Travel-associated chikungunya acquired in Myanmar in 2019

Eighteen cases of chikungunya virus infection in travellers returning from Myanmar were reported to the GeoSentinel Surveillance Network, its subnetwork EuroTravNet and TropNet in 2019, reflecting an ongoing local outbreak. This report reinforces the importance of travellers as sentinels of emerging arboviral outbreaks and highlights the importance of vigilance for imported cases, due to the potential for dissemination of the virus into areas with competent local vectors and conducive environmental conditions.

Eighteen cases of chikungunya virus infection in travellers returning from Myanmar were reported to the GeoSentinel Surveillance Network, its subnetwork EuroTravNet and TropNet in 2019, reflecting an ongoing local outbreak. This report reinforces the importance of travellers as sentinels of emerging arboviral outbreaks and highlights the importance of vigilance for imported cases, due to the potential for dissemination of the virus into areas with competent local vectors and conducive environmental conditions. According to a newspaper report, the Myanmar Department of Public Health identified an outbreak of CHIKV infection in 2019, mainly in Nay Pyi Taw, Kachin State, and Tanintharyi Region, Myanmar [1]. In the previous 8 years however, no cases of CHIKV infection had been officially reported in the country [2,3].
We describe travellers with imported CHIKV infection from Myanmar who were diagnosed in 2019 (January to November) and identified at GeoSentinel, EuroTravNet and TropNet sites. Some epidemiological and clinical information as well as places visited by the travellers are presented.

Case finding
GeoSentinel is a global surveillance network for emerging infectious diseases that has 68 sites in 28 countries; EuroTravNet is its European subnetwork; TropNet represents a separate European surveillance entity represented by 75 specialised tropical medicine centres in Europe.
For chikungunya surveillance, GeoSentinel follows case definitions proposed by the World Health Organization (WHO) Regional Office for Southeast Asia [4]. Such definitions were used in the current report. A possible case was a patient with acute onset of fever >38.5°C and severe arthralgia/arthritis not explained by other medical conditions. A probable case was a patient meeting both the previously mentioned clinical criteria and the following epidemiological criteria: residing or having visited epidemic areas, having reported transmission within 15 days prior to the onset of symptoms. A confirmed case had to meet one or more of the following laboratory criteria, irrespective of the clinical presentation: (i) virus isolation in cell culture or animal inoculations from acute phase sera, or (ii) presence of viral RNA in acute phase sera as demonstrated by RT-PCR, or (iii) presence of virus-specific IgM antibodies in single serum sample in acute or convalescent stage, or (iv) fourfold increase in virus-specific IgG antibody titre in samples collected at least 3 weeks apart.
For this study, cases were excluded if they were not in Myanmar at the likely time of exposure, which was inferred from their date of symptom onset and the typical incubation period (defined as 3 to 7 days) [5] or if the travellers had more than one potential travel destination exposure based on the incubation period.

Description of cases
Epidemiological and clinical details for each patient were collected from reporting sites.
The 18 cases reported in the current study had a median age of 51 years (range: 19-68 years) and 10 were male (Table 1). Among the overall cases, one had visited friends and relatives in Myanmar at the likely time of exposure, two were there on a business trip and three were expatriates. The rest of the cases, which represented the majority (n = 12), were tourists who had experienced a median length of stay of 13 days in Myanmar (range: 6-65 days).
The majority of cases were imported to Europe (n = 13) and Asia (n = 4), with one case in the Americas. Fewer than half (7/16) of travellers with available information, received pre-travel advice. Destinations visited in Myanmar are shown in the Figure. Among the 18 cases, acute symptoms reported included fever (n = 18), arthralgia (n = 16) and rash (n = 14) ( Table  2). A median of 9.5 days (range: 1-240 days) elapsed between symptom onset and diagnosis. Most travellers (n = 13) were identified at reporting sites between August and October 2019. Seven cases were confirmed by PCR, including five who were also found IgM positive, and nine cases were confirmed by a positive IgM only; the remaining two patients had probable diagnoses based on a single IgG positive serology. No acute coinfections with other arboviruses were described in the 16 patients tested for additional pathogens. Asterisks represent the places where travellers in the current study were possibly exposed to chikungunya virus. Some travellers visited the same destinations.

Table 2b
Clinical and diagnostic characteristics of confirmed and probable chikungunya cases among returning travellers from Myanmar, 2019 (n = 18) Four patients were hospitalised, and 15 needed nonsteroidal anti-inflammatory drugs; four had corticosteroids added to manage symptoms ( Table 2). As at 20 October 2019, fourteen patients still had persistent symptoms, predominantly arthralgia.

Discussion
Chikungunya is often a mild self-limited illness, but severe and life-threatening complications have been described [6]. Persistent polyarthralgia can affect up to 40% of infected individuals and may last for months or years [7,8]. A high proportion of cases in our case series (14 of 18) had persistent sequelae (mainly arthralgia) that interfered with their daily life. Given the potential for long-term morbidity, the absence of a curative treatment or a preventive vaccine, detailed pre-travel counselling should focus on mosquito-bite prevention, particularly for those at high risk of incapacitating complications such as women older than 40 years, people with underlying rheumatic diseases, and professional athletes [9].
Considering the short duration of stay in different parts of Myanmar of many of the currently reported travellers, as well as the usual incubation period of chikungunya, the specific place where they were infected cannot be determined with certainty. Most patients in the case series acquired chikungunya from August to October, likely due to enhanced vector activity at the end or after the monsoon season, which occurs between May and August.
CHIKV has been detected in South and South East Asia since the 1950s [3], but its current epidemiology is poorly understood [10]. In Myanmar, CHIKV infection was first described in 1973 [11]. Since this time, the virus has caused outbreaks in 1998, 2006 and 2008 [3]. The number of reported cases subsequently declined until 2010, when local surveillance identified the last reported case [3]. Thereafter, from 2011 to 2018 no CHIKV cases were officially reported in the country, however a review of travellers who acquired chikungunya in Myanmar in the GeoSentinel database from 2000 to October 2019 revealed two cases in 2015 and 2016, one probable and one confirmed.
In 2019, Myanmar's National Health Laboratory detected chikungunya cases again [1].  [14]. Certain areas of the country are, however, difficult to access and there could have been underreporting due to limitations of surveillance capacity.
Myanmar has geographical and epidemiological characteristics that put it at risk for CHIKV epidemics: a highly susceptible population [15], a long border with two large neighbouring endemic countries (India and Thailand) with cross-border population movement for trade and travel purposes [16], and competent vectors (both Aedes aegypti and Ae. albopictus) [17]. According to the Thai Ministry of Public Health, CHIKV is currently circulating in Thailand with an increasing incidence: as at 30 October 2019, the country has reported 8,744 cases this year [18]. Coincidentally, GeoSentinel recently documented a rise in the number of travelassociated CHIKV infections from Thailand [19]. Without further epidemiological and phylogenetic evidence, it remains difficult to establish whether any links exist between cases in Thailand and those in Myanmar.
It should be noted that Myanmar is an increasingly popular tourist destination, with more than 3.5 million visitors in 2017 [20]. Hence, although we report more cases exported from Myanmar in 2019 than in previous years, a surge in tourism to this country may have led to an apparent increase of the number of exported cases. GeoSentinel data are moreover not populationbased so rates or risk estimates cannot be derived. Also, diagnostics depend on local site interpretation and reporting.
Imported CHIKV infection by viraemic travellers returning to their home countries raises the possibility of virus spread to these countries if competent vectors are present, mainly in hot seasons, as European Ae. albopictus is affected by seasonal temperature and undergoes a winter diapause [21]. There is also a potential risk of transmission by other routes, such as blood donation [22]. In Europe, some countries (Italy in 2007 and 2017, France in 2010, 2014 and 2017) have experienced autochthonous transmission of CHIKV through viraemic travellers [23]. Surveillance and early detection of both imported and autochthonous CHIKV infections is therefore relevant in areas with competent vectors, as well as close vector monitoring and rapid public health response.
This report of imported CHIKV infections reinforces the importance of travellers as sentinels of local outbreaks, particularly in settings with limited public health surveillance and reporting infrastructure.