How does the Eurytrema life cycle vary by region and which intermediate hosts are present locally?
Executive summary
Eurytrema pancreaticum and related Eurytrema spp. complete a heteroxenous (two‑intermediate‑host) life cycle that varies by region primarily through the identity of local terrestrial snails (first intermediate hosts) and orthopteran insects (second intermediate hosts), with environmental conditions such as temperature, rainfall and wet soils governing where those hosts and thus transmission can persist [1] [2]. Published field and laboratory studies from Asia and South America document Bradybaena spp., Acusta despecta and other land snails as first hosts, and Conocephalus spp., tettigoniid grasshoppers or crickets as second hosts, while some regional differences in host species, cycle length and seasonality have been explicitly recorded [3] [4] [5] [6].
1. Life cycle basics — the global template and where it changes
The canonical Eurytrema life cycle begins with eggs containing miracidia passed in ruminant feces, ingestion by a terrestrial snail in which sporocysts develop and release cercariae, and then transfer of daughter sporocysts/cercariae to an orthopteran insect where metacercariae encyst before accidental ingestion by grazing ruminants [3] [7]; this heteroxenous pattern is consistent across continents, but the species that fill the snail and insect roles differ by locality and thereby alter transmission dynamics [1] [8].
2. Asia — snails beyond Bradybaena and tettigoniid grasshoppers on islands and mainlands
Asian investigations from Korea, China, Japan and Vietnam show variation: on Chejudo (Quelpart) Island, Acusta despecta was identified as the first intermediate host and tettigoniid grasshoppers (Conocephalus maculatus, C. gladiatus) as second hosts in lifecycle studies there [4] [9], while broader Asian surveys and molecular studies emphasize Bradybaena spp. as common first hosts and Conocephalus or Oecanthus (cricket) species as second hosts in other regions [3] [6]. These differences reflect local malacofauna and orthopteran communities and mean that island or micro‑regional ecology can materially change where and when ruminants are at risk [4] [6].
3. South America — the Bradybaena–Conocephalus partnership and seasonal patterns
In Brazil and neighboring countries, multiple studies report Bradybaena similaris as a widespread first intermediate host and Conocephalus grasshoppers as the dominant second host, with high prevalences documented in cattle and strong geographic clustering tied to local climate and farm conditions [5] [1] [10]. Authors there link abundant rainfall, soggy ground and wetlands to greater snail and insect survival and even suggest shortened parasite developmental timing under warm, wet climates that favor host availability [2] [5]. Seasonal variation in infection prevalence has been observed across Brazilian states, although not uniformly in every study [5].
4. Ecological drivers and practical implications for local transmission
Environmental variables—annual mean temperature, rainfall, and soil moisture—determine where candidate snail and orthopteran hosts can persist and therefore where the Eurytrema cycle completes; regions lacking suitable terrestrial snails or compatible grasshoppers/crickets will have broken transmission chains [2] [1]. Control implications follow: measures that reduce snail habitat or alter grazing behavior can lower accidental ingestion of infected insects, but the literature warns against generalized insecticide campaigns and notes few labeled anthelmintics for Eurytrema, underscoring that local host ecology must guide interventions [11] [1].
5. Limits of current evidence and conflicting interpretations
Although a clear two‑intermediate‑host model is supported by multiple regional studies, some confusion in the literature has led to misinterpretations—most notably mistaken mixing of Eurytrema life cycle diagrams with those of other flukes—so caution is required when extrapolating one region’s host identities to another [7]. Many studies are regional case reports or slaughterhouse surveys (Brazil, Argentina, Vietnam, Korea) and while molecular work is expanding geographic resolution, there remain gaps in mapping which specific snail and orthopteran species serve as hosts in many countries and habitat types [6] [10].