Laboratory of Aquatic Pathobiology, Department of Veterinary and Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Stigbøjlen 7, 1870, Frederiksberg C, Denmark.
Department of Dermatology and Allergy Centre, Odense University Hospital, J.B. Winsløws Vej 4, 5000, Odense C, Denmark.
Acta Vet Scand. 2020 Sep 15;62(1):54. doi: 10.1186/s13028-020-00553-z.
Cercarial dermatitis (swimmer's itch) caused by bird schistosome cercariae, released from intermediate host snails, is a common disorder also at higher latitudes. Several cases were observed in the artificial Danish freshwater Ringen Lake frequently used by the public for recreational purposes. The lake may serve as a model system when establishing a risk analysis for this zoonotic disease. In order to explain high risk periods we determined infection levels of intermediate host snails from early spring to late summer (March, June and August) and elucidated the effect of temperature and light on parasite shedding, behavior and life span.
Field studies revealed no shedding snails in March and June but in late summer the prevalence of Trichobilharzia szidati infection (in a sample of 226 pulmonate Lymnaea stagnalis snails) reached 10%. When investigated under laboratory conditions the cercarial shedding rate (number of cercariae shed per snail per day) was positively correlated to temperature raising from a mean of 3000 (SD 4000) at 7 °C to a mean of 44,000 (SD 30,000) at 27 °C). The cercarial life span was inversely correlated to temperature but the parasites remained active for up to 60 h at 20 °C indicating accumulation of cercariae in the lake during summer periods. Cercariae exhibited positive phototaxy suggesting a higher pathogen concentration in surface water of the lake during daytime when the public visits the lake.
The only causative agent of cercarial dermatitis in Ringen Lake detected was T. szidati. The infection risk associated with aquatic activities is low during spring and early summer (March-June). In late summer the risk of infection is high since the release, behavior and life span of the infective parasite larvae have optimal conditions.
由从中间宿主蜗牛中释放出来的鸟类血吸虫尾蚴引起的尾蚴性皮炎（游泳者瘙痒症），在高纬度地区也很常见。在丹麦人工淡水 Ring 湖中，经常有公众进行娱乐活动，观察到几例这种疾病。该湖可以作为建立这种人畜共患疾病风险分析的模型系统。为了解释高发期，我们测定了从早春到晚夏（三月、六月和八月）期间中间宿主蜗牛的感染水平，并阐明了温度和光照对寄生虫脱落、行为和寿命的影响。
实地研究表明，三月和六月没有脱落的蜗牛，但在夏末， Trichobilharzia szidati 感染的流行率（在 226 只陆生 Lymnaea stagnalis 蜗牛样本中）达到 10%。在实验室条件下进行调查时，尾蚴脱落率（每只蜗牛每天脱落的尾蚴数）与温度呈正相关，从 7°C 时的平均 3000（SD 4000）升高到 27°C 时的平均 44000（SD 30000）。尾蚴的寿命与温度呈负相关，但寄生虫在 20°C 时仍能活跃长达 60 小时，表明夏季期间寄生虫在湖中积累。尾蚴表现出正趋光性，这表明在公众访问湖泊的白天，湖表水中的病原体浓度更高。
在 Ring 湖中检测到的唯一引起尾蚴性皮炎的病原体是 T. szidati。在春季和初夏（三月至六月），与水接触活动相关的感染风险较低。在夏末，感染风险很高，因为感染性幼虫的释放、行为和寿命都处于最佳条件。
