Genetic diversity of dengue virus serotype 1 associated with rare dengue ophthalmic syndrome in Reunion Island, Southwestern Indian Ocean, 2020-2022

A study of 447 hospitalized patients in Reunion Island (2020–2022) suggests that a specific genetic cluster of dengue virus serotype 1, distinct from other co-circulating strains, may be responsible for the rare ocular complications observed during the outbreak.

The Gist

🌴 The Big Picture: A Tropical Storm with a Strange Side Effect

Imagine Réunion Island (a tropical paradise in the Indian Ocean) as a busy airport. For years, it had been dealing with a specific type of "traveler" called Dengue Fever, a virus carried by mosquitoes. Usually, this virus causes a bad fever, but in 2020–2022, something weird happened.

While the virus was spreading, doctors noticed a strange new side effect: people were losing their vision or seeing spots (scotomas). It was like the virus wasn't just attacking the body; it was sneaking into the "camera" (the eyes) and fogging up the lens.

The scientists wanted to know: "Is there a specific 'bad apple' strain of the virus causing these eye problems, or is it just bad luck?"

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🔍 The Investigation: Sorting the Suspects

The researchers acted like detectives at a crime scene. They gathered blood samples from 447 sick people. Some had the usual fever, but 43 of them had the scary eye symptoms.

They took the virus from these blood samples and looked at its genetic code (its DNA blueprint). Think of the virus as a car, and the genetic code as the engine manual. They wanted to see if the "eye-sick" cars had a different engine manual than the "fever-only" cars.

The Findings: Two Different Models

They found that two different "models" of the Dengue virus were driving around the island at the same time:

1. Model A (Genotype I): The most common one.
2. Model B (Genotype V): A rare one that had just arrived from Asia.

The Smoking Gun:
Every single person with the eye problems was infected with Model A.
The people infected with Model B only had the fever; their eyes were fine.

Analogy: Imagine a city where two types of delivery trucks are driving around. One type (Model A) is dropping off packages that accidentally knock over people's glasses. The other type (Model B) just drops off packages normally. The scientists realized that only the Model A trucks were the ones knocking over the glasses.

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🔬 The Deep Dive: Finding the "Defect" in the Blueprint

Since all the eye-sick people had Model A, the scientists zoomed in even closer. They looked at the specific letters in the virus's genetic code to see if there was a tiny typo that made it dangerous to eyes.

They compared the "Model A" viruses from the sick people against other Model A viruses that didn't cause eye problems.

What they found:
They discovered 11 specific "typos" (mutations) in the virus's code that were present in the eye-sick group but missing in the others.

• Most of these typos were small changes, like swapping a red Lego brick for a slightly different shade of red.
• One big typo stood out: A change in a part of the virus called NS5 (which acts like the virus's engine). This change swapped a "polar" part for a "non-polar" part.
  • Analogy: Imagine the virus's engine is a machine that usually uses water to cool down. This specific typo changed a water-cooled part to an oil-cooled part. It's a fundamental change that might make the engine run hotter or differently, potentially allowing the virus to sneak into the eye more easily.

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🧩 The Conclusion: It's Not Just One Thing, But a Clue

The scientists concluded that while they can't say 100% "This one typo causes blindness," the evidence is very strong.

1. The Culprit: The specific strain of Dengue virus (Genotype I) that took over Réunion Island in 2020–2022 is the one linked to the eye problems.
2. The Mechanism: This strain has a unique set of genetic "tools" (mutations) that other strains don't have. One of these tools, located in the virus's engine, might be the key that unlocks the door to the eye.
3. The Future: The virus is like a master thief who changed its lock-picking tools. The scientists now know what the new tools look like. The next step is to build a lab experiment to prove exactly how these tools break into the eye.

🏁 The Takeaway for Everyone

This study is like finding a specific fingerprint at a crime scene. It tells us that not all Dengue viruses are the same. Some have evolved to be sneakier and target our eyes. By identifying the specific genetic "fingerprint" of this dangerous strain, doctors and scientists can now:

• Watch out for this specific strain in the future.
• Develop better tests to spot it early.
• Work on treatments that stop this specific "engine" from attacking our vision.

In short: The virus didn't just get stronger; it got smarter and found a new way to hurt us. Science has now caught a glimpse of how it did it.

Technical Summary

1. Problem Statement

During the 2018–2022 dengue outbreak on Reunion Island, clinicians observed an unusual surge in ocular complications (ophthalmic syndrome) among patients, including scotoma, sudden visual acuity loss, and macular involvement. While ocular complications in dengue are known globally, their incidence in Reunion was notably high, and the viral determinants driving this specific tropism were unknown. Previous studies lacked full-genome sequencing or genotyping of strains associated with these cases. The study aimed to investigate whether specific genetic variants of Dengue Virus Serotype 1 (DENV-1) were responsible for these rare ophthalmic manifestations.

2. Methodology

The researchers employed a retrospective cohort study design combined with advanced genomic sequencing and phylogenetic analysis.

• Cohort and Sampling:
  • Source: The CARBO cohort (Cohort of patients infected by an ARBOvirus) established at the University Hospital of Reunion Island.
  • Population: 447 hospitalized patients (2019–2021), including 81 with confirmed ophthalmic manifestations.
  • Selection: A subset of 129 cases (43 ophthalmic, 86 non-ophthalmic) met criteria for molecular investigation. Samples were collected on average 6.6 days post-symptom onset.
• Molecular Diagnosis & Sequencing:
  • RT-qPCR: Used for viral RNA detection and serotyping (DENV-1 vs. others).
  • Genome Amplification: Full-genome amplification via amplicon tiling (targeting 4 overlapping regions) and E-gene amplification for samples where full genomes failed.
  • Sequencing Platform: Oxford Nanopore Technologies (Mk1C device, R9.4.1 flowcells).
  • Data Processing: Basecalling with Guppy, alignment to reference GCF_000862125.1, and consensus generation using minimap2 and Medaka.
  • Output: Generated 23 complete coding sequences (CDS) and 10 E-gene sequences from the 2021 cohort, plus 8 previously published sequences.
• Phylogenetic & Mutation Analysis:
  • Tree Construction: Maximum-likelihood trees built using IQ-Tree (GTR+F+I+G4 model) for both CDS and E-gene datasets.
  • Clustering: Hierarchical Bayesian Population Structure analysis (hierBAPS) to group sequences by nucleotide similarity.
  • Mutation Screening: Compared the "ophthalmic ingroup" (strains from the 2019–2022 outbreak cluster) against "outgroups" (closely and distantly related genotype 1 strains) to identify conserved amino acid (AA) substitutions present in ≥95% of the ingroup and absent/infrequent in outgroups.

3. Key Contributions

• First Genomic Characterization: This is the first study to perform whole-genome sequencing of DENV-1 strains specifically linked to ophthalmic complications in the Indian Ocean region.
• Epidemiological Insight: It documents the shift from DENV-2 dominance to DENV-1 dominance in Reunion Island and characterizes the genetic heterogeneity of DENV-1 introductions over the last two decades.
• Genotype-Specific Association: It establishes a strong correlation between a specific DENV-1 Genotype I cluster and the observed ophthalmic syndrome, distinguishing it from co-circulating Genotype V strains.

4. Results

A. Genomic Diversity and Phylogeny

• Co-circulation: Two distinct genotypes of DENV-1 were identified: Genotype I (predominant) and Genotype V (Asian lineage).
• Genotype I (Outbreak Group): 32 sequences from 2021 and 7 from 2019–2020 clustered within a highly homogeneous Genotype I group (>99.99% AA identity). This group is phylogenetically distinct from earlier Reunion strains and closely related to strains from continental Asia and Sri Lanka.
• Genotype V: Only 2 sequences were found, closely related to strains from Mayotte (2019) and Reunion (2020).
• Clinical Correlation: 100% of the sequenced ophthalmic cases were infected with the Genotype I outbreak cluster. In contrast, the two Genotype V cases showed no ocular manifestations.

B. Mutation Analysis

• Unique Mutations: The study identified 11 conserved amino acid mutations specific to the Genotype I outbreak group (the "ophthalmic ingroup").
• Location: All mutations were located in non-structural proteins (NS1, NS2B, NS3, and NS5).
  • NS5 (RNA-dependent RNA polymerase): 6 mutations, including the most significant S566A.
  • Other proteins: NS1 (R214K, N293S), NS2B (I44M, V106I), NS3 (E12K).
• Significance of S566A: The S566A mutation in the RdRp domain is a non-conservative substitution (polar serine to non-polar alanine), potentially affecting viral replication fidelity, enzymatic function, or host interaction.
• Distribution: These mutations were absent or infrequent in the outgroups (closely related and distant Genotype I strains), suggesting they are specific to the 2019–2022 outbreak lineage.

C. Clinical Demographics

• Ophthalmic cases were significantly more likely to be severe (83.7% severe vs. 30.2% in non-ophthalmic cases).
• The median age of ophthalmic patients was lower (35 years) compared to non-ophthalmic patients (54 years).

5. Significance and Limitations

Significance:

• Viral Determinants: The findings suggest that the rare ophthalmic complications in Reunion are not random but are associated with a specific viral genetic cluster (Genotype I) harboring unique mutations in non-structural proteins.
• Mechanism Hypothesis: The presence of non-conservative mutations in the RdRp (NS5) suggests a potential mechanism for altered viral fitness or tissue tropism, specifically targeting ocular tissues.
• Public Health: Highlights the need for genomic surveillance to detect emerging viral variants that may cause atypical or severe clinical presentations.

Limitations:

• Sample Size: The number of sequenced ophthalmic cases (n=12 from 2021) and Genotype V cases (n=2) was small, limiting statistical power.
• Causality: The study identifies an association, not causation. Functional studies (e.g., reverse genetics) are required to confirm if these mutations directly cause ocular tropism.
• Host Factors: The study did not fully account for host immune status (e.g., secondary infection via Antibody-Dependent Enhancement), which is a known risk factor for severe dengue and ocular involvement.

Conclusion:
The study provides compelling molecular evidence linking the 2019–2022 DENV-1 Genotype I outbreak in Reunion Island to a cluster of rare ophthalmic complications. The identification of 11 specific mutations, particularly the non-conservative S566A in the NS5 protein, offers a target for future functional research to understand the mechanisms of dengue-associated ocular disease.