Introduction: Prototheca are colorless algae that can cause mastitis in dairy cattle. They are widespread in housing areas, pens and pastures used by dairy cattle. Most infections are clinical and remain as chronic infections. Prototheca are often associated with wet areas containing decaying manure and plant matter. So, the aim of this survey was to identify, isolate and determine the frequency of the Prototheca spp. in milk samples from cows suffering from mastitis in Mashhad city, northeast Iran. Material and methods: The milk samples were obtained from 400 dairy cattle with clinical and subclinical mastitis from 10 dairy cattle herd in suburb of Mashhad, Iran. All samples were cultured in Sabouraud dextrose agar containing chloramphenicol. Plates were then incubated aerobically at 27°C and examined daily for a 7-day period. Prototheca colonies were identified on the basis of macro- and micromorphological characteristics, and on the basis of physiological profile. Results: Of the 400 samples, 93 (23/25%) were positive for Prototheca spp. Our results are considered to be the first report on the high prevalence of Prototheca spp. in milk samples from bovine mastitis in Mashhad, Iran. Conclusion: Protothecosis is a zoonotic disease, which can be transmitted to the human by consuming milk and cause intestinal infections and enteritis because of its resistance to pasteurization. As a result, it is important and crucial to consider and identify these microorganisms in milk, because they can be potentially harmful to human and animal health.

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