7% identity to Muellerius capillaris sequences in the GenBank database.

Diagnoses and Case Summary

Multifocal eosinophilic and granulomatous interstitial pneumonia and alveolar fibrosis, with eosinophilic granulomas and intralesional third-stage larvae (L3) of nematodes, consistent with the lungworm M capillaris; multifocal eosinophilic granulomas in mesentery and liver; multifocal eosinophilic granulomas with intralesional nematodes (L3) in a mesenteric lymph node; and mammary carcinoma transformation from adenoma in a 12-year-old Finn-Shetland crossbreed ewe.

Comments

Mammary tumors are uncommon in sheep, and the unique pathologic findings of this particular case have been summarized previously.3 Postmortem findings revealed parasitism was the primary cause of multiple lesions in multiple organs in this case. The lung, liver, mesentery, and mesenteric lymph nodes all revealed similar parasitic eosinophilic granulomas, some of which revealed causative, nondegenerative nematode worms, supportive of the diagnosis of visceral larval migrans, most consistent with that caused by second-stage larvae or L3, on the basis of the larval width measurements.4 Although Ascarids, such as Ascaris ovis, are a more common class of nematode that produces this type of migratory lesion, strongyles (eg, M capillaris, Protostrongylus rufescens, and Parelaphostrongylus tenuis, in decreasing order of likelihood) and strongyloides (eg, Strongyloides papillosus) can produce similar pathology in livestock.

Deworming practices had not been routinely performed in this flock, as regular annual monitoring via fecal flotation for parasite eggs failed to reveal clinically important egg burdens, as might be expected with ascarid infections. However, with strongyle and strongyloides infections, first-stage larvae (L1) could have passed in the feces and have been missed on a fecal flotation assessment. The more definitive diagnostic procedure needed to detect L1 of these parasites would require a Baermann technique, which was not performed in this case.

On small hobby farms, where sheep are raised as pets, for wool, or both, M capillaris is not considered to cause serious clinical disease, even with high worm burdens. Where sheep are raised for meat, lungworms are a cause of substantial economic loss due to the identification of pulmonary lesions at the abattoir.5,6, Lung lesions vary from emphysematous areas, small firm nodules, and raised white consolidated areas as seen in the ewe of the present report to the finding of worms in the distal bronchioles.5,7,8 Histologically, various stages of L1 development may be found in the pulmonary parenchyma, surrounded by eosinophils and mixtures of lymphocytes, plasma cells, and macrophages and accompanied by variable bronchitis and diffuse interstitial pneumonia.5 Data from a large abattoir survey6 of sheep determined that the right lung (387/497 [92%]) and the cranial lobes (383/497 [77%]) were the most commonly affected portions of lung. Additionally, pathology may vary depending on the parasite, with Protostrongylus infections giving rise to fewer granulomas than species of Muellerius or Cystocaulus.7,8 Additionally, a poorer body condition score has been associated with the presence of lungworm.9 Recommended treatment strategies include pasture rotation, testing and deworming, and use of different anthelmintics if laboratory or field test results indicate administration of a different anthelmintic.5,9,

Lungworms of importance in small ruminants worldwide include M capillaris, P rufescens, Cystocaulus ocreatus, and Dictocaulus filaria.10 Muellerius capillaris is the most common lungworm in sheep.10 Infections are usually subclinical, especially if the infection occurs gradually and sheep develop resistance.10 Adult lungworms live in the lung, as do the L1, which can be passed from the definitive host in the feces. Development from L1 to L3 occurs in the gastropod intermediate host. When the gastropods are ingested, the terminal larvae migrate through lymphatics of the sheep until they reach the alveoli.10 The lymphatic route of transport may help explain the presence of larvae in the mesenteric lymph nodes and mesentery in this case.

For the ewe of the present report, M capillaris was diagnosed on the basis of classic gross lung lesions, the presence of nematode larvae within the histologic lesions, and the results of PCR assay and DNA sequencing of the lung tissue. The presence of similar histologic lesions in liver and lymph tissue was a less common presentation and captured the migratory L3 of the parasite. The ewe was euthanized for an unrelated mammary neoplasm and without evidence of respiratory signs suggestive of the parasite-induced lesions. This case highlighted the importance of fecal testing beyond the classical Sheather sugar flotation technique to detect the burden of lungworm within an animal and reminded us of the potential for migratory lesions in multiple tissues before lungworms populate their final host tissue.

References

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