JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2023

P-P02

 

Signalment (JPC #3134519): 12-year-old female mixed breed dog

 

HISTORY: The dog was kept in a kennel and died spontaneously overnight.

 

HISTOPATHOLOGIC DESCRIPTION: Lung: There are multifocal to coalescing granulomas that efface the pulmonary architecture. The granulomas are composed of a central core of eosinophilic necrotic debris admixed with myriad metastrongyle larvae and eggs surrounded by epithelioid macrophages and multinucleated giant cells (foreign body type), which are further rimmed by fibroblasts, concentrically arranged mature collagen (fibrosis), lymphocytes, and plasma cells. The larvae are elongated with a thin eosinophilic cuticle and a primitive intestinal tract. The thin-walled eggs are ovoid, 50-60 μm in diameter, and contain either a morula or larva. The bronchial lumina contain numerous eggs and larvae intermixed with few histiocytes and multinucleated giant cells, sloughed epithelial cells, and abundant mucus. Multifocally, pulmonary arteries are severely dilated and the tunica intima is thickened by moderate amounts of fibrous connective tissue and few lymphocytes and plasma cells (proliferative endarteritis).  Within the lumen there are few cross sections of two morphologically distinct nematode adults. The larger adult nematode is 0.5-1 mm in diameter with a thin eosinophilic cuticle, internal cuticular ridges, tall polymyarian-coelomyarian musculature, and a small intestinal tract lined by cuboidal uninucleated cells (consistent with Dirofilaria immitis). The cross section of the second smaller adult nematode is 100 to 250 μm in diameter with smooth 3-4 μm cuticle, thin polymyarian-coelomyarian musculature, a large intestinal tract lined by few multinucleated cells, and a male reproductive tract. Intravascular adult nematodes are surrounded by thrombi composed of lamellar bands of fibrillar eosinophilic fibrin which multifocally are adhered to the vessel wall. Few vessels are occluded with organized fibrous connective tissue that contains multiple small blood-filled channels (organized thrombus with recanalization). The tunica media of numerous pulmonary arteries is markedly thickened with increase smooth muscle (medial hypertrophy).  

 

MORPHOLOGIC DIAGNOSIS: 1. Lung, arteries: Endarteritis, proliferative, chronic, multifocal, marked, with organizing thrombosis, dilatation, and few intravascular adult metastrongylid and filarid nematodes, etiology consistent with Angiostrongylus vasorum and Dirofilaria immitis.

2. Lung: Pneumonia, granulomatous, multifocal to coalescing, marked, with hemorrhage, fibrosis, and many nematode larvae and eggs, consistent with Angiostrongylus vasorum.

 

ETIOLOGIC DIAGNOSIS: Pulmonary angiostrongyliasis and dirofilariasis

 

ETIOLOGY: Angiostrongylus vasorum and Dirofilaria immitis

 

GENERAL DISCUSSION: 

• Depending on stage, produce different pulmonary lesions

• Adults: Chronic arteritis that leads to pulmonary hypertension, pulmonary arterial thrombosis, interstitial (eosinophilic) granulomatous pneumonia, pulmonary interstitial fibrosis, congestive right-sided heart failure, and eventually caudal vena caval syndrome; less commonly pleural petechial hemorrhages and diffuse pulmonary hemosiderosis and multifocal pulmonary infarcts
• Larvae and eggs of Angiostrongylus vasorum cause alveolar injury, thickening of alveolar walls with eosinophils and lymphocytes (interstitial pneumonia), and multifocal or coalescing granulomas with giant cells (parasitic granulomas)

 

• Dirofilaria immitis

• See C-P03 (cardiovascular) for additional discussion
• Spirurid, filarial nematode (family Onchocercidae)
• “American heartworm”
• Canine is the only mammal commonly infected and only significant reservoir (definitive host); other species can be infected but have low or absent microfilaria
• Has temperate, subtropical, and tropical distribution

• Angiostrongylus vasorum 

• Metastrongylid nematode (superfamily Metastrongyloidae, family Angiostrongyloidae)
• “French lungworm” because it was first reported in France in the 1800’s 
• Most pathogenic lungworm of dogs; causes mild to severe respiratory disease
• Adults inhabit pulmonary arteries and right ventricle of dogs and foxes
• Reported in Europe, South America, Newfoundland, and Africa; recent reports in West Virginia, USA, Nova Scotia, and Canada in coyotes (Kotwa J Vet Diagn Invest 2021); geographical range is expected to continue to expand

 

PATHOGENESIS:

• D. immitis

• Primarily a pulmonary vascular disease
• The bacterial endosymbiont Wolbachia sp. harbored by D. immitis results in more severe vascular lesions
• Right heart failure: inflammatory reaction to nematodes à myointimal proliferation mediated by platelet derived growth factor (PDGF) à fibromuscular vascular wall hyperplasia à pulmonary hypertension à right heart failure
• Vena caval syndrome: Large number of adult nematodes in right atrium and vena cava à venous obstruction à decreased venous return à shock 
• Renal: Immune response to all forms of the parasite à immune complex formation à deposition in glomeruli à membranoproliferative glomerulonephritis

• A. vasorum 

• Right heart failure and extensive pulmonary lesions are a consequence of egg embolization and pulmonary arterial thrombosis

• Both can induce pulmonary hypertension which may result in cor pulmonale (i.e. right heart failure secondary to pulmonary hypertension)
• Both can induce pulmonary thromboembolism and/or DIC of undetermined pathogenesis, possibly triggered by parasite proteins or endothelial damage from adult and larval nematodes
  • D. immitis may also induce thrombotic state due to immune complex deposition in glomeruli à immune complex glomerulonephritis à loss of anti-thrombin III into urine à hypercoagulable state

 

LIFE CYCLE

• D. immitis

• L1, L2, and early L3 larvae are obligate parasites of Aedes, Culex, and Anopheles spp. Mosquitoes à larvae enter definitive host (dog) when the mosquito feeds à larvae mature and reach the right ventricle in 3-4 months à adults reside in pulmonary vasculature and release microfilaria into the bloodstream

• A. vasorum 

• Indirect life cycle: Intermediate host is a gastropod such as aquatic and terrestrial snails (penetrate muscular foot)
• Adults shed eggs into pulmonary circulation à eggs deposited in pulmonary parenchyma where they develop and hatch à first stage larva penetrate the alveoli and are coughed up and swallowed à excretion in feces and ingestion by gastropod à maturation into L3 in gastropods à ingestion of gastropods by definitive host à L3 penetrate the GI tract à migration through mesenteric lymph nodes while developing into immature adults à migration through the portal circulation and reach maturity in pulmonary arteries

 

TYPICAL CLINICAL FINDINGS: 

• D. immitis

• Asymptomatic to severe systemic disease including vena caval syndrome
• Cough and exercise intolerance
• Vena caval syndrome (young dogs with large burden): weakness, anorexia, bilirubinuria, hemoglobinuria, and anemia (microangiopathic anemia with schistocytes)
• The number of circulating microfilaria does not correlate to the number of adult worms or severity of disease

• A. vasorum 

• Gagging, coughing, dyspnea, and exercise intolerance to bleeding and fatal cardiopulmonary disease
• Clinical pathology: Hypercalcemia, coagulation abnormalities suggestive of DIC (increased aPTT or PT, thrombocytopenia, elevated D-dimer, elevated fibrin degradation products) are second most common clinical manifestation

 

TYPICAL GROSS FINDINGS:  

• D. immitis

• Females are 25-31 cm in length x 1-1.3 mm in diameter; males are smaller, 12-20 cm in length x 0.7-0.9 mm in diameter
• Adult heartworms in pulmonary arteries and right ventricle; less frequently in the left ventricle and vena cava in heavy infestations (> 50 heartworms)
• Caudal lobar pulmonary arteries most severely affected
• Ascites, rarely cylothorax
• Hepatic chronic passive congestion

• A. vasorum 

• Adult, 14-21mm long worms in pulmonary arteries
• During prepatent period: Few 1-2 mm red nodules (aggregates of eosinophils and mononuclear cells)
• At time of patency: Red or golden-brown nodular or confluent areas of hemorrhage, edema, and firmness at periphery of lung 
• Chronic heart failure 
• Adults have “barber pole” appearance due to helically arranged red digestive tract and white reproductive tract

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

• D. immitis

• Pulmonary: Most common pulmonary parenchymal lesions are arterial thrombosis and periarterial and interstitial eosinophilic and granulomatous inflammation and fibrosis; diffuse pulmonary hemosiderosis; vasculitis
• Cardiovascular: Eosinophilic and proliferative endarteritis with infiltration of eosinophils and neutrophils
• Adult parasites are found in pulmonary artery and right heart and are characterized by thin eosinophilic cuticle, prominent lateral cords, evenly spaced lateral internal cuticular ridges, thick very well developed coelomyarian musculature, and a small (proportional to body size) intestine lined by few multinucleated epithelial cells
• Renal: Membranoproliferative glomerulonephritis

• A. vasorum: 

• Lungs: Eosinophilic and granulomatous pneumonia due to embolized eggs and larvae (coalescing granulomas)
• Pulmonary arteries: Proliferative endarteritis in response to adult worms (inflammation of the tunica intima and thickening of the of tunica intima by fibromuscular tissue); thrombosis; medial hypertrophy; infiltration of eosinophils, lymphocytes and plasma cells; chronic cases may have fibrosis and vascular recanalization of arterial thrombi
• Adults: Thin coelomyarian musculature, large strongylid intestine composed of few multinucleated cells, and uterus with eggs; found in pulmonary arteries and right heart
• All metastrongyles have polymyarian-coelomyarian musculature and L1 larvae have a distinctive kinked tail and small dorsal spine
• Aberrant migration of larva can cause granulomas with or without association of larvae and eggs in multiple organs including: brain, kidney, tracheo-bronchial lymph nodes, adrenal gland, skin, liver, pancreas, pericardial sac, urinary bladder, femoral artery, intestinal tract, thyroid gland, pituitary gland, skeletal muscle, heart, and eye

 

ADDITIONAL DIAGNOSTIC TESTS:  

• D. immitis

• Antigen SNAP test
• Antibody and modified knotts test
• A. vasorum: Identification of L1 via fecal examination using the Baermann technique or examination of bronchiolar lavage samples; serum ELISA for circulating antibody or PCR on BAL material

 

DIFFERENTIAL DIAGNOSIS:

Distinguishing Angiostrongylus and Dirofilaria:

cells, and eggs in the uterus

a uterus containing microfilariae.

Other pulmonary parasites in dogs:

• Crenosoma vulpis (P-P14): Chronic coughing due to infection of bronchioles, bronchi, and trachea, differentiate on cytology using morphology of tails (A. vasorum tails longer with indentation on dorsal and ventral surface, and dorsal spine)
• Eucoleus aerophilous (P-P15, formerly Capillaria aerophila): Trichurid nematode; adult embeds in respiratory epithelium; causes inapparent to catarrhal inflammation; the distinguishing morphologic feature is eggs with bipolar plugs; features of adults include segmental thickenings of hypodermis (bacillary bands), a basophilic esophageal gland (stichosome), and embryonated eggs in the uterus
• Oslerus osleri (P-P14, formerly Filaroides osleri): Single to multiple 1-10 mm, firm submucosal nodules in the trachea and bronchi which are most prominent at the tracheal bifurcation; granulomatous to pyogranulomatous inflammatory nodules containing adult parasites or fifth stage larvae
• Filaroides hirthi: Direct life cycle with most infections from the dam; adults live in alveoli and respiratory bronchioles; typically incidental findings at necropsy unless immunosuppressed; gray-tan to black-green nodules in subpleural regions of the lungs; nodules may be white or have cystic centers
• Andersonstrongylus milksi (P-P14, formerly Filaroides milksi): Similar lesions as F. milksi
• Parastrongylus cantonensis (P-P10, formerly A. cantonensis - rat lungworm): Accidental hosts; infection results in ascending paralysis and lumbar hyperalgesia due to an obligate neural migration cycle 

Other Dirofilaria spp.: (40 recognized species total)

• D. repens: affects subcutaneous tissues of dogs and carnivores, rarely humans
• D. tenuis: affects subcutaneous tissues of the raccoon in the southern US
• D. striata: affects bobcats, reported in dogs in Florida

 

COMPARATIVE PATHOLOGY:  

• D. immitis

• Cats: Usually infected with low numbers of adults (often one) and frequently male-only infections
• NHPs: heart and abdominal cavity of orangutans; pulmonary in rhesus monkeys
• Rabbits can be aberrant hosts; develop in pulmonary arteries, but die and cause organized thrombi
• Dogs: inconsistently found in cases of pulmonary eosinophilic granulomatosis (Abbott J Vet Diagn Invest 2020)
• Reported in bears (less severe), red pandas (death has been reported), sea lions, ferrets, and occasionally humans

• A. vasorum: Wild canids (foxes, coyotes, wolves, badgers); red panda; red foxes are the natural definitive host and an important reservoir for infection in domestic dogs

 

REFERENCES:  

1. Abbott DEE, Allen AL. Canine eosinophilic pulmonary granulomatosis: case report and literature review. J Vet Diagn Invest. 2020;32(2):329-335.
2. Boes KM. Respiratory System. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023:212, 227-228. 
3. Bourque AC, Conboy G, Miller LM, Whitney H. Pathological findings in dogs naturally infected with

Angiostrongylus vasorum in Newfoundland and Labrador, Canada. J Vet Diagn Invest. 2008;20(1):11-20.

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