The Incidence of Marine Toxins and the Associated Seafood Poisoning Episodes in the African Countries of the Indian Ocean and the Red Sea

The occurrence of Harmful Algal Blooms (HABs) and bacteria can be one of the great threats to public health due to their ability to produce marine toxins (MTs). The most reported MTs include paralytic shellfish toxins (PSTs), amnesic shellfish toxins (ASTs), diarrheic shellfish toxins (DSTs), cyclic imines (CIs), ciguatoxins (CTXs), azaspiracids (AZTs), palytoxin (PlTXs), tetrodotoxins (TTXs) and their analogs, some of them leading to fatal outcomes. MTs have been reported in several marine organisms causing human poisoning incidents since these organisms constitute the food basis of coastal human populations. In African countries of the Indian Ocean and the Red Sea, to date, only South Africa has a specific monitoring program for MTs and some other countries count only with respect to centers of seafood poisoning control. Therefore, the aim of this review is to evaluate the occurrence of MTs and associated poisoning episodes as a contribution to public health and monitoring programs as an MT risk assessment tool for this geographic region.


Introduction
The occurrence of Harmful Algal Blooms (HABs) in marine ecosystems can be one of the great threats to public health due to their capacity to produce marine toxins (MTs) as secondary metabolites [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. MTs can be accumulated by distinct marine organisms such as fish, mollusks and crustaceans [15][16][17][18][19][20][21][22][23][24] which are the basic diet of coastal human populations. Suspected or confirmed episodes of human poisoning caused by MTs have been reported worldwide in the last century [20,21,. The occurrence of episodes of human poisoning occurs via ingestion of contaminated marine food due to the lack of monitoring programs in some countries or violations of national health authorities' regulations imposing the closure of harvesting areas and theformation of blooms in this geographical area, there are insufficient data related to their occurrence and toxin production [50]. This review analyses the occurrence of MTs and their producers along the African Indian and the Red Sea coasts (from Egypt to South Africa) and associated human poisoning episodes. The existence of monitoring programs of MTs will be also highlighted and finally, some suggestions for the control and prevention of marine toxins in this area will be presented.

Marine Toxins and Their Producers
Chemically, toxins can be grouped according to their polarity, lipophilic and hydrophilic. Concerning MT monitoring, analysis and quantification methods in seafood are described in Table 1, including bioassays, immunoassays, and analytical chemistry methods. The bioassay methods (Mouse Bioassay (MBA), Rat Bioassay (RBA)) are no longer in use due to ethical reasons according to Directive 86/609/EEC [51] and procedural variation [52](e.g., use of different extraction solvents and consequently shortcomings). Chemical methods, mainly liquid chromatography coupled to mass spectrometry, are considered as the most promising since they are fully validated and standardized to replace bioassays in many organizations worldwide. Further information related to each toxin group such as syndromes, producers, common vectors, symptoms, detections methods in seafood, limit of detection (LOD) and quantification (LOQ) and permitted limit used in some parts of the world is also described in Table 1.

Domoic Acid and Analogs
Domoic acid (DA) ( Figure 8) and analogs are polar cyclic amino acid toxins of diatom origin Pseudo-nitzschia spp. [126] and red algae: Chondria armata [127] (Table 1). They present three carboxylic acid groups and the most reported DA analogs include epi-domoic acid (epi-DA), domoic acid C5′diastereomer and isodomoic acids A, B, C, D, E, F, G and H [iso-DA A-H] [128,129]. DA is the representative molecule of the DA-group that is responsible for amnesic shellfish poisoning (ASP) syndrome [130]. The characteristic symptomology of ASP is detailed in Table 1.

Domoic Acid and Analogs
Domoic acid (DA) ( Figure 8) and analogs are polar cyclic amino acid toxins of diatom origin Pseudo-nitzschia spp. [126] and red algae: Chondria armata [127] (Table 1). They present three carboxylic acid groups and the most reported DA analogs include epi-domoic acid (epi-DA), domoic acid C5 -diastereomer and isodomoic acids A, B, C, D, E, F, G and H [iso-DA A-H] [128,129]. DA is the representative molecule of the DA-group that is responsible for amnesic shellfish poisoning (ASP) syndrome [130]. The characteristic symptomology of ASP is detailed in Table 1
H H H Figure 9. Chemical structures of STX group.

Marine Cyanotoxins
Most marine toxins reported are produced mainly by microalgae (composed basically by dinoflagellates, diatoms, and marine bacteria), while cyanobacteria are reported as toxin producers in fresh, brackish waters and terrestrial habitats. Recently, cyanotoxins typical from freshwater have been identified in the marine environment [162]. Thus, this section will be focused on the description of the most reported marine cyanotoxins involved in seafood poisoning, their producers and mode of action ( Table 1).
One of the most relevant groups of marine cyanotoxins is themicrocystin group (MCs) [163] (Figure 12). MCs are produced by cyanobacteria of genus Pseudoanabaena, Phormidium, Spirilia [164], Leptolyngbya, Oscillatoria, Geitlerinema [165], Trichodesmium [166] and Synechococcus [167] and their occurrence have been reported in many parts of the world, namely: the central Atlantic coast of Portugal [168], Canary Islands Archipelago [166], Brazilian coast [169], Amvrakikos Gulf (Greece) [167] and Indian Ocean [170]. To date, MCs is regulated in freshwater habitats but should be extended to the marine environments since there are reports of these hepatotoxins in marine environments [162].

Marine Cyanotoxins
Most marine toxins reported are produced mainly by microalgae (composed basically by dinoflagellates, diatoms, and marine bacteria), while cyanobacteria are reported as toxin producers in fresh, brackish waters and terrestrial habitats. Recently, cyanotoxins typical from freshwater have been identified in the marine environment [162]. Thus, this section will be focused on the description of the most reported marine cyanotoxins involved in seafood poisoning, their producers and mode of action ( Table 1).

Incidence of Harmful Algal Blooms MarineToxins and Consequent Poisoning Incidents along African Indian and the Red Sea Coasts
The main geographical focus of this review is the African Indian and the Red Sea coasts, including surrounding islands (Figure 14). The marine environment of this area is understudied due to a lack of monitoring infrastructure. There is a high rate of poverty in local communities, and the local population is vulnerable to natural disasters [including HABs, tropical storms]. The exponential increase in population accompanied by industrialization and climate change contributes to eutrophication in coastal areas [295,296]. This study area is characterized as subtropical to tropical climate with a water temperature above 20 • C [297]. Eutrophication and the transportation of cysts [through maritime traffic] are considered the main factors contributing to large phytoplankton blooms, including those comprised of HAB species and/or pathogenic bacteria [295,296]. Countries with monitoring programs of marine environments related to control of seafood poisoning are listed in Table 2. A few of these programs have noted the presence of MTs ( Figure 14) and HAB species [dinoflagellates, cyanobacteria, diatoms], some of which [HAB species] were detected/confirmed by microscopic techniques and some confirmed by partial 16 S rRNA genes analysis [12,13,.
Due to the absence of legislation regarding CTXs, currently, there is an absence of monitoring programs regarding this group in South Africa.Since the Indian Ocean is considered an endemic site of CTXs, this is a matter of major importance.

Mozambique
Studies related to HAB occurrence in Mozambique are very scarce and the few published works indicate the occurrence of dinoflagellates of the genus Alexandrium [313] and species of cyanobacteria (Phormidium ambiguum, Lyngbya majuscula, and Lyngbya cf. putealis) [307]. To date, due to the absence of a Monitoring Program and trained health staff to recognize specific symptoms of seafood poisoning in humans, there are no records of published data of MT occurrence or reports of seafood poisoning cases in this country.

Tanzania
Published studies indicate the occurrence of cyanobacteria, namely: Pseudanabaena sp., Spirulina labyrinthiformis, Spirulina sp., Leptolyngbya sp., Phormidium sp., Oscillatoria sp., Lyngbyaaestuarii, Lyngbya sp., Lyngbya majuscula, Nodularia sp., Synechococcus sp., Microcystis sp.; Dinoflagellates: Gambierdiscus toxicus, Procentrum sp. and diatoms: Pseudo-nitzschia sp., Pseudo-nitzschia pungens, P. seriata and P. cuspidate [335][336][337][338][339][340][341]. Data related to MTs and seafood poisoning episodes are very scarce in Tanzania. In 2003, the Tanzanian government created guidelines for investigation and control of foodborne diseases and the regulatory institution is the Tanzania Food and Drugs Authority (TFDA) ( Table 2) [325]. The main objective of TFDA is to regulate matters related to food quality and safety for consumers through the dissemination of the information related to causative agents, latency period [duration], principal symptoms, typical vectors, and prevention of poisoning as measures of public health protection [325]. Among several foodborne disease sources, MTs such as CTXs, TTXs, DA, and PSTs are described by TFDA. The creation of alert and monitoring programs is an effective way to prevent poisoning episodes caused by MT-contaminated seafood.

Madagascar
Madagascar is the country with more records of published data regarding MT occurrence ( Figure 14) and consequently, many reported cases of seafood poisoning [36,47,49,343]. The seafood poisoning cases in Madagascar have been registered since 1930 mainly after the consumption of fish of the family Sphyrnidae, Cacharinidae, Clupeidae (herrings, sardines), and marine turtles species (Eretmochelys imbricata and Chelonia mydas) [36,47,49,343]. The main marine poisoning causative agents reported are CTXs, TTXs, and PlTXs [18,344] (Table 4). To reduce the number of seafood poisoning events, the MadagascarMinistry of Health has created a Seafood Poisoning National Control Program (Table 2) based on the setting of an epidemiological surveillance network, prevention of the communities through educational programs and the development of research on marine eco-environment [327].

Indian Ocean French Islands
Mayotte, Europa, Banc du Geyser, Bassas da India, Glorioso, Juan de Nova, Reunion and Tromelin islands administratively make part in the French government but since they are in the Indian Ocean, were considered for the present study. In these islands, there are reports of the occurrence of HABs and cases of seafood poisoning linked to MTs. The reported HAB forming species include: dinoflagellates (Prorocentrum lima, P. convacum, Ostreopsis ovata, Gambierdiscus toxicus, Alexandrium spp.), cyanobacteria (Hydrocoleum sp., Lyngbya majuscula, Phormidium sp., Leptolyngbya sp. and Oscillatoria sp.) [70,300,317,319,345]. The recorded human intoxications were due to DSTs and TTXs [35,328] ( Table 4). Centers of Disease for control and Preventing is the organization responsible for National Biomonitoring Program of toxins (PSTs) in these islands [35,328] (Table 2).

Somalia and Seychelles
There are no published studies related to the occurrence of HABs and MTs in Somalia and Seychelles. While there are no published reports of HABs or MTs in Somalia and Seychelles waters, the proximity to other countries with such reports and currents in the area suggest that investigations are necessary to avoid potential seafood poisoning events [62].   Several research works related to MTs are carried out in the Red Sea but are very limited on the African coast. Saudi Arabia is the country with the most published studies related to the occurrence of HABs along the Red Sea [13,308,311,316,321,322,352,353]. The Dinoflagellates (Alexandrium sp., Dinophysis sp., Prorocentrum sp., Pyrodinium sp., Gymnodinium sp.), cyanobacteria (Lyngbya sp., Oscillatoria sp., Trichodesmium sp.) and diatoms (Pseudonitzschia spp.) are the most reported marine producer species [13,308,311,316,321,322,352,353]. The bacteria Vibrio paraehemolyticus, producer of TTX, was detected in shrimp (Penaeus latisulcatus) in the Suez Gulf [299]. MTs reported in the Red Sea, mainly the Egyptian coast, described in Tables 3 and 4, include CTXs, TTXs, PSTs detected in puffer fish such as Pleuranacanthus sceleratus and Lagocephalus sceleratus [13,316,[349][350][351][352][353]. Cases of seafood poisoning caused by CTXs and TTXs were reported, and according to the Poison Control Center, affiliated with Ain Shams University (Cairo, Egypt), CTXs are the third most responsible agents that induce food poisoning in Egypt [354]. Puffer fish poisoning has been recorded since ancient Egyptian times [42]. In Egypt, there is monitoring ofHABs in aquatic ecosystems since 1994 when Egypt became a member of the Convention on Biological Diversity although the Nature Conservation Sector, Egyptian Environment Affairs Agency and the Ministry of State for Environmental Affairs (Table 2) are focal points [330]. There are no reports of HABs and MT occurrence in coastal areas of Djibouti, Eritrea, and Sudan.

Final Considerations and Recomendations
African Indian Ocean and the Red Sea coasts have a subtropical and tropical climate, considered optimal for the development and transportation of several HAB-forming species, and consequently, the production of MTs. Paradoxically, studiesrelated to the occurrence and incidence of HABs and MTs are very limited, from South Africa to Egypt. From a few data available in this zone, most describe only the genus and not the full species, making it very difficult to evaluate the occurrence of the toxic species. The most reported HAB phytoplanktons in this region are cyanobacteria, followed by dinoflagellates, and diatoms as potential MT producers. Relative to MTs, the most reported and involved in seafood poisoning episodes include CTXs, PSTs, and TTXs. The scarcity of the data related to MTs suggests the need for further studies and the creation of specific monitoring programs of HABs, particularly for dinoflagellates and diatoms since these constitute the phytoplankton that produces more fatal MTs, though in recent years several genera of bacteria have been described as producers of a potent group of marine toxins, TTXs, which have already been detected on the African coasts of the Indian Ocean and Red Sea. The main MTs that must be monitored in shellfish are presented in Table 5. Analytical techniques such as LC-MS/MS are advised and recommended as determination and quantification methods due to their higher reproducibility, specificity, sensitivity and capacity to discriminate analogs of given toxins in the sample. The permitted limit of a toxin in shellfish can be adopted from other countries as an example to follow such as the EU region, USA, Japan, Australia, and New Zealand. For the success of the MT monitoring programs, the integration and intercollaboration of environmental, public health and researches institutions and universities of the all African Countries of the Indian Ocean and the Red Sea is crucial.