1. INTRODUCTION
Free-living amoebae (FLA) are cosmopolitan and ubiquitous microorganisms widely distributed in the environment and can be opportunistic and/or pathogenic (Visvesvara et al., 2007). They have been isolated from many natural and anthropogenic environmental matrices, including plants, soil, air conditioning dust, bottled mineral water, drinking water treatment and distribution system, and cooling towers (Landell et al., 2013; Maschio et al., 2015; Javanmard et al., 2017; Soares et al., 2017; Wopereis et al., 2020; Pazoki et al., 2020). They have also been isolated from contact lenses, swimming pools, and other recreational waters (Fabres et al., 2016; Santos et al., 2021; Fabros et al., 2021).
Among its representatives with importance for human health, the generaAcanthamoeba , Naegleria and Balamuthia stand out.Acanthamoeba spp. can cause illnesses in healthy people, such asAcanthamoeba keratitis (AK) which primarily affects contact lens (CL) wearers, usually due to lens wear while swimming or improper lens cleaning (Dos Santos et al., 2018). In immunosuppressed individuals, it can cause Granulomatous Amebic Encephalitis (GAE), which can be fatal (Visvesvara et al., 2007; Sarink et al., 2022). Acanthamoeba spp. has also been reported to cause skin infections (Murakawa et al., 1995; Paltiel et al. 2004). In addition, was isolated from 26% (17/63) of critically ill patient urine samples (Santos et al., 2009); similarly,Acanthamoeba (T4) was isolated from 22% (11/50) of urine samples collected from patients with recurrent urinary tract infection (Saberi et al., 2021).
Naegleria fowleri is known as a “brain-eating amoeba”, and primarily affects healthy young people using recreational waters, causing primary amoebic meningoencephalitis (PAM) (Fowler and Carter, 1965). PAM is a serious and usually fatal disease if adequate treatment is not initiated at the onset of symptoms (Król-Turmińska & Olender, 2017). The rapid deterioration in the health status of patient affected by PAM, combined with the ease of being confused with bacterial meningoencephalitis (since the symptoms are similar), as well as erratic or late diagnosis, contribute to a high prevalence of deaths (> 97%) (Capewell et al., 2010; Johnson et al., 2016).Balamuthia mandrillaris and Sappinia pedatta also cause encephalitis (Gelman et al., 2001; Visvesvara et al., 2007; Cope et al., 2019) however, there are no reports of the isolation of S. diploidea pedatta from swimming pools and recreational waters.
The FLA essentially have two forms of life, namely, the trophozoite form (with or without flagellum) which is the active form of the protozoan, in which it feeds, reproduces and expresses pathogenicity; and the form of cysts (which is the form of environmental resistance). Cysts have a double-layer wall made essentially of cellulose (Garajová, et al., 2019) that protects the protozoan against unfavorable conditions (e.g., food shortages, dissection, extreme pH and temperatures) or antimicrobial agents (e.g., NaCl, chlorine, drugs, UV, heat) (Aksozek et al., 2002; Thomas et al., 2008; Chaúque and Rott, 2021; Chaúque et al., 2021). FLA are considered the ”Trojan Horse” of the microbial world, as phylogenetically diverse microorganisms including bacteria, fungi and viruses survive and multiply within them; these microorganisms are called amoeba-resistant microorganisms (ARM) (Greub & Raoult, 2004; Scheid 2014 ; Delafont et al., 2016; Hubert et al., 2021; Rayamajhee et al. 2021). A wide range of pathogens of public health importance have been described as being ARM, including Legionella pneumophila, Mycobacterium leprae, Pseudomonas spp., Candida auris and various viruses (Maschio et al., 2015; Staggemeier et al., 2016; Balczun & Scheid, 2017; Turankar et al., 2019; Nisar et al., 2020; Hubert et al., 2021). All these aspects that characterize the profile of FLA constitute the main attributes that determine the great importance of these protozoa for human health and the environment.
Although increasingly prevalent, diseases caused by FLA remain rare; however, the presence of these protozoa, especially in the aquatic environment, is well documented (Stapleton, 2021; Saburi et al., 2017; Caumo et al., 2009). The presence of FLA in swimming pools and other recreational waters is of concern, as they can be pathogenic or opportunistic and/or lead to the persistence of non-amoebic pathogens in the water, including waters treated with chlorine-based disinfectants (Siddiqui & Khan, 2014; Kiss et al., 2014; Dey et al., 2021). It was determined that the prevalence of Naegleria spp in different water sources around the world (considering data from 35 countries) was 26.42%, in recreational water it was 21.27% (10.80 - 34.11) and in swimming pools was 44.80% (16.19 - 75.45 ) (Saberi et al., 2020), however, the global prevalence of AFL in swimming pools and recreational waters remains to be determined. The present systematic review and meta-analysis aimed to determine the prevalence of FLA in swimming pools and recreational waters worldwide.