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).