Viral, Clinical, and Epidemiological Characteristics of Oropouche Fever in Southeastern Brazil

Abstract

Background

Oropouche virus (OROV), an arbovirus causing acute febrile illness, was mostly restricted to the Amazon basin until 2023, when a reassortant lineage spread across Latin America. Increasing numbers of cases have subsequently been reported in extra-Amazonian regions of Brazil. However, follow-up and detailed clinical description is limited. This study aimed to describe viral, clinical, and epidemiological characteristics of OROV cases in the Brazilian states of Rio de Janeiro and Minas Gerais.

Methods

This longitudinal study enrolled adults with OROV exposure. Clinical and laboratorial data were assessed, and serum, urine, and saliva samples were tested for OROV RNA and sequenced.

Results

From December 2024 to May 2025, 55 OROV cases were recruited (median age: 46, 65% female). The novel OROV reassortant was confirmed by RNA sequencing. The acute phase was characterized by headache (87%), malaise (87%), fever (82%), myalgia (75%), and rash (45%). Recurrence of symptoms occurred in one-third of participants, including malaise (53%), fever (41%), arthralgia (41%), and chills (41%), but without resurgence of viral load. Viral RNA in serum and saliva was primarily detected in the first week of disease, and beyond the third week in urine.

Conclusions

Cases appeared in clusters and rash was frequently observed. Symptoms returned after 1 week, indicating the importance of patient follow-up. Cases either lived near banana plantations or participated in recreational activities at waterfalls, raising concerns about ecotourism in the Atlantic Forest. Since OROV RNA was detectable in urine for a prolonged period, urine samples may be useful for diagnosis.

A novel Oropouche virus reassortant has spread beyond the Amazon, with Southeastern Brazilian cases showing high prevalence of rash, symptom recurrence without viremia, and persistent viral genome detection in urine, potentially enhancing molecular diagnostic sensitivity.

Graphical Abstract

Graphical Abstract.

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Since its first identification in Trinidad and Tobago in 1955, the Oropouche virus (OROV) has been responsible for recurrent outbreaks of acute febrile illness in limited areas of South and Central America, historically restricted to the Amazon basin [1]. However, the 2023–2024 epidemic in the Brazilian Amazon was followed by outbreaks across multiple regions of Brazil and other Latin American countries [2, 3]. This geographic expansion has been linked to the emergence of a novel reassortant viral lineage originating in the Brazilian Amazon [4].

A distinctive pattern of OROV presence in extra-Amazon areas has been observed, predominantly affecting small rural municipalities while sparing large urban centers. Agricultural settings, particularly banana plantations have been implicated as potential ecological drivers, fostering the proliferation of Culicoides paraensis, the primary vector of OROV [5].

Previous descriptions of Oropuche disease indicate a median incubation period of 3.2 days post-infection [6]. Symptoms resemble those of other arboviruses such as dengue, Zika, and chikungunya, with abrupt onset of fever, severe headache, arthralgia, myalgia, nausea, vomiting, chills, and photophobia [7, 8]. In many cases, the disease follows a biphasic pattern: an acute phase lasting 2–4 days, followed by an asymptomatic interval, and a recurrence phase 7–10 days after this [9]. Although generally self-limited, myalgia and fatigue may persist for up to 30 days [10]. Rare complications, including hemorrhages, meningitis, meningoencephalitis and death have been reported [11–13]. Currently, there are no specific antiviral therapies or vaccines; thus, patient management relies on supportive care [14].

Laboratory diagnosis primarily relies on reverse transcription quantitative polymerase chain reaction (RT-qPCR) targeting OROV RNA segments (S or M). Serological assays are rarely available in routine practice. IgM detection-based diagnosis is most effective when performed after 5 days from symptom onset, following the peak viremia. Other immunological tests include detection of IgG seroconversion by enzyme-linked immunosorbent assay (ELISA), immunofluorescence, and neutralizing antibody assays [15]. Nevertheless, little is known about the clinical and virological characteristics of OROV infections caused by newly emerging viral clades outside the Amazon region.

This study aimed to investigate the genetic characteristics of OROV spreading in Rio de Janeiro and Minas Gerais and to describe the clinical and epidemiological characteristics of Oropouche fever caused by this clade. We also assessed OROV detection in multiple clinical specimens (serum, urine, and saliva), and evaluated viral kinetics throughout the course of illness. These findings may contribute to improved clinical recognition and diagnostic strategies in newly affected areas.

METHODS

In this longitudinal observational study, we enrolled adults (≥ 18 years-old) seeking care at a reference center for acute febrile illness in Rio de Janeiro, Brazil, who resided in or had visited an area with confirmed OROV circulation, and whose first visit occurred within 2 weeks of symptom onset.

In November 2024, an epidemic suspected to be caused by a dengue-negative arbovirus emerged in the municipality of Piau, Minas Gerais state (Supplementary Figure 1), a rural area where banana cultivation is a major economic activity. From December 2024 to February 2025, our team conducted periodic field visits in Piau to evaluate new cases, collect clinical and epidemiological data, and obtain samples from both the acute and convalescent phases (blood, urine, and saliva). Patient follow-up was carried out in collaboration with local healthcare professionals and via telemedicine.

Individuals testing positive for OROV by RT-qPCR, according to a previously described protocol [16], in serum, urine, or saliva samples were included after providing written informed consent. Upon enrollment, detailed demographic information, medical and travel history, and clinical findings were recorded in case report forms. Travel history included inquiries about visits to forested areas in Rio de Janeiro state (Supplementary Figure 1).

During the acute phase (1–14 days after symptom onset), each participant attended at least 2 study visits for collection of clinical samples. Additional visits were scheduled to identify disease recurrence and collect samples for up to 3 months after symptom onset.

The study was approved by the Committee of Ethics in Research of the Instituto Nacional de Infectologia Evandro Chagas, Rio de Janeiro, Brazil, under the protocol number 88551218.6.0000.5262. Access to the genetic heritage of the OROV under investigation is registered in the National System for the Management of Genetic Heritage and Associated Traditional Knowledge (SisGen A0C0D2F).

Laboratory Testing and Genome Sequencing

Quantitative RT-qPCR for OROV was performed with RNA extracted from serum, saliva, and urine samples using Thermo Scientific™ KingFisher™ Flex Purification System [16]. RT-qPCR was carried out with the GoTaq Probe 1-Step RT-qPCR System and run in the Applied Biosystems 7500 Real-Time PCR System. Cycle threshold (Ct) values less than 38 and sigmoid curves were considered positive. Exclusion of concurrent infection with other arboviruses commonly circulating in the investigated areas (ZIKV, CHIKV, and DENV) was performed by RT-qPCR in serum collected during the acute phase of the disease, using a commercial kit (IBMP Biomol ZDC—Zika, Dengue e Chikungunya, Paraná Institute of Molecular Biology—IBMP, Brazil). Urine and saliva samples were also assessed and tested negative.

Total RNA from OROV-positive samples was also used for OROV genome sequencing and phylogenetic analysis. The detailed protocol is described in the Supplementary material.

OROV IgM detection was performed by an in-house ELISA using hamster serum antigen, as previously described, which shows 93% sensitivity and 99% specificity [17]. Confirmed cases of Oropouche fever were defined as positive by RT-qPCR in serum, saliva, and/or urine, while cases with only OROV IgM positive results were considered probable. Hematological and biochemical parameters were also evaluated by routine laboratory analysis.

Statistical Analysis

Data were collected and managed using Research Electronic Data Capture. The sociodemographic variables were described using frequencies and proportions for categorical variables and medians and interquartile ranges (IQRs) for continuous variables [18]. Symptom frequencies between the acute and recurrent phases were explored by testing for a difference in proportions using Fisher's exact test. In addition, we compared headache intensity between phases using a χ² test. OROV viral load based on Ct values was compared between the acute and recurrent phases with Mann–Whitney test. Life tables were constructed to tally seropositive samples by week since symptom onset. Missing data were handled by restricting the analysis to variables for which less than 10% of observations were missing. In addition, we assessed whether participants who reported particular symptoms or were seropositive in the acute phase were more likely to experience symptom recurrence by calculating the hazard ratio for the acute phase variables. Calculations were performed with R software v4.1.2 (https://cran.r-project.org/) and SPSS 19.

RESULTS

From 20 December 2024, to 8 May 2025, 55 participants were included in the study. Of these, 35 patients (64%) reported exposure from either residing in or visiting forested areas in the Rio de Janeiro state. The remaining 20 patients were from Piau, Minas Gerais.

To investigate the genetic characteristics of OROV spreading in the states of Rio de Janeiro and Minas Gerais, we sequenced the complete viral genome in 6 cases included in the present clinical study. We also incorporated sequences of 6 cases obtained from genomic surveillance of the states as a proxy for the genetic characteristics of OROV currently circulating in the same municipalities (Figure 1). Regional and local OROV clusters were defined as highly supported (approximate likelihood-ratio test based on the Shimodaira–Hasegawa-like procedure > 0.80) monophyletic lineages mostly (>90%) composed by sequences from individuals living in the Brazilian Southeast region or in specific states (Rio de Janeiro or Minas Gerais) of the Southeast region. Maximum likelihood (ML) phylogenetic analysis with concatenated segments (L, M, and S) showed that sequences recovered from Rio de Janeiro and Minas Gerais originated from 2 highly supported (aLRT > 0.95) introductions into the Southeast region, designated here as SE-I and SE-II, from different clades circulating in the Amazon region (Supplementary Figure 2). Within clades SE-I and SE-II, we identified well-supported subclades (aLRT > 0.8) composed of sequences from specific municipalities, highlighting the ongoing genetic divergence of OROV during its spread throughout the Southeast region. Despite the ongoing diversification of OROV, few amino acid mutations were observed at the ancestral nodes of clades SE-I and SE-II, or within their nested subclades (Figure 1). Thus, the OROV clades circulating in the Southeast region closely resemble their parental clades from the Amazon region and are identical to those circulating in Espírito Santo.

Figure 1.

Characterization of complete OROV genomes from Rio de Janeiro and Minas Gerais. The figure illustrates the genomic structure and molecular features of 2 clusters within the OROV_{BR-2015–2024} lineage, inferred from concatenated complete-genome sequences and encompassing isolates from both states: A, SE-I and B, SE-II. In both phylogenies, tip symbols are colored according to the geographic origin shown in the adjacent maps on the left, by sampling state (Espírito Santo [ES] and Pernambuco [PE]) or by municipality (Rio de Janeiro [RJ] and Minas Gerais [MG]). Municipalities in Rio de Janeiro are color-coded by sampling region, with sampled municipalities shown in stronger hues and unsampled municipalities in lighter tones. In both trees, key node support values (aLRT) are indicated, alongside synapomorphic mutations defining the clusters. Genomes with associated clinical data are marked with asterisks. All trees are scaled according to the bars shown at the bottom of each panel.

The main sociodemographic and epidemiological characteristics of the evaluated cases are described in Table 1. Most individuals were females, aged 20–49 years, and approximately half identified as White. The racial distribution differed from the demographic profile of the affected municipalities, with individuals identifying as multiracial underrepresented, corresponding to 20% of the study population compared to 49% of the local population according to the 2022 Brazilian Census (Fisher's exact test, P = .0097). Exposures refer to the 4 weeks preceding symptom onset. Most participants (69%) reported knowing other people with similar symptoms during the onset of acute illness, which were primarily family members or friends who resided in or visited the same location. Slightly less than half of participants had a history of past DENV infection (n = 22, 40%), and 5% (n = 3) of CHIKV or ZIKV infections.

Table 1.

Sociodemographic and Epidemiological Characteristics of the Study Population, December 2024 to May 2025, Southeast Region, Brazil (55 Patients)

Characteristics	Subcategory	N	%
Sex	Female	31	56.4
	Female (pregnant)	5	9.1
	Male	19	34.5
Age	< 20 y	2	4.0
	20 to 29 y	8	15.0
	30 to 39 y	10	18.0
	40 to 49 y	13	24.0
	50 to 59 y	12	22.0
	≥ 60 y	10	18.0
Median age (IQR)	46 y (32–58)	…	…
Race/ethnicity	White	28	51.0
	Multiracial	16	29.0
	Black	11	20.0
Education	Primary school	10	18.0
	High school	18	33.0
	University	27	49.0
Exposure variables	Knowing other people with similar symptoms during the period of acute illness	38	69.0
	Rural resident/commuter	33	60.0
	Visited creek/woods/waterfall	30	55.0
	History of recent in-state travel	27	49.0
	Uses mosquito repellent regularlya	23	42.0
	Outdoor animal let into house	10	18.0
	Animal, flea, or tick exposure	8	15.0
	Exposure to animal carcass	2	4.0
	Rural resident/commuter or Recent in-state travel or Visited creek/woods/waterfall	55	100.0
More than 1 type of exposure to OROV	Rural resident/commuter or Visited creek/woods/waterfall	50	91.0

IQR, interquartile range.

^aThe use of mosquito repellent regularly was considered positive when the participant answered “yes” or “sometimes,” and negative when answered “rarely,” or “no” to the question.

The most prevalent comorbidities were diabetes mellitus (n = 7, 13%), systemic arterial hypertension (n = 5, 9%), depression (n = 5, 9%), anxiety (n = 4, 7%), cardiovascular disease (n = 3, 5%), and obesity (n = 3, 5%). Most participants reported no comorbidities (n = 29, 53%).

Clinically, the disease presented an acute phase, with recurrence of symptoms in approximately one-third of participants (n = 17, 31%). During the acute phase, the most frequent symptoms were headache (87%), malaise (87%), fever (82%), myalgia (75%), and retro-orbital pain (55%) (Table 2). For those participants who experienced recurrence of symptoms, the most frequent during this phase were malaise (53%), fever (41%), arthralgia (41%), and chills (41%) (Table 2). Indeed, headache (P < .001), myalgia (P = .007), malaise (P = .007), anorexia (P = .007), rash (P = .03), nausea (P = .04), and fever (P = .040) were symptoms more likely presented in the acute phase than during recurrence (Table 2). An early, non-pruritic, centripetal maculopapular rash was observed, sometimes petechial, but resolved rapidly within 2–5 days (Figure 2).

Table 2.

Symptom Frequency by Phase of Illness, December 2024 to May 2025, Southeast Region, Brazil

Sign/Symptom	Acute Phase (n = 55)			Recurrent Phase (n = 17)		P valuea
	N	%		N	%
Headache	48	87.0	…	6	35.0	<.001
Malaise	48	87.0	…	9	53.0	.005
Fever	45	82.0	…	7	41.0	.004
Myalgia	41	75.0	…	6	35.0	.007
Retro-orbital Pain	30	55.0	…	5	29.0	.097
Rash	25	45.0	…	2	12.0	.020
Anorexia	25	45.0	…	1	6.0	.003
Nausea	24	44.0	…	2	12.0	.021
Arthralgia	21	38.0	…	7	41.0	1.000
Chills	20	36.0	…	7	41.0	.778
Lower Back Pain	20	36.0	…	5	29.0	.773
Photophobia	17	31.0	…	2	12.0	.206
Diarrhea	14	25.0	…	1	6.0	.100
Diaphoresis	14	25.0	…	1	6.0	.100
Pruritus	12	22.0	…	4	24.0	1.000
Cervicalgia	12	22.0	…	4	24.0	1.000
Dysgeusia	10	18.0	…	1	6.0	.440
Paresthesia	8	15.0	…	2	12.0	1.000
Abdominal pain	8	15.0	…	1	6.0	.676
Nasal congestion	8	15.0	…	2	12.0	1.000
Ocular burning sensation	8	15.0	…	3	18.0	.714
Coryza	7	13.0	…	2	12.0	1.000
Vomiting	7	13.0	…	1	6.0	.671
Edema	7	13.0	…	–	–	.187
Cough	6	11.0	…	2	12.0	1.000
Productive cough	5	9.0	…	1	6.0	1.000
Odynophagia	5	9.0	…	2	12.0	.665
Otalgia	3	5.0	…	–	–	1.000
Presyncope	3	6.0	…	3	18.0	.139
Dysuria	2	4.0	…	–	–	1.000
Choluria	1	2.0	…	1	6.0	.419
Metrorrhagia	2	4.0	…	1	6.0	.560
Petechial hemorrhage	1	2.0	…	–	–	1.000

^aSymptom frequency between the acute and recurrent phases were explored by testing for a difference in proportions using Fisher's exact test. Significant differences (P < .05) are bolded.

Figure 2.

Maculopapular and petechial rash in the acute period of Oropouche fever. A common clinical sign observed in patients from Piau, Minas Gerais, and municipalities in Rio de Janeiro state.

Differences in symptom presentation were also investigated according to SE-I and SE-II OROV clades identified. The clinical manifestation that differed the most between sites was fever, with 94% prevalence at SE-I versus 60% at SE-II (Supplementary Table 1). The other symptoms that differed significantly between sites were retro-orbital pain (more frequent at SE-I) and cervicalgia (more frequent at SE-II) (Supplementary Table 1).

Headache intensity was often moderate (n = 23/48, 48%), responding only to opioid analgesics, followed by high-intensity (n = 16/48, 33%), characterized by persistence and lack of response to analgesic therapy, and mild headache (n = 9/48, 19%), which responded to common non-opioid analgesics. In the recurrence of symptoms, headache was reported only by 6 participants (n = 6/17, 35%), predominantly of milder intensity (n = 4/6), followed by moderate (n = 1/6) and high (n = 1/6) intensities.

By assessing the dynamics of the disease, average duration of acute phase symptoms was 3 days, followed by a median asymptomatic period of 8 days (IQR 6–8), with recurrence of symptoms (when present) for another 3 days. Duration of disease phases according to clinical signs and symptoms is shown in the Supplementary material (Supplementary Table 2).

Pregnant participants (n = 5) reported more rash (80%, P = .043), ocular burning sensation (80%, P < .001), photophobia (80%, P = .028), and back pain (80%, P = .043) than non-pregnant women. Moreover, as expected, pregnant women reported nausea more frequently (100%) than those who were not pregnant (42%) (P = .010).

Participants who reported nausea/vomiting (P = .003), hemorrhage (P = .020), anorexia (P = .040), myalgia (P = .040), and rash (P = .040) in the acute phase were more likely to experience symptom recurrence (Supplementary Table 3). Further analysis of the influence of nausea, hemorrhage, and OROV IgM in the acute phase on the probability of disease recurrence is shown in Supplementary material (Supplementary Figure 3).

Hematological and serum biochemical data were largely within normal ranges in the acute phase of the disease (Supplementary Table 4). In complete blood counts, only the number of white blood cells was altered, with slight leukopenia in the first week of symptoms, resolving in the second week. Additionally, atypical lymphocytes showed a significant increase in the second week of symptoms. Elevated serum concentration of factors associated with liver function were also observed but mildly increased in most cases. Participants with nausea in the acute phase had higher transaminase levels than those not reporting nausea, but elevation was again mild (67 [IQR: 18–212] vs 47 [18–109] U/L, P = .02). C-reactive protein (CRP) was the most sensitive marker of inflammation, with a considerable increase in the first week of symptoms, largely returning to normal levels in the second week. The main laboratory changes observed in the acute phase of the disease are shown in the Supplementary material (Supplementary Table 4).

Red blood cell counts and associated parameters—including hemoglobin concentration, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, red cell distribution width-coefficient of variation—and the levels of total serum proteins, blood urea nitrogen, creatinine, bilirubin, and creatine kinase were normal along the entire disease course. In the recurrent phase, laboratory tests were performed on blood of 5 patients, but no changes were identified.

Serum OROV IgM was detected in 32 patients, ranging from 3 to 57 days after the onset of symptoms. Seven patients were seropositive within the first week of symptoms. OROV RNA was detected in 118 samples (65 urine, 22 sera, and 31 saliva). Detection of viral RNA in serum and saliva occurred primarily in the first week of the disease, and remained detectable in the urine beyond the third week after symptom onset, as shown in Figure 3A. Viral RNA was undetectable (Ct > 45) in most serum and saliva samples by the time of disease recurrence, and therefore significantly lower than levels detected in the acute phase of disease. However, no significant difference was observed between the viral load in urine samples collected during the acute phase and recurrence (Figure 3B).

Figure 3.

Dynamics of RT-qPCR cycle threshold (Ct) values in serum, urine, and saliva. A, Samples were collected during the acute phase and throughout patients’ follow-up and tested for the presence of OROV RNA. B, Ct values in serum, urine, and saliva during the acute and recurrent phases of the disease were compared by unpaired Mann–Whitney test. Ct = 45 (The maximum number of amplification cycles in RTq-PCR) was attributed to samples with undetectable OROV RNA levels. P-values < .05 were considered significant.

DISCUSSION

Our study supports previous reports of the expansion of a reassortant OROV lineage from the Amazon into Southeastern Brazil [5], including in municipalities such as Piau (Minas Gerais state), where the economy is largely based on banana cultivation and related products. A novel contribution of our findings is the suggestion that waterfalls may serve as exposure sites for the OROV's vector, which may have implications for ecotourism in Rio de Janeiro state and similar regions.

Although there is no consensus regarding OROV infection risk by sex or age, studies suggest that younger individuals are more susceptible due to the lack of prior exposure [8, 19]. In a Brazilian study, no sex differences were observed, but most cases occurred among working-age participants (20–49 years-old) [20]. In our cohort, 65% of cases were women, and most were aged 20–49 years. This is in agreement with PAHO/WHO data, showing that the highest proportion of cases recorded from January 1 to 27 July 2025, in Brazil were in the 30–39 age group [3]. However, although our study did not include children, reflecting the age group seeking care in our outpatient clinic, we believe that investigation of OROV cases among children is still needed to define the prevalence and clinical presentation in this group. The racial distribution differed from that of the general population in the affected municipalities. However, given the limited sample size, this difference may be attributed to sampling variability or reporting bias. Nevertheless, outdoor exposure to midges appeared to be a stronger indicator of infection risk than demographic characteristics. Participants frequently reported the presence of midges in their households, at work (eg, banana plantation), or in tourist areas such as waterfalls. Several case clusters were linked to ecotourism groups, contrasting with the isolated cases typically reported in periurban areas. However, whether waterfalls represent a breeding ground for Culicoides paraensis and a potential environment of OROV transmission needs further investigation.

Regarding clinical presentation, headache, malaise, and fever were the most frequent symptoms, followed by myalgia, retro-orbital pain, anorexia, and nausea, consistent with previous reports [21]. Notably, 45% of patients in our study developed rash during the acute phase, compared with the pooled prevalence of 11% reported in a recent meta-analysis of cases from 1999 to 2023 in the Amazon region [21]. This difference is unlikely to be explained by viral clade variation, as a prior study of the same reassortant lineage found a rash prevalence of only 1.2% [22]. Instead, methodological differences may explain the discrepancy: our prospective, longitudinal design allowed multiple patient assessments, increasing the likelihood of detecting rash. In extra-Amazon regions, symptom overlap between OROV and dengue may complicate empirical diagnosis. Odynophagia and abdominal pain—previously suggested as distinguishing features for dengue—were not useful discriminators in our cohort [7, 23]. Moreover, distinct clinical presentation associated with genetic differences between the 2 OROV clades identified in the study (SE-I and SE-II) should be investigated further due to the limited sample size.

Although no participant progressed to severe disease, symptom recurrence occurred in 30% of cases, consistent with previous reports [24, 25]. Literature on recurrence is limited in both clinical detail and timing. In our study population, recurrence occurred after an asymptomatic interval of 6–8 days and was characterized by malaise, fever, arthralgia, chills, headache, and myalgia with lower frequency and intensity than in the acute phase. Nausea, hemorrhage and the presence of OROV IgM antibodies during the acute phase were predictors of recurrence. Nausea was accompanied by elevated liver enzymes, indicating liver involvement. Viral loads in serum and saliva were higher during the acute phase than during recurrence, when viremia was mostly undetectable. In this recurrent phase, OROV was more frequently detected in urine than in other specimens. Whether recurrence reflects persistent viral replication—detectable in urine—or an inflammatory response to the initial infection remains unclear.

Other laboratory findings during the acute phase included leukopenia, and increased CRP. These are nonspecific and not particularly useful for distinguishing OROV from other arboviruses [26, 27]. However, the absence of thrombocytopenia may help in differentiating Oropouche fever from dengue, which remains the arboviral disease with the highest epidemiological burden. In the recurrent phase, the only notable laboratory finding was an increased frequency of atypical lymphocytes. Further research is needed to identify clinical and laboratory biomarkers for OROV.

RT-qPCR is the gold standard for OROV detection, with sensitivity dependent on the day of illness. It can detect OROV RNA in up to 93.3% of acute phase samples [28]. In our study, OROV was detectable in serum/plasma for up to 21 days after symptom onset, in saliva for over 30 days, and in urine for more than 40 days. To our knowledge, these are the longest detection periods in saliva and urine reported to date [29–32]. These findings in saliva and urine may favor the extension of OROV diagnostic window when combined with serum assessment, potentially improving diagnostic sensitivity in the later stages of illness. However, this persistence of viral RNA fragments does not confirm active viral replication and/or the presence of infectious virus particles. Therefore, studies on virus isolation are still needed to confirm this.

With respect to serology, a previous study detected OROV IgM and IgG antibodies 1–2 weeks after symptoms onset [33]. In contrast, we found IgM in over half of participants (58.2%), between 3 and 109 days after disease onset. A notable finding was that 7 (13%) patients living in Piau, Minas Gerais, thus permanently exposed, tested positive for IgM within the first week of illness. These patients had no history of travel to other regions where OROV was circulating, and this may indicate that they had a prior undiagnosed infection. Furthermore, 12 patients (21.8%) were seropositive within 2 weeks of symptom onset. Currently, the US Centers for Disease Control and Prevention recommends using RT-PCR to diagnose OROV in the first week of illness and neutralization antibody tests subsequently [34]. Due to the limited availability of neutralization tests, perhaps it could be useful to incorporate binding antibody IgM tests into the diagnostic algorithm. This would require collecting more data on IgM kinetics from cohort studies to provide an evidence base for recommendations about laboratory diagnostics.

Our limitations include the small sample size, variations in data collection resulting from fieldwork constraints, and the inability to follow pregnant women due to the study timeline. Strengths of the study include its prospective design, structured data collection, and active follow-up, which likely improved the accuracy and completeness of clinical monitoring compared with reliance on passive surveillance. Another strength was the longitudinal analysis of multiple biological specimens highlighting the underappreciated diagnostic utility of urine.

CONCLUSIONS

In our study, Oropouche fever cases appeared in clusters linked to individuals who either lived in or visited forest areas, particularly near banana plantations and waterfalls. The prevalence of rash during the acute phase was high. Symptom recurrence was not associated with a second peak in viral load or with hematological or hepatic abnormalities. Extending specimen collection to a broader range of sample types and time points after onset could improve diagnostic yield. These findings may guide clinical management and inform preventive strategies for stakeholders in ecotourism and agriculture to help control OROV transmission.

Supplementary Data

Supplementary materials are available at Open Forum Infectious Diseases online. Consisting of data provided by the authors to benefit the reader, the posted materials are not copyedited and are the sole responsibility of the authors, so questions or comments should be addressed to the corresponding author.