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Case History: An adult male 160-g Common Grackle (Quiscalus quiscula) was found dead as part of a mortality event involving 30 grackles in Ohio, U.S.A.

Clinical signs included birds falling as they attempted to fly out of trees, and once on the ground, wing flapping and scooting with arched heads. Death occurred in under an hour and AvitrolTM poisoning was suspected and later confirmed.

Gross Findings: There were no significant external findings. There was a moderate amount of subcutaneous, visceral, and epicardial fat. Lungs were diffusely red and floated in formalin. The esophagus contained three pieces of corn and crushed corn filled the ventriculus.


Numerous microfilariae were present in pulmonary blood vessels (Fig. 1A) and occasionally within airways (Fig. 1 B). Lungs were diffusely congested. Rare microfilariae were observed in the cerebellar parenchyma (Fig. 1C). There were multiple adult filarial nematodes in the lateral ventricle of the brain (Fig. 1D) and within the wall of the pulmonary artery (Fig. 1E, F).

Photomicrographs from a Common Grackle showing lesions in pulmonary blood vessel and nematodes in brain and pulmonary artery.
Figure 1. Photomicrographs from a Common Grackle (Quiscalus quiscula) found dead in Ohio, U.S.A. H&E stains. (A) Longitudinal (arrow) and cross sections (arrowheads) of microfilariae in a pulmonary blood vessel. (B) Microfilariae (arrows) within a parabronchus. (C) Microfilaria (arrow) within the cerebellar parenchyma with no associated inflammation. (D) Adult filarial nematodes in brain ventricle.  (E) Adult filarial nematodes in the wall of the pulmonary artery.  (F) Higher magnification of adult filarial nematodes in the wall of the pulmonary artery.

Etiologic diagnosis: Pulmonary, cerebral, and cardiac filariasis

Etiology: Splendidofilaria spp. microfilariae occur in the lungs and adults are reported in great vessels of the heart, specifically pulmonary arteries. Other possible genera include Aproctella, and Cardiofilaria spp. which can occur in association with the heart. Chandlerella quiscali microfilariae and adults are found in the brain.

Distribution: Little research exists to document the geographic distribution of filarial nematodes in wild avian species, so the complete geographic range is yet to be determined. However, filarial nematodes are well documented in wild passerines throughout the United States.

Host range: Filarial nematodes have a wide host range. Specifically, Splendidofilaria spp. and Chandlerella spp. are reported in 11 and 13 avian orders, respectively.

Transmission: Filarial nematodes have a definitive vertebrate host and an intermediate invertebrate host. Adult male and female nematodes mate, and female nematodes produce microfilariae in the definitive host. Microfilariae migrate to the blood or skin in vertebrate hosts, where they are picked up by invertebrates during a blood meal. Microfilariae then develop in the invertebrate host into infective 3rd stage larvae, which are deposited into vertebrate hosts during blood meals. Microfilariae develop into 4th stage larvae and then adults in the vertebrate hosts, and the cycle continues. In the case of Chandlerella and Splendidofilaria, Culicoides (biting midges) serve as the intermediate host.

Clinical signs: There are no reported clinical signs of filariasis in grackles.

Pathology: Filarial nematodes are generally regarded as non-pathogenic in their respective hosts. However, when adult filarids are present in specific structures, such as the wall or lumen of the aorta or pulmonary arteries, compromised cardiac output is possible. Adult Splendidofilaria passerina in pulmonary arteries of house sparrows (Passer domesticus domesticus) was associated with arterial wall thickening due to fibrosis, degeneration, and necrosis of the tunica media. Additionally, presence of adult filarial nematodes in abnormal host species can result in disease. Adult Chandlerella quiscali were associated with torticollis and progressive ataxia in a flock of emu. Treatment with ivermectin resolved clinical signs of neurologic disease. Adult C. quiscali were identified in the brain of a northern crested caracara (Caracara cheriway) that presented with hind-limb paresis and was subsequently euthanized.

Diagnosis: Microfilariae can be identified in lung, skin, and blood via a wet mount or in tissues using histopathology. It is important to note that most avian microfiliariae have a nocturnal periodicity and infections can be missed with daytime sampling.

Public health concerns: Avian filarial nematodes are not zoonotic and do not pose a significant human health risk.

Wildlife population impacts: Filarial nematodes are considered non-pathogenic and an incidental finding in common grackles, and therefore are not considered to have a significant impact on wildlife population health.

Management: Management of this disease in wildlife is complicated by the life cycle of the parasite that includes vectors, and is thus not practical.


  • Bartlett, CM. 2008. Filarioid Nematodes. In: Parasitic Diseases of Wild Birds. Atkinson CT, Thomas NJ, and Hunter DB, editors. Blackwell Publishing, Iowa, pp. 439–462.
  • Edwards EE, Dangoudoubiyam S, Hoppes SM, Porter BF. 2017. Granulomatous filarial encephalomyelitis caused by Chandlerella quiscali in a northern crested caracara (Caracara cheriway). J Zoo Wild Med: 48(1):237–240.
  • Huizinga HW, Cosgrove GE, Koch CF. 1971. Pulmonary arterial filariasis in the house sparrow. J Wildl Dis: 7(3):205–212.
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  • Law JM, Tully TN, Stewart TB. 1993. Verminous encephalitis apparently caused by the filarioid nematode Chandlerella quiscali in emus (Dromaius novaehollandiae). Avian Dis: 37:597–601.
  • Vaughan JA, Hinson J, Andrews ES, Turell MJ. 2021. Pre-existing microfilarial infections of American robins (Passeriformes: Turdidae) and common grackles (Passeriformes: Icteridae) have limited impact on enhancing dissemination of West Nile virus in Culex pipiens Mosquitoes (Diptera: Culicidae). J Med Entomol: 58(3):1389–1397.

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