Guidelines for Canadian recreational water quality: Microbiological pathogens and biological hazards: Other biological hazards

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3.0 Other biological hazards

This section provides guidance on other organisms that may affect the recreational value of natural waters by rendering them unsafe, aesthetically objectionable or otherwise unusable, and thus interfering with users’ health, physical comfort and enjoyment. These organisms are free-living species that occur naturally in recreational waters. Guidance is provided here so that authorities responsible for managing recreational waters and the general public have information on the possible hazard posed by these organisms and steps for reducing potential exposure. This list is not intended to be exhaustive. The responsible authorities may wish to provide information on other organisms of regional or local significance.

3.1 Schistosomes causing swimmer’s itch

Swimmer’s itch (cercarial dermatitis) is caused by human reaction to dermal penetration by parasitic flatworms or “schistosomes” which belong to the Schistosomatidae family and may infect certain waterfowl and aquatic rodent species (Manitoba Water Stewardship, 2007). The species known to cause swimmer’s itch include members of the genera Austrobilharzia,Trichobilharzia, Dendritobilharzia, Gigantobilharzia and Schistosomatium (Levesque et al., 2002; CDC, 2004a; Gordy et al., 2018). Cercarial dermatitis should be clearly distinguished from human schistosomiasis, a far more serious human infection caused by species of the genus Schistosoma which is typically restricted to tropical regions of the world (WHO, 2003).

The species of schistosome that cause swimmer’s itch have a two-host life cycle, consisting of a definitive host (waterfowl or aquatic rodents) and an intermediate host (certain species of aquatic snails). It is the aquatic snails that produce the form of the parasite (i.e., the cercariae) that can impact humans. Once a snail is infected (e.g., in late spring by migrating waterfowl), it generally takes up to 1.5 months for the cercariae to emerge. Water temperature can have an effect on the release of mature cercariae by infected snails, with higher concentrations occurring in warmer waters (Verbrugge et al., 2004). Warmer waters and the parasite life cycle may partially explain why infections are encountered more frequently during the summer months. Summer is also a time when recreational activities increase in Canada, increasing potential exposure to this parasite. Schistosome cercariae are encountered in areas with dense snail beds. Outbreak data indicate that dense beds are typically found in shallow waters along the shoreline, particularly where there are large numbers of aquatic plants (Levesque et al., 2002; Leighton et al., 2004; Verbrugge et al., 2004). Onshore winds can also direct cercariae towards shorelines where snails are absent (Rudko et al., 2018; Sckrabulis et al., 2020).

Humans are an accidental or dead-end host for the cercariae. When cercariae come in contact with humans in the water, they may penetrate the outer layer of skin but then quickly die as they cannot develop any further. The presence of the cercariae beneath the skin causes an allergic reaction (i.e., cercarial dermatitis) with symptoms such as an initial tingling, itching or burning sensation. Small, reddish pimples typically appear within 12 hours after infection. These can progress to larger secondary blisters or rashes, which can be accompanied by an even stronger itching sensation. The effects of swimmer’s itch may be felt shortly after swimming, in some cases in as little as a few minutes. Although the infection is self-limiting, typically lasting from 2 to 5 days, symptoms can persist for as long as 2 weeks. Swimmer’s itch is not contagious and cannot spread from person to person. However, individuals who develop an allergic reaction to cercariae can experience an increased sensitivity to subsequent infections. The symptoms become more intense and develop much more rapidly in these instances (British Columbia Ministry of Health, 2018). Sensitivity can vary considerably between different individuals; some may show a strong allergic reaction, whereas others show no signs of infection. Individuals who have a severe reaction are advised to seek medical treatment from a health professional. Treatments suggested for alleviating itching symptoms include bathing in Epsom salts, baking soda or colloidal oatmeal; and using cold compresses; anti-itch medications such as corticosteroid creams or calamine lotion; or oral antihistamines (Manitoba Water Stewardship, 2007; British Columbia Ministry of Health, 2018). Affected individuals should refrain from scratching because it increases the potential for secondary bacterial infection (CDC, 2004b).

 For most recreational waters in Canada, the risk of contracting swimmer’s itch through recreational activity is considered to be low. A review of reported episodes of swimmer’s itch across Canada found very few documented cases (only 280) over a more than 60-year period, and indicates that only one or two outbreaks of swimmer’s itch occur every decade (Gordy et al., 2018). However, many cases of swimmer’s itch go unreported because the symptoms are typically benign and users may not seek medical attention. To better capture the amount of underreporting and assess the real incidence of swimmer’s itch, Gordy et al. (2018) implemented a voluntary online questionnaire, which was available across the country from 2013 to 2017. The survey captured 3,882 cases of swimmer’s itch over the five summers the website was active. The reported cases occurred in every province in Canada except for Prince Edward Island. This work confirms that although the risk of contracting swimmer’s itch is low considering the number of individuals that engage in recreational activities each year, recreational users should be made aware of the potential risks in impacted areas.

3.1.1 Managing health risks from schistosomes

The species of schistosomes that cause swimmer’s itch are considered to occur naturally in Canadian surface waters. They are not related to fecal pollution. As a result, their presence is not detected during standard water quality testing for the recommended indicators of fecal contamination. The presence of these organisms in natural waters is dependent upon a number of factors, both biological and environmental. This makes it very difficult to predict when and where swimmer’s itch might become a problem. Certain areas may report a problem where none appeared to exist previously. Areas in which swimmer’s itch has been reported will not necessarily always remain a problem. Propagation of the parasites responsible for causing swimmer’s itch requires that both the primary and intermediate host be present in sufficient numbers.

A management strategy combining actions to control the extent of the water quality hazard and steps to limit exposure during periods or in areas perceived to be at greater risk is recommended to reduce the risk of human exposure to these schistosomes in recreational waters. Actions that may help control the presence of schistosomes include not feeding waterfowl, and where possible, removing organic wastes within the main snail habitat.

Warning signs that clearly notify the public of the risk of exposure should be posted in recreational water areas where cases of swimmer’s itch have been reported. A swimming advisory may be issued at the discretion of the responsible authority. Further details on the posting of information in recreational water areas can be found in Understanding and Managing Risks in Recreational Waters (Health Canada, in publication-d).

Another risk reduction approach can include distributing educational materials outlining steps the public can take to reduce their personal risk of exposure and the severity of symptoms in the event of infection. Guidance provided in communication materials intended for the general public could include the following:

3.2 Aquatic vascular plants and algae

Aquatic vascular plants (macrophytes) and algae can affect recreational water use. It is difficult to estimate the magnitude of their impact in terms of their degree of interference with recreational pursuits and the potential risks to the health of recreational water users.

The presence of these organisms can present a safety risk to recreational water users. Swimmers may become entangled in the fronds of aquatic plants. Dense growths can obstruct the view of the bottom and of underwater hazards and may impair the ability of safety personnel to see persons in distress. Algal mats attached to rocks and other substrates (i.e., periphyton) can cause slippery conditions that may lead to unintentional immersions or injuries.

Excessive growth of plants and algae can also create aesthetic problems for recreational water areas. Macrophytes can reach high population densities and make nearshore and shallow regions unsuitable for any recreational purpose (Priyadarshi, 2005). Dislodged rafts or mats of plants and algal material can be washed ashore and left to rot, fouling beaches. In addition to being unsightly, these masses can further detract from user enjoyment by producing offensive odours and restricting access to shorelines. They may also pose a public health risk because they can attract undesirable animals to the area and provide breeding grounds for a variety of species of insects and bacteria (Whitman et al., 2003). The most notorious organism in this respect has been the green algal species Cladophora (Priyadarshi, 2005). There have been many accounts of beaches and shorelines fouled by rotting, stinking masses of this alga. Cladophora mats can also provide a secondary habitat for bacteria that can adversely affect water quality in affected swimming areas (Whitman et al., 2003; Ishii et al., 2006; Englebert et al., 2008; Verhougstraete et al., 2010) and for bacteria linked to bird die-offs (avian botulism) (Lan Chun et al., 2015). Blooms of other non-toxic algal species can also cause aesthetic problems and potentially be mistaken for cyanobacteria blooms. Cyanobacteria blooms are a public health concern as they can contain cyanotoxins and contact with bloom material may cause skin irritation and gastrointestinal upset. Further information on cyanobacteria can be found in the Guidelines for Canadian Recreational Water Quality – Cyanobacteria and their toxins technical document (Health Canada, 2022b).

The presence of excess nutrients in the water may increase plant and algae growth; this is known as eutrophication. Various nutrient sources, including agricultural runoff, domestic sewage and industrial effluent, contribute phosphorus and nitrogen to aquatic systems and can lead to eutrophication. Impaired water quality due to eutrophication can reduce recreational opportunities (Chambers et al., 2001; Watson et al., 2017). Canadian water quality guidelines have been developed for both phosphorus and nitrogen to protect the aquatic environment from the buildup of these nutrients and their effects on aquatic organisms (CCME, 1999).

3.2.1 Managing health risks

Recreational activities should not be pursued in areas where there are such large quantities of aquatic plants and algae that the responsible authorities consider that their presence poses a potential health or safety risk to recreational water users. An environmental health and safety survey should be conducted at the start of each swimming season to identify potential safety hazards in a given recreational water area. If problems are identified, signs may be posted to warn users of potential visibility or entanglement risks from these organisms. Further information on the posting of warning signs can be found in Understanding and Managing Risks in Recreational Waters (Health Canada, in publication-d).

Improved beach clean-up procedures to remove masses of plants and algal material that have washed up on shorelines can be effective in reducing potential risks to recreational users. Management actions that involve trying to remove these organisms from natural waters are discouraged. Removal may be harmful to the aquatic environment and is generally not effective from the perspectives of practicality (plants quickly grow back) and financial resources (many hours of paid labour). Many aquatic plants and algae also provide an important habitat for fish and other aquatic biota. The application of pesticides to combat these organisms is not recommended as it may create a health hazard for recreational water users if the products are used incorrectly. Furthermore, the application of pesticides could cause the release of cyanobacterial toxins if toxin producing cyanobacteria are present. Longer-term actions that may reduce the impact of these organisms may include identifying the major nutrient inputs within the watershed and implementing strategies for their control.

3.3 Additional organisms

Numerous other organisms can interfere with the safe and enjoyable use of recreational waters in Canada. For example, at some coastal beaches, jellyfish can cause painful and possibly serious stings to recreational water users who come into contact with them. Similarly, leech “bites” can occur in leech-infested areas, and sea urchins and mussel shells can cause painful injuries when stepped on by users of recreational waters. As these organisms are often of local or regional significance, it is recommended that the responsible authority provide appropriate guidance to recreational water users on these subjects where necessary. This may include providing information on the potential risks posed by these organisms and on steps for reducing the risk of exposure for individuals.

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