Swimmer's itch control: Timely waterfowl brood relocation significantly reduces an avian schistosome population and human cases on recreational lakes

Swimmer's itch (SI) is a dermatitis in humans caused by cercariae of avian and mammalian schistosomes which emerge from infected snails on a daily basis. Mitigation methods for SI have long been sought with little success. Copper sulfate application to the water to kill the snail hosts is the historically employed method, but is localized, temporary, and harmful to many aquatic species. Here, we test an alternative method to control Trichobilharzia stagnicolae, a species well-known to cause SI in northern Michigan and elsewhere in North America. Summer relocation of broods of the only known vertebrate host, common merganser (Mergus merganser), greatly reduced snail infection prevalence the following year on two large, geographically separated lakes in northern Michigan. Subsequent years of host relocation achieved and maintained snail infection prevalence at ~0.05%, more than an order of magnitude lower than pre-intervention. A Before-After-Control-Intervention (BACI) study design using multiple-year snail infection data from two intervention lakes and three control lakes demonstrates that dramatic lake-wide reduction of an avian schistosome can be achieved and is not due to natural fluctuations in the parasite populations. The relevance of reducing snail infection prevalence is demonstrated by a large seven-year data set of SI incidence in swimmers at a high-use beach, which showed a substantial reduction in SI cases in two successive years after relocation began. In addition, data from another Michigan lake where vertebrate-host based intervention occurred in the 1980's are analyzed statistically and show a remarkably similar pattern of reduction in snail infection prevalence. Together, these results demonstrate a highly effective SI mitigation strategy that avoids the use of environmentally suspect chemicals and removes incentive for lethal host removal. Biologically, the results strongly suggest that T. stagnicolae is reliant on the yearly hatch of ducklings to maintain populations at high levels on a lake and that the role of migratory hosts in the spring and fall is much less significant.

Use of qPCR-Based Cercariometry to Assess Swimmer's Itch in Recreational Lakes

Swimmer's itch (cercarial dermatitis) is a nuisance encountered by bathers and recreational water users worldwide. The condition is caused by the penetration of larval digenean trematodes (cercariae) of the family Schistosomatidae, into the skin, following their release into freshwater from pulmonate snails that serve as the intermediate hosts for these parasites. This study utilizes qPCR-based cercariometry to monitor and quantify cercariae from water samples collected at 5 lakes in northern Michigan. The resolution provided by qPCR facilitated assessment of the environmental and biological drivers of swimmer's itch-causing cercariae concentrations, allowing us to demonstrate that cercarial abundance is greatest at the top of the water column, in locations with prevailing on- and alongshore winds.

Impairment of retinal function in yellow perch (Perca flavescens) by Diplostomum baeri metacercariae

Histologic studies of fish from Douglas Lake, Cheboygan County, Michigan, USA show that Diplostomum spp. infect the lens of spottail shiners (Notropis hudsonius) and common shiners (Luxilus cornutus). In contrast, infection was confined to the choroidal vasculature of yellow perch (Perca flavescens), and the morphology of the pigment epithelium and retina in regions adjacent to the metacercariae was abnormal. The difference in location of metacercariae within the host suggested that different Diplostomum species may infect shiners and perch in Douglas Lake. Species diversity was investigated by sequencing the barcode region of the cytochrome oxidase I gene of metacercariae. Four species of Diplostomum were identified, all four of which were present in shiner lenses; however, only Diplostomum baeri was present in the perch choroid. To determine whether infection of perch eyes affects the response of the retina to a light stimulus, electroretinograms (ERG) were recorded. The amplitude of the b-wave of the ERG was reduced and the b-wave latency was increased in infected perch, as compared to uninfected eyes, and the flicker-fusion frequency was also reduced. Infection of the yellow perch choroid by Diplostomum baeri, which shows strong host and tissue specificity, has an adverse effect on retinal function, lending support to the hypothesis that parasite-induced impairment of host vision may afford Diplostomum baeri the evolutionary benefit of increasing the likelihood of transmission, via host fish predation, to its definitive avian host.

Efficacy of praziquantel in treating natural schistosome infections in common mergansers

Fifty-one common mergansers were captured on Douglas Lake (Cheboygan County, Michigan) and their avian schistosome loads were determined by fecal examination. Each bird was given a single dose of 0, 40, or 200 mg/kg of body weight of praziquantel and released. All birds were recaptured within 10 days of drug administration to determine posttreatment schistosome loads. Only the highest dose of praziquantel was found to significantly reduce avian schistosome loads. The potential use of praziquantel in swimmer's itch control programs is discussed.

Impairment of the chemical defence of the beetle, Tenebrio molitor, by metacestodes (cysticeroids) of the tapeworm, Hymenolepis diminuta

The defensive glands of beetles, Tenebrio molitor, infected with metacestodes (cysticercoids) of Hymenolepis diminuta are everted less frequently upon stimulation, and contain less toluquinone (methylbenzoquinone) and m-cresol, than glands of uninfected controls. These differences, as shown in predation trials with wild rats, increase the likelihood that both cysticercoids and beetles will be ingested by the tapeworm's definitive host. This is the first documented case of a parasite inhibiting the chemical defence of an intermediate host, and one of only a few reports of parasite-induced manipulation of host biology supported by empirical evidence implicating facilitated parasite transmission between host species.

Biosynthesis of a defensive insect alkaloid: epilachnene from oleic acid and serine

The biosynthesis of the azamacrolide epilachnene by the coccinellid beetle Epilachna varivestis has been studied with 2H-labeled oleic acid, 2H-labeled L-serine, and 13C,15N-labeled L-serine. The incorporation of these precursors into epilachnene defines the origin of the alkaloid's entire carbon/nitrogen skeleton. GC/MS and GC/IR studies of alkaloid produced by Epilachna fed with deuteriated oleic acid show that oleic acid loses four carbon atoms from its carboxyl end during the biosynthesis. Other details, including the mechanism of carbon-nitrogen bond formation between the fatty acid and amino acid moieties, remain to be established.

Azamacrolides: a family of alkaloids from the pupal defensive secretion of a ladybird beetle (Epilachna varivestis)

Defensive droplets from glandular hairs of the pupa of the Mexican bean beetle, Epilachna varivestis, contain a group of structurally novel alkaloids, the azamacrolides. The major constituent of this secretion, epilachnene, is shown to be (5Z)-11-propyl-12-azacyclotetradec-5-en-14-olide. The secretion also contains an epilachnadiene and trace amounts of three closely related components.