Blastocystis is an anaerobic intestinal parasite of humans and a wide range of animals (Tan 2008). It is the commonest worldwide parasite reported in human stool samples, where more than a billion individuals are estimated to be infected (Scanlan 2012). Four morphological forms of Blastocystis were identified in stool or cultures: vacuolar, granular, amoeboid and cyst forms, from which the vacuolar form is the most easily recognized. The infective stage is the cyst form which is environment-resistant and transmitted by fecal-oral route, then undergoes excystation in the large intestine and develops into vacuolar, amoeboid or granular forms, and then encystation occurs while crossing along the colon (Tan 2008; Suresh et al. 2009).

Blastocystis prevalence rate ranges from 30 to 50% (Windsor et al. 2002; Tan 2004) or even up to 60% in some developing countries compared to the developed ones (Khoshnood et al., 2015). In Egypt, the prevalence varies in different governorates. In Cairo, the prevalence rate was 34.5% and up to 54.2% in the iron deficiency anemia patients (El Deeb and Khodeer 2013). In other Egyptian governorates, the prevalence rates were 22.4% in Dakahlia (El-Shazly et al. 2006), 33.3–35.7% in Ismailia (Rayan et al. 2007; Mokhtar and Youssef 2018), 53% in Gharbeya (EL-Marhoumy et al., 2015), 67.4%, 52% and 54.17% in Alexandria in 2016, 2019 and 2021 respectively (Eassa et al. 2016; Elsayad et al. 2019; Mossallam et al. 2021), 16.7% in Sohag (El-Nazer et al. 2017), 42.3% in Sharkia (Farghaly et al. 2017), 53.6% in Beni-Suef (Hamdy et al. 2020), 39% in Kafr El-Sheikh (Abdo et al. 2021).

The high prevalence of Blastocystis spp., especially in developing countries, may be attributed to poor hygiene practices, close animal contact and consumption of contaminated food or water (Wawrzyniak et al. 2013), as well as low standard of living, poor environmental sanitation and waste disposal system (Hamdy et al. 2020). Blastocystis has been correlated with unfiltered drinking water-borne outbreaks (Tan, 2008), so it was included in the Water Sanitation and Health programs of the World Health Organization (WHO2008). Moreover, Blastocystis spp. were detected in sewage in Malaysia and Scotland, and many studies provided evidence for waterborne transmission of the parasite (Leelayoova et al. 2004; Lee et al. 2012; Efstratiou et al. 2017). Zoonotic waterborne transmission was supported by detecting Blastocystis spp. in a stool sample of a patient with chronic blastocystosis and at the same time in fountain water samples fecally contaminated by livestock (Angelici et al. 2018). Blastocystis can be found in healthy asymptomatic patients (Souppart et al. 2009; Scanlan et al. 2014). However, Blastocystis infection is associated with non-specific gastrointestinal disorders called blastocystosis, including diarrhea, abdominal pain, flatulence, nausea, vomiting, constipation, weight loss or fatigue (Tan et al. 2010; Roberts et al. 2014), iron deficiency anaemia (El Deeb et al. 2012), irritable bowel syndrome and ulcerative colitis (Poirier et al. 2012; Azizian et al. 2016; Kök et al. 2019), acute and chronic urticaria (Eida et al. 2008; Abdel Hameed et al. 2011; Zuel-Fakkar et al. 2011; Wawrzyniak et al. 2013). Also, the parasite was incriminated in the pathogenesis of colorectal carcinoma (Steer 2007; Padukone et al. 2017; Mohamed et al. 2017; Habib et al. 2018). Furthermore, Blastocystis was listed as one of the commonly reported opportunistic parasites in immunocompromised hosts (Hassanein and Fanaky 2021).

The different degrees of B. hominis pathogenicity could be attributed to the high genetic diversity of the parasite (Stensvold and Clark 2016). According to small subunit ribosomal RNA gene (SSU-rRNA), different Blastocystis spp. are designated as subtypes (STs) (Stensvold et al. 2007a; Rene et al. 2009). There are 26 STs of which ST1-9 were found in human infections (Maloney et al. 2019; Stensvold and Clark 2020).

Metronidazole (MTZ) is the first-line prescribed drug for Blastocystis infection (Stensvold et al. 2010; Adao and Rivera 2018). Despite MTZ effectiveness in many individuals, it has been shown to produce side effects and resistance in others (Raman et al. 2016; Kurt et al. 2016; Rajamanikam et al. 2019). The variations in drug susceptibility and response to treatment can be mostly due to different genetic STs of the parasite (Maloney et al. 2019; Stensvold and Clark 2020). Many authors reported Blastocystis as an emerging pathogen (Tan et al. 2010; Poirier et al. 2012; Scanlan 2012) due to the pathogenic potential of Blastocystis spp., and the increasing resistance to MTZ, as well as the water-borne transmission. So, finding an effective prophylactic water-disinfecting solution may be beneficial. Ionizing radiation has been used in food safety (Osterholm and Norgan 2004). Disinfection of food and water by γ-irradiation has been shown to inactivate many bacterial pathogens contaminating food (Clavero et al. 1994), Toxoplasma gondii oocysts on fruits (Dubey et al. 1998), Cryptosporidium parvum in water (Yu and Park 2003) and Giardia lamblia (Lenaghan and Sundermann 2003; Fikry et al. 2017). Moreover, γ-irradiation had a damaging effect on the ultrastructure of Plasmodium falciparum blood stages (Okley et al. 2012). Additionally, γ-irradiation was proved to induce apoptosis in Echinococcus granulosus metacestodes (Alam Eldin and Badawy 2015). Thus, the aim of the present study is to evaluate the destructive effect of cytotoxic γ-irradiation combined with and compared to MTZ on Blastocystis spp. in vitro.