Respiratory infection in a Harris’s hawk (Parabuteo unicinctus) by Cyathostoma spp.

Felipe Reggeti, Jacob Avula, Ginger Louws

Animal Health Laboratory, University of Guelph, Guelph, ON (Reggeti, Avula); Campus Estates Animal Hospital, Guelph, ON (Louws)

AHL Newsletter 2025;29(3):12.

A 10-year-old female Harris’s hawk was presented to the referring veterinarian with a history of coughing and sneezing. The patient had been suspected of having aspergillosis the previous year. On clinical examination, a bloody mucoid discharge was observed. A complete blood count revealed a mild leukocytosis of 25.6 × 10⁹/L (reference interval [RI]: 5–15×10⁹/L) and a mild eosinophilia of 6.4×10⁹/L (RI: 0–4×10⁹/L). The heterophils were well-granulated, with no evidence of a left shift or toxic changes (Fig. 1). These hematological findings were non-specific, although eosinophilia can be associated with fungal infections, parasitism, hypersensitivity reactions, mixed inflammation, and, less commonly, paraneoplastic syndromes.

Figure 1. Blood smear examination showing eosinophilia. Eosinophils (E) can be distinguished from heterophils (H) by their smaller round eosinophilic-staining granules. Wright’s stain, 100x.

Fluid collected from the endotracheal tube (ETT) was submitted to the Animal Health Laboratory, University of Guelph, for cytological examination. The sample consisted of mixed inflammatory cells within a mucoid background, with low numbers of red blood cells, unremarkable epithelial cells, a mixed population of free bacteria, and a few contaminants (plant material). Numerous large, blue-green staining oval structures, suggestive of parasitic ova, were scattered throughout the sample. A few “vesicular” structures with granular contents were also noted, possibly environmental contaminants (Fig. 2).

Figure 2. Cytology smears of endotracheal tube fluid. 2A. Several oval structures are observed within a proteinaceous background with mixed inflammatory cells. Wright’s stain, 10x. 2B. Oval structures compatible with parasite ova (arrow) and vesicular structure of unknown significance with granular content (asterisk). Wright’s stain, 40x.

Additional diagnostic testing included centrifugal fecal flotation for parasite ova identification, and fungal culture, considering the patient’s history of suspected aspergillosis and the presence of unidentified structures resembling fungal spores in the cytology of the ETT fluid. Fecal flotation revealed unioperculated parasite ova with morphometric range of 84-96 µmx48-53 µm which were identified as Cyathostoma spp. (Fig. 3). The fungal culture was negative, suggesting that the spore-like structures observed were probably environmental contaminants.

Figure 3. Fecal flotation. Unioperculated parasite ova with morphological features consistent with Cyathostoma spp. 3A. Ova recovered on fecal float. 3B. Ova stained with methylene blue showing the single polar operculum.

The patient was treated with ivermectin at 0.3 mg/kg orally, administered twice at a three-week interval. Clinical symptoms resolved, and follow-up radiographs showed resolution of the parabronchial pattern and improved air sac definition. A repeat fecal test was negative for parasite ova.

Tracheal worms of birds are parasitic nematodes belonging to the family Syngamidae which includes two genera: Syngamus and Cyathostoma. Cyathostoma spp. has been reported in predatory birds in Canada, including hawks and owls. Adult parasites reside in the trachea and produce eggs that are coughed up, swallowed, and subsequently passed in the feces. In the environment, infective larvae develop within the eggs, which may be ingested by paratenic (carrier) invertebrates such as earthworms. These invertebrates can transmit the infection when consumed by other animals. Interestingly, raptors typically do not feed directly on such invertebrates, but are likely infected through predation on rodents with infected intermediate hosts in their gastrointestinal tracts. Wild birds infected with Cyathostoma spp. are often asymptomatic but may present with emaciation and respiratory signs such as sneezing, dyspnea, and abnormal respiratory sounds. These signs are associated with air sacculitis and the presence of parasites in the trachea. Antemortem diagnosis can be achieved via fecal flotation by identifying morulated eggs measuring approximately 75–90 µm×45–60 µm with a single polar operculum.

Reference

1.Fernando MA, Barta JR. Tracheal worms. In: Parasitic Diseases of Wild Birds. Atkinson CT, Thomas NJ, Hunter DB, eds. John Wiley & Sons, 2009:343–354.