Merck Manual

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Professional Version

Lung Nematodes in Small Animals


Caroline C. Tonozzi

, DVM, DACVECC, Mission Veterinary Partners

Reviewed/Revised Feb 2022 | Modified Oct 2022

Aelurostrongylus abstrusus

Aelurostrongylus abstrusus, the most common lungworm of cats, is found in many parts of the world, including the US, Europe, and Australia. They are small parasites (males 7 mm, females 10 mm), deeply embedded in the lung tissues. The eggs are forced into alveolar ducts and adjacent alveoli, where they form small nodules and hatch. Once the larvae escape, they are coughed up, swallowed, and passed in the feces. The larvae seen in the feces of infected animals are tightly coiled, have an undulating tail with a spine, and are < 400 μm long. The life cycle includes snails or slugs as first intermediate hosts, and frogs, lizards, birds, or rodents as transport hosts of encysted larvae. When one of these transport hosts is eaten, the larvae migrate from the stomach to the lungs via the peritoneal and thoracic cavities. They reach the lungs within 24 hours and are seen in the feces in ~1 month.

Although not uncommon, clinical and diagnostic signs are often lacking. Chronic wasting, cough, dyspnea, and pulmonary wheezes may be seen. Thoracic radiographs may show a diffuse miliary or bronchial pattern with nodular interstitial changes early, advancing to alveolar disease in the later stages. The lungs usually have solidified, gray, raised nodules 1–10 mm in diameter; generalized alveolar disease has been seen in chronic cases. Treatment recommended for A abstrusus is ivermectin 0.4 mg/kg, SC, once, and repeated in 2 weeks, or selamectin, 6 mg/kg, topically, once.

Capillaria aerophila (Eucoleus aerophilus)

Although usually parasites of the frontal sinuses, trachea, bronchi, and rarely nasal cavities of foxes, C aerophila are found in dogs and other carnivores. They are 25–35 mm long. The females produce eggs with bipolar plugs that resemble those of whipworms; however, their shells are colorless to greenish and pitted. The eggs are laid in the lungs, coughed up and swallowed, and passed in the feces. The eggs can be identified from either tracheal washes and bronchoalveolar lavage or fecal flotation. The life cycle is direct; dogs become infected through consumption of feed or water contaminated with larvated eggs. After hatching in the intestine, the larvae reach the lungs and bronchi via the circulatory system. They mature ~40 days after infection.

Clinical signs include coughing, sneezing, and nasal discharge. Treatment may be attempted using fenbendazole (50 mg/kg/day, PO, for 10–14 days) or ivermectin (200 mcg/kg, SC, twice at a 3-week interval).


Oslerus osleri are tracheal worms of dogs, usually found in thin-walled nodules around the bronchial bifurcation. They have been found in the US, South Africa, New Zealand, India, Great Britain, France, and Australia. The males are ~5 mm long, and the females 10–15 mm. The life cycle is direct, and an infected bitch can transfer larvae in her saliva to her pups while licking and cleaning them. On ingestion, the larvae pass to the blood and are carried to the lungs and bronchi.

A persistent, dry cough is the most common clinical sign. Coughing may later become severe, with respiratory distress. Finding larvae in the feces is diagnostic, but because these larvae are lethargic and few in number, bronchoscopy is a better method. Surgical excision of the nodules combined with administration of fenbendazole, levamisole, or thiabendazole has effectively treated infected dogs.

Filaroides hirthi is similar to O osleri but is found in the lung parenchyma. The females are ovoviviparous. Adults are found in nests in the lung parenchyma, where a focal granulomatous reaction occurs. Diagnosis of low-grade infection can be difficult. Zinc sulfate flotation is usually more successful than using a Baermann apparatus. Treatment with fenbendazole (50 mg/kg/day, PO, for 10–14 days) or less preferably albendazole (25 mg/kg, PO, twice a day for 5 days and repeated in 2 weeks) has reportedly been effective. Ivermectin (200 mcg/kg, SC, twice at a 3-week interval) may also be effective.

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