# First evidence of human borreliosis local transmission in Cambodia

Sony Yean, Pierre-Olivier Maquart, Gauthier Delvallez, Sébastien Boyer

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Sony Yean, Pierre-Olivier Maquart, Gauthier Delvallez, Sébastien Boyer. First evidence of human borreliosis local transmission in Cambodia. International Journal of Infectious Diseases, 2026, 163, pp.108208. 10.1016/j.ijid.2025.108208. hal-05424747

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Short Communication

# First evidence of human borreliosis local transmission in Cambodia

Sony Yean<fnref n="1" />, Pierre-Olivier Maquart<fnref n="1" />,<fnref n="2" />, Gauthier Delvallez<fnref n="3" />, Sébastien Boyer<fnref n="1" />,<fnref n="4" />,*

<figure><img src="image_2.png" /></figure>

ARTICLE INFO

Article history:

Received 10 October 2025

Revised 10 November 2025

Accepted 11 November 2025

Keywords:

Borreliia

Neuropathy

Cambodia

Southeast Asia

A B S T R A C T

We report the first cases of confirmed human local transmission of borreliosis in Cambodia. Four patients, including cases with no travel history, presented typical clinical signs and tested positive for *Borrelia burgdorferi* antibodies. These findings confirm the presence, circulation, and local transmission of Borrelia in Cambodia. These results are important to convince for the need and the development of surveillance of tick-borne diseases in Southeast Asia.

© 2025 The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious Diseases. This is an open access article under the CC BY license ([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/))

Introduction

Borreliosis, caused by *Borreliia* spirochaetes, is primarily transmitted through tick bites of the *Ixodes* genus. The most prevalent *Borreliia* species responsible for human infections are *Borreliia burgdorferi*, *Borreliia afzelii*, and *Borreliia garinii*. These infections have become a growing public health concern, predominantly affecting humans and animals in North America, Europe, North Africa, and Northern Asia, with the highest prevalence reported in the United States [1]. Although well-documented in the northern hemisphere, borreliosis has also been reported in humans in Asia, including Japan, China, South Korea, Nepal, and northern India [2], as well as in parts of Southeast Asia, notably Indonesia, Singapore, and Malaysia [3].

Methods

Between 2017 and 2020, four patients presenting classical clinical symptoms of a borreliosis infection (fever, joint and muscle pain, headache) and an erythema migrans following tick bites were referred by their local general practitioners to the Institut Pasteur du Cambodge in Phnom Penh for testing. Serum samples were sent to a certified laboratory in France for enzyme immunoassay testing. All samples tested positive for IgM or IgG antibodies against

*B. burgdorferi sensu lato* (Table 1). According to the CSTE/CDC case definition matrix [4], the combination of level 2 clinical criteria and positive serology confirmed all four patients as borreliosis cases.

Results and discussion

Prior to their infections, all patients had spent time in forested areas and had experienced recent tick bites. Two of the four patients, both 26-year-old French tourists, had hiked for 10 days in the Cardamom Mountains of southwestern Cambodia and developed symptoms within a few days of returning. The third patient had stayed in the mountainous and forested Mondulkiri province in eastern Cambodia, while the fourth resided in Koh Kong province, near the Cardamom Mountains. Notably, the last two patients had no travel history outside Cambodia, indicating local transmission.

Serological testing was performed using the Enzygnost Borrelia Lyme IgM and IgG assays (Siemens/DADE Behring, Marburg, Germany). The IgM assay uses inactivated *B. afzelii* antigens, while the IgG assay combines inactivated *B. afzelii* antigens with recombinant VIsE proteins from *B. afzelii*, *B. garinii*, and *B. burgdorferi sensu stricto* [5]. Both assays detect antibodies against *B. burgdorferi sensu lato*, although cross-reactivity (e.g., with *Treponema pallidum*, EBV, or CMV) has been described [6]. In the present cases, such infections were not clinically suspected. Therefore, while serological cross-reactivity cannot be entirely excluded, it is unlikely to account for the observed results.

<footnote n="1">¹ Medical and Veterinary Entomology Unit, Institut Pasteur du Cambodge, Phnom Penh, Cambodia</footnote>

<footnote n="2">² Institut de Recherche pour le Développement, Gif sur Yvette, France</footnote>

<footnote n="3">³ Medical Biology Laboratory, Institut Pasteur du Cambodge, Phnom Penh, Cambodia</footnote>

<footnote n="4">⁴ Ecology and Emergence of Arthropod-Borne Pathogens Unit, Department of Global Health, Institut Pasteur, CNRS, Paris, France</footnote>

<footnote>* Corresponding author: Sebastien Boyer, Institut Pasteur du Cambodge, 5 Bd Monivong, 12201 Phnom Penh, Cambodia.</footnote>

<footnote>E-mail addresses: sebastien.boyer@pasteur.fr, sboyer@pasteur-kh.org, drsebastien.boyer@gmail.com (S. Boyer).</footnote>
<table><caption>**Table 1**
Epidemiological data and test result of the patients.</caption><thead><tr><th>Case</th><th>Year</th><th>Age</th><th>Sex</th><th>IgM<sup>a</sup></th><th>IgG<sup>b</sup></th><th>Pathogen</th><th>Tick bite</th><th>Comment</th></tr></thead><tbody><tr><td>1</td><td>2017</td><td>26</td><td>M</td><td>Positive</td><td>Negative</td><td>Borrelia burgdorferi sensu lato</td><td>Yes</td><td>Hiked in Cardamom</td></tr><tr><td>2</td><td>2017</td><td>26</td><td>M</td><td>Negative</td><td>Positive</td><td>B. burgdorferi sensu lato</td><td>Yes</td><td>Mountains</td></tr><tr><td>3</td><td>2017</td><td>38</td><td>M</td><td>Positive<sup>c</sup></td><td>Negative</td><td>B. burgdorferi sensu lato</td><td>Yes</td><td>Hiked in Mondulkiri</td></tr><tr><td>4</td><td>2020</td><td>69</td><td>F</td><td>Positive<sup>d</sup></td><td>Negative</td><td>B. burgdorferi sensu lato</td><td>Yes</td><td>Stayed in Koh Kong</td></tr></tbody></table>

Serum from the third patient was further tested by Western Blot (Meridian Bioscience), revealing a single IgM band (p39) and confirming IgM positivity [7]. Interestingly, the fourth patient showed a high IgM level that increased over a 1-month period without IgG seroconversion, which may be attributed to early macrolide antibiotic treatment and the resulting favorable clinical outcome.

In conclusion, these four patients provide the first confirmed evidence of borreliosis infection in human in Cambodia. Human borreliosis may have remain undetected in the country due to the frequent use of broad-spectrum antibiotics [8] and general lack of knowledge about tick-borne diseases. These significant findings highlight the need for increased awareness and the establishment of specific guidelines for public health authorities in Cambodia and neighboring countries to address the circulation of borreliosis and other tick-borne diseases.

## Funding

The study has been financed by Institut Pasteur du Cambodge. The funding source had no involvement in the conduct of the study, the analysis, or the writing of this article.

## Author contributions

All authors contributed to the acquisition, analysis and interpretation of data, such as the drafting, revising and approving the current version.

## Ethical approval

We obtained consent for publication of the case details from the four patients.

## Declaration of competing interest

Authors declare no conflict of interest.

## References

[1] Ji Z, Jian M, Yue P, Cao W, Xu X, Zhang Y, et al. Prevalence of Borrelia burgdorferi in Ixodidae tick around Asia: a systematic review and meta-analysis. *Pathogens* 2022;11(2):143.

[2] Dong Y, Zhou G, Cao W, Xu X, Zhang Y, Ji Z, et al. Global seroprevalence and sociodemographic characteristics of Borrelia burgdorferi sensu lato in human populations: a systematic review and meta-analysis. *BMJ Global Health* 2022;7(6):e007744.

[3] Teh KJ, Tang HY, Lim LS, Pung HS, Gan SY, Lai NS. Mini review: surveillance of Lyme borreliosis in Southeast Asia and method of diagnosis. *Eur Rev Med Pharmacol Sci* 2023;27(10):4378-85.

[4] MDSS 2022, Guidelines for the reporting of Lyme disease cases using the Michigan Disease Surveillance System [cited 2025 Nov 29]. Available from: https://www.michigan.gov/-/media/Project/Websites/emergingdiseases/2022_Lyme_Disease_Guidelines_-_Full.pdf?rev=dbb2fb7ed8e24483aded6236a5e8dfdc

[5] ANSM 2016, Contrôle du marché d'après les notices des réactifs de sérologie de la borréiose de lyme (hors techniques de biologie moléculaire) [cited 2025 Nov 29]. Available from: https://ansm.sante.fr/uploads/2020/10/29/20201029-rapport-lyme-controle-marche-2017.pdf

[6] Grażewska W, Holec-Gąsior L. Antibody cross-reactivity in serodiagnosis of Lyme disease. *Antibodies* 2023;12(4):63.

[7] Hauser U, Lehnert G, Wilske B. Validity of interpretation criteria for standardized western blots (immunoblots) for serodiagnosis of lyme borreliosis based on sera collected throughout Europe. *J Clin Microbiol* 1999;37(7):2241-7.

[8] Om C, Daily F, Vlieghe E, McLaughlin JC, McLaws ML. Pervasive antibiotic misuse in the Cambodian community: antibiotic-seeking behaviour with unrestricted access. *Antimicrob Resist Infect Control* 2017;6:30.

<footnote>a IgM: Enzygnost Borrelia Lyme IgM (Siemens/DADE Behring, Marburg, Germany).</footnote>

<footnote>b IgG: Enzygnost Borrelia Lyme link VlsE/IgG (Siemens/DADE Behring, Marburg, Germany).</footnote>

<footnote>c Tested by Western Blot (Meridian Bioscience): IgM anti-p39 confirmed.</footnote>

<footnote>d Confirmed on two different samples, 1 month apart.</footnote>