A parasitic worm that normally completes its life cycle in rats and snails has emerged as a significant cause of meningitis in northern Vietnam, and a new case series from Hanoi suggests that one of the disease’s hallmark laboratory findings is less reliable than many clinicians assume. In a retrospective review published in Acta Parasitologica, researchers at Bach Mai Hospital, the country’s national tertiary referral centre, documented 30 consecutive patients treated for probable neuroangiostrongyliasis between January 2018 and December 2021. The infection, caused by the rat lungworm Angiostrongylus cantonensis, is a leading cause of eosinophilic meningitis across Southeast Asia and the Pacific, yet detailed clinical data from northern Vietnam have remained scarce. The new series offers one of the most granular pictures to date of how the disease presents in this region, and it carries a practical warning: absence of eosinophils in the cerebrospinal fluid does not rule the diagnosis out.
The parasite behind the disease has an unusual life cycle that explains why humans become accidental and unfortunate hosts. Adult worms live in the pulmonary arteries of rats, where they lay eggs that hatch into first-stage larvae. These larvae are expelled in rat faeces, then ingested by snails and slugs, which act as intermediate hosts. Within those molluscs, the larvae develop into third-stage larvae, the infectious form for humans. People typically acquire the infection by eating raw or undercooked snails, slugs, or contaminated fresh produce, or sometimes by consuming paratenic hosts such as freshwater prawns or crabs that have carried the larvae without supporting their development. Once ingested, the larvae penetrate the intestinal wall, enter the bloodstream and migrate to the central nervous system, where they die in the brain and spinal cord and trigger a fierce inflammatory response.
That inflammatory response is what defines the disease clinically. Unlike bacterial meningitis, which is dominated by neutrophils, or viral meningitis, which is dominated by lymphocytes, neuroangiostrongyliasis produces a striking influx of eosinophils, the white blood cells best known for fighting parasites and mediating allergic disease. Eosinophilic pleocytosis in the cerebrospinal fluid has long been considered the diagnostic signature of the infection, and it underpins most case definitions used in endemic regions. The headache that brings patients to hospital is typically severe and unrelenting, often accompanied by neck stiffness, and in more severe cases the migrating larvae and the inflammation they provoke can cause focal neurological deficits, paralysis, or death. There is no universally validated rapid diagnostic test in most endemic settings, and molecular confirmation by polymerase chain reaction remains unavailable in many hospitals where the disease is most common.
The Hanoi team, led by Tra Thu Doan of the Institute for Tropical Medicine at Bach Mai Hospital, assembled their series using a composite case definition. Patients had to show clinical meningitis, eosinophilia in the peripheral blood or the cerebrospinal fluid, and a reactive serum enzyme-linked immunosorbent assay for antibodies specific to A. cantonensis, with alternative diagnoses excluded. During the study period, the hospital lacked the capacity to confirm the parasite by cerebrospinal fluid polymerase chain reaction, and no patient in the series met a definite standard based on recovery of larvae or molecular detection. That limitation matters, and the authors are explicit about it: their findings describe probable cases, and the proportions they report are conditioned on the sensitivity of the case definition and of routine laboratory differentials.
The demographic and clinical profile of the 30 patients is instructive. Their mean age was 39.2 years, with a standard deviation of 17.1, and just over half, 16 of 30, were men. More than half, 17 patients, came from Nghe An or Thanh Hoa, two north-central provinces that appear to be hotspots for exposure. Headache was universal, present in all 30 patients, and neck stiffness occurred in 25, or 83.3 percent. Eleven patients, 36.7 percent, presented with focal neurological deficits, a reminder that the disease is not always a benign self-limiting headache syndrome. Exposure histories were abstracted retrospectively from routine admission notes, so the authors caution that they cannot identify a specific transmission route, though the geographic clustering points to where future surveillance should concentrate.
The laboratory findings reveal the diagnostic subtlety at the heart of the paper. Peripheral blood eosinophilia, defined as more than 8 percent eosinophils, was present in 19 of 30 patients, or 63.3 percent. But the most striking figure concerns the cerebrospinal fluid: in six patients, 20 percent of the series, no eosinophils at all were recorded on the routine differential at presentation. The authors report a 95 percent confidence interval of 9.5 to 37.3 percent for that proportion, and their conclusion is unambiguous. A normal or lymphocyte-predominant cerebrospinal fluid differential at presentation should not exclude neuroangiostrongyliasis in a patient with compatible symptoms and reactive serology. The finding echoes earlier work from northern Vietnam, including a 2020 study in PLoS Neglected Tropical Diseases that described meningitis patients with confirmed A. cantonensis infection who lacked cerebrospinal fluid eosinophilia.
Other cerebrospinal fluid parameters painted a picture of a disease that is inflammatory but rarely metabolic in the way tuberculosis meningitis can be. Hypoglycorrhachia, a low cerebrospinal fluid glucose below 2.2 millimoles per litre, was uncommon, seen in only five patients, 16.7 percent. Low cerebrospinal fluid chloride, below 120 millimoles per litre, was more frequent, present in 13 of 30 patients, or 43.3 percent, a finding that could mislead clinicians toward a diagnosis of tuberculous meningitis if interpreted in isolation. Perhaps the most encouraging laboratory trend was temporal: across serial lumbar punctures, cerebrospinal fluid cell counts, eosinophil percentages and protein levels all fell progressively, providing an objective measure of recovery that clinicians can track over the course of treatment.
Treatment in the series followed a combined approach. Every patient received albendazole, an anthelminthic drug that kills the larvae, together with methylprednisolone, a corticosteroid intended to dampen the inflammatory reaction triggered by dying parasites. This combination reflects international guidelines updated in 2021, which recommend concurrent corticosteroids with anthelminthic therapy on the theory that killing larvae without suppressing inflammation can paradoxically worsen symptoms. The outcomes were broadly favourable: 26 of 30 patients, 86.7 percent, recovered fully, three patients, 10 percent, relapsed within a six-month follow-up window, and none died. No serious adverse event was attributed to either drug. The authors are careful, however, to note that without a comparator group, no inference can be drawn about the independent contribution of either albendazole or methylprednisolone to those outcomes, a caveat that applies to virtually all published treatment data in this field.
The study’s limitations are those inherent to any retrospective case series. The case definition admits patients on peripheral eosinophilia with reactive serology alone, and the sensitivity of a routine clinical differential is imperfect, so the 20 percent figure for absent cerebrospinal fluid eosinophils is best understood as a property of this cohort and its methods rather than a universal constant. Serological tests for A. cantonensis vary in specificity, and cross-reactivity with other parasitic infections remains a concern in endemic settings. Exposure histories recorded in routine notes are a blunt instrument for epidemiology. Yet the clinical signal is consistent with a growing body of evidence from across the region, including a 2017 study in Clinical Infectious Diseases that established A. cantonensis as an important cause of eosinophilic meningitis in southern Vietnam and molecular confirmations of the parasite in cerebrospinal fluid from Laos.
For clinicians working in Vietnam and neighbouring countries, the practical messages are concrete. Neuroangiostrongyliasis should be considered in any patient with subacute severe headache and meningism, particularly when focal deficits are present, even if the cerebrospinal fluid differential shows no eosinophils on first examination. Repeat lumbar puncture may reveal the characteristic eosinophilic pleocytosis as the disease evolves, and serial measurements offer a way to monitor response. For public health authorities, the concentration of cases from Nghe An and Thanh Hoa argues for prospective surveillance and investment in molecular diagnostic capacity in the north-central provinces, where the ecological conditions that sustain the parasite, invasive Pomacea snails among them, are well documented. As the rat lungworm continues its global spread, documented as far afield as Brazil and Hawaii, the Vietnamese series adds an important data point: the disease is present, treatable, and easier to miss than its textbook description suggests.
Subject of Research: Human neuroangiostrongyliasis caused by Angiostrongylus cantonensis in northern Vietnam
Article Title: Human Neuroangiostrongyliasis Caused by Angiostrongylus cantonensis in Northern Vietnam: a Clinical–Epidemiological Case Series of Eosinophilic Response and Disease Manifestations
Article References: Thu Doan, T., Khanh Thi Nguyen, L., Minh Nong, V., Phuong Thi Ngo, N., Van Do, T., Xuan Dao, C., Xuan Do, C., & Van Vu, G. (2026). Human Neuroangiostrongyliasis Caused by Angiostrongylus cantonensis in Northern Vietnam: a Clinical–Epidemiological Case Series of Eosinophilic Response and Disease Manifestations. Acta Parasitologica, 71(5), Article 222. https://doi.org/10.1007/s11686-026-01412-z
Image Credits: AI Generated
DOI: 10.1007/s11686-026-01412-z
Keywords: Angiostrongylus cantonensis, neuroangiostrongyliasis, eosinophilic meningitis, rat lungworm, Vietnam, eosinophils, cerebrospinal fluid, albendazole, methylprednisolone, case series, parasitic disease, tropical medicine
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Ophelia Keating. (September 24, 2026). Rat Lungworm Disease in Northern Vietnam: Headache, Hidden Eosinophils and a Diagnostic Warning. Scienmag. https://scienmag.com/rat-lungworm-disease-in-northern-vietnam-headache-hidden-eosinophils-and-a-diagnostic-warning/
Ophelia Keating. “Rat Lungworm Disease in Northern Vietnam: Headache, Hidden Eosinophils and a Diagnostic Warning.” Scienmag, 24 September 2026, https://scienmag.com/rat-lungworm-disease-in-northern-vietnam-headache-hidden-eosinophils-and-a-diagnostic-warning/. Accessed 24 September 2026.
Ophelia Keating. “Rat Lungworm Disease in Northern Vietnam: Headache, Hidden Eosinophils and a Diagnostic Warning.” Scienmag. September 24, 2026. https://scienmag.com/rat-lungworm-disease-in-northern-vietnam-headache-hidden-eosinophils-and-a-diagnostic-warning/
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Tags: albendazoleAngiostrongylus cantonensiscase seriescerebrospinal fluidcerebrospinal fluid analysis limitationsclinical features of Angiostrongylus cantonensisdiagnostic challenges in parasitic meningitiseosinophilic meningitiseosinophilic meningitis diagnosiseosinophilsheadach and neurological symptoms in parasitic infectionsmethylprednisoloneneuroangiostrongyliasisnorthern Vietnam parasitic infectionsparasitic diseaserat and snail life cycle in disease transmissionrat lungwormRat lungworm diseaseretrospective case series on neuroangiostrongyrole of eosinophils in meningitisSoutheast Asia parasitic disease epidemiologytropical medicineVietnam
