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Pathology Case of the Month - Northern Pintail Duck

November 1, 2020

Case History: An adult male Northern Pintail Duck (Anas acuta) was found dead on a refuge in Arizona, US. Collectively at this location, 29 birds including Snow Geese (Chen caerulescens), Canada Geese (Branta canadensis), Mallard Ducks (Anas platyrhynchos), and Northern Pintail Ducks (Anas acuta) were found dead with no reported clinical signs.

Gross Findings: On external examination, conjunctivae were pale and a small amount of blood was present in the oral cavity. On internal examination, the bird was in fair body condition with small adipose stores. There were multiple foci of hemorrhage in the subcutaneous tissue over the pectoral muscle measuring up to 1.5 cm in diameter and several foci of hemorrhage within the underlying pectoral muscle. Also, within the pectoral muscle were numerous tan, tubular rice grain-like structures. There were multiple hemorrhages on the epicardium measuring up to 1 cm in diameter. A small amount of blood was present within the coelomic cavity and in the lumen of the esophagus and intestines. There was a focus of hemorrhage at the junction between the proventriculus and the ventriculus. The proventriculus and ventriculus were empty.

Photograph of pectoral muscle from a Northern Pintail Duck (Anas acuta) found dead in Arizona, U.S.
Figure 1. Photograph from a Northern Pintail Duck (Anas acuta) found dead in Arizona, US.  (A) The pectoral muscle contains numerous sarcocysts (arrowheads) of Sarcocystis sp. that are grossly visible as tan rice grain-like structures. An area of hemorrhage (*) is also shown.

Histopathological Findings: Hemorrhages are observed at the junction of the proventriculus and ventriculus, in the pericardial adipose tissue, intestine, and in the trachea. There are moderate numbers of lymphocytes and plasma cells and macrophages with intracytoplasmic brown granular material and occasional clear, acicular clefts in the lamina propria of a secondary bronchus. At the junction of a secondary and tertiary bronchus, there were large numbers of lymphocytes and plasma cells and foreign body and Langhans type multinucleated giant cells which occasionally contained clear acicular clefts that were birefringent under polarized light. Special stains for bacteria and fungi were negative. Large sarcocysts with thick eosinophilic walls filled with zoites are present in skeletal muscle.

Photomicrographs of pectoral muscle from a Northern Pintail Duck (Anas acuta) found dead in Arizona, US.
Figure 2. Photomicrographs from a Northern Pintail Duck (Anas acuta) found dead in Arizona, US.  (A) Low magnification of a Sarcocystis sp. cyst in the pectoral muscle of a Northern Pintail Duck. H&E stain. (B) Higher magnification of the cyst showing the cyst wall (arrowheads) and septations (arrow), a zone containing zoites (solid line) and a zone containing degenerative material (*).

Diagnostic Test Results: Routine culture of the liver yielded light mixed bacterial growth of no significance. A mouse neutralization test for botulinum Type C using heart blood was negative. No viruses were isolated from the liver, kidney or spleen. An Avian Influenza Virus matrix RT-PCR screen using tracheal and cloacal swabs was negative. Hemorrhages were also observed in other birds in this case. Bacterial cultures, PCR to detect viruses, virus isolation, and toxicologic testing did not reveal a cause for the observed hemorrhages in this mortality event. Sarcocystis sp. was an incidental finding in this duck. 

Morphologic Diagnosis/es:

  1. Multi-organ hemorrhage, acute
  2. Pectoral muscle sarcocysts
  3. Pulmonary granulomatous inflammation with intralesional foreign material

Disease: Sarcocystosis (Rice-breast)

Etiology: Suspect Sarcocystis rileyi. Macroscopic sarcocysts in waterfowl in North America have historically been considered to be S. rileyi. However, ducks are also intermediate hosts for other species of Sarcocystis, and it is not currently known if these macroscopic sarcocysts in ducks represent one or more species of Sarcocystis.

Distribution: Found in waterfowl throughout North America.

Seasonality: Year round.

Host range: Most commonly found in dabbling ducks. Less commonly found in other species of ducks, swans, geese, and wading birds.

Transmission: The definitive hosts of Sarcocystis sp. are carnivores; the definitive host of S. rileyi specifically in North America is the striped skunk. Carnivores are infected by eating prey muscle with the encysted form of the parasite. The parasite matures within the carnivore and oocysts are subsequently shed in carnivore feces. Oocysts are ingested by birds (the intermediate host) in contaminated food/water. Oocysts develop into sporozoites in the intestine of the bird and then migrate into arterioles, where they multiply and transform into merozoites. Merozoites travel through the bloodstream to skeletal muscle, where they encyst.  The lifecycle begins again when carnivores consume prey muscle containing cysts.

Clinical signs: Usually none, but in severe cases muscle damage can lead to weakness, lameness, and paralysis.

Pathology: Sarcocysts appear grossly as tan, tubular rice grain-like structures in the muscle of affected animals. Usually most noticeable in the pectoral muscles but may also be present in other skeletal and cardiac muscle. Tissue around the cysts is frequently calcified, giving tissue a gritty texture when cut.

Diagnosis: Visual detection of sarcocystis cysts in muscle tissue on necropsy/histology. PCR to confirm genus and species.

Public health concerns: No known human health concerns. May cause hunters to discard meat.

Wildlife population impacts: As most infections are incidental/asymptomatic, it is not thought that sarcocystosis has significant impacts on wild bird populations.

Management: No current management strategies for control of S. rileyi are in place.


Dubey JP, Rosenthal BM, Felix TA. 2010. Morphologic and molecular characterization of the sarcocysts of Sarcocystis rileyi (Apicomplexa: Sarcocystidae) from the mallard duck (Anas platyrhynchos).  J Parasitol 96:765–770.

Dubey JP, Cawthorn RJ, Speer CA, Wobeser GA. 2003. Redescription of the Sarcocysts of Sarcocystis rileyi (Apicomplexa: Sarcocystidae). J Eukaryot Microbiol 50:476–482.

U.S. Geological Survey. 1999. Field manual of wildlife diseases: general field procedures and diseases of birds, Accessed October 2019.

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