ABSTRACT
Despite ‘Swimmers Itch’ having been first reported in the Transactions of the Royal Society of South Australia in 1941, and shortly afterwards in the New Zealand Medical Journal in 1944, there remain few publications in the Australasian dermatology journals.
1. Introduction
Cercarial dermatitis, aka ‘Swimmers’ or ‘Duck’ itch, is an acute dermatitis caused by an immunological reaction to the cercarial stage of non‐human schistosome parasites [1].
Adult flatworms of the dermatitis‐producing schistosomes are usually blood parasites of waterfowl. The life cycle begins with the hatching of the eggs in the blood vessels of the intestinal walls of certain waterfowl. Ova excreted in the birds' faeces are then released into water, where they hatch into ciliated larvae (miracidiae). Specific species of snails become infected upon contact with the miracidia, the snails serving as an intermediate host. After an incubation period in the snail, the infectious schistosome cercariae are released into the surrounding water. Upon exposure to these cercariae, the definitive bird host, or occasionally a mammalian host, is infected and the parasites mature in the vascular system of the hosts. The life cycle is completed when the adult worms produce eggs (Figure 1). Cercarial dermatitis in humans occurs from accidental introduction into the life cycle of these parasites, often by swimming in shallow water; whilst the cercaria can penetrate through human epidermis, they quickly die, inducing an immune reaction.
FIGURE 1.
Life cycle of schistosomes causing cercarial dermatitis.
Although cercarial dermatitis can occur in both fresh and salt water, it is different to ‘Sea bathers eruption’ which is caused by the larval jellyfish and/or sea anemones, which have become trapped under swimwear.
Despite ‘Swimmers Itch’ having first been reported in the Transactions of the Royal Society of South Australia in 1941 [2], and shortly afterwards in the New Zealand Medical Journal in 1944 [3], there remain few publications in the Australasian dermatology journals.
We report a case series of cercarial dermatitis in open‐water swimmers in New Zealand lakes.
2. Main Text
The index case was a 68‐year‐old open water swimmer who swam on two consecutive days in mid‐summer at Lake Tarawera, New Zealand. The first day he swam 2 km (40 min), followed by a 3 km swim (60 min) the next day. He wore goggles, swim cap and a short summer wetsuit on both occasions. Sixty minutes after each emergence from the water, he noted a bite‐like sensation on his neck, left side face (he breathes to the right‐side) and posterior hair margin, as well as on the back of both hands, wrists, lower arms, dorsum of the feet, ankles, and lower legs; all areas that had been exposed to the lake water. Symptoms continued for approximately 60 min, with each individual itch‐bite symptom lasting only 1–2 s. Over the next 2–3 days, he developed a pruritic, papular eruption at the sites of the previous itch/bite. The severity of the itch was reported as 3 out of 10. The rash gradually diminished over the following 12 days with no treatment, leaving post‐inflammatory pigmentation which resolved over 6 weeks. A number of aquatic birds were seen during both swims.
A confident diagnosis of ‘Swimmer's Itch’ was made. Discussion with fellow open‐water swimmers indicated his was not an isolated case.
In March 2025 a survey of open‐water swimmers in New Zealand lakes identified 25 further cases of Cercarial dermatitis. The survey was performed using a structured questionnaire on Google Forms that was posted on Auckland City Triathlon Club and New Zealand Open Water Swimming Facebook pages. Specific details regarding each incident was collected. Respondents provided information on the date and location of their swim, their age, the type of activity and swim gear used (e.g., wetsuit vs. swimming costume), the duration and distance of the swim, the physical distribution of the resulting rash, the timing and severity of symptoms, and whether any birds were present in the area.
The mean age (and range) of the cases (including the index case) was 45 years (5–68 years). Approximately half (12/26) wore some style of wet suit during their swim. The average contact time in the water was 84 min (range 30–270 min), with an average distance swum being 3.5 km (0.1–12.5 km). The 26 cases all occurred in the summer months of January to March. Cases were reported from 6 different New Zealand lakes in both the north and south islands: Lake Taupō (10 cases), Lake Whakatipu (9), Lake Wanaka (4), and Lakes Pupuke, Ruataniwha and Tarawera (1 each).
The rash had a limited distribution in 5/26 cases, but affected most of the exposed skin in the remaining cases (21/26). The main anatomic areas involved were: limbs (22 cases) (Figure 2), trunk (16), and face (15) (Figure 3), which reflected whether a wetsuit was worn or not. The mean time to 1st symptom was 3 h (range 0.1–48 h), time to first visible lesion was 1 day (0.1–2 days), with resolution over 10.6 days (3–28 days). The mean itch severity was 7/10 (range 3–10), with 7/19 reporting post‐inflammatory pigmentation.
FIGURE 2.
Photograph of forearm with cercarial dermatitis. Location of papules correspond to the area of exposed skin.
FIGURE 3.
Photograph of Rt‐side face with cercarial dermatitis. Location of papules corresponds to the area of exposed skin.
Eighteen respondents identified the following waterfowl as being present whilst swimming: Mallards/Rakiraki (14 reports), New Zealand Scaups/Pāpango (7), Eurasian Coots (5), Black swans/Kakīānau (4), Seagulls (4), Crested Grebes/Pūteketeke (2), Cormorants (2), Paradise duck/Pūtangitangi. Analysis of water samples was not performed as part of this study.
3. Discussion
Previous New Zealand studies have indicated the presence of four species of Trichobilharzia in various New Zealand lakes: three of these use the same snail host, Austropeplea tomentosa. In these studies, the common definitive waterfowl hosts were identified as the New Zealand scaup, Aythya novaeseelandiae , which hosts Trichobilharzia longicauda, and the New Zealand shoveler (Spatula rhynchotis), which hosts T. querquedulae [4]. They have also been found in mallards and paradise shelduck in New Zealand [5], but worldwide, Trichobilharzia has been found in a wide number of birds and some mammals [6, 7].
Cercarial dermatitis is considered an emerging disease with almost 100 species of schistosome, 70 being avian schistosome, distributed around the world [6, 7, 8]. Lakes represent high‐risk areas for cercarial dermatitis, as they are attractive for a large number of people, particularly in summer, but it has been reported in salt water as well. Longer exposure in water increases cercarial penetration, particularly in shallow water, as the highest risk of infection occurs in shallow, warm, and vegetation‐rich shore areas, where the snails accumulate. Swimming in deeper water may reduce the infection risk. Season and time of day are important as most cases of cercarial dermatitis correlate with high air and water temperatures during the summer months, but they can occur in geothermally heated water at any time of year.
Treatment is symptomatic itch control with antihistamines and topical corticosteroids. Repeated exposure can, unfortunately, result in more severe immune reactions, although the development of tolerance has also been reported. It is possible that immediate showering and/or towelling dry, to remove any parasites on the skin, may prevent cercarial dermatitis developing.
Global warming is thought to be contributing to the increase in numbers of outbreaks of cercarial dermatitis worldwide [8]. In Australia, cases of cercarial dermatitis have been reported from most states [9, 10].
Funding
The authors have nothing to report.
Disclosure
AI was not used in the preparation of this manuscript.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgements
We are very grateful to Dr. Norman Davis, of Otago, for his insights into the cercarial dermatitis associated with schistosomes and definitive hosts of New Zealand. We also thank Vince Sesto for help recruiting survey participants.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.