Trichomoniasis (also called canker or frounce) is a disease of the gastrointestinal tract, especially of the crop, which is caused by protozoa of the order Trichomonadida. In particular, the flagellates are transmitted through the crop milk or through contaminated drinking water. It is most notably pigeons and finches, but also budgerigars, cockatiels and sometimes other parrots and canary birds that become infected. In pigeons, older animals are often persistently infected, clinically inapparent carriers. Trichomonas gallinae is a pear-shaped flagellate of 5 to 18 µm in size that uses small lesions in the mucous membrane to penetrate into the tissue and triggers the characteristic focal, yellowish tumours there. Occurrence of the disease is often associated with stress, vitamin deficiency or other illnesses and in some cases it can lead to the colonisation of inner organs such as the liver and the heart. Clinical signs often include regurgitation of undigested food, but diarrhoea can be an indicator, too. In case of a longer duration of the disease, the animals lose weight and become apathetic. In young birds, the mortality rate can be up to 40%.

If there are predisposing factors, trichomonads can contribute to enteritis in reptiles.