Tritrichomonas foetus is a microscopic single-celled flagellated protozoan parasite that has traditionally been identified as a cause of reproductive disease in cattle (infertility, abortion and endometritis). It has been found all over the world, but the widespread use of artificial insemination in breeding cattle has led to the virtual elimination of this organism from the cattle population in many countries including the UK and much of Europe. Tritrichomonas foetus may also be an important cause of diarrhoea in cats. It can infect and colonises the large intestine, and can cause prolonged and intractable diarrhoea.