CHAPTER 41 HARMFUL ALGAL BLOOMS INCLUDING Cyanobacterial toxicosis

KJELL HANDELAND¹ AND DOLORES GAVIER-WIDEN²

¹Department of Animal Health, Section of Wildlife Diseases, National Veterinary Institute, Oslo, Norway ²National Veterinary Institute (SVA), and Swedish University of Agricultural Sciences, Uppsala, Sweden

INTRODUCTION

A number of aquatic microalgal species are known to cause harmful toxic blooms and animal intoxications (phyco­toxicosis).

Toxic cyanobacterial blooms have long been associated with intoxication events in domestic animals all over the world, and they are also posing a human health hazard (e.g.⁽³⁾). Some reports mention that wild mammals and birds were also affected. Specific reports of suspected cya­notoxic die-offs in wildlife are, however, mostly restricted to mass mortality events in birds⁽⁴’⁵⁾. Owing to the acciden­tal nature of exposure and limitations of diagnostic meth­odology, the general frequency of cyanotoxicosis in wildlife is likely to have been greatly underestimated in the past.

AETIOLOGY

Cyanobacteria (Class Cyanophyceae) are prokaryotic (no nucleus) photosynthetic microorganisms containing chlorophyll-a⁽⁶⁾. They also contain accessory photosyn­thetic pigments such as phycocyanin, allophycocyanin and phycoerythrin. Cyanobacteria are important primary pro­ducers that may flourish in fresh, brackish and marine water ecosystems. Growth is dependent on carbon dioxide, inorganic compounds and light, and may take place both in the pelagic (water column) and benthic (sediment) water domains.

Cyanobacterial toxins constitute a diverse group of compounds from both a chemical and toxicological point of view⁽²⁾. They can be classified according to their mode of toxicity into three different groups:

1. hepatotoxins, including microcystins and nodularins

2. neurotoxins, including anatoxins and saxitoxins

3. dermatotoxins.

Microcystins include 80 structural variants of cyclic pep­tides, among which microcystin- RR and -YR are recog­nized as the most toxic. Microcystins are produced by a relatively wide range of fresh and brackish water cyanobac­teria, the most important belonging to the genera M icro- cystis, Anabaena, Planktothrix and Oscillatoria. Production of nodularins is restricted to the species Nodularia spumi- gena, which occurs in brackish waters and is an important species in the Baltic Sea.

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

The anatoxins involved in animal cyanotoxicoses are anatoxin-a, homoanatoxin-a and anatoxin-a(s). The ana­toxins are mainly produced by cyanobacteria within the genus Anabaena, but also by other genera, including Microcystis, Oscillatoria and Planktothrix.

The dinoflagellate Karenia brevis produces brevitoxin, a potent marine neurotoxin.

EPIDEMIOLOGY

GEOGRAPHICAL DISTRIBUTION AND HOSTS

Most cases of cyano-intoxication associated with cyanobac- terial blooms have been reported from livestock and dogs. In Europe, domestic animal intoxication has been reported in Germany, Denmark, Sweden, Norway, England, Scot­land, Ireland and Switzerland (e.g.(3)).

Mortalities in wild waterfowl associated with cyanobac- terial bloom have been registered in several European countries including Denmark(^9,10); Belgium⁽¹¹⁾, Spain⁽¹²’¹³⁾, Sweden and Finland¹-⁵). The Danish outbreaks were associ­ated with anatoxin- a(s) producing A nabaena blooms in freshwater lakes. The Spanish outbreaks occurred in the Donana National Park, which is a major waterfowl refuge in Southern Europe. Here, many mass mortality events have occurred since 1973, and at least two were associated with heavy cyanobacterial blooms. In the first recorded outbreak, a large number of waterfowl, especially greater flamingos (Phoenicopterus ruber), died⁽⁵⁾. This outbreak was associated with a mixed bloom, consisting of Micro­cystis aeruginosa and Anabaena flos-aquae. Laboratory examination revealed hepatotoxicosis and high levels of microcystins in the liver of dead birds. The second out­break was associated with a bloom of Microcystis aerugi­nosa, and at least 6,000 waterfowl belonging to 47 species were killed(¹³). This outbreak initially affected herbivorous waterfowl and fish, followed by a die- off in piscivorous birds. Laboratory findings included typical liver lesions in dead birds, and the presence of high concentrations of microcystins in the livers of both dead birds and fish. The authors suggested this cascade of waterfowl die-offs could be explained by accumulation of cyanotoxins in the food web: firstly affecting herbivorous avian species consuming cyanobacterial bloom water (scum), and subsequent poi­soning of piscivorous birds feeding on fish that had bio­accumulated the toxins through predation on contaminated zooplankton and aquatic invertebrates.

Episodes of mortality associated with brevetoxicosis affected endangered Florida manatees ( Trichechus manatus latirostris) and bottlenose dolphins ( Tursiops truncatus) that had consumed toxic seagrass and fish, respectively¹-¹⁵). Southern sea otters (Enhydra lutris nereis), a threatened species, were reported to have died of cyanotoxicosis in the Pacific coast of California¹-¹⁶). Mortality of marine mammals caused by phycotoxicosis has not been reported in Europe.

ENVIRONMENTAL FACTORS

Planktonic (pelagic) cyanobacterial blooms are character­ized by water discoloration and formation of a blue or green surface scum. Blooms occur in eutrophic waters rich in mineral nutrients (phosphorous, nitrate) during summer or autumn, when water temperature is high and light conditions favourable for photosynthesis. Cyanobacteria are slow-growing compared with other competing plank­tonic microalgae, and require stagnant water to proliferate in large numbers. They have, however, the advantage of needing less light (accessory photosynthetic pigments) than other microalgae, allowing them to outcompete other species under turbid water conditions. Under stagnant and turbid water conditions one or a few species of cyanobac­teria may completely dominate, and if the dominating species include one or several toxin-producing cyanobac­teria, intoxication events may occur (Figure 41.1). Toxic cyanobacterial blooms occur mostly in freshwater ecosys­tems (lakes, ponds, rivers) and have been reported from a large number of European countries⁽¹⁷⁾. In Europe, some 53% of fresh water lakes are considered eutrophic and thus constitute potential sites of toxic blooms(¹⁸). Brackish (marine) ecosystems may also be eutrophic and develop

FIGURES 41.1 Mixed cyanobacterial bloom of Microcystis aeruginosa (A) and Anabaena flos-aquae (B) occurring in lake Ostensjovannet (C), Oslo, Norway in July 1989.

algal blooms, e.g. the Baltic Sea⁽⁶⁾. Although some water sources are naturally eutrophic, in many others the excess nutrient levels are caused by human activity, resulting from wastewater discharges or run-off from agricultural areas.

Animals are most often exposed to toxic amounts of algae as a result of drinking water or scum from a lake or pond in which prevailing winds have concentrated the bloom. Cyano-intoxication may also occur following water consumption from smaller water sources such as water­holes, small dams and dugouts containing stagnant water, in which thick coats of algae may form⁽¹⁹⁾. There are indi­cations that animals do not avoid cyanobacterium- contaminated drinking water⁽²⁰⁾.

Intoxication may also be associated with benthic aggre­gations (coherent mats) of cyanobacteria. During periods of high photosynthesis, production of oxygen bubbles may increase the buoyancy of the mats, causing them to tear loose and rise to the surface, where if ingested, they may cause animal intoxication. This type of cyanotoxicosis has been reported in domestic animals in Scotland⁽²¹⁾ and Switzerland⁽²²⁾. Another route of intoxication is through consumption of organisms (fish, molluscs, zooplankton) that have themselves fed on cyanobacteria or otherwise bio-accumulated cyanotoxins⁽²⁾. In this way, mortality can occur weeks after the disappearance of the bloom.

PATHOGENESIS AND PATHOLOGY

Cyanobacterial toxins are released following algal cell destruction in the animal’s stomach, but may also exist free in the water body as a result of cyanobacterial cell autoly­sis.

The toxicokinetics of the neurotoxic anatotoxins have not been described. However, based on the peracute clini­cal course of this intoxication, intestinal absorption must be rapid. Anatoxin- a and homoanatoxin- a act as potent cholinergic agonists at nicotinic acetylcholine receptors in neurons and neuromuscular junctions, leading to respira­tory paralysis. Anatoxin- a(s) is an irreversible acetylcho­linesterase inhibitor, which leads to increased concentrations of acetylcholine in the synapse, resulting in neuromuscular blockage and respiratory arrest. The anatoxins do not cause cell-morphological changes.

FIGURE 41.2 Liver from a microcystin- intoxicated roe deer, showing diffuse hepatocyte dissociation, degeneration and necrosis and perisi- nusoidalhaemorrhage.T = portal tract; V = central vein. Haematoxylin- eosin stain. From Handeland & 0stensvik, 2010(⁸).

Animals found dead after drinking toxic bloom may show evidence of green- or blue-coloured cyanobacteria on the hair or feathers. Gross pathological findings reported following acute microcystin and nodularin intoxication include a pale, swollen and sometimes haemorrhagic liver, as well as ascites and widespread small haemorrhages. Microscopic liver lesions include hepatocellular dissocia­tion, degeneration and necrosis, as well as areas of perisi- nusoidal haemorrhage or massive bleedings (Figure 41.2). No specific gross or microscopic lesions are found in animals dying from anatoxin intoxications.

CLINICAL SIGNS

Clinical signs of cyanotoxicosis may be linked to either hepatotoxic or neurotoxic effects. However, it is important to keep in mind that mixed cyanobacterial blooms, with multiple associated toxins, are common, and may compli­cate the clinical picture. Moreover, the effect of interac­tions between the different toxins involved is not clearly understood. Thus, the clinical signs resulting from natu­rally occurring intoxication events may be complex.

The clinical course of fatal microcystin and nodularin hepatotoxicoses is acute, with death normally occurring within a few hours post- exposure. Clinical signs include general weakness, vomiting, diarrhoea, pallor and shock. Animals living beyond a few hours post-exposure may develop hyperkalaemia, hypoglycaemia, nervousness, recumbancy and convulsions.

The course of fatal neurotoxicoses caused by anatoxin-a, homoanatoxin-a and anatoxin-a(s) is peracute, with death due to respiratory paralysis. Death normally occurs within a few minutes to a few hours post-exposure. Clinical signs following anatoxin-a and homoanatoxin-a ingestion include rigidity, muscle tremor, cyanosis and paralysis and, in birds, opisthotonos. Clinical signs following anatoxin- a(s) intoxication include tremor, inco-ordination, and con­vulsion, as well as excess salivation (‘s’ stands for salivation), lacrimation and urinary incontinence.

DIAGNOSIS

Specific diagnosis of cyanobacterial toxicosis has been dif­ficult, as a result of the lack of appropriate diagnostic procedures, and remains challenging. In the past, diagnosis was based upon algal bloom identification and disease history, possibly in combination with toxin analyses of algae and bioassays in experimental animals. Toxin pro­duction is, however, not constant during a bloom, and many algal blooms, even when dominated by species known to be capable of producing toxicosis, may not be hazardous.

A definitive diagnosis of cyanotoxicosis should be based upon a thorough post mortem examination, followed by demonstration of high tissue cyanotoxin levels. The post mortem examination should aim to demonstrate the presence of typical liver lesions by cyanobacterial hepato- toxicosis or absence of morphological lesions by cyanobac­terial neurotoxicosis, as well as consideration of other possible aetiologies. One important differential diagnosis in avian events is botulism, a common cause of waterfowl death in eutrophic waters. Methodologies for the determi­nation of cyanotoxins in post mortem tissues include liquid chromatography and mass Spectrometric systems, as well as enzymatic assays and immunoassays¹-³). Liquid chroma­tography and mass spectrometric methods have been employed on tissue samples for quantitative analyses of specific cyanotoxin accumulation or intoxication in wild birds and mammals in Europe)^8,14,25). Commercial kits are also available for determination of cyanotoxins (microcys­tin) in the tissues and have been used to confirm cyano­toxicosis in birds)^12,13).

MANAGEMENT, CONTROL AND REGULATIONS

The temporary management of phycotoxicosis is based on mitigation and/or removal of the bloom, by applying algi­cides, by physical removal of surface scums and by mixing of the water column. However, possible adverse effects on the ecosystem have to be considered. Exposure to toxic water, by domestic animals for example, should be avoided whenever possible.

Process- based and statistical models have been devel­oped to predict cyanobacterial bloom occurrence, and their concentration and distribution in water bodies can be monitored.

Anthropogenic input of nutrients into sensitive waters largely affects the rate of blooming of harmful algae. Effec­tive long-term control and management of blooms requires reducing eutrophication, including constraints of human- made supply of nutrients, mostly nitrogen and phospho­rus. The European Commission, Institute for Environment and Sustainability (IES) Global Environment Monitoring Unit has developed two eutrophication indices, which allow the assessment of the coastal and marine ecosystem’s status and physical sensitivity to eutrophication, as part of the Research Action 2121 (Monitoring and Assessment of Marine Ecosystems))²⁶).

PUBLIC HEALTH CONCERN

Mass populations of toxic cyanobacteria present important environmental and human health hazards. Phycotoxicoses in humans are mainly associated with consumption of contaminated shellfish and include, among others, para­lytic shellfish poisoning (caused by a saxitoxin), neurotoxic shellfish poisoning (caused by brevetoxin), and amnesic shellfish poisoning (caused by domic acid). Algal derma­totoxins and gastrointestinal toxins also affect humans.

Hepatotoxins, such as microcystin, cause major liver damage in humans. Nodularin is a carcinogen. Cases may occur simultaneously in humans and animals when both are exposed to the same source.

SIGNIFICANCE AND IMPLICATIONS

FOR ANIMAL HEALTH

Phycotoxicosis may cause important mortality events in wild animal populations, in particular in wild waterfowl.

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