Parasite discovery in sea eagle

Tammy Shadbolt

Wildlife Veterinarian & Research Associate

The white-tailed sea eagle is the largest bird of prey in the United Kingdom (UK) with an incredible wingspan of up to 2.5 metres.  Birds have a brown plumage with distinct pale head and neck feathers and a pale, short, wedge-shaped tail.  These birds are however a very rare sight.

The species went extinct in the UK during the early 20th century most likely due to persecution, although it was subsequently reintroduced to Scotland.  In 2019 a conservation initiative established by the Roy Dennis Wildlife Foundation and Forestry England acquired juveniles from Scotland under licence issued by Scottish Natural Heritage.  Six birds were translocated and, after a protected period, released on the Isle of Wight under licence from Natural England.  The white-tailed sea eagle was once again seen soaring English skies.

White-tailed sea eagle flying through the air

As part of post-release health surveillance (PRHS) the Disease Risk Analysis and Health Surveillance (DRAHS) team at the Institute of Zoology (IOZ), ZSL carry out post-mortem examinations (PME) of rare and endangered British species found dead.  A PME can help determine what might have caused the demise and death of an individual animal, and also allows emerging infectious and non-infectious threats to endangered populations to be identified early.

Shortly after release, one satellite-harnessed and radio-tracked white-tailed sea eagle was found dead on the west coast of the Isle of Wight and was submitted for PME.  DRAHS conducted a PME, carried out radiography of the carcase and took tissue samples for tests including parasitology, bacteriology, toxicology and histopathology.  There were no signs of disease on external and internal examination of the carcase and a definite cause of death could not be elicited from extensive testing.

Examining the tissues of the bird under a microscope however revealed the presence of encysted protozoan parasites in the breast and heart muscle (see image below).  Further investigation using a molecular technique called polymerase chain reaction (PCR) confirmed the presence of a Sarcocystis species phylogenetically similar to Sarcocystis wobeseri.

Sarcocyst in a section of breast muscle viewed under the microscope
Sarcocyst in a section of breast muscle viewed under the microscope. Scale bar indicates 20 µM.

Sarcocystis protozoa are intracellular parasites which can infect mammals, reptiles and birds and have a two-host life cycle.  Studying parasites found in animals is important for understanding which parasitic species are commensal (carried without causing harm) and which may cause disease. 

Infection with Sarcocystis species such as Sarcocystis falcatula and Sarcocystis calchasi has been associated with disease in intermediate avian hosts manifesting in clinical signs such as anorexia, weakness, difficulty breathing and neurological signs. Sarcocystis wobeseri has been identified in a number of avian intermediate hosts but has not previously been found in a raptor species.  (An intermediate host refers to an animal in which a parasite undergoes development in order to complete its lifecycle.)

This white-tailed sea eagle most likely acquired infection via a faecal-oral route through accidental ingestion of water contaminated with oocysts-sporocysts. This was the first time that Sarcocystis wobeseri-like infection had been detected in the white-tailed sea eagle revealing a new intermediate host species for the parasite.

In this white-tailed sea eagle there was no evidence that the presence of Sarcocystis wobeseri-like parasites had caused the demise and death of the bird.  Sarcocystis protozoa remain an important finding however in any wildlife species found dead, especially one of conservation concern. We encourage all wildlife health professionals to investigate unusual findings revealed through PME as part of PRHS in order to broaden our understanding of ecological parasites affecting rare and endangered species.


Further information

The full research paper by Shadbolt, T, Pocknell, A, Sainsbury, AW, Egerton-Read, S & Blake DP (2021) is available to subscribers of Parasitology Research here.

A White-tailed Sea Eagle perches on a tree


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