3.2 Disease spread via invasive golden apple snails (Pomacea spp.) in Taiwan

The primary mode of spread of Angiostrongylus cantonensis are through their hosts, particularly rats and gastropods, some of which are highly invasive, readily transport the parasite beyond its endemic zones. Although a broad range of species can act as intermediate hosts, recent studies have found that some hosts naturally hold moreAngiostrongylus cantonensis larvae than others (e.g.,Parmarion martensi ), (Medeiros et al., 2020; Rollins et al., 2021), however prevalence of larva may also depend on size, availability, physiological and immunological capability of the hosts. The frequency of association of Angiostrongylus cantonensis withPomacea spp., as well as other vector gastropods such asAchatina fulica differs among countries; for example,Pomacea spp. has low contribution to parasite prevalence in Thailand (Vitta et al., 2016), and Brazil (Carvalho et al., 2012), however, in contrast Pomacea spp., particularly Pomacea canaliculata have been shown to have considerable parasite prevalence in Vietnam and Cambodia (Lv et al., 2018), and also be major contributors of the spread of neuroangiostrongyliasis in China (Song et al., 2016) and Taiwan (Tsai et al., 2013).
Another species of Pomacea , P. maculata has also been involved in spreading the disease, as for example, a study in the USA reported that P. maculata shows notable prevalence and exhibits competence in acting as intermediate host to Angiostrongylus cantonensis (Teem et al., 2013). The Pomacea spp. were introduced in many places as a result of global trade of snails in Asia, and they turned invasive. However, due to the cryptic shell morphology with P. canaliculata , P. maculata has often been misidentified and thus this association has been less evaluated worldwide. In Taiwan, both species of Pomacea are now widely distributed (Banerjee et al., 2022), thus causing a potential threat of increased infection. According to the reviewed articles, approximately 15.5% of cases have been known to be caused by Pomacea spp. and 23% of cases have an unknown cause, which underlines the dire situation growing populations of invasive species have on the spread of neuroangiostrongyliasis (Table 1).
Studies conducted by Howe et al. (2019) and Modrý et al. (2021) held further implications for transmission of the parasite via the aquaticPomacea spp. The infectivity of L3Angiostrongylus cantonensis larvae released into water from drowned gastropod hosts have demonstrated that the parasite has potential for spread without involving a host’s body, to some extent. Additionally, the process of intermediesis being successfully demonstrated for Angiostrongylus cantonensis larvae suggests greater range and frequency of parasite transmission, particularly in aquatic habitats (Modrý et al., 2021). This greatly impacts control strategies to reduce the prevalence of the parasite in the environment. However, the characteristics of such water-based transmission modes with live (uninjured) aquatic gastropod hosts such as Pomacea spp. have not been adequately investigated.