# Food and Drinking Water as Sources of Pathogenic Protozoans: An Update
## 1 Introduction  
Protozoans are unicellular eukaryotes once classified in the subkingdom Protozoa [1] and currently reclassified in different clades of the lineage Eukaryota [2]. Among protozoans, some are waterborne and foodborne parasites of humans and animals that can invade host intestinal cells through mechanical intrusion, attachment, and enzymatic digestion [3]. The oocysts and cysts of protozoans are the infectious stages of these organisms, resistant to disinfectants and other external agents, released into the environment by the feces of infected hosts. Consequently, they are widely distributed in the environment, in water, and in food and persist for a long time in different conditions [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14].  
In 1987, the World Health Organization (WHO) designated some of the protozoan parasites commonly transmitted through contaminated food and water--namely _Cryptsporidium_ spp., _Entamoeba histolytica_, and _Giardia intestinalis_ (basonyms _G. lamblia_ and _G. duodenalis_[15])--as a heavy public health concern since they cause malabsorption in children mainly in developing countries [16]. These microorganisms cause intestinal parasitic infections (IPIs) with prolonged diarrhea that impairs children's growth [17]. Based on data from 7800 children infected by intestinal protozoan parasites enrolled in the Global Enteric Multicenter Study (GEMS) in South Asia and Sub-Saharan Africa, the growth parameters length/height-for-age (HAZ), weight-for-age (WAZ), and weight-for-length/height (WHZ) determined after 60 days from infection significantly decreased in children 0-11 months of age infected with _G. intestinalis_, _Cryptsporidium_ spp. or _E. histolytica_[17].  
The Food and Agricultural Organization of the United Nations (FAO) and World Health Organization (WHO) published a multi-criteria-based ranking of foodborne parasites that included, in descending order of pathogenicity, protozoans transmissible by the oral route, namely _Toxoplasma gondii_, _Cryptosporidium_ spp., _E. histolyticaa_, _T Trypanosoma cruzi_, _G. intestinalis_, _Cyclospora cayetanensis_, _Balantioides coli_ (basonym _Balantiidium coli_[2]), and _Sarcocystis_ species [18]. Other species of protozoans causing foodborne infections with lower prevalence in humans or less defined pathogenic roles are _Cystoisospora belli_ (basonym _Isospora belli_[19]), _Blastocystis hominis_, _Dientamoeba fragilis_, _Endolimax nana_, and _Pentotrichomonas hominis_[8; 20; 21; 22; 23; 24; 25].  
These organisms can cause acute, chronic infections and sequelae [17; 26; 27; 28; 29; 30; 31; 32; 33]. Moreover, some studies reported a significantly higher prevalence of _B. hominis_ and _Cryptosporidium_ spp. in colorectal cancer (CRC) patients and of _C. parvum_ also in patients with other gastrointestinal cancer types [34; 35]. The latter favored the development of CRC in a murine model [36].  
This review aimed to collect the latest information on global prevalence, diseases caused, risk factors, infection sources, and prevention measures for protozoan infections acquired through the dietary route. Only scientific articles published since 2019 were included, except for references to older taxonomy, definitions, norms, and official public health acts. Protozoan pathogens considered in this literature survey were those officially indicated as major public health concerns [16; 18] and those of wide distribution with evidence of detrimental effects on human health [8; 20; 21; 22; 23; 24; 25]. The criteria of choice of the literature sources are the clear or most probable connection of infections and severe manifestations with contaminated food and drinking water, the occurrence in these sources, their origin, and measures for prevention.