Tag: flatworms

Swimmer’s Itch: what causes this neglected snail-borne disease?

A new study suggests that a cercarial dermatitis outbreak in South Thailand was caused by the blood fluke Schistosoma indicum.

Cercarial dermatitis, also known as swimmer’s itch or clam-digger’s itch, is caused by the larvae of blood flukes that are parasites of birds or mammals. When these larvae, called cercariae, penetrate human skin, they trigger an allergic reaction within 10-15 hours that takes about a week to heal. Unable to mature into adults, the larvae then die on the skin. The gravity of an outbreak depends on how humans and birds or mammals come in contract with the aquatic environment, but people engaged in water activities, such as farmers, fishermen, and agricultural workers, are most likely to be affected.

Cercarial dermatitis cases from Chana district, October 2020

Between August and October 2020, a cercarial dermatitis outbreak with 359 confirmed cases occurred in Chana district, Songkhla Province, South Thailand. It mostly affected rice farmers from the area, who were busy with cultivation during the rainy season. Following a short investigation, three cases of patients were confirmed to be cercarial infections by skin biopsy (Bureau of Epidemiology, Department of Disease Control, Ministry of Public Health, Thailand).

“The study of intermediate host and definitive host in the outbreak area are important for the control program of snail-borne disease,” the researchers argue in their research paper, which was published in the open-access scientific journal Evolutionary Systematics.

Having studied six snail species from the area, they found out that two were infected, each with three different species of flatworms. The cercarial dermatitis outbreak was due to ruminant parasites, such as the blood fluke Schistosoma indicum, which often uses domestic animals as its host.

Collected snails from five locations of cercarial dermatitis outbreak area. a. *Filopaludina s. peninsularis* b. *Filopaludina s. polygramma* c. *Indoplanorbis exustus* d. *Filopaludina m. cambodjensis* e. *Bithynia s. siamensis* f. *Pomacea canaliculat*a (Scale bar: 1 cm).

“Ruminant-infecting trematodes, namely, S. indicum and S. spindale, cause a hepato-intestinal schistosomiasis resulting in reduced milk yield,” the authors explain. “This occurrence of S. indicum and S. spindale implies the spread of cattle blood fluke cercariae in aquatic environments.”

“Additionally, these species of the S. indicum group primarily cause cercarial dermatitis in humans, which has become an important public health issue for people living in endemic regions.”

“In South India and Southeast Asia, where S. indicum and S. spindale have been reported to be widespread, they caused major pathology and mortality to livestock, leading to welfare and socio-economic issues, predominantly among poor subsistence farmers and their families.”

Image of *Schistosoma indicum* Montgomery, 1906 (Syn. *S. nasalis* Rao, 1933) a. Head organ of cercaria stained with 0.5% neutral red (DIC microscopy) b. Body part of cercaria stained with 0.5% neutral red (DIC microscopy) c. Image of unstained cercaria (DIC microscopy) d. Images of cercaria stained with 0.5% neutral red (DIC microscopy) e. Drawing of cercaria structure f. Images of sporocyst stained with 0.5% neutral red (light microscopy) Abbreviations: c: cercaria, eb: excretory bladder, ep: esophagus, fu: furca, h: head organ, i: intestine, pg: penetration gland, sp: sporocyst, ta: tail, vs: ventral sucker.. (Scale bars: 100 μm).

Some of the other worm species they found parasitized the intestines of fish, mammals, or birds, while others caused anemia and even death in ruminant animals.

“The results of this study will provide insights into the parasite species that cause cercarial dermatitis and may improve our understanding of public health problems in the outbreak and agricultural vicinity areas,” the authors of the study say. “In addition, the sequence data generated here are the first S. indicum DNA sequences from Thailand, which will be useful for further genetic study of the other blood flukes in this region.”

Research article:

Krailas D, Namchote S, Komsuwan J, Wongpim T, Apiraksena K, Glaubrecht M, Sonthiporn P, Sansawang C, Suwanrit S (2022) Cercarial dermatitis outbreak caused by ruminant parasite with intermediate snail host: schistosome in Chana, South Thailand. Evolutionary Systematics 6(2): 151-173. https://doi.org/10.3897/evolsyst.6.87670

Images by Professor Dr. Duangduen Krailas.

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Agents of food-borne zoonoses confirmed to parasitise newly-recorded in Thailand snails

Parasitic flatworms known as agents of food-borne zoonoses were confirmed to use several species of thiarid snails, commonly found in freshwater and brackish environments in southeast Asia, as their first intermediate host. These parasites can cause severe ocular infections in humans who consume raw or improperly cooked fish that have fed on infected snails. The study, conducted in South Thailand by Thai and German researchers and led by Kitja Apiraksena, Silpakorn University, is published in the peer-reviewed open-access journal Zoosystematics and Evolution.

“Trematode infections are major public health problems affecting humans in southeast Asia. Trematode infections depend not only on the habit of people, but also on the presence of first and second intermediate host species, resulting in the endemic spread of parasites, such as intestinal and liver flukes in Thailand”.
explain the scientists.

The snails of concern belong to the genus Stenomelania, have elongated and pointed shells and can be found near and in the brackish water environment of estuaries in the Oriental Region, from India to the Western Pacific islands. Worryingly enough, science does not know much else about these snails to date. Further, these species are hard to distinguish from related trumpet snails, because of the similarities in their shell morphology.

In order to provide some basic knowledge about the parasitic worms in Thailand and neighbouring countries, the research team collected a total of 1,551 Stenomelania snails, identified as four species, from streams and rivers near the coastline of the south of Thailand in Krabi, Trang and Satun Provinces. Of them, ten were infected with trematodes. The parasites were found at seven of the studied localities and belonged to three different species. In Krabi Province, the researchers observed all three species.

Speculating on their presence, the scientists suspect that it could be related to the circulation of sea currents, as the flow of water along the Andaman coast is affected by the monsoon season.

In conclusion, the researchers note that it is a matter of public health that further research looks into the biodiversity and biology of these snails, in order to improve our knowledge about the susceptibility of Stenomelania snails to food-borne zoonotic.

“This finding indicated that the resulting parasitic diseases are still largely neglected in tropical medicine, so further studies should be performed on the prevalence of various trematode-borne diseases in locations with snail occurrences in Thailand,”
they say.

Research article:

Apiraksena K, Namchote S, Komsuwan J, Dechraksa W, Tharapoom K, Veeravechsukij N, Glaubrecht M, Krailas D (2020) Survey of Stenomelania Fisher, 1885 (Cerithioidea, Thiaridae): The potential of trematode infections in a newly-recorded snail genus at the coast of Andaman Sea, South Thailand. Zoosystematics and Evolution 96(2): 807-819. https://doi.org/10.3897/zse.96.59448

Hidden diversity: 3 new species of land flatworms from the Brazilian Araucaria forest

A huge invertebrate diversity is hidden on the forest floor in areas of the Araucaria moist forest, Brazil. Land flatworms constitute a numerous group among these invertebrates occurring in the Neotropical region. Flatworms are considered to be top predators within the soil ecosystem, preying on other invertebrates.

The Araucaria moist forest is part of the Brazilian Atlantic Rain Forest and is considered a hotspot of land flatworm diversity, harboring many yet undescribed species. Study recently published in the open access journal ZooKeys describes three new species from areas covered by Araucaria moist forest in South Brazil, which belong to the Neotropical genus Cratera.

Land flatworms lack a water retention mechanism and have a low tolerance to intense changes in temperature and humidity. Their low vagility leads to the existence of a high number of endemic species. Thus, they are considered good bioindicators of the degree of impact on their habitat.

The new species are named after characteristics of their color pattern and are probably endemic for the study areas. Besides differing from each other, as well as from other species of the genus, by their characteristic color pattern, they also show other distinguishing features in the reproductive system. The study provides an identification key to the species of the genus.

The work was conducted by the south Brazilian research group on triclads, led by Dr. Ana Leal-Zanchet, of the Universidade do Vale do Rio dos Sinos (UNISINOS), in southern Brazil. The study was supported by the Brazilian Research Council (CNPq).

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Original source:

Rossi, I, Leal-Zanchet, A. (2017) Three new species of Cratera Carbayo et al., 2013 from Araucaria forests with a key to species of the genus (Platyhelminthes, Continenticola). ZooKeys 643 (2017): 1-32. doi: 10.3897/zookeys.643.11093