Cyanotoxins, biosynthetic gene clusters, and factors modulating cyanotoxin biosynthesis

Cyanobacterial harmful algal blooms (CHABs) are a global environmental concern that encompasses public health issues, water availability, and water quality owing to the production of various secondary metabolites (SMs), including cyanotoxins in freshwater, brackish water, and marine ecosystems. The frequency, extent, magnitude, and duration of CHABs are increasing globally. Cyanobacterial species traits and changing environmental conditions, including anthropogenic pressure, eutrophication, and global climate change, together allow cyanobacteria to thrive. The cyanotoxins include a diverse range of low molecular weight compounds with varying biochemical properties and modes of action. With the application of modern molecular biology techniques, many important aspects of cyanobacteria are being elucidated, including aspects of their diversity, gene-environment interactions, and genes that express cyanotoxins. The toxicological, environmental, and economic impacts of CHABs strongly advocate the need for continuing, extensive efforts to monitor cyanobacterial growth and to understand the mechanisms regulating species composition and cyanotoxin biosynthesis. In this review, we critically examined the genomic organization of some cyanobacterial species that lead to the production of cyanotoxins and their characteristic properties discovered to date.

Meta-analysis reveals cyanotoxins risk across African inland waters

Global eutrophication and climate warming exacerbate production of cyanotoxins such as microcystins (MCs), presenting risks to human and animal health. Africa is a continent suffering from severe environmental crises, including MC intoxication, but with very limited understanding of the occurrence and extent of MCs. By analysing 90 publications from 1989 to 2019, we found that in various water bodies where MCs have been detected so far, the concentrations were 1.4-2803 times higher than the WHO provisional guideline for human lifetime exposure via drinking water (1 µg/L) in 12 of 15 African countries where data were available. MCs were relatively high in the Republic of South Africa (averaged 2803 μg/L) and Southern Africa as a whole (702 μg/L) when compared to other regions. Values were higher in reservoirs (958 μg/L) and lakes (159 μg/L) than in other water types, and much higher in temperate (1381 μg/L) than in arid (161 μg/L) and tropical (4 μg/L) zones. Highly significant positive relationships were found between MCs and planktonic chlorophyll a. Further assessment revealed high ecological risk for 14 of the 56 water bodies, with half used as human drinking water sources. Recognizing the extremely high MCs and exposure risk in Africa, we recommend routine monitoring and risk assessment of MCs be prioritized to ensure safe water use and sustainability in this region.

Co-Occurrence of Taste and Odor Compounds and Cyanotoxins in Cyanobacterial Blooms: Emerging Risks to Human Health?

Cyanobacteria commonly form large blooms in waterbodies; they can produce cyanotoxins, with toxic effects on humans and animals, and volatile compounds, causing bad tastes and odors (T&O) at naturally occurring low concentrations. Notwithstanding the large amount of literature on either cyanotoxins or T&O, no review has focused on them at the same time. The present review critically evaluates the recent literature on cyanotoxins and T&O compounds (geosmin, 2-methylisoborneol, β-ionone and β-cyclocitral) to identify research gaps on harmful exposure of humans and animals to both metabolite classes. T&O and cyanotoxins production can be due to the same or common to different cyanobacterial species/strains, with the additional possibility of T&O production by non-cyanobacterial species. The few environmental studies on the co-occurrence of these two groups of metabolites are not sufficient to understand if and how they can co-vary, or influence each other, perhaps stimulating cyanotoxin production. Therefore, T&Os cannot reliably serve as early warning surrogates for cyanotoxins. The scarce data on T&O toxicity seem to indicate a low health risk (but the inhalation of β-cyclocitral deserves more study). However, no data are available on the effects of combined exposure to mixtures of cyanotoxins and T&O compounds and to combinations of T&O compounds; therefore, whether the co-occurrence of cyanotoxins and T&O compounds is a health issue remains an open question.

A review and assessment of cyanobacterial toxins as cardiovascular health hazards

Eutrophicated waters frequently support bloom-forming cyanobacteria, many of which produce potent cyanobacterial toxins (cyanotoxins). Cyanotoxins can cause adverse health effects in a wide range of organisms where the toxins may target the liver, other internal organs, mucous surfaces and the skin and nervous system. This review surveyed more than 100 studies concerning the cardiovascular toxicity of cyanotoxins and related topics. Over 60 studies have described various negative effects on the cardiovascular system by seven major types of cyanotoxins, i.e. the microcystin (MC), nodularin (NOD), cylindrospermopsin (CYN), anatoxin (ATX), guanitoxin (GNTX), saxitoxin (STX) and lyngbyatoxin (LTX) groups. Much of the research was done on rodents and fish using high, acutely toxin concentrations and unnatural exposure routes (such as intraperitoneal injection), and it is thus concluded that the emphasis in future studies should be on oral, chronic exposure of mammalian species at environmentally relevant concentrations. It is also suggested that future in vivo studies are conducted in parallel with studies on cells and tissues. In the light of the presented evidence, it is likely that cyanotoxins do not constitute a major risk to cardiovascular health under ordinary conditions met in everyday life. The risk of illnesses in other organs, in particular the liver, is higher under the same exposure conditions. However, adverse cardiovascular effects can be expected due to indirect effects arising from damage in other organs. In addition to risks related to extraordinary concentrations of the cyanotoxins and atypical exposure routes, chronic exposure together with co-existing diseases could make some of the cyanotoxins more dangerous to cardiovascular health.

Genomic insights into the biosynthesis and physiology of the cyanobacterial neurotoxin 3-N-methyl-2,3-diaminopropanoic acid (BMAA)

Cyanobacteria are an ancient clade of photosynthetic prokaryotes, present in many habitats throughout the world, including water resources. They can present health hazards to humans and animals due to the production of a wide range of toxins (cyanotoxins), including the diaminoacid neurotoxin, 3-N-methyl-2,3-diaminopropanoic acid (β-N-methylaminoalanine, BMAA). Knowledge of the biosynthetic pathway for BMAA, and its role in cyanobacteria, is lacking. Present evidence suggests that BMAA is derived by 3-N methylation of 2,3-diaminopropanoic acid (2,3-DAP) and, although the latter has never been reported in cyanobacteria, there are multiple pathways to its biosynthesis known in other bacteria and in plants. Here, we used bioinformatics analyses to investigate hypotheses concerning 2,3-DAP and BMAA biosynthesis in cyanobacteria. We assessed the potential presence or absence of each enzyme in candidate biosynthetic routes known in Albizia julibrissin, Lathyrus sativus seedlings, Streptomyces, Clostridium, Staphylococcus aureus, Pantoea agglomerans, and Paenibacillus larvae, in 130 cyanobacterial genomes using sequence alignment, profile hidden Markov models, substrate specificity/active site identification and the reconstruction of gene phylogenies. Most enzymes involved in pathways leading to 2,3-DAP in other species were not found in the cyanobacteria analysed. Nevertheless, two species appear to have the genes sbnA and sbnB, responsible for forming the 2,3-DAP constituent in staphyloferrin B, a siderophore from Staphylococcus aureus. It is currently undetermined whether these species are also capable of biosynthesising BMAA. It is possible that, in some cyanobacteria, the formation of 2,3-DAP and/or BMAA is associated with environmental iron-scavenging. The pam gene cluster, responsible for the biosynthesis of the BMAA-containing peptide, paenilamicin, so far appears to be restricted to Paenibacillus larvae. It was not detected in any of the cyanobacterial genomes analysed, nor was it found in 93 other Paenibacillus genomes or in the genomes of two BMAA-producing diatom species. We hypothesise that the presence, in some cyanobacterial species, of the enzymes 2,3-diaminopropionate ammonia-lyase (DAPAL) and reactive intermediate deaminase A (RidA) may explain the failure to detect 2,3-DAP in analytical studies. Overall, the taxonomic distribution of 2,3-DAP and BMAA in cyanobacteria is unclear; there may be multiple and additional routes, and roles, for the biosynthesis of 2,3-DAP and BMAA in these organisms.

Genomic insights into the biosynthesis and physiology of the cyanobacterial neurotoxin 2,4-diaminobutanoic acid (2,4-DAB)

Cyanobacteria are an ancient clade of photosynthetic prokaryotes, whose worldwide occurrence, especially in water, presents health hazards to humans and animals due to the production of a range of toxins (cyanotoxins). These include the sometimes co-occurring, non-encoded diaminoacid neurotoxins 2,4-diaminobutanoic acid (2,4-DAB) and its structural analogue β-N-methylaminoalanine (BMAA). Knowledge of the biosynthetic pathway for 2,4-DAB, and its role in cyanobacteria, is lacking. The aspartate 4-phosphate pathway is a known route of 2,4-DAB biosynthesis in other bacteria and in some plant species. Another pathway to 2,4-DAB has been described in Lathyrus species. Here, we use bioinformatics analyses to investigate hypotheses concerning 2,4-DAB biosynthesis in cyanobacteria. We assessed the presence or absence of each enzyme in candidate biosynthesis routes, the aspartate 4-phosphate pathway and a pathway to 2,4-DAB derived from S-adenosyl-L-methionine (SAM), in 130 cyanobacterial genomes using sequence alignment, profile hidden Markov models, substrate specificity/active site identification and the reconstruction of gene phylogenies. In the aspartate 4-phosphate pathway, for the 18 species encoding diaminobutanoate-2-oxo-glutarate transaminase, the co-localisation of genes encoding the transaminase with the downstream decarboxylase or ectoine synthase - often within hybrid non-ribosomal peptide synthetase (NRPS)-polyketide synthases (PKS) clusters, NRPS-independent siderophore (NIS) clusters and incomplete ectoine clusters - is compatible with the hypothesis that some cyanobacteria use the aspartate 4-phosphate pathway for 2,4-DAB production. Through this route, in cyanobacteria, 2,4-DAB may be functionally associated with environmental iron-scavenging, via the production of siderophores of the schizokinen/synechobactin type and of some polyamines. In the pathway to 2,4-DAB derived from SAM, eight cyanobacterial species encode homologs of SAM-dependent 3-amino-3-carboxypropyl transferases. Other enzymes in this pathway have not yet been purified or sequenced. Ultimately, the biosynthesis of 2,4-DAB appears to be either restricted to some cyanobacterial species, or there may be multiple and additional routes, and roles, for the synthesis of this neurotoxin.

Challenges of using blooms of Microcystis spp. in animal feeds: A comprehensive review of nutritional, toxicological and microbial health evaluation

Microcystis spp., are Gram-negative, oxygenic, photosynthetic prokaryotes which use solar energy to convert carbon dioxide (CO₂) and minerals into organic compounds and biomass. Eutrophication, rising CO₂ concentrations and global warming are increasing Microcystis blooms globally. Due to its high availability and protein content, Microcystis biomass has been suggested as a protein source for animal feeds. This would reduce dependency on soybean and other agricultural crops and could make use of "waste" biomass when Microcystis scums and blooms are harvested. Besides proteins, Microcystis contain further nutrients including lipids, carbohydrates, vitamins and minerals. However, Microcystis produce cyanobacterial toxins, including microcystins (MCs) and other bioactive metabolites, which present health hazards. In this review, challenges of using Microcystis blooms in feeds are identified. First, nutritional and toxicological (nutri-toxicogical) data, including toxicity of Microcystis to mollusks, crustaceans, fish, amphibians, mammals and birds, is reviewed. Inclusion of Microcystis in diets caused greater mortality, lesser growth, cachexia, histopathological changes and oxidative stress in liver, kidney, gill, intestine and spleen of several fish species. Estimated daily intake (EDI) of MCs in muscle of fish fed Microcystis might exceed the provisional tolerable daily intake (TDI) for humans, 0.04 μg/kg body mass (bm)/day, as established by the World Health Organization (WHO), and is thus not safe. Muscle of fish fed M. aeruginosa is of low nutritional value and exhibits poor palatability/taste. Microcystis also causes hepatotoxicity, reproductive toxicity, cardiotoxicity, neurotoxicity and immunotoxicity to mollusks, crustaceans, amphibians, mammals and birds. Microbial pathogens can also occur in blooms of Microcystis. Thus, cyanotoxins/xenobiotics/pathogens in Microcystis biomass should be removed/degraded/inactivated sufficiently to assure safety for use of the biomass as a primary/main/supplemental ingredient in animal feed. As an ameliorative measure, antidotes/detoxicants can be used to avoid/reduce the toxic effects. Before using Microcystis in feed ingredients/supplements, further screening for health protection and cost control is required.

Co-Occurrence of Cyanobacteria and Cyanotoxins with Other Environmental Health Hazards: Impacts and Implications

Toxin-producing cyanobacteria in aquatic, terrestrial, and aerial environments can occur alongside a wide range of additional health hazards including biological agents and synthetic materials. Cases of intoxications involving cyanobacteria and cyanotoxins, with exposure to additional hazards, are discussed. Examples of the co-occurrence of cyanobacteria in such combinations are reviewed, including cyanobacteria and cyanotoxins plus algal toxins, microbial pathogens and fecal indicator bacteria, metals, pesticides, and microplastics. Toxicity assessments of cyanobacteria, cyanotoxins, and these additional agents, where investigated in bioassays and in defined combinations, are discussed and further research needs are identified.

Analysis of Cyanobacterial Toxins by Physicochemical and Biochemical Methods

Cyanobacteria (blue-green algae) produce a wide range of low molecular weight metabolites that include potent neurotoxins, hepatotoxins, and cytotoxins. The accumulation of such toxins in freshwaters, and in brackish and marine waters presents hazards to human and animal health by a range of exposure routes. A review is presented of developments in the detection and analysis of cyanobacterial toxins, other than bioassays, including application of physicochemical, immunoassays, and enzyme-based methods. Analytical requirements are considered with reference to recently derived guideline levels for the protection of health and to the availability, or otherwise, of purified, quantitative cyanobacterial toxin standards.

Environmental distribution of the neurotoxin l-BMAA in Paenibacillus species

The environmental distribution of the neurotoxic amino acid, 3-N-methyl-2,3-diaminopropanoic acid (BMAA), first isolated in 1967, was initially believed to be limited to tropical and subtropical plants of the genus Cycas. The seeds of one such species, which had been used historically on the Pacific island of Guam as a foodstuff, had a reputation for neurotoxicity. Some 40 years later the amino acid was detected in terrestrial and aquatic cyanobacteria and in other aquatic organisms. Overlooked was the discovery of BMAA in peptides of bizarre structure that had been isolated in 1975 from Paenibacillus pulvifaciens during a search for antibiotics. More recently (2014), peptides of similar structure were isolated from Paenibacillus larvae; this organism is causative of American Foulbrood, a lethal disease of honeybee colonies. These are interesting chemical and environmental observations, but knowledge of the bacterial distribution of BMAA is limited to just these two species of Paenibacillus, while more than 200 Paenibacillus spp. are known. Paenibacillus spp. are ever present naturally in the environment and are used agriculturally; recent research reports that some species infect human foods - including cow's milk - and have been isolated from human body fluids. We wish to stimulate interest in the environmental distribution of the neurotoxic BMAA in Paenibacillus spp. by drawing together previously isolated streams of research and by proposing experimental approaches by which this matter might be resolved.

Cyanobacterial effects in Lake Ludoš, Serbia - Is preservation of a degraded aquatic ecosystem justified?

Cyanobacteria are present in many aquatic ecosystems in Serbia. Lake Ludoš, a wetland area of international significance and an important habitat for waterbirds, has become the subject of intense research interest because of practically continuous blooming of cyanobacteria. Analyses of water samples indicated a deterioration of ecological condition and water quality, and the presence of toxin-producing cyanobacteria (the most abundant Limnothrix redekei, Pseudanabaena limnetica, Planktothrix agardhii and Microcystis spp.). Furthermore, microcystins were detected in plants and animals from the lake: in macrophyte rhizomes (Phragmites communis, Typha latifolia and Nymphaea elegans), and in the muscle, intestines, kidneys, gonads and gills of fish (Carassius gibelio). Moreover, histopathological deleterious effects (liver, kidney, gills and intestines) and DNA damage (liver and gills) were observed in fish. A potential treatment for the reduction of cyanobacterial populations employing hydrogen peroxide was tested during this study. The treatment was not effective in laboratory tests although further in-lake trials are needed to make final conclusions about the applicability of the method. Based on our observations of the cyanobacterial populations and cyanotoxins in the water, as well as other aquatic organisms and, a survey of historical data on Lake Ludoš, it can be concluded that the lake is continuously in a poor ecological state. Conservation of the lake in order to protect the waterbirds (without urgent control of eutrophication) actually endangers them and the rest of the biota in this wetland habitat, and possibly other ecosystems. Thus, urgent measures for restoration are required, so that the preservation of this Ramsar site would be meaningful.

Metabolic solutions to the biosynthesis of some diaminomonocarboxylic acids in nature: Formation in cyanobacteria of the neurotoxins 3-N-methyl-2,3-diaminopropanoic acid (BMAA) and 2,4-diaminobutanoic acid (2,4-DAB)

The non-encoded diaminomonocarboxylic acids, 3-N-methyl-2,3-diaminopropanoic acid (syn: α-amino-β-methylaminopropionic acid, MeDAP; β-N-methylaminoalanine, BMAA) and 2,4-diaminobutanoic acid (2,4-DAB), are distributed widely in cyanobacterial species in free and bound forms. Both amino acids are neurotoxic in whole animal and cell-based bioassays. The biosynthetic pathway to 2,4-DAB is well documented in bacteria and in one higher plant species, but has not been confirmed in cyanobacteria. The biosynthetic pathway to BMAA is unknown. This review considers possible metabolic routes, by analogy with reactions used in other species, by which these amino acids might be biosynthesised by cyanobacteria, which are a widespread potential environmental source of these neurotoxins. Where possible, the gene expression that might be implicated in these biosyntheses is discussed.

Massive fish mortality and Cylindrospermopsis raciborskii bloom in Aleksandrovac Lake

This paper presents a case study of a massive fish mortality during a Cylindrospermopsis raciborskii bloom in Aleksandrovac Lake, Serbia in mid-December 2012. According to a preliminary investigation of the samples taken on November 6 before the fish mortalities and to extended analyses of samples taken on November 15, no values of significant physicochemical parameters emerged to explain the cause(s) of the fish mortality. No industrial pollutants were apparent at this location, and results excluded the likelihood of bacterial infections. Even after freezing, the dissolved oxygen concentration in the water was sufficient for fish survival. High concentrations of chlorophyll a and phaeophytin occurred in the lake, and phytoplankton bloom samples were lethal in Artemia salina bioassays. A bloom of the cyanobacterium C. raciborskii was recorded during November. Although the A. salina bioassays indicated the presence of toxic compounds in the cyanobacterial cells, the cyanotoxins, microcystins, cylindrospermopsin and saxitoxin were not detected.

Occurrence of microcystins in water, bloom, sediment and fish from a public water supply

Microcystin (MC) accumulation was determined in the liver and muscle of two omnivorous fish species which are consumed and are economically important, and in a planktivorous-carnivorous fish from Lake Eğirdir, Turkey. Free extractable MCs in fish tissue samples were detected by enzyme-linked immunosorbent assay (ELISA) with confirmation by high performance liquid chromatography with photodiode array detection (HPLC-PDA). MC-LA and -YR, were detected in both liver and muscle, followed by MCs -LY, -LF, -RR and -LR respectively. The MC concentrations varied between 0.043 and 1.72μg/g dry weight in liver and muscle tissues. MCs were also determined in samples of water, sediment and a bloom sample of Microcystis aeruginosa from the lake by HPLC-PDA. MC-LY and -YR were most commonly identified in water samples, with total MC concentrations ranging from 2.9±0.05 to 13.5±2.3μg/L. Sediment analyses, showed that MC-YR was present in samples between 7.0 and 17.6μg/g dw, especially in October, November and December when no MC-YR was recorded in water, followed by MC-LW. The findings indicate that water and sediment contained MCs, and more importantly that fish were contaminated with MCs that may pose an MC-associated human health risk.

The 2000 Canine Distemper Epidemic in Caspian Seals (Phoca caspica): Pathology and Analysis of Contributory Factors

More than 10,000 Caspian seals ( Phoca caspica) were reported dead in the Caspian Sea during spring and summer 2000. We performed necropsies and extensive laboratory analyses on 18 seals, as well as examination of the pattern of strandings and variation in weather in recent years, to identify the cause of mortality and potential contributory factors. The monthly stranding rate in 2000 was up to 2.8 times the historic mean. It was preceded by an unusually mild winter, as observed before in mass mortality events of pinnipeds. The primary diagnosis in 11 of 13 seals was canine distemper, characterized by broncho-interstitial pneumonia, lymphocytic necrosis and depletion in lymphoid organs, and the presence of typical intracytoplasmic inclusion bodies in multiple epithelia. Canine distemper virus infection was confirmed by phylogenetic analysis of reverse transcriptase-polymerase chain reaction products. Organochlorine and zinc concentrations in tissues of seals with canine distemper were comparable to those of Caspian seals in previous years. Concurrent bacterial infections that may have contributed to the mortality of the seals included Bordetella bronchiseptica (4/8 seals), Streptococcus phocae (3/8), Salmonella dublin (1/8), and S. choleraesuis (1/8). A newly identified bacterium, Corynebacterium caspium, was associated with balanoposthitis in one seal. Several infectious and parasitic organisms, including poxvirus, Atopobacter phocae, Eimeria- and Sarcocystis-like organisms, and Halarachne sp. were identified in Caspian seals for the first time.

The common bloom-forming cyanobacterium Microcystis is prone to a wide array of microbial antagonists

Many degraded waterbodies around the world are subject to strong proliferations of cyanobacteria - notorious for their toxicity, high biomass build-up and negative impacts on aquatic food webs - the presence of which puts serious limits on the human use of affected water bodies. Cyanobacterial blooms are largely regarded as trophic dead ends since they are a relatively poor food source for zooplankton. As a consequence, their population dynamics are generally attributed to changes in abiotic conditions (bottom-up control). Blooms however generally contain a vast and diverse community of micro-organisms of which some have shown devastating effects on cyanobacterial biomass. For Microcystis, one of the most common bloom-forming cyanobacteria worldwide, a high number of micro-organisms (about 120 taxa) including viruses, bacteria, microfungi, different groups of heterotrophic protists, other cyanobacteria and several eukaryotic microalgal groups are currently known to negatively affect its growth by infection and predation or by the production of allelopathic compounds. Although many of these specifically target Microcystis, sharp declines of Microcystis biomass in nature are only rarely assigned to these antagonistic microbiota. The commonly found strain specificity of their interactions may largely preclude strong antagonistic effects on Microcystis population levels but may however induce compositional shifts that can change ecological properties such as bloom toxicity. These highly specific interactions may form the basis of a continuous arms race (co-evolution) between Microcystis and its antagonists which potentially limits the possibilities for (micro)biological bloom control.

Cyanobacteria and cyanotoxins in fishponds and their effects on fish tissue

Cyanobacteria can produce toxic metabolites known as cyanotoxins. Common and frequently investigated cyanotoxins include microcystins (MCs), nodularin (NOD) and saxitoxins (STXs). During the summer of 2011 extensive cyanobacterial growth was found in several fishponds in Serbia. Sampling of the water and fish (common carp, Cyprinus carpio) was performed. Water samples from 13 fishponds were found to contain saxitoxin, microcystin, and/or nodularin. LC-MS/MS showed that MC-RR was present in samples of fish muscle tissue. Histopathological analyses of fish grown in fishponds with cyanotoxin production showed histopathological damage to liver, kidney, gills, intestines and muscle tissues. This study is among the first so far to report severe hyperplasia of intestinal epithelium and severe degeneration of muscle tissue of fish after cyanobacterial exposure. These findings emphasize the importance of cyanobacterial and cyanotoxin monitoring in fishponds in order to recognize cyanotoxins and their potential effects on fish used for human consumption and, further, on human health.

Hepatotoxic microcystin removal using pumice embedded monolithic composite cryogel as an alternative water treatment method

Microcystins are the most commonly encountered water-borne cyanotoxins which present short- and long-term risks to human health. Guidelines at international and national level, and legislation in some countries, have been introduced for the effective health risk management of these potent hepatotoxic, tumour-promoters. The stable cyclic structure of microcystins and their common production by cyanobacteria in waterbodies at times of high total dissolved organic carbon content presents challenges to drinking water treatment facilities, with conventional, advanced and novel strategies under evaluation. Here, we have studied the removal of microcystins using three different forms of pumice particles (PPs), which are embedded into macroporous cryogel columns. Macroporous composite cryogel columns (MCCs) are a new generation of separation media designed to face this challenging task. Three different MCCs were prepared by adding plain PPs, Cu(2+)-attached PPs and Fe(3+)-attached PPs to reaction media before the cryogelation step. Column studies showed that MCCs could be successfully used as an alternative water treatment method for successful microcystin removal.

Grazing livestock are exposed to terrestrial cyanobacteria

While toxins from aquatic cyanobacteria are a well-recognised cause of disease in birds and animals, exposure of grazing livestock to terrestrial cyanobacteria has not been described. This study identified terrestrial cyanobacteria, predominantly Phormidium spp., in the biofilm of plants from most livestock fields investigated. Lower numbers of other cyanobacteria, microalgae and fungi were present on many plants. Cyanobacterial 16S rDNA, predominantly from Phormidium spp., was detected in all samples tested, including 6 plant washings, 1 soil sample and ileal contents from 2 grazing horses. Further work was performed to test the hypothesis that ingestion of cyanotoxins contributes to the pathogenesis of some currently unexplained diseases of grazing horses, including equine grass sickness (EGS), equine motor neuron disease (EMND) and hepatopathy. Phormidium population density was significantly higher on EGS fields than on control fields. The cyanobacterial neurotoxic amino acid 2,4-diaminobutyric acid (DAB) was detected in plant washings from EGS fields, but worst case scenario estimations suggested the dose would be insufficient to cause disease. Neither DAB nor the cyanobacterial neurotoxins β-N-methylamino-L-alanine and N-(2-aminoethyl) glycine were detected in neural tissue from 6 EGS horses, 2 EMND horses and 7 control horses. Phormidium was present in low numbers on plants where horses had unexplained hepatopathy. This study did not yield evidence linking known cyanotoxins with disease in grazing horses. However, further study is warranted to identify and quantify toxins produced by cyanobacteria on livestock fields, and determine whether, under appropriate conditions, known or unknown cyanotoxins contribute to currently unexplained diseases in grazing livestock.