Spotting sarcocystosis in wild ducks

Have you ever had a cracking morning flight, proudly returned home with your bag, and started prepping your ducks ready to stock the freezer, only to find what look like grains of rice embedded in the breast muscle? If not, the below will prepare you in case you do come across it.

It is not uncommon to ask what these white cysts are, whether the meat is safe to eat once cooked, and whether you would notice them if the bird was cooked whole. Here’s what we know so far.

What is 'rice breast?'

The cysts are caused by a parasite from the genus sarcosystis, leading to a disease called sarcocystosis, or rice breast as it’s more commonly known (owing to its appearance resembling grains of rice).

Several species of sarcocystis exist, each with its own unique host cycle. Research published in 2020 found that Sarcocystis rileyi is one of the most commonly found forms of the parasite in wild ducks in the UK, though others are also present.

The parasite forms whitish, grain-like cysts in the muscles tissue of waterfowl, primarily the breast. The cysts are protective capsules where the parasite develops. If a predator, such as a fox, eats the infected meat, the parasite enters its reproductive phase in the predator’s intestines. The resulting oocysts (essentially parasitic eggs) are shed into the environment via faeces, contaminating water and feeding areas where more ducks may pick them up.

This process generally does not affect predators (the end hosts) and does not appear to cause mortality or physical signs of disease in waterfowl.

Is it harmful to humans

At present, there’s no evidence that sarcocystis spp. (ie. multiple species of sarcocystis parasite) found in waterfowl in the UK poses a health risk to humans.

Studies suggest that many sarcocystis species, and other parasites, become inactive at:

freezing temperatures from –4°C to –18°C
cooking temperatures of 60°C to 75°C (internal)

However, because species and conditions may vary, we do not recommend consuming infected birds, even if cooked or frozen.

Once cooked, the cysts become much harder to detect due to changes in the colour and texture of the meat, especially if they’re deeper in the muscle. This makes visual inspection before cooking all the more important. If you spot cysts in the muscle it’s best to discard the affected bird.

Shooters are often the first to spot sarcocystosis, as it’s only visible when the breast muscle is exposed. Your vigilance and reports are helping us monitor the spread and this parasite.

If you encounter a suspected case, please submit details and photo if possible to our team here. Together, we can build a clearer picture of sarcocystosis in the UK and support management of wildfowl health.

Take home messages

Do not eat infected birds, even if cooked or frozen. While the risk to humans is low, it’s better to err on the side of caution.
Never feed raw infected meat to dogs or other carnivores. This can continue the cycle of the parasite within the environment.
Dispose of infected carcasses responsibly. Bag the, and place them in general waste, do not leave them in their environment.
Report suspected cases. Your observations are vital to improving our understanding of sarcocystosis in UK waterfowl.

Your observations in the field are invaluable. Whether it’s behaviour, signs of disease, or changes in population numbers, your contributions help shape our understanding of wildfowl health and inform conservation efforts.

By sharing what you see, you’re actively supporting research and helping ensure the long-term sustainability of our waterfowl populations. For more information, visit our sarcocystosis (rice breast) project page.

Sophie Stafford

Sophie is one of BASC's scientific advisors. She has a BSc in Zoology with Conservation from Bangor University and conducted experimental research in Finland following this. She has a broad range of research interests including ecosystem ecology, drivers of migration, and behavioural evolution.