Eosinophilic meningitis epidemiological data from a national database in Thailand’s Department of Disease Control: a pragmatic, retrospective analytical study

EOM had a steady prevalence over the six-year period, with the highest prevalence rate in Northeast Thailand. The common age groups were 25–54 years, and farmers and laborers were the two most common occupations. Note that the number of infected cases may be higher than reported in the literature (1,083 cases from 2014 to 2019 vs. 1,337 cases from 1945 to 2008) [1].

This study revealed that EOM is prevalent in Northeast Thailand, where consuming uncooked freshwater snails is a common eating habit. As previously reported, the most common age of EOM patients was in the third decade of life [4]. The youngest EOM patients were infants (Table 1). The transmission route for children may be from crawling or playing with contaminated snail or eating contaminated food or drink from parents’ hands [15,16,17]. As stated previously, people in Northeast Thailand commonly eat raw freshwater snails, particularly in the rice fields [7]. Thus, the highest incidence was found in farmers (45%), followed by laborers and students. The latter two occupations are believed to be infected from the same eating habit as farmers. Students in Northeast Thailand must work with their parents in the rice fields on weekends or semester breaks.

EOM in Thailand is reported throughout the year with a wax and wane pattern. The common months of EOM may be related, once again, to rice harvest season: September to January. As shown in Table 3, the numbers of EOM cases dropped in February to May, which is post-rice harvest. In Thailand, the rice harvest starts again in June and reaches the peak after September. An ecological study of EOM showed that the incidence of EOM cases was significantly related to wind speed negatively [18]. This rather peculiar phenomenon is explained as follows: strong wind reduces social and family sharing of food in the rice field. Additionally, it is a cultural habit of farmers to consume raw freshwater snails in the rice fields. Therefore, the prevalence of EOM may be highest in the rice harvest months [7]. Snails may reproduce more during the raining season as well.

During the study period, there was no case of death due to EOM. This finding may be explained by the definition of reported case. The 506 report form does not include the severe form of EOM, namely eosinophilic meningoencephalitis. This severe form has been found to have a high mortality rate: approximately 80% [10]. The conventional EOM reported in this study is less severe and may be self-limited, though it can take approximately two months to recover from severe headache. EOM patients may develop the severe form if they are left untreated or are misdiagnosed [11]. Ingestion of A. cantonensis larva by eating raw freshwater snails, shrimp, or contaminated salad may be possible transmission routes worldwide. To prevent this disease, providing education, washing produce, and refraining from consuming contaminated foods are crucial. In this report, sex data are not available on the website. From a previous report, cases among males are more prominent than among females, at a ratio of approximately 2–3:1, likely due to the higher frequency of males eating uncooked food [3, 8, 19].

Even though these data were collected only from Thailand, other countries with reported cases of EOM may replicate this study in their own setting to evaluate the epidemiological data. This study included data for six years. Data for the following six years will be reported, thus showing the trend of EOM.

There are some limitations in this study. First, the 506 report form does not include the severity of disease or the treatment provided. Second, patients with EOM who were misdiagnosis or who did not undergo a lumbar puncture may not be reported to the system. As mentioned earlier, EOM may be missed due to physicians’ lack of knowledge. Therefore, the prevalence indicated here may be under reported. Third, no causal relationship of personal factors or systematic review was studied [20,21,22,23,24,25]. Finally, there are no data regarding the total number of farmers in each zone or in Thailand overall.

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