JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2026 P-P14

HISTOPATHOLOGIC DESCRIPTION:

Lung: Diffusely, bronchial, bronchiolar, and alveolar lumina contain tangential and cross-sections of adult and rare larval nematodes. Adult nematodes are 100 µm in diameter and have a thin cuticle, coelomyarian-polymyarian musculature, and a pseudocoelom that contains a large intestine lined by a few multinucleated cells and that often contains golden-brown granules and reproductive tracts with developing larvae or sperm. Larvae are 20-30 µm in diameter with a thin eosinophilic cuticle containing many basophilic nuclei and a tapered end. Bronchial, bronchiolar, and alveolar lumina are filled with an exudate composed of plasma cells, lymphocytes, eosinophils, macrophages that are sometimes hemosiderin-laden, multinucleated giant cells (foreign body or Langhans type), and neutrophils admixed with small amounts of fibrin, hemorrhage, edema, and karyorrhectic and cellular debris (necrosis). Alveolar septa are mildly expanded by lymphocytes, macrophages, fibrin, and edema.

HISTOPATHOLOGIC DESCRIPTION:

Trachea and peritracheal tissue: Diffusely infiltrating the tracheal submucosa, forming inflammatory nodules in the peritracheal adventitia, and extending into adjacent adipose tissue and skeletal muscle are eosinophils, lymphocytes, plasma cells, and fewer histiocytes and neutrophils admixed with fibrin, edema, and hemorrhage. Inflammatory nodules are multifocally centered on lymphatic vessels that contain intraluminal tangential and cross sections of adult nematodes. Adult nematodes are 80µm in diameter and have a 4µm thick cuticle, coelomyarian-polymyarian musculature, and a pseudocoelom that contains an intestinal tract lined by few multinucleated cells and a reproductive tract that either contains developing larvae or sperm. There is multifocal attenuation, erosion and ulceration of the tracheal epithelium.

Lymph node, tracheobronchial: The subcapsular and medullary sinuses contain few to moderate numbers of eosinophils and few neutrophils and histiocytes that occasionally contain intracytoplasmic erythrocytes (erythrophagocytosis) as well as fibrin, hemorrhage, and mild edema.

HISTOPATHOLOGIC DESCRIPTION:

Lung: Multifocally bronchiolar and bronchial lumina contain cross and tangential sections of adult nematodes that are 350-400 µm diameter and have a thin cuticle, coelomyarian-polymyarian musculature, and a pseudocoelom that contains a prominent digestive tract lined by a few multinucleated cells and a reproductive tract containing larvae or clusters of eosinophilic sperm. Larvae are rarely noted within lumina of the bronchi, bronchioles, and alveoli; they are 10-15 µm in diameter with a thin eosinophilic cuticle and a basophilic granular interior. Multifocally, the lumina of bronchi and bronchioles are filled with a dense cellular exudate composed of viable and necrotic neutrophils, eosinophils, macrophages, and fewer plasma cells and lymphocytes admixed with necrotic karyorrhectic and cellular debris, eosinophilic fibrillar material (fibrin), edema, scattered foci of mineralization, few sloughed epithelial cells, and hemorrhage. The peri-bronchial and peri-bronchiolar connective tissue is moderately expanded by the previously described inflammatory cells, fibrin, and edema. There is multifocal stacking of respiratory epithelial cells (hyperplasia). Multifocally, alveolar lumina contain hemorrhage, fibrin, edema, and rare aggregates of pigment laden macrophages (hemosiderin and anthracosilicosis). There is multifocal type II pneumocyte hyperplasia. Alveolar septa are multifocally discontinuous (septal necrosis) with resultant confluent alveolar spaces (emphysema).There is increased perivascular clear space (edema).

GENERAL DISCUSSION:

• Superfamily Metastrongyloidea:
  • Family Filaroididae:
    • Filaroides hirthi
    • Oslerus osleri (formerly Filaroides osleri)
  • Family Angiostrongylidea
    • Andersonstrongylus milksi (F. milksi in some references)
• Angylostongylus vasorumFamily Crenosomatidae
  • Crenosoma vulpis
• Filaroides/Oslerus/Andersonstrongylus: Respiratory parasites found in canidae (including wolves, foxes, dingos, and coyotes), mink, raccoons, monkeys and red pandas
  • Filaroides is occasionally seen in lab beagles used for pharmaceutical/toxicity studies where it can obscure/confound interpretation; endemic in some institutions
• Crenosoma: Respiratory parasite found in canidae, raccoons; has been reported in badgers, wolverines, black bears; C. vulpis is the fox lungworm

LIFECYCLE:

• Similar for F. hirthi and O. osleri (direct life cycle)
  • Adults in nodules in tracheal mucosa (O. osleri) or in bronchiolar/ alveolar lumina (F. hirthi) → produce thin walled embryonated eggs → eggs coughed up and swallowed → passed in feces (most having ‘hatched’) → L1 swallowed → migrate via hepatic portal circulation and/or mesenteric lymphatic drainage to the trachea or lung > 4 molts > adult (~5weeks after infection)
  • Unlike other metastrongyloids, this family does not have an intermediate host
  • <1yr most affected; puppies often acquire infection from dam’s sputum, grooming, regurgitative feeding, or via fecal-oral transmission
• A. milksi lifecycle is unknown; molluscan intermediate host proposed
• Crenosoma sp. requires a snail/slug intermediate host
  • 18-21 day prepatent period → adults live in small bronchi/bronchioles → deposit larva → coughed up and swallowed → passed in feces → eaten by snail/slug → definitive host eats snail/slug

TYPICAL CLINICAL FINDINGS:

• Dogs (and other infected mammals) are asymptomatic; rarely have a chronic non-productive cough (‘retching’)
• Immunocompromised dogs may develop severe pneumonia that can be fatal

TYPICAL GROSS FINDINGS:

• F. hirthi/A. milksi (Lung): Gray-tan or black-green, 1-5 mm diameter nodules scattered widely in subpleural regions of the lung; may have clear cystic center or be white and firmO. osleri (Trachea): Single or multiple 1-10 mm diameter, firm, gray-pink, sessile submucosal nodules in trachea and bronchi, tracheal bifurcation; coiled worms are visible through the intact overlying mucosa
  • Nodules are detectable at 10 weeks, well developed by 18 weeks post-infection
• C. vulpis (Lung): Grey consolidation of caudodorsal lung; adult worms live in bronchioles and small bronchi

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Metastrongyles: Coelomyarian-polymyarian musculature, accessory hypodermal cords, intestine is large and lined by few multinucleated cells, larvae or embryonated eggs within uterus
• Greenish hue of Filaroides can be attributed to eosinophils; eosinophilic granulomas from alveolar interstitium
• Live worms produce minimal reaction
• Dead worms result in severe granulomatous and eosinophilic inflammation, with giant cells
• C. vulpis: Catarrhal, eosinophilic bronchitis and bronchiolitis; alveolar granulomatous reaction with aspirated larvae; may see bronchiolar goblet metaplasia with mucus plugs causing lobar atelectasis
  • Lack a dorsal spine or “kink” to differentiate from A. vasorum
• Cannot distinguish eggs/larvae of Oslerus or Filaroides from each other; use location as a hint

ADDITIONAL DIAGNOSTIC TESTS:

• Fecal examination (Zinc sulfate flotation > Baermann technique)
• Tracheal wash/bronchoalveolar lavage to identify larvae or see eosinophilc exudate
• Bronchial brushing
• Radiography (visualize nodules); may resemble neoplasia
• Bronchoscopy: Nodule sample and biopsy
• Crush preparations of nodules (Oslerus)

DIFFERENTIAL DIAGNOSIS:

Pulmonary parasites of dogs:

• Eucoleus aerophilus (Capillaria aerophila): Aphasmid of the trachea and bronchi of foxes, dogs, and cats
• Angiostrongylus vasorum (Angiostrongylidae): Metastrongyle in pulmonary arteries and right ventricle of dogs/foxes
• Dirofilaria immitis: Filarid nematode in pulmonary arteries with associated arteritis and thrombosis; other sequela include eosinophilic granuloma formation and eosinophilic pneumonitis; With A. vasorum both have barber-pole shape in pulmonary a. and right ventricle, but A. vasorum has larger intestine with multinucleate cells and eggs in uterus

Tracheal parasites of dogs:

• Eucoleus aerophilus (Capillaria aerophila)

COMPARATIVE PATHOLOGY:

Selected respiratory metazoan parasites (not comprehensive):

REFERENCES:

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