3.3 Clinical manifestations and diagnosis

In most cases for patients suffering from neuroangiostrongyliasis, a presumptive diagnosis has to be made by using several observations such as; symptoms, history of travel and diet, analysis of CSF or serum samples, and serological tests (Ansdell et al., 2021). Nevertheless, in regions where the rat lungworm is not endemic or it has been very recently introduced, there may be a delay in considering neuroangiostrongyliasis as diagnosis by the medical practitioners, which may hinder treatment effectiveness (Cowie et al., 2022; Luo et al., 2023).
Generally, the incubation period of Angiostrongylus cantonensisis 1-2 weeks; however it may vary. The symptoms associated withAngiostrongylus cantonensis may also vary; nevertheless, some common indicators are headache, neck stiffness, Brudzinski’s sign/ Kernig’s sign, hyperesthesia/paresthesia, fever, muscle weakness, facial palsy and paralysis of the external muscles (Cowie et al., 2022; Tseng et al., 2011). The symptoms and severity of the disease may be dependent on parasite strain (Lee et al., 2014), parasite load, and the patient’s tolerance level towards their immune response (Watthanakulpanich et al., 2021). For example, severe headache was the most common symptom in Thailand, fever and neck stiffness in China, and headache and fever in Taiwan (Khamsai et al., 2020). Whether the variation in symptoms sheds light on the severity of the infection is unclear; the majority of cases resolve spontaneously, though more serious cases may result in death (McAuliffe et al., 2019). Nevertheless, given that none of the symptoms are distinctive enough to be diagnostic, most cases require laboratory testing to obtain a definitive diagnosis.
In Taiwan, a study by Lee et al. (2014) investigated the genetic differences among Angiostrongylus cantonensis populations, and discovered the presence of two primary strains showing infectivity in non-permissible hosts such as humans. The more common strain showed low genetic diversity and high infectivity, whereas the new strain isolated from Hualien showed considerable genetic difference from the other strains, and much lower infectivity in their mouse models. This is corroborated by another metagenomic study concluding that there is low genetic diversity in infective invasive Angiostrongylus cantonensis strains globally (Červená et al., 2019). Hence, the large variety of symptoms may possibly arise from other factors such as larval development in the human host, and interaction with the individual immune systems.
Definitive diagnosis is possible with visual confirmation ofAngiostrongylus cantonensis larvae in the blood or cerebrospinal fluid (CSF); however it is rarely successful (Ansdell et al., 2021). Detection of blood eosinophilia, and studying CSF characteristics e.g., high CSF pressure, CSF white blood cell count (especially eosinophils), and CSF proteins are often used as initial indicators (Wang et al., 2011). Radiography such as magnetic resonance imaging (MRI) and computed tomography (CT) scans are also used as supplementary tests, particularly MRI of the brain where lesions, leptomeningeal enhancement, or increased signal intensity in the subcortical white matter may be detected after the first few weeks of infection (Ansdell et al., 2021). MRI of the spine is recommended for patients with myeloradicular symptoms. Furthermore, CT scans of the chest have shown nodular lesions, indicative of migratory larvae or young adult parasites, but is ineffective in most cases. Nevertheless, it is recommended to undergo a chest CT scan only if there are respiratory symptoms (Ansdell et al., 2021).
Serological methods such as dot immunogold filtration assay (DIGFA) developed by Eamsobhana et al. (2021), are already in use in hospitals of endemic regions, such as Thailand. The disadvantage of serological methods is the delay in formation of antibodies; however serological tests are still a common procedure, both as a method of diagnosis, as well as to rule out the presence of other parasites (Carvalho et al., 2022).
Detection of parasite DNA in CSF or serum of patient by polymerase chain reaction (PCR) is currently used for conclusive results (Carvalho et al., 2022). Molecular techniques have a number of benefits over other methods (e.g., visual detection or serological methods) where early detection can be done from parasite fragments without need of antibody formation (Qvarnstrom et al., 2016). There are a number of molecular assays that are currently in use or under development which can be used for high sensitivity or specificity (Carvalho et al., 2022). There is a possibility of a negative PCR result despite strong clinical suspicion if the sample is taken from early infection stages, however it is recommended that the test be repeated in 5-10 days in such a scenario (Ansdell et al., 2021). Recently, a novel method, metagenomic next-generation sequencing (mNGS) used for the presence of multiple parasites simultaneously, has also been used to detectAngiostrongylus cantonensis in some case studies (Carvalho et al., 2022). Additionally, the AcanR3990 qPCR assay developed by Sears et al. (2021), shows potential for success as a diagnostic tool for neuroangiostrongyliasis, with greater accuracy and sensitivity than conventional PCR with ITS primers (Jarvi et al., 2023). Genetic characterization of the obtained A. cantonensis larvae as done by Dumidae et al. (2023) provides substantial reference data for more robust molecular identification methods.
In Taiwan, the current method of Angiostrongylus cantonensisdetection is by use of PCR on CSF samples (Luo et al., 2023). In addition, alternative markers for easier detection ofAngiostrongylus cantonensis infection are being tested, such as IgE antibody levels in Angiostrongylus cantonensis infected mouse models (Lee et al., 2023), and microRNA upregulation in meningoencephalitis caused by Angiostrongylus cantonensisinfection (Chen and Lai, 2023).