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The purpose of this paper is to explore one form of vector borne diseases, swimmer’s itch, what it is, what are the causes and if there are any remedial methods to  mitigate the problem.

Cercarea Dermatitis or Swimmer’s itch is a benign form of dermatitis transmitted to human while wading or swimming in shallow lake water. It causes the skin to become itchy and small red dots form on the exposed areas. The skin rashes often look like mosquito bites. It is more annoying that it is serious and will disappear quickly, however it does compromise the recreational value for those who like to swim outdoors. Those most affected happen to be young children, simply because they are the ones most likely to wade and swim in shallow lake water.

Initial location of interest:

Memphrémagog LakeThe primary area of interest is Lake Memphremagog. “Lake Memphremagog is a fresh water glacial lake located between Newport Vermont and Magog Quebec.  It is 27 miles long (43Km) and roughly 75% of its watersheds are in Vermont. The coordinates for Lake Memphremagog are: 45o05’N, 72o16”W.”  Specific to Lake Memphremagog, this paper, in part looks at an outbreak of swimmer’s itch in the small community of Georgeville. It is important to mention that a few kilometers from the Georgeville beach area is a duck sanctuary. As we shall see, birds are the primary hosts for the transmission of “swimmer’s itch”.

Children registered in a summer day camp program were affected by this dermatitis. Those most susceptible were children under the age of seven and inexperienced swimmer’s, this group of children would wade in the shallow end of the dock in Georgeville only to come out rather itchy some were in such discomfort, they asked to be sent home. This outbreak has gone on for three consecutive summer’s 2007, 08 and 09 causing frustration to staff, parents and children. The problem became so prevalent that on warm sunny days when it would be ideal to go swimming a decision was taken not to allow the young ones in the water. To compensate for this unfortunate situation, the staff made use of a water slide and ran a water hose so that the children may benefit from water activities.

Cyclical pattern of cercariae dermatitis

During those summers other beach areas of Lake Memphremagog had similar experiences. Magog Conservation Inc. had done research on the matter and has documented the cause and life cycle of swimmer’s itch. The initial culpritHow parasites infect humans of swimmer’s itch is a type of parasitic flatworm known as trichobilharzia. These types of worms are ingested by birds, the birds then release cercariae into the water were eggs pass through the feces; once the eggs hatch they release a larva which eventually gets ingested by snails. The snails then transfers the parasite to the swimmer’s skin leading to the dermatitis. This is a cyclical transfer of a parasite where the main hosts are birds, gulls, swans and ducks. The snail becomes the intermediate hosts of the parasite and seeks to transfer it to another body.

Historical overview of Cercariae Dermatitis and literary review:


The first known documented study of cercariae Dermatitis in North America dates as far back as the early 1930’s. The outbreak was first studied from lakes in Northern Michigan. In the early 1930’s there were 17 different types of snails found at Douglas Lake. It was from this study that feces contaminated snails were first examined and associated with an annoying dermatitis known as swimmer’s itch.

30 years later, in the early 1960’s, when the study was looked at again, a biomedical research center discovered that the amount of species of snails had dropped from 17 to 7 but that cases of swimmer’s itch still remained. The researchers hypothesized that the drop in species prevalence was likely due to human influence such as industrial construction along shore lines and the use of water motor craft, and the migration of birds.

It would appear that feces infected snails happens to be a natural byproduct of nature and could not necessary be associated with cyanobacteria or eutrophication because in the early 1930’s eutrophication was not much of a concern.

Interestingly when the study was once again examined in the 1990’s species prevalence had halted, that is from the 1960’s onward, still, 7 different types of snails were identified however recovery of the snails’ species lost did not occur. It was naturally concluded that only the most adapted species of snails survived the changes to their aquatic environment that occurred between the 1930’s and the 1960’s.

Other studies determined, swimmers’ itch initially originated in Europe and that parasitic contaminated snails seem to be spreading both with latitudinal and with altitude, certain areas in the Andes have documented  clusters or prevalence of  freshwater snails carrying vector borne diseases. The cause of this spread is thought to be climate change.

Relationship between Climate Change and infections transmitted by snails

Swimmer’s itch

Taking into consideration that the issues of climate change are no longer up for debate this portion of the literary review discusses the effects of how climate change is affecting the prevalence of vector borne diseases. An article by Santiago et al have determined the following relationships: “temperate environments are more receptive to vector borne diseases because higher temperatures are thought to increase the invertebrate metabolic rate, egg production and amount of feeding frequency…. and that disease transmission is modulated by the survival reproduction rate and daily activity of the vector…this will most affect species of R-strategists type, that is species with short generation time and rapid population growth”. The article goes on to say that the amount of larvae output from snails is primarily a function of species type. Some snail types are able to transmit more larvae than others when the temperatures increase, and observed that the amount of parasitic larvae is finite in most snails. With relationship to climate change, this implies that once a specific snail type has reached their own threshold of output an increase in temperature will not affect their release has they have emitted the maximum they possibly could.

The most ambient temperature for snails to reproduce and thus emit parasites is at about 20oC, if temperatures drop below that snail reproduction is significantly reduced, while temperatures above 30o C are too warm and the snails tend to die off.

Remembering that primary or definitive hosts are herbivorous mammals i.e. ducks, they too are also affected by variations in climate. As temperatures increase, these mammals will excrete more of their feces, which in turn will affect the intermediate host’s population dynamics as well as their parasitic release potential.

Climate change tends to affect the migratory patterns of birds. With warmer temperatures, the birds that usually migrate south might tend to move on later in the season due to warming trends.


It is also believed that with global warming trends there will be increase in precipitation.  This is because warm air tends to rise which will eventually cool adiabatically and subsequently form clouds. This additional rain fall will also likely affect the intermediate host’s population dynamics.

Another vital point to address is that with increasing temperatures the contaminated snail population is likely to expand geographically. “Fascioliasis (genus of liver flukes), has become the vector borne disease presenting the widest latitudinal, longitudinal and altitudinal distribution known”. It is expected that the lower latitudes will be most affected by this vector reproduction and potential parasitic transmission.

A program funded by the Rockefeller Foundation as deemed the study of snail borne vector diseases important enough to warrant research and a database is currently being set up specially to monitor Schistosomiasis. “Schistosomiasis, is an infection caused by parasitic flukes of the genus schistosome occurring commonly in Eastern Asia and in tropical regions and transmitted to humans through feces contaminated fresh water or snails.  Symptoms commonly include pain, anemia and malfunction of the infected organ”. As noted by the definition Schistosomiasis, also referred to as “snail fever” is potentially more harmful than Swimmer’s itch, yet both diseases are transmitted by feces contaminated fresh water snails.  It is only when one travels south of the equator that contamination becomes more serious. A possible reason for this could be attributed to results found from a study conducted by Brant & Locker who looked at Trichobilharzia, a sub class of parasitic flatworms in North America thought to be the initial culprit in the transmission of swimmer’s itch. They noted that when they compared North American studies with other regions on species specimens of host birds, “none of the North American adult worms was found in host nasal chambers, a location inhabited by some worm species in Europe, Australia, Islam and Africa….this is of note from a public health perspective because the nasal dwelling worm species (T. Regenti) migrates via both peripheral nerves, and the central nervous system to reach its preferred site of infection. This has been shown to cause anomalous behavior in both experimentally infected birds and mammals and has a potential to present similar consequences in humans”.

The aim of the program funded by the Rockefeller foundation is to “create a global network database in collaboration with health workers and earth scientists dedicated for improved control programs for schistosomiasis”. “This global network is assembled using GIS methods and a web site known as http://www.gnosisGIS.org (GIS network on Snail-borne infections with special reference to schistosomiasis) was established to create health maps and monitor disease impacts and city water treatment projects associated with climate variations”.

World Health Organization logoThe project consists of linking four central resource groups: World Health Organization (WHO), the Food and Agriculture Organisation (FAO), Louisiana State University (LSU) and the Danish Bilharziasis Laboratory (DBL) with regional GIS networks in Asia, the Jiangsu Institute of Parasitic Diseases; East Africa, the Addis Ababa University; South America, the University of Natal; West Africa, the University of Mali, and South America, the Federal University of Bahia. The initial goal is to establish what is termed a Minimum Medical GIS Database, to construct this database digital geospatial models will be used such as: 1) GIS methods,  2) Sensor Data, 3) Disease prevalence data, 4) the distribution and abundance of snail hosts, 5) soil land use and topographic maps and 6) Global climate data. Once, implemented it is believed that GIS and satellite surveillance will act as a powerful tool as predictive models for the spread of vector borne diseases. Though the Data base does not include North America as a prime location it is none the less interesting to note that the World Health Organization and other prominent institutions deem the study of snail borne diseases important enough to monitor globally.

Relationship between gastropods (snails) and cyanobacteria

An article presented by Lance & al examining the interaction between snails and cyanobacteria were interested in identifying whether toxic ingestion would have a negative impact of the life history traits of L. Lymnaea snails.  The method used to answer this question involved incubating snails for a five week gestation period.  The snails were divided into four different groups. One group of were fed a density of cyanobacteria comparable to those found in eutrophic waters.  Another was fed lettuce a non-toxic food, one set was starved and another was fed both lettuce and cyanobacteria. With this method the authors hypothesize that both accumulation and detoxification of cyanobacteria will affect the life-history traits and physiology of the L. Lymnaea snail.  The authors point out that “crawling of aquatic gastropods constitutes the most costly form of locomotion in the animal kingdom, due to their need of mucus secretion, which is highly energy demanding” they have hypothesized that “a decrease in locomotry activity may allow the release of energy which can be used for the detoxification process”. The main assumptions discussed were that snails play a key role in our ecosystem as herbivorous grazers, should they become at risk,  this could have a negative effect on the populations of predator organisms (i.e. crayfish, leeches, and insects). The overall results of the research indicate that generally snails are robust.  When exposed to cyanobacteria, results show a reduction in fecundity and growth rate of L. Lymnaea but not a complete absence. Exposures to levels of cyanobacteria were not lethal to the snails. They eat the toxins and their metabolic system can easily recover when put under conditions of detoxification. The authors suspect that this survival rate is overestimated they maintain that “probability of survival of L. Stagnalis in an environment polluted with toxic cyanobacteria is likely to be less than predicted by our results”. They base this assumption on the possibility that they have not exposed the snails to enough of the toxins.

With respect to swimmer’s itch there would appear to be no real causal relationship between feces infected snails and cyanobacteria; snails seem to cope with both types of infestations differently.  One by ingesting cercariae and then looking for another host to send the parasite to, the other by consuming algae and slowing the down the snails level of fecundity and overall growth rate.  Snail population growth and activity appears to result more from the extra energy these gastropods gain with warming temperatures.

Remedial efforts: Habitat Disturbance

Lac Beauport

This portion of the literary review looks at two separate studies conducted in Canada, One at Cultus Lake British Columbia, the other at Lac Beauport in the Quebec City region.  These studies were conducted to address outbreaks of cercariae dermatitis or “swimmer’s itch in those respective regions. Both studies propose habitat disturbance to mitigate the problem. For the area at Cultus Lake, the study was done over a three year period in order to gain a better understanding of the cyclic relationship between the definitive primary hosts, and their intermediate snail host. The authors also wanted to test new environmental management techniques which would lead to a reduction in the number of schistosome dermatitis cases at Cultus Lake. To achieve this end they “studied the life cycles of the different schistosome species at Cultus Lake, the biology of their respective host snail population, and the behavior of the definitive host birds”.  The methods and materials used included: recording surface water temperature, from April through October, analyzing surface wind measurements. “Eight sites along the Lake were selected for studying the prevalence of infected snails. Snail population density was measured by taking monthly collection using seven randomly placed, 1.0m2 quadrates in less than 1 m of water” pp10. Birds were also monitored along the shoreline where their behavior and roosting sites were observed. Fecal samples of birds was also collected and examined. Once the samples and materials were collected snails were incubated for a 20 day periods and were exposed to birds’ fecal samples. The snails were later examined for the presence of schistosome dermatitis. Human volunteers were used to by placing their forearm in water containing 10 schistosome cercarial. This was done to determine their epidemiological reaction to the parasite. To investigate environmental protection and management options, the snail habitat of four of the eight sites were disturbed by using a rototiller mounted on a motor vessel. Treatment and control sites were sampled for egg masses before and after treatment.  The results of this three year experiment yield the following:  “Egg laying began when the water temperature reached 12oC…, eggs hatched when the water temperature reached between 14 and 16oC…, and the population density peeked in the shallow waters in mid-July….”. “The most common bird carrying the schistosome eggs was the Merganser. These type of birds roots on logs in areas where the incidence of schistosome infection rates in snails is high”. Snail population density decreased by 96% at the treatment sites, that is those sites using rock rakes or boat mounted rototillers.  The disturbed habitat attracted a popular small cyprinid fish, which readily fed on the exposed or loose snail eggs”. “It is hypothesized that wind driven surface currents at Cultus Lake, probably carried schistosome cercariae from hyper endemic areas to beaches at both ends of the lake”.

A study from Lévesque & al looks at an outbreak of cercarial dermatitis (swimmer’s itch) that occurred in the Quebec City region in the summer of 1999. 450 families who made use of Lake Beauport during the summer of 1999 were studied and cases of cercarial dermatitis were reported. Results from the survey indicated that 69% of the cases reported occurred at the same beach. Interestingly the location reporting the most cases identified was the only area with a population of snails known as gastropods. A clean up of the snail population was done after which there were no cases of swimmer’s itch reported.

Following the first reported outbreaks snail samples were taken from various sites on the lake. One of the research methods used was “single blind”, that is the biologist who selected the samples did not know from which areas the documented cases came from. Once the snail samples were collected they were studied for a 24 hour period in a Petri dish under incandescent light, and classified according to their anatomical characteristics and swimming behavior.  Once the general collection was done, then the second set of samples were taken from the known sites thought to be the locations causing the dermatitis, typically these areas where public beaches  The second method used to collect and analyze the situation was a questionnaire sent to citizens of the area. The question about what cercarial dermatitis is was clearly defined and people were asked to report the skin rash, the number of lesions they had, the swimming date and location, as well as the type of swimming and bathing equipment used.  The results from both the questionnaires and the biological data came up with the following conclusions: Symptoms were reported mainly during the months of July and August. This could be in part due to the increase of swimmer’s during the summer season. Most reported cases are that of children under the age of 10. Also most of the cases reported come from citizens swimming in shallow water. From a biological point of view shallow water is a common habitat for aquatic molluscs and also since young children are just learning to swim they are the ones who most often will swim in shallow waters. The recommendations were two fold.  Since it has been determined that the primary hosts are waterfowl in this case Mallard ducks, and that citizen were often feeding the ducks thereby attracting them to the area. Citizens were asked not to feed the ducks near shorelines.  Finally, since the area was an isolated location in shallow water.  It was thought reasonable to disturb the snail habitat. This was done by using a device commonly used for cleaning septic tanks. It is a suction pump equipped with a large diameter hose connected to a tank.  A team of diver’s did the work, which was completed in 8 hours. This seems to have been effective because no case of dermatitis occurred at this location during the subsequent summer.

Analysis and recommendations:

It would interesting to do a follow up study of both Cultus Lake in British Columbia and Lac Beauport in Quebec in order to investigate the quality of the disturbed environments before recommending snail habitat disturbance for all reported areas of swimmer’s itch outbreaks. It would be important to ascertain with a reasonable degree of certainty that snail habitat disturbance would not disturb the natural ecosystems of shallow bodies of water.  It would also be important to investigate further the ecological value of snails. As mentioned above these are our primary grazers on the food chain, and their role is important in protecting other species.

Below is an estimated budget of possible cost for one week stay at respective sites:

Cultus Lake British Columbia                                                                  Lac Beauport  Quebec
Travel & lodging:                 $ 2000.00 Travel & lodging:                   $700.00
Water quality analysis:         $   200.00 Water quality analysis:           $200.00
Research Fees:                      $ 1750.00 Research Fee:                        $1750.00
   Random snail sampling     Random snail sampling
   Laboratory analysis     Laboratory analysis
   Citizen interviews     Citizen interviews
Total:                                    $ 3950.00 Total:                                      $2650.00

In terms of the initial site of study, the Georgeville Dock. Magog Conservation Inc. (MCI) has proposed that citizens refrain from feeding the ducks, that showers be installed near the beach area and that swimmer’s towel dry as soon as they come out of the water. The city of Magog has also a form that citizen can fill out and return to the city, this form allows for documentation of swimmer’s itch. It is recommended the Georgeville Recreation Program obtain these forms and have parents fill out and document the cases of swimmer’s itch of the children as they arise and submit the information back to the city of Magog. In doing so, both the MCI and the city of Magog will have a better measure of the cases of swimmer’s itch. Finally though this outbreak does not pose a serious health risk to the population it is none the less an annoyance and does devalue the enjoyment obtained from swimming in shallow bodies of water, however since we must share our lakes with other denizens of this planet it is best we find ways to adapt so that all may benefit.


Brant Sara V., Locker Eric S. (2009) Molecular systematic of the avian          schistosome    genus trichobilharzia (Trematoda: schistosomatidae) in North America J. Parasitol    (95) 4. Pp.941-963

CBC news Montreal, July 2008 Memphremagog Lake itch blamed on       snailshttp://www.cbc.ca/canada/montreal/story/2008/07/22/qc-      memphremagogitch0723.html

Keas E. Brian, Blankespoor Harvey D,(1997) The prevalence of Cercariae from Stagnicola emarginata (Lymnaeidae) over 50 years in Northern Michigan. The journal of parasitology vol. 83 NO.3 pp.536-540

Lance Emilie, Paty Chrystelle, Bormans Myriam, Brient Luc, Gérard Claudia      (2006) Interaction between cyanobacteria and gastropods II.  Impact of toxic Planktothrix agardhii on the life-history traits of Lymnaea stagnalis. Aquatic Toxicology 81 (2007) pp. 389-396

Leighton Bruce, Zervos Sandra, Webster John M. (2000) Ecological factors in schistosome transmission, and an environmentally benign method for controlling snails in a recreational lake with a record of schistosome dermatitis, Parasitology International 49 pp.9-17

Lévesque B., Giovenazzo P.Guerrier P., Laverdière D., Prud’homme H. (2002). Investigation of an outbreak of cercarial dermatitis. Epidemiology and Infection, Vol.129, No 2 pp.379-386

Malone J.B. et al (2001) A global network for the control of snail-borne disease using satellite surveillance and Geographic Information Systems pp.7-12

Memphremagog Conservation Inc. (MCI) www.memphremagog.org

Santiago Mass-Coma, Valero Maria Adela, Burges Maria Dolores (2009) Climate Change effects on trematodes, with emphasis on zoonotic fasciolisasis and shistosomiasis. Veterinary Parasitology, Volume 163 issue 4 pp.264-80

Verbrugge Lois M, Rainey, Jeanette J., Reimink Ronald L., Blankespoor Harvey D. (2004) Prospective Study of Swimmer’s Itch Incidence and Severity. The Journal of Parasitology, Vol. 90 No 4. Pp.697-704