Chapter 57: Respiratory Parasites

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Respiratory Parasites

Various parasites, including nematodes, trematodes, protozoa, and arthropods, have a predilection for invading the nasal cavity, airways, lung parenchyma, and pulmonary arteries of dogs and cats. This chapter focuses on primary parasites of the respiratory system that live in the nasal cavity, airways, or lung tissue. Intestinal parasites that transiently migrate through the lung during development such as Toxocara canis and Ancylostoma caninum are not discussed here. Systemic parasitic infections that occasionally cause pulmonary disease such as infection with the protozoan Toxoplasma gondii also are not included. In addition, cardiovascular parasites that live primarily in the heart or pulmonary arteries (Dirofilaria immitis, Angiostrongylus vasorum) are beyond the scope of this chapter, although these are important causes of secondary cardiopulmonary disease (see Chapters 183 and 184).

Nasal Parasites

Nasal parasites of clinical significance include worms, mites, and Cuterebra fly larvae. These cause nonspecific signs of acute or chronic rhinitis that are difficult to distinguish from more common causes of nasal disease.

Eucoleus boehmi

Eucoleus boehmi is a small (1.5 to 4 cm long) capillarid nematode that parasitizes the epithelial lining of the nasal passages, turbinates, and sinuses of dogs. Eggs are passed into the environment in nasal discharges or in the feces when nasal secretions are swallowed. The life cycle of E. boehmi is not well understood, but infection likely occurs by direct transmission through coprophagia or may involve earthworms as transport hosts. Infection can spread among dogs kenneled together, and this has been recognized in racing greyhound kennels.

Clinical Signs and Diagnosis

Eucoleus infection may be clinically inapparent or it may cause signs of chronic rhinitis, including sneezing, mucopurulent or blood-tinged nasal discharge, nasal congestion, and reverse sneezing. Aberrant migration through the cribriform plate caused intracranial infection and seizures in one dog (Clark et al, 2013). The diagnosis is confirmed by the microscopic identification of characteristic clear to golden barrel-shaped eggs with clear bipolar plugs on routine fecal flotation or in nasal flushes. Egg excretion may be cyclical and easily overlooked, so multiple fecal examinations may be needed to detect infection. E. boehmi eggs are misidentified easily as eggs of Eucoleus aerophilus (see section on E. aerophilus later in the chapter) because of their similarity in appearance; however, E. boehmi eggs are slightly smaller (50 to 60 µm × 30 to 35 µm) and have a distinctive finely pitted shell surface with readily visible spaces between the eggshells and embryonated contents. Eggs of E. boehmi also resemble those of the whipworm Trichuris vulpis, but whipworm eggs are larger and have a smooth outer shell. Adult E. boehmi nematodes attached to the nasal mucosa occasionally are identified during rhinoscopy and in nasal mucosal biopsy specimens.


Effective treatments for E. boehmi include an extended course of fenbendazole (Panacur; 50 mg/kg q24h PO for 14 days), milbemycin oxime (Interceptor; 2 mg/kg PO repeated at 1- to 2-week intervals for a total of three doses), or ivermectin (Ivomec; 0.2 to 0.4 mg/kg SC or PO repeated at 2-week intervals for a total of two or three doses). Ivermectin should not be administered to collies, shelties, and other herding breeds without first determining safety by performing an MDR1 (multidrug resistance 1) genotype test (available at A single topical spot-on application of moxidectin (2.5 mg/kg) in a formulation with imidacloprid (Advantage Multi) was effective in a dog. During and after treatment, feces should be removed from the dog’s environment to prevent reinfection through coprophagia. Successful treatment is confirmed by clinical response and follow-up fecal examinations. Relapses can occur after initial treatment, necessitating additional courses of therapy.

Mammomonogamus ierei

Mammomonogamus ierei is a small nematode, ranging in length from 0.5 to 2 cm, that is widespread in cats in Puerto Rico, Saint Kitts, and other islands of the Caribbean, where it is found attached to the mucosal lining of the nasal cavity and nasopharynx and causes mild chronic inflammation. Clinical signs generally are minimal. The life cycle is unknown. The diagnosis is confirmed by fecal flotation to identify the typical large embryonated eggs (84 to 100 µm × 48 to 52 µm), which appear very similar to hookworm eggs except that Mammomonogamus eggs are larger and have a thicker shell. Eggs also can be detected in rhinoscopic cytologic specimens and nasal flush samples. Treatment of Mammomonogamus in cats has not been well documented, but fenbendazole (dosed as for Eucoleus infection) is suggested.

Pneumonyssoides caninum

Pneumonyssoides caninum is a small mite (1 mm in length) found worldwide that infects the nasal cavity and sinuses of dogs. The parasite is endemic in Scandinavia with over 20% of dogs infected in some regions. Nasal mite transmission is presumed to occur through direct contact with infected animals, and infection can spread among dogs confined together in the same household or kennel.


Cuterebra spp. larvae are the larval maggot forms (bots) of numerous species of arthropod flies that infect mostly rabbits and rodents. Cuterebra occasionally can infect outdoor dogs and cats in spring and summer and migrate especially in the subcutis of the face, head, and neck region. Rarely Cuterebra larvae can cause respiratory disease when they migrate in the nasopharynx, causing sneezing and unilateral bloody nasal discharge, or in the wall of the cervical trachea, causing clinical signs of cough and airway obstruction. The diagnosis is made by direct endoscopic visualization of the Cuterebra larvae embedded in the nasopharyngeal or upper airway mucosa. An enzyme-linked immunosorbent assay (ELISA) for Cuterebra-specific immunoglobulin G antibodies looks promising as a serologic test for presumptive diagnosis of invasive cuterebriasis (Davis et al, 2013). Airway Cuterebra can be treated by endoscopic extraction or by administration of a single dose of ivermectin (0.2 to 0.4 mg/kg SC; see precautions in the section on E. boehmi). Before endoscopic extraction is performed, injection of an antihistamine and corticosteroid is advisable to prevent anaphylaxis if the Cuterebra ruptures. The use of prednisone (1 mg/kg q12h PO for 2 weeks) may reduce the local inflammatory reaction to the parasite.

Linguatula serrata

Linguatula serrata, a pentastomid parasite, is a large crustacean-like arthropod that firmly attaches to the mucosa of the nasal passages of dogs, causing signs of chronic rhinitis (sneezing, nasal discharge) and nasal obstruction. Infected dogs have been reported in parts of Europe, the Middle East, North Africa, South America, and Australia. Adult females grow up to 10 cm long and males are 2 cm long. The wormlike body is tan and annulated. After a prepatent period of 6 months, eggs are shed in the nasal secretions of dogs and are ingested by an intermediate host, especially ruminants such as sheep. The larvae hatch and develop as nymphs in the viscera of the intermediate host. The life cycle is completed when a dog ingests sheep offal and the parasite migrates from the dog’s pharynx into its nasal passages. L. serrata is diagnosed by visualization of the adult parasite in the nasal passages or by identification of yellow oval eggs (80 µm in length), which contain four-legged larvae, in nasal secretions. Treatment usually is by physical removal of the parasite, but ivermectin (0.2 to 0.4 mg/kg SC; see precautions in the section on E. boehmi) may also be effective.

Bronchopulmonary Parasites

Most bronchopulmonary parasitic infections are caused by metastrongyloid nematodes such as Oslerus osleri, Filaroides hirthi, Filaroides milksi, and Crenosoma vulpis in dogs and Aelurostrongylus abstrusus in cats. Both dogs and cats also can be infected with the capillarid lungworm Eucoleus aerophilus (also known as Capillaria aerophila) and the trematode lung fluke Paragonimus kellicotti.

Lungworm and lung fluke infections are relatively uncommon; young animals are most susceptible. Many infections are inapparent or subclinical; however, when clinical signs occur, chronic persistent cough is typical. Severe infections can cause dyspnea or be complicated by secondary bacterial bronchopneumonia. Abnormal bronchopulmonary infiltration patterns often are found on routine thoracic radiographs in animals that manifest overt clinical signs. An increase in circulating eosinophils sometimes is seen on a complete blood count, but this is not a consistent finding. Results of other routine laboratory evaluations usually are unremarkable. Definitive diagnosis requires identification of parasite eggs or first-stage larvae in the feces or in respiratory tract cytologic specimens. Standard benchtop fecal flotation generally is not reliable for detection of larvae and operculated eggs of respiratory parasites. Fecal examination techniques such as zinc sulfate (specific gravity, 1.18) centrifugal flotation (Oslerus, Filaroides, Eucoleus), Baermann procedure (Crenosoma, Aelurostrongylus), or fecal sedimentation (Paragonimus) are more effective.

Oslerus osleri

Oslerus osleri is a metastrongyloid nematode (6.5 to 13.5 mm in length) that lives in granulomatous nodules located on the mucosal surface of the distal trachea, tracheal bifurcation, and first-division bronchi of dogs, especially in young dogs less than 2 years of age. Wild canid species such as the North American coyote and Australian dingo have a high prevalence of O. osleri infection and may serve as natural reservoirs of infection, although one study found that dogs exposed to infective larvae derived from coyotes failed to develop O. osleri infection.

O. osleri has a direct life cycle that does not require an intermediate host; thus infective first-stage larvae that are coughed up, swallowed, and passed in the feces can be transmitted directly to another animal through coprophagia. Saliva may be more important than feces as a means of transmission. Larvae that are expelled from the trachea with respiratory secretions can be transmitted through saliva from dam to puppy during maternal licking and regurgitative feeding. In the new host, tracheal nodules develop 10 weeks after exposure, which coincides with the arrival of immature worms in the trachea. The prepatent period ranges from 12 to 18 weeks.


Bronchoscopy is the most reliable way to detect the distinctive tracheobronchial nodular lesions (granulomas) caused by O. osleri (Web Figure 57-1). In some dogs these nodules also can be identified on thoracic radiographs or computed tomographic scans as large, space-occupying mucosal masses protruding into the lumen near the bifurcation (Web Figure 57-2). The diagnosis of O. osleri infection is confirmed by identification of thin-walled larvated eggs (80 by 50 µm) and coiled larvae (230 to 267 µm with kinked tail) in airway washings or bronchoscopic biopsy specimens. Biopsy samples of parasitic nodules often reveal numerous O. osleri larvae, eggs, and adult worms embedded in granulomatous inflammatory tissue (Web Figure 57-3).

Feces also can be examined for larvae, but this is less reliable than examination of airway specimens. Larvae passed in feces are larger (326 to 378 µm). For detection of O. osleri larvae in feces, zinc sulfate centrifugal flotation is preferred over the Baermann technique; however, a negative result on fecal examination by either method is inconclusive, and fewer than one third of active infections are detected by fecal testing. This may be attributable to the presence of few larvae and an intermittent pattern of larval shedding. The larvae of O. osleri are morphologically indistinguishable from those of Filaroides spp.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 57: Respiratory Parasites
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