Swimmers Itch

at Horne Lake

August 2003, Prepared for the Horne Lake Community Association, by Bruce J. Leighton

EXECUTIVE SUMMARY

An outbreak of schistosome dermatitis in Horne Lake in 2002 prompted a study of the problem. Site visits were made to the Lake in June and July 2003 and included surveys of the aquatic snails and faecal examination of probable bird hosts. It was concluded that a well documented schistosome system was present in the Lake and that this system was responsible for the dermatitis cases that were occurring. The system involves the schistosome Trichobilharzia physellae which cycles between the snail Physa sp. and Common Mergansers that are resident on the Lake through the summer. The cercarial stage of the schistosome which is released by the snails mistakes humans for its bird hosts and causes dermatitis when it enters the skin and dies. The human immune reaction to the foreign protein results in dermatis. This report includes recommendations for further research and options for control.

ACKNOWLEDGEMENTS

The author would like to thank Murray Hamilton for organizing the site visits and providing equipment and support for the study, and to thank both Murray and Isabel Hamilton for their kind hospitality. Thanks to Grant Ladouceur of DFO who provided information and maps of Horne Lake. Thanks also to the council and all the residents who shared information and opinions on the swimmers’ itch problem at the Lake.

CONTENTS

• 1.0 INTRODUCTION
  • 1.1 Schistosome Dermatitis
  • 1.2 Outbreak Horne Lake
• 2.0 OBJECTIVES
• 3.0 METHODS
• 4.0 RESULTS
  • 4.1 Field observations
  • 4.2 Snails
  • 4.3 Schistosome Infections in Snails
  • 4.4 Final Hosts
• DISCUSSION
  • 5.1 Field observations
  • 5.2 Snails
  • 5.3 Schistosome Infections in Snails
  • 5.4 Final Hosts
• 6.0 RECOMMENDATIONS FOR FURTHER RESEARCH
• 7.0 OPTIONS FOR CONTROL
• Literature Cited
• Tables and Figures
  • Table 1: Schistosome infections rates in Physa sp
  • Figure 1: Map of Horne Lake showing study sites
  • Figure 2: Physa sp
  • Figure 3: Stagnicola sp
  • Figure 4: Size frequency distribution and infection rates of Physa sp
  • Figure 5: The cercaria of the schistosome, Trichobilharzia physellae
  • Figure 6: Schistosome eggs from the faeces of Common Mergansers

1.0 INTRODUCTION

1.1 Schistosome Dermatitis

Schistosome Dermatitis or Swimmers’ Itch is an acute dermatitis caused by the cercarial stage of a schistosome, a flatworm parasite of birds and mammals. Schistosomes reproduce asexually in aquatic and marine snails (intermediate hosts) and sexually in birds and mammals (final hosts). The cercariae are the transmission stage of the parasite between snails and final hosts. Cercariae of the schistosomes of wild birds and mammals often mistake swimming or wading humans (accidental hosts) for final hosts, burrow into their skin and die, causing dermatitis when the human undergoes a sensitization reaction to the foreign proteins of the parasite.

For humans, initial exposure results in mild, if any, clinical signs or symptoms. However, within a few days to two weeks sensitization is acquired. Subsequent exposures result in the typical “Swimmers’ Itch” eruption.

As the exposed skin dries, cercariae penetrate the skin. This may be associated with itching. Within a few hours, there is progression from an itchy erythematous or petechial rash to macules, papules, vesicles or even pustules. Maximum intensity is reached two or three days after exposure. If not secondarily infected, signs and symptoms usually resolve within seven to ten days, though in severe cases may last for weeks (Mandell et al. 2000).

Bayssade-Dufour et al. (2001) note that in humans, only the human skin and its underlying layers have been explored by biopsies and that, by 10 to 60 hours after exposure to avian schistosome cercariae, no alive or dead schistosomula is seen in skin biopsies. Noting that no direct data concerning the presence of avian schistosomes in deeper tissues in humans are available, and in response to reports of cases of cercarial dermatitis associated with respiratory complications in asthmatic children, Bayssad-Dufour et al (2001) developed an animal model to try to document any migration of schistosomes in inadvertent hosts. They concluded that there is “a body of presumptions, suggesting a penetration of avian bilharziae (schistosomes) into deep viscera in Humans, but no direct proof”.

By contrast, in proper final hosts (eg. birds or rodents) the schistosomes pass through the skin into the circulatory system. They mature and mate in the arterial system surrounding the intestine and produce eggs that enter the intestine and are released in the host faeces. These eggs hatch in water, producing a miracidium larvae that seeks, enters and parasitizes the snail hosts. In this way the cycle continues.

Swimmers’ Itch is a natural phenomenon that occurs in many lakes in the temperate zones of the Earth. The attacks by bird schistosomes on humans are accidental and of no advantage to the parasite. In tropical areas, there are schistosomes that do infect humans and cause debilitating disease known as schistosomiasis or bilharzia in humans.

1.2 Outbreak at Horne Lake

In the summer of 2002 there was an apparent increase in reports of dermatitis at Horne Lake with cases occurring at many locations around the shoreline. By the time of the first site visit in June 2003 there had already been a number of cases, and reports of dermatitis continued throughout July. Most of the reported cases were contracted by people swimming or wading in the shallows, and there were some cases of dermatitis associated with swimmers rubbing up against docks or logs.

There is a history of swimmers’ itch at Horne Lake but there is no data on the number of cases. Recent changes in the organization and status of the residential land at Horne Lake (change to Strata title) have improved communications between residents and a questionaire has been delivered to residents to improve the recording of dermatitis cases.

2.0 OBJECTIVES

1.To analyse the cause of swimmers’ itch outbreaks at Horne Lake, identify the causative agents and examine the factors influencing the severity of outbreaks of swimmers’ itch. These studies include field, laboratory and literature research.

2.To make recommendations for further research leading toward management of the problem.

3.0 METHODS

Visits were made to Horne Lake June 20-24 and July 21-23 2003 to examine the physical aspects of the Lake and to survey and test the local birds and molluscs for schistosome infections. Faecal samples of the common water fowl and shorebirds were collected and collections of snails were made from 12 shallow water sites on the Lake that represented the range of habitat types. The snails were collected by hand and with dip nets in water less than 1 meter depth. Some simple snail traps consisting of a short PVC pipe closed with a rubber stopper at one end and baited with fish food were tested in deeper water.

In the laboratory, snails were held overnight in plastic cups of lake water to observe the release of the cercarial (infective) stage. The snails were measured (shell length), then the shells were removed and the soft parts crushed between glass plates and examined under a dissecting microscope for undeveloped (prepatent) infections. The gut contents of a few snails were examined. Snails that were held for longer periods were maintained on finely ground fish food and lettuce.

The behaviour of the cercariae from these snails was observed with a dissecting microscope. Some cercariae were stained with neutral red and examined under a compound microscope to better observe their morphology. Sporocyst stage parasite larvae and immature cercariae from crushed snails were observed and specimens of sporocysts and cercariae were fixed in Formalin-Acetic acid-Alcohol (FAA) and stored in 75% ethanol.

Samples of faeces were collected from Canada Geese (18 samples), Common Mergansers (4 samples), swallows (2 samples), an unidentified sandpiper species (21 samples) and a number of unidentified shorebirds(13 samples) to check for the presence of schistosome eggs. The samples were diluted, homogenized, filtered though cheesecloth and each sample was examined directly, using a dissecting microscope for the characteristically shaped schistosome eggs.

4.0 RESULTS

4.1 Field Observations

Horne Lake is a deep, poorly productive lake with a surface area of 131.6 hectares and a perimeter of 21,284 metres. The limestone substrate and the low flow rate of water through the lake give the lake a high mineral content and a high pH (8.6). The shoreline has a low slope in many areas and the substrate in the shallows includes stone outcrops, mud, sand, clay, cobbles imbedded in mud, stacked cobbles and areas covered in organic detritus, mainly wood debris and leaves. There are few aquatic plants in the shallows but there are some areas with grasses or reeds or the Floating-leaved Pondweed, Potamogeton natans which is found growing in muddy substrates.

The long irregular shape of the lake and the surrounding mountains make for highly variable wind directions at the surface of the Lake. Recent changes at the lake include logging of some of the slopes above the Lake and changes in the control of the water level. In 2002 the water level was exceptionally low through the summer and fall.

4.2 Snails

Three species of snail were found in the 2003 surveys at Horne Lake. All were pulmonate snails including the physid snail, Physa sp., the lymnaeid snail, Stagnicola sp. and the planorbid snail, Menetus cooperi. As the survey of snails was largely limited to the shallows and to two short visits, the list of snail species may not yet be complete for Horne Lake.

Physa sp. (Fig. 2) was the most abundant snail in the shallows at Horne Lake and was found at all 12 study sites. This snail is characterized by the extended spire, the sinistral shell (aperture on the left hand side when the shell is held spire up) and a shell length that is up to 25 mm (1 inch). There is considerable morphological variation in the shell structure of these snails and it is possible that there is more than one species of Physa present. Newly hatched snails which are approximately 1 mm in shell length could not be detected with these collection methods. On both site visits, the population density of Physa sp. in the shallows was less than 1 snail/m2.

Physa sp. was found in the shallows in greater numbers in July than in June. The large crescent shaped, gelatinous egg masses of this species were found on the upper surfaces of large stones and large wood debris and were more apparent in June than in July. There appeared to be an increase in the numbers of small snails and a decrease in the number of larger snails in July (Fig. 4.). There were also more empty shells of Physa sp. in July suggesting the beginning of a die-off of the older snails. Some small (4-5 mm) Physa sp. were caught in traps that were in 5 meters of water at site 7.

In the shallows these snails were seen actively grazing on the algal coating on large cobbles and sunken logs. The snails were also found on packed mud substrates and on loose mud substrates with the Floating-leaved Pondweed, located in areas sheltered from wave action. Physa sp. appeared to avoid the areas with sandy or clay substrates, or organic detritus such as leaves or wood debris. The gut contents of Physa sp. was mainly sand with algae and detritus. In captivity the snails actively fed on flaked fish food and lettuce.

Physa sp. was found to be host to schistosomes and a number of trematode parasites including strigeids and the echinostome Echinostoma revolutum.

The lymnaeid snail, Stagnicola sp. (Fig 3.) was found in large numbers at Site 8 on stacked cobble and at Site 11 on clay and were occasionally found at other sites. These snails are smaller than Physa sp. with a shell length from 9 – 15 mm. They are dextral (right-hand aperture) and the shell is tall and narrow. The gut contents appear to be similar to Physa. The egg masses are small and oval.

No schistosomes were found in this species (110 specimens tested) although lymnaeid snails are commonly schistosome hosts. Some of the larger specimens were infected with a strigeid trematode with a cercarial stage that superficially resembles schistosome cercariae but does not cause dermatitis in humans.

Only two specimens of the snail, Menetus cooperi were captured in a trap that was set in 5 meters of water off the dock at Site 7. These small snails have flat spiral, dextral shells with rapidly expanding whorls and a keel (carina) on the shoulder of the outer whorl. The specimens were 4.3 and 5.3 mm in diameter and were on a wood debris substrate. This species is not known to be a host for schistosomes.

Only two specimens of the snail, Menetus cooperi were captured in a trap that was set in 5 meters of water off the dock at Site 7. These small snails have flat spiral, dextral shells with rapidly expanding whorls and a keel (carina) on the shoulder of the outer whorl. The specimens were 4.3 and 5.3 mm in diameter and were on a wood debris substrate. This species is not known to be a host for schistosomes.

4.3 Schistosome Infections in Snails

Site	June	July
1	0/10 (0%)	10/32(31%)
4	3/12 (25%)	7/24 (29%)
5	-	1/10 (10%)
6	-	6/20 (30%)
7	7/26 (27%)	4/14 (29%)
8	7/20 (35%)	-
9	8/24 (33%)	8/20 (40%)
10	2/18 (11%)	-
11	-	3/24 (13%)
12	-	7/24 (29%)
Table 1. Schistosome infection rates in Physa sp. at all sites where a minimum of 10 Physa sp. were collected.

Physa sp. were the only snails collected in the survey that were infected with schistosomes. These were identified as the bird schistosome, Trichobilharzia physellae based on the description by Talbot (1936). The overall infection rate for Physa sp. collected in June was 27/123 = 22% and in July 47/167 = 28%. The infection rates for each site where a minimum of 10 Physa sp. were collected are listed in Table 1. Larger snails were more likely to be infected with schistosomes (see Fig. 4.).

Cercariae of the schistosome T. physellae (Fig. 5) were released immediately from many of the infected snails when they were placed in plastic cups of lake water, while other snails released cercariae only after exposure to light after being in the dark for a period. The cercariae were active swimmers and were attracted to the lighted side of the glass, where they attached themselves to the plastic by their front sucker. These schistosome cercariae readily produced schistosome dermatitis symptoms when placed on the skin of a human.

4.4 Final Hosts

The examination of bird faeces at Horne Lake was limited to those birds commonly seen near the water and to faecal samples from these birds that were collected on the beaches, wharves, and on stones and logs close to the water. A single schistosome infection was found in the samples and this was from Common Merganser faeces collected in June. All other samples were negative for any parasite eggs. The trematode eggs in the merganser faeces were fully embryonated and their shape and size was characteristic of the schistosome, T. physellae (Fig. 6).

Common Mergansers were not observed in large numbers during the survey. In the June site visit, two females with broods and 4 lone females were observed and in July only 3 non-breeding yearlings were seen.

5.0 DISCUSSION

5.1 Field observations

Winds and currents on the lake may be an important part of the outbreaks as the cercariae can travel considerable distances in the surface currents and can be concentrated on some swimming beaches. The irregular shape of the lake and the variable wind makes it difficult to predict wind drift concentration effects. This most likely would occur along the long axis of the lake.

The low water event that occurred over the summer and fall of 2002 may have been a factor in the outbreak of swimmers’ itch at the Lake. Low water levels in Horne Lake create more extended shallow areas for snails to breed and may have brought Common Mergansers into closer proximity with the snails, causing a higher infection rates in both snails and mergansers.

5.2 Snails

Physid snails that hatch in the shallows over the summer will overwinter under detritis in deeper water and will generally return to the shallows in the spring and summer to mate and lay eggs in the warmer water near the surface. While the majority of snails will be found in the shallows, there will likely be some present in deeper water throughout the year (Howard and Walden 1965).

At Horne Lake in 2003 the snails were laying eggs in the shallows in June and July and very few young snails were found in June, which is late in the season compared to other lakes in southern British Columbia. The low water level late in the season in 2002 may have caused the snails to overwinter in areas in the Lake, such that, when the water returned to normal levels, left them much deeper than in normal years. This may have delayed their return to the shallows for breeding.

It was noted that the egg masses of Physa sp. were laid on the upper surfaces of stones and sunken logs. In other lakes physid eggs are nearly always hidden under stones or other objects. This behavior may indicate the absence of predators that feed on snail eggs.

The population density of Physa sp. was less than 1 snail/m2 which was very low compared with Cultus Lake in the Fraser valley where snail population densities in the shallows over the summer were frequently 100-200 snails/m2 (Leighton et al. 2000).

There were only three species of snails found in the survey of Horne Lake compared to 6 species at Cultus Lake (Leighton et al. 2000) and 7 species at St. Mary Lake, Saltspring Island (Leighton et al. unpublished).

5.3 Schistosome Infections in Snails

The schistosome T. physellae was the only species of schistosome found in snails at Horne Lake and Physa sp. was the only snail host of T. physellae at the Lake. Infected snails were found at almost all the study sites. The overall infection rates were very high (22% in June and 28% in July) compared with other lakes where infection rates of 1% in the snails are sufficient to cause outbreaks. Infection rates of 10% were considered very high at Cultus Lake (Leighton et al. 2000). T. physellae is well known as a cause of swimmers’ itch across North America and physid snails are the only hosts of this species.

The young snails become infected over the summer and may begin to produce cercariae by late summer. Infected snails overwinter and return to the shallows in the spring and early summer where they begin to release cercariae. The spring and fall release may be correlated with seasonal bird migrations.

The behavior of T. physellae cercariae could be an important factor. Unlike some schistosome cercariae which attach themselves to the surface layer of the water, T. physellae are active swimmers which swim near the surface and tend to attach themselves to solid surfaces. It is possible that large numbers of cercariae may attach themselves to wharves and logs and swimmers may encounter concentrations of cercariae when they brush against these surfaces.

The wide distribution of infected snails around the Lake makes it difficult to determine if locally released cercariae, or cercariae carried or concentrated by surface currents are responsible for causing swimmers’ itch. As each infected snail can produce thousands of cercariae each day, it is possible that, in calm water, just a few infected snails could produce enough cercariae to cause a local problem.

The infection of Physa sp. with other parasites may be important as some of these parasites may compete or be predatory on the schistosome stages in the snails. At site 11, the schistosome infection rate of 13% was lower than many other sites (see Table 1.) but the infection rate of these snails with other trematode parasites was 81%.

5.4 Final Hosts

A Common Merganser was the only final host of T. physellae at Horne Lake that was found in the survey. While there have been a number of birds recorded as hosts of T. physellae, Common Mergansers are frequently the significant host at recreational lakes because they are residents though the spring and summer, and feed in the shallows in close proximity to the snails. Infected Common Mergansers may pass 3,120 schistosome eggs/day in their faeces (H.D. Blankespoor, personal communication).

6.0 RECOMMENDATIONS FOR FURTHER RESEARCH

1. A more complete study of the seasonal behavior and population dynamics of the snails is needed to better understand the numbers of snails involved and how their behavior influences their infection rates. This should include sampling in deep water by SCUBA or other means.
2. Examine the population dynamics of Common Mergansers at Horne Lake and discuss with wildlife authorities, possible control or treatment measures to reduce their role in the transmission of schistosomes.
3. Examine the behavior of cercariae on wharves and logs to determine if they are concentrating on these surfaces.
4. Continue to develop a system of monitoring cases of dermatitis through communications with residents. Compared with other lakes that lack a governing body, the Horne Lake Community Association provides an excellent means for monitoring. It would be valuable to have records of when and where cases occur and be able to detect real changes in rates over the seasons. This system could be used to assess the effectiveness of control methods. Lindblade (1998) developed a method for surveillance and risk assessment for swimmers’ itch at a lake using telephone interviews.
5. Test the efficacy of water-proof sun lotions or other products as a barrier to swimmers’ itch.

7.0 OPTIONS FOR CONTROL

Horne Lake is a beautiful natural asset. Any attempt to control swimmers’ itch would have to be appropriate for the aesthetic values of the owners and users, and within the regulations of the fisheries and wildlife agencies . The goal of a control program would be to reduce the probability of contracting swimmers’ itch and the components of this program would include public education on prevention along with reduction of the presence of cercariae in areas frequented by humans.

1. Personal protection: The use of water-proof sun lotions as a barrier against swimmers’ itch and immediate toweling off after leaving the water including under the bathing suit can provide some protection. Avoid staying in the water for periods longer than 10 minutes before towelling off.
2. If reduction of the presence of cercariae was based on management of the snail hosts: No suitable or registered chemicals exist for the control of aquatic snails in British Columbia and there are no biological controls that are developed for these snail species. Habitat manipulation (Leighton et al. 2000) which has been used to control freshwater snails might be considered in limited areas, but the wide distribution of infected snails around the lake and the diverse substrate types, may make it ineffective.
3. Blankespoor and Reimink (1991) have researched the use of the drug praziquantel to treat ducks, including mergansers for schistosome infections. The drug is very effective but must be administered orally once a month to avoid reinfection. Recapture of the mergansers for treatment can be challenging. They have also suggested moving host ducks to lakes with less human use. Any treatment or moving of Common Mergansers would have to be approved by wildlife authorities.

LITERATURE CITED

Bayssade-Dufour C., C. Martins and P.M. Vuong 2001. Histopathlologie pulmonaire D’un modèle mammifère et dermatite cercarienne humaine. Mèd. Mal. Infect. 31: 713-722.

Blankespoor, H.D. and R.L. Reimink 1991. The control of swimmers’ itch in Michigan: Past, Present and Future. Michigan Academician 24: 7-23.

Howard, T.E. and C.C. Walden 1965. An ecological study of the snail hosts for the agent of schistosome dermatitis in Cultus Lake, British Columbia. J. Appl. Ecol. 2: 121-135.

Leighton, B.J., S. Zervos and J.M. Webster 2000. Ecological factors in schistosome transmission and an environmentally benign method for controlling snails in a recreational lake with a record of schistosome dermatitis. Parasitol. Int. 49: 9-17.

Lindblade, K.A. 1998. The epidemiology of cercarial dermatitis and its association with limnological characteristics of a northern Michigan lake. J. Parasitol. 84: 19-23.

Mandell, G., J. Bennet and R. Dolin 2000. Principles and practices of infectious diseases. 5th ed. Churchill Livingstone Inc.

Talbot, S.B. 1936. Studies on schistosome dermatitis. II. Morphological and life history studies on three dermatitis-producing schistosome cercariae, C. elvae Miller, 1923, C. stagnicolae n.sp., and C. physellae n.sp. Amer. J. Epidemiol. 23: 372-384.