JPC Systemic Pathology

Respiratory System

August 2026 P-P15

Histopathological Description

Trachea: Diffusely and circumferentially, the submucosa is thickened up to 700µm by macrophages, lymphocytes, plasma cells, with fewer eosinophils and neutrophils, and increased clear space (edema). Multifocally, the epithelium exhibits one of the following changes: piles up to 8-10 cell layers deep (hyperplasia); is attenuated, eroded, and often lacks cilia (degeneration); is infiltrated by numerous neutrophils, lymphocytes, and plasma cells, admixed with necrotic debris and sloughed epithelial cells (necrosis); or has undergone squamous metaplasia. Within the superficial epithelium there are several cross and tangential sections of adult nematodes that are up to 80 µm in diameter with a 2 µm thick, eosinophilic cuticle, coelomyarian-polymyarian musculature, bacillary bands, and a pseudocoelom that contains an intestinal tract including a stichosome (basophilic stichocytes surrounding the esophagus) esophagus and a reproductive tract. Submucosal glands are often ectatic, lined by attenuated epithelial cells, and contain small amounts of mucin.

Lung: Multifocally affecting approximately 60% of this section, bronchial and bronchiolar epithelium is hyperplastic with cells piled up to 6 layers deep, forming papillary projections into the lumen. Bronchial and bronchiolar lumina are variably filled with an exudate composed of sloughed epithelial cells, abundant mucin, eosinophils, intact and necrotic neutrophils, macrophages, fewer lymphocytes, necrotic cellular debris, and few of the previously described adult nematodes. Inflammation extends through the bronchial and bronchiolar walls and surrounds hyperplastic bronchial glands. Multifocally in the adjacent pulmonary parenchyma, alveolar lumina are filled with an exudate composed of neutrophils and macrophages admixed with necrotic cellular debris, colonies of 2 µm coccobacilli, fibrin, hemorrhage, and edema; alveolar septa often effaced in these areas. Less affected alveoli are filled with eosinophilic edema fluid and septa are thickened up to three times normal with congestion, macrophages, lymphocytes, and plasma cells. There is marked perivascular edema and mild hemorrhage with increased clear space and dilated lymphatics.

General Discussion

• Capillaria spp. are divided into: Eucoleus (airways), Aonchotheca (intestine), and Pearsonema (urinary bladder)
• Capillaria has wide variety of hosts: Canids (dogs, foxes, coyotes, wolves), cats, birds, frogs, fish, rabbits
• Eucoleus aerophilus are nematodes in Trichuridae family (stichosome esophagus), subclass Aphasmid
• Aphasmids differ from phasmid nematodes in the following ways:
  • Possess a row of basophilic esophageal gland cells (stichocytes) that form a stichosome and surround the esophagus - one of the most characteristic features
  • Lack a tiny pair of sensory papillae (phasmids) on their caudal end (not readily identifiable histologically) and lack lateral cords
  • Have bacillary bands (hypodermal bands with associated nuclei)
  • Adult females have only one genital tract (most phasmids have two)
• E. aerophilus parasitizes trachea and bronchi of wild canids, domestic dogs, and cats; can also infect the nasal passages of dogs
• Possible cause of eosinophilic bronchopneumopathy in dogs

Pathogenesis

• The body mounts an inflammatory response against developing adults in epithelium (bronchitis, bronchiolitis) > decreased mucociliary clearance > +/- secondary bacterial infection > pneumonia
• Heavy infections may cause sufficient inflammation to obstruct airways

Life Cycle

• Life cycle is indirect - earthworms are intermediate hosts
• Host ingests earthworm containing infective larvae > infective larvae penetrate intestinal wall > larvae migrate to lungs (via blood) > develop into adults in mucosa of trachea and bronchi > eggs laid in lumen, coughed up, swallowed, passed out with feces, become infective in 5 to 7 weeks (prepatent period)

Typical Clinical Findings

• Dogs and cats are usually asymptomatic
• Wheezing, chronic cough, weight loss

Typical Gross Findings

• Adult worms (20-40 mm long) embedded in mucosa of trachea and bronchi or nasal mucosa less frequently (dogs)

Typical Light Microscopic Findings

• E. aerophilus adult: Reduced polymyarian-coelomyarian musculature, two bacillary bands (hypodermal bands with associated nuclei), a stichosome esophagus (deeply basophilic esophageal gland cells [stichocytes] that surround the esophagus), a spiny sheath, and oval, embryonated, bioperculate eggs (approximately 70 µm x 35 µm) within airways
• Subacute to chronic mild tracheobronchitis; mild catarrhal inflammation most common

Additional Diagnostic Tests

• Fecal float, zinc sulfate flotation, Baerman technique, tracheal wash
• Cytology: Ova are bipolar thick-walled, pigmented golden brown, 58–79 μm length, 29–40 μm width; can be found in TW/BAL in conjunction with increased eosinophils (hypersensitivity)

Differential Diagnosis

• Common lungworms in cats: Aelurostrongylus abstrusus (P-P09, metastrongyle; most common nematode parasite in domestic cats), Troglostrongylus subcrenatus, Filaroides rostratus
• Bioperculate eggs: Capillaria feliscati, C. putorii, Trichuris spp., Pearsonema plica

Comparative Pathology

• Poultry: Capillaria annulata and C. contorta infect crop and esophagus
• Raptors: Eucoleus contortus and E. dispar have been diagnosed in hawks, falcons, eagles, harriers, and Osprey
• Wild canids including foxes: E. aerophilus mainly affects trachea and bronchi
• Eucoleus boehmi, related trichurid found in mucosal surface of nasal cavity and sinuses of wild canids and rarely domestic dogs and cats
• Hedgehogs: Recent European study identified Capillaria (Eucoleus) aerophilus as the most common respiratory parasite with 82% prevalence in the study cohort; larynx, trachea, bronchi, and bronchioles most affected; verminous pneumonia was the cause of death in 25% of cases in this study (Lehmann, 2024)

References

1. Boes KM. Respiratory Tract. In: Raskin RE, Meyer DJ, Boes KM, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023:225-226.
2. Caswell JL, Williams KJ. Respiratory System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 2. 7th ed. St. Louis, MO: Elsevier; 2026:578.
3. De Terlizzi R, English K, Cowell RL, Tyler RD, Meinkoth JH. Transtracheal and Bronchoalveolar Washes. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. Elsevier Mosby; 2020:257, 264.
4. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: Armed Forces Institute of Pathology; 1999: 40-43.
5. Keel MK, Terio KA, McAloose D. Canidae, Ursidae, and Ailuridae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:246.
6. Lehmann S, Dervas E, Ruiz Subira A, et al. Verminous pneumonia in European hedgehogs (Erinaceus europaeus). Vet Pathol. 2024;61(2):256-268.
7. Lopez A, Martinson SA. Respiratory System, Thoracic Cavities, Mediastinum, and Pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. Elsevier; 2022:637.