Case of the Month - March 2026

Signalment and History

This adult, male rock dove was found sitting in the road. On the admission the animal was ataxic and dull. Due to the worsening of the clinical signs the animal was euthanized 2 days after admission. 

Gross Findings

The bird was underweight. There was acute hemorrhage into subcutis of the upper neck and the caudal third of the right pectoral muscle.

Histopathology

Brain (forebrain, brainstem and cerebellum): The leptomeninges and adjacent neuropil are diffusely expanded by a marked inflammatory infiltrate predominantly composed of lymphocytes and plasma cells with fewer macrophages. Areas of neuroparenchymal pan-necrosis are present. Multifocally, the inflammation extends into the neuroparenchyma, resulting in disruption and rarefaction of the neuropil. Scattered glial nodules and perivascular cuffs composed of lymphocytes, plasma cells, and macrophages are present. Few protozoal cysts with basophilic zoites are scattered within the neuropil.

Skeletal muscle (and heart; not shown): The cytoplasm of numerous myofibers contain elongated to oval protozoal tissue cysts. The cysts have thin walls and are populated by large numbers of small basophilic zoites arranged in parallel to haphazard clusters.

Morphologic Diagnosis

• Underweight
• Brain, meningoencephalitis, lymphoplasmacytic and histiocytic, marked with rare protozoal cysts.
• Skeletal muscle, sarcocystosis, moderate
• Heart, sarcocystosis, milt to moderate.
• Skin (neck), subcutaneous hemorrhage, acute, moderate.
• Pectoral muscle (right), hemorrhage, acute, focally extensive.

Etiologic Diagnosis

Presumptive Sarcocystis calchasi infection.

Discussion

Sarcocystis calchasi is an emerging protozoal parasite associated with severe neurologic disease in domestic pigeons (Columba livia). The parasite was first described in Germany in 2009 as the cause of a highly fatal neurologic disease in racing pigeons, and its first documented occurrence in North America was in 2011 in a feral rock dove (aka. rock pigeon) from St. Paul, Minnesota. In that case, the pigeon presented with progressive neurologic signs, similar to this case. Histologically, the most significant lesion was severe lymphoplasmacytic and granulomatous meningoencephalitis, with multiple granulomas in the leptomeninges of the cerebrum and brainstem. Numerous slender protozoal cysts were present within skeletal muscle fibers, while only rare cysts were observed in the myocardium, and no parasitic structures were identified in the brain. PCR and sequencing confirmed S. calchasi in skeletal muscle and the brain.

A notable feature of S. calchasi infection is the apparent dissociation between the site where the parasite is most readily detected and the site of the clinically important lesion. Tissue cysts are typically identified in skeletal muscle, whereas the major lesion responsible for clinical disease is meningoencephalitis. In naturally and experimentally infected pigeons, organisms are often absent or difficult to demonstrate within brain lesions, and the pathomechanism of the central nervous system inflammation remains uncertain but metabolites or cytokines have been presumed to be involved in these cases. 

The life cycle of S. calchasi is indirect and involves a predator-prey relationship. Domestic pigeons act as intermediate hosts, developing intramuscular sarcocysts, whereas raptors serve as definitive hosts and shed infective sporocysts in feces. In Europe, northern goshawks (Accipiter gentilis) and European sparrowhawks (Accipiter nisus) have been identified as definitive hosts. In North America, the definitive host range appears broader than initially recognized. A recent molecular survey of predatory birds in California detected S. calchasi in intestinal samples from Cooper’s hawks (Accipiter cooperii) and red-tailed hawks (Buteo jamaicensis), with sporocysts/oocysts observed histologically in the intestinal mucosa of most PCR-positive birds. These findings support Cooper’s hawks and red-tailed hawks as likely definitive hosts in North America.

The recognition of S. calchasi in North America is epidemiologically important because this parasite has now been associated with neurologic disease in free-ranging rock pigeons, psittacines, and Brandt’s cormorants. The detection of likely definitive hosts in urban-adapted raptors may help explain outbreaks in urban pigeon populations, where rock pigeons and raptors overlap. 

References

1. Wünschmann, A., Armien, A.G., Reed, L., Gruber, A.D. and Olias, P. (2011), Sarcocystis calchasi-associated neurologic disease in a domestic pigeon in North America. Transboundary and Emerging Diseases, 58: 526-530. https://doi.org/10.1111/j.1865-1682.2011.01254

2. Olias, P., Maier, K., Wuenschmann, A., Reed, L., Armi´ en, A.G., Shaw, D.P., Gruber, A.D., Lierz, M., 2014. Sarcocystis calchasi has an expanded host range and induces neurological disease in cockatiels (Nymphicus hollandicus) and North American rock pigeons (Columbia livia f. dom.). Vet. Parasitol. 200, 59–65. 10.1016/j.vetpar.2013.11.012

3. Krysta H. Rogers, David Arranz-Solís, Jeroen P.J. Saeij, Stephany Lewis, Aslı Mete, Sarcocystis calchasi and other Sarcocystidae detected in predatory birds in California, USA, International Journal for Parasitology: Parasites and Wildlife, Volume 17, 2022, Pages 91-99. https://doi.org/10.1016/j.ijppaw.2021.12.008