Background

Oropouche virus is an arthropod-borne virus that has caused outbreaks of Oropouche fever in central and South America since the 1950s. This study investigates virological factors contributing to the re-emergence of Oropouche fever in Brazil between 2023 and 2024.

Methods

In this observational epidemiological study, we combined multiple data sources for Oropouche virus infections in Brazil and conducted in-vitro and in-vivo characterisation. We collected serum samples obtained in Manaus City, Amazonas state, Brazil, from patients with acute febrile illnesses aged 18 years or older who tested negative for malaria and samples from people with previous Oropouche virus infection from Coari municipality, Amazonas state, Brazil. Basic clinical and demographic data were collected from the Brazilian Laboratory Environment Management System. We calculated the incidence of Oropouche fever cases with data from the Brazilian Ministry of Health and the 2022 Brazilian population census and conducted age–sex analyses. We used reverse transcription quantitative PCR to test for Oropouche virus RNA in samples and subsequently performed sequencing and phylogenetic analysis of viral isolates. We compared the phenotype of the 2023–24 epidemic isolate (AM0088) with the historical prototype strain BeAn19991 through assessment of titre, plaque number, and plaque size. We used a plaque reduction neutralisation test (PRNT₅₀) to assess the susceptibility of the novel isolate and BeAn19991 isolate to antibody neutralisation, both in serum samples from people previously infected with Oropouche virus and in blood collected from mice that were inoculated with either of the strains.

Findings

8639 (81·8%) of 10 557 laboratory-confirmed Oropouche fever cases from Jan 4, 2015, to Aug 10, 2024, occurred in 2024, which is 58·8 times the annual median of 147 cases (IQR 73–325). Oropouche virus infections were reported in all 27 federal units, with 8182 (77·5%) of 10 557 infections occurring in North Brazil. We detected Oropouche virus RNA in ten (11%) of 93 patients with acute febrile illness between Jan 1 and Feb 4, 2024, in Amazonas state. AM0088 had a significantly higher replication at 12 h and 24 h after infection in mammalian cells than the prototype strain. AM0088 had a more virulent phenotype than the prototype in mammalian cells, characterised by earlier plaque formation, between 27% and 65% increase in plaque number, and plaques between 2·4-times and 2·6-times larger. Furthermore, serum collected on May 2 and May 20, 2016, from individuals previously infected with Oropouche virus showed at least a 32-fold reduction in neutralising capacity (ie, median PRNT₅₀ titre of 640 [IQR 320–640] for BeAn19991 vs <20 [ie, below the limit of detection] for AM0088) against the reassortant strain compared with the prototype.

Interpretation

These findings provide a comprehensive assessment of Oropouche fever in Brazil and contribute to an improved understanding of the 2023–24 Oropouche virus re-emergence. Our exploratory in-vitro data suggest that the increased incidence might be related to a higher replication efficiency of a new Oropouche virus reassortant for which previous immunity shows lower neutralising capacity.

Funding

São Paulo Research Foundation, Burroughs Wellcome Fund, Wellcome Trust, US National Institutes of Health, and Brazilian National Council for Scientific and Technological Development.

Translation

For the Portuguese translation of the abstract see Supplementary Materials section.

中文翻译：

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2023 年至 2024 年巴西 Oropouche 病毒再次出现：一项观察性流行病学研究

 背景

Oropouche 病毒是一种节肢动物传播的病毒，自 1950 年代以来已在中美洲和南美洲引起 Oropouche 热的爆发。本研究调查了导致 2023 年至 2024 年间巴西再次出现 Oropouche 热的病毒学因素。

 方法

在这项观察性流行病学研究中，我们结合了巴西 Oropouche 病毒感染的多个数据源，并进行了体外和体内表征。我们收集了在巴西亚马逊州马瑙斯市获得的血清样本，这些血清样本来自疟疾检测呈阴性的 18 岁或以上的急性发热性疾病患者，以及来自巴西亚马逊州科里市的既往 Oropouche 病毒感染者的样本。从巴西实验室环境管理系统收集基本临床和人口统计数据。我们使用巴西卫生部和 2022 年巴西人口普查的数据计算了 Oropouche 热病例的发病率，并进行了年龄-性别分析。我们使用逆转录定量 PCR 检测样本中的 Oropouche 病毒 RNA，随后对病毒分离株进行测序和系统发育分析。我们通过评估滴度、斑块数量和斑块大小，将 2023-24 流行病分离株 （AM0088） 的表型与历史原型菌株 BeAn19991 进行了比较。我们使用噬菌斑减少中和试验 （PRNT₅₀） 来评估新型分离株和 BeAn19991 分离株对抗体中和的敏感性，无论是在先前感染 Oropouche 病毒的人的血清样本中，还是在接种过任何一种菌株的小鼠的血液中。

 发现

从 2015 年 1 月 4 日至 2024 年 8 月 10 日，实验室确诊的 10 557 例奥罗普切热病例中有 8639 例 （81·8%） 发生在 2024 年，是 147 例年中位数 （IQR 73-325） 的 58·8 倍。所有 27 个联邦单位都报告了 Oropouche 病毒感染，其中 10 557 例感染中有 8182 例 （77·5%） 发生在巴西北部。我们在 2024 年 1 月 1 日至 2 月 4 日期间在亚马逊州 93 名急性发热病患者中的 10 名 （11%） 中检测到 Oropouche 病毒 RNA。AM0088 在哺乳动物细胞感染后 12 h 和 24 h 的复制率显著高于原型菌株。AM0088 在哺乳动物细胞中具有比原型更具毒性的表型，其特征是斑块形成较早，斑块数量增加 27% 至 65%，斑块大 2·4 倍至 2·6 倍。此外，2016 年 5 月 2 日和 5 月 20 日从先前感染过 Oropouche 病毒的个体中收集的血清显示，与原型相比，对重排菌株的中和能力至少降低了 32 倍（即，BeAn19991 的中位 PRNT₅₀ 滴度为 640 [IQR 320–640] 与 AM0088 的 <20 [即低于检测限]）。

 解释

这些发现对巴西的 Oropouche 热进行了全面评估，并有助于更好地了解 2023-24 年 Oropouche 病毒再次出现。我们的探索性体外数据表明，发病率的增加可能与新的 Oropouche 病毒重排剂的较高复制效率有关，而先前的免疫力显示较低的中和能力。

 资金

圣保罗研究基金会、Burroughs Wellcome Fund、Wellcome Trust、美国国立卫生研究院和巴西国家科学技术发展委员会。

 译本

有关摘要的葡萄牙语翻译，请参阅补充材料部分。