Global dimension of the problem of cyanobacterial blooms; Cyanobacterial blooms in Serbia – Epidemiological studies and health risk assessment.

Abstract

Cyanobacteria inhabit almost every aquatic and terrestrial environment. In aquatic ecosystems, under favourable conditions, they may grow excessively and form cyanobacterial blooms. Cyanobacterial blooms are classified as “harmful” when they cause negative environmental impacts such as mortality, ecosystem instability and cyanotoxin production. Cyanotoxins can be classified as hepatotoxins, neurotoxins, cytotoxins, and skin and gastrointestinal irritants (Codd et al., 2005). The best studied and most often detected cyanotoxins are hepatotoxic microcystins (MCs), and for most toxic variant MC-LR tolerable daily intake (TDI) of 0.04 μg per kg b.w. per day was established (WHO, 1998). MCs can cause acute illnesses (Jochimsen et al., 1998; Annadotter et al., 2001) and may have the potential capability to initiate cancer through high doses and promote tumors through chronic exposure (Svirčev et al., 2010). Exposure to cyanotoxins can occur via ingestion of contaminated drinking water, dermal contact and inhalation during recreational activities, consumption of plants contaminated during irrigation, consumption of aquatic organisms from contaminated waters, consumption of cyanobacterial dietary supplements and through hemodialysis (Drobac et al., 2013). On a global level in 659 studied ecosystems, MCs were the most distributed cyanotoxins (74%) and had the greatest distribution in rivers and lakes across Europe (70%). Consequential intoxications of humans and animals with cyanotoxins were described on every continent: in Europe (Annadotter et al., 2001; Svirčev et al., 2013; 2014a), Asia (Ueno et al., 1996), Africa (Gunnarsson and Sanseovic, 2001), Australia (Falconer et al., 1983) and America (Jochimsen et al., 1998). MCs were the most common toxins causing poisonings (33%). Percentage of ecosystems where humans were poisoned was 33%.Research on algae and cyanobacteria in Serbia has begun 130 years ago. During that period around 70 species of cyanobacteria were detected, and 24 of them bloomed. The most frequently observed were Microcystis aeruginosa and Aphanizomenon flos-aquae (Svirčev et al., 2014b). MCs were found in various examined aquatic ecosystems, with the highest concentrations up to 650 μg/L. MCs were also detected in fish tissues in which histopathological changes were observed. Health risks can arise when cyanobacterial blooms occur in reservoirs for drinking water supply (Drobac et al., 2016). During December 2013 Planktothrix rubescens bloomed in Vrutci reservoir for water supply of Užice city, and 70 000 inhabitants were exposed. Cyanobacterial cells were present in the reservoir (107 900 per ml), treated water (10 000 per l) and piped-water network (1 000 per l).The use of tap water was prohibited. Analyses showed toxicity of cyanobacterial biomass (Artemia salina bioassay) and the presence of different MC variants in biomass, water from the reservoir, tap water, and fish from Vrutci reservoir (LC-MS/MS). A questionnaire survey and an epidemiological study showed that diseases of the digestive system and skin were increased even before December 2013, indicating possibility that blooms occurred before the incident. This was confirmed by detecting MC in fish caught in Vrutci before the perceived blooming (Svirčev et al., 2017). Based on epidemiological data in Serbia, it was found that the high incidence of primary liver cancer (PLC) in the Nišavski, Toplički, and Šumadijski districts correlates with the appearance of water blooms and the presence of MCs in reservoirs used for drinking water supply of this districts (Svirčev et al., 2013), while there was no correlation with main risk factors- incidence of chronic viral hepatitis (HBV and HCV) and liver cirrhosis mortality in Central Serbia from 1999 to 2008. The lowest PLC incidences in Central Serbia were seen in the Rasinski and Zaječarski districts, which are also supplied with drinking water from frequently blooming reservoirs but use ozonation process in the preparation of drinking water. Therefore, it could be presumed that ozonation is a solution for the problem with cyanotoxins in drinking water. However, the high incidence of follicular non-Hodgkin’s lymphoma in these districts should be clarified (Svirčev et al., 2013). Further epidemiological investigation of other cancers in Serbia also showed that Nišavski, Toplički and Šumadijski districts had significantly higher incidences of ten cancers (brain, heart, mediastinum and pleura, ovary, testicular, gastric, colorectal, retroperitoneum and peritoneum, leukemia, malignant melanoma of skin, and PLC) and the districts may thus be considered critical (Svirčev et al. 2014a). Additional research is required to prove whether cyanobacterial toxic metabolites, as novel and not sufficiently recognised risk factors, act synergistically with other risk factors causing the higher incidence of the examined cancers. 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