3.5 Recent advancements in treatment and control measures

3.5.1 Treatment

There is no official treatment protocol, however, the treatment is generally performed according to the symptoms (Carvalho et al., 2022). Hence, courses of analgesics and regular lumbar punctures to relieve the severe headaches typical of neural angiostrongyliasis cases is a common course of action (Cowie et al., 2022). Corticosteroids, such as prednisolone, are effective in management of inflammation that causes headaches, and further reduce duration of headache persistence. Other anti-inflammatory drugs, such as nonsteroidal anti-inflammatory drug (NSAID) options may have increased risk of gastrointestinal and intracranial bleeding, particularly if used concomitantly with corticosteroids (Ansdell et al., 2021). Anthelmintics as treatment must be administered carefully, because the increased death rate of the parasites in the tissues may trigger an increased immune response, particularly for more severe infections. However, in reality, the use of ivermectin, levamisole, albendazole (which shows highest absorption among benzimadoles) have shown improvement of symptoms in many cases (Ansdell et al., 2021; McAuliffe et al., 2019). Nevertheless, albendazole has displayed decreased efficacy when used after L4 stage larval development (McAuliffe et al., 2019). In Taiwan, research is currently being conducted in order to protect the brain from the harmful inflammatory effects of albendazole being administered in the later infection stages. In BALB/c mouse models, Chen et al. (2023b) investigated the combined effects of benzaldehyde with albendazole; Lu et al. (2023) explored the efficacy of an active compound, calycosin, extracted from a herb commonly used in Chinese medicine known for its anti-inflammatory effects. Therapeutic use of Tanshinone IIA (TSIIA) to reverse cell damage to mouse astrocytes are being investigated using the excretory-secretory products of A. cantonensis as an effective way to induce host immune responses to the infection (Chen et al., 2023a).
There is scarce information available about management of chronic symptoms that emerge in some cases of neural angiostrongyliasis (McAuliffe et al., 2019). However, there are cases with debilitating sequelae after treatment such as erectile dysfunction, strabismus, intellectual disability, prolonged severe pain, hyperalgesia, and even death (Carvalho et al., 2022; McAuliffe et al., 2019). Treatment varies case-by-case; however the corticosteroids may cause adrenal suppression and albendazole may elevate liver transaminases if used long-term, hence they are unsuitable for chronic care (Ansdell et al., 2021). Thus, more research is required to develop effective management strategies for sequelae.

3.5.2 Control

The broad range of hosts, as well as the invasive nature of many of them, has resulted in the widespread occurrence of Angiostrongylus cantonensis (Carvalho et al., 2012; Kim et al., 2014; Morassutti et al., 2014; Thiengo et al., 2022). Nevertheless, some factors limit the spread of the disease; such as rainfall, temperature and close association/interactions between definitive and intermediate hosts (Gamiette et al., 2023; Rollins et al., 2021). Given the nematode is tropical by nature, their spread has been limited to tropical zones with warm temperatures and humidity. The gastropod hosts commonly associated with Angiostrongylus cantonensis display a preference for a similar climate (Cowie et al., 2022; Rollins et al., 2021). Therefore, the spread of the nematode can be considered to be severely restricted by the availability of its hosts. The life cycle ofAngiostrongylus cantonensis involves direct transfer of larvae from rat to the gastropod via rat feces, and vice versa via consumption of the gastropod by rats, instead of indirect transmission via gastropod mucus (Kramer et al., 2018). Nevertheless, despite the general containment of the parasite within its hosts’ bodies, parasitic contamination of water from dead/injured infected hosts is quite possible, and must be accounted for (Howe et al., 2019; Modrý et al., 2021).
The intermediate hosts precede the definitive hosts in priority during survey, as the infectious larval stage of the parasite (L3) develops in the intermediate hosts (Lv et al., 2018). This is particularly important for Taiwan, where snails form part of a regular diet. The invasive nature of Pomacea spp. andAchatina fulica imply increased chances for accidental consumption, particularly considering their proximity to agricultural fields. In general, human infection is seen to occur via three modes: (i) consumption of intermediate or paratenic hosts; (ii) indirect contact by handling infected gastropods or rat feces; and (iii) usage of contaminated water (Carvalho et al., 2022). Widespread surveys of gastropods, testing for the prevalence of Angiostrongylus cantonensis, have taken place in numerous regions/countries where cases of infection have been reported, such as China (Lv et al., 2009), Brazil (Carvalho et al., 2012), Thailand (Vitta et al., 2016), Vietnam (Lv et al., 2018), Hawaii (Kim et al., 2014), French Polynesia (Fontanilla and Wade, 2012), USA (Teem et al., 2013), Spain (Martin-Alonso et al., 2015), and Taiwan, where surveys have investigated the prevalence of the nematode in gastropods, particularly Pomacea canaliculata (Yen et al., 1990) (Banerjee et al., unpublished data), Achatina fulica(Lee et al., 2014), and rats (Yang and Lu, 2000).
Another major source of infection in Taiwan is the consumption of infected paratenic hosts, such as frogs (Tseng et al., 2011). Other paratenic hosts, eaten raw (e.g. shrimp, crabs, monitor lizards and frogs) in some other countries, are also quite frequent sources of infection (Carvalho et al., 2022; Turck et al., 2022). Therefore, any raw animal products intended for direct consumption should be regulated carefully for presence of parasites. Cooking has been shown to eliminate the infectivity of the Angiostrongylus cantonensis larvae, and there should be a general caution around eating raw foods (Hollingsworth et al., 2013).
Regarding the eradication of the nematode, there are particular considerations that need to be made in case of the aquatic snail hostPomacea spp. The general mode of control of this invasive snail is via hand-picking of the snails, commercially available pesticides, pesticidal agents made from plant extracts or by-products, and biocontrol using ducks or fish (Yang et al., 2013). However, given the chances of increased parasite spread from injured or dead gastropod hosts in aquatic media (Modrý et al., 2021), special care needs to be taken to remove the dead gastropod bodies from the field and dispose safely. Additionally, with regards to biocontrol, birds and fish have been known to get infected or act as paratenic hosts for the disease, thus aiding in spreading the disease (Burns et al., 2014; Turck et al., 2022).
Genetic characterization and phylogenetic study have been performed onAngiostrongylus cantonensis populations in multiple studies to understand the distribution and aid in control measures (Gamiette et al., 2023; Lee et al., 2014; Lv et al., 2020). In Taiwan, Lee et al. (2014) had discovered two major strains present in Taiwan; the more common and infectious P strain, and the less infectious H strain, which had been discovered only in Hualien. Further analyses of the distribution of the parasite and its hosts may aid in developing a model to predict the spread of parasite, and result in targeted eradication strategies (Lv et al., 2020).