Dengue Outbreak in Europe, Rise in Local Cases Raises Concerns

In a surprising turn of events, 2023 witnessed an alarming surge in locally transmitted cases of dengue fever on the European mainland. While travel-related cases are not uncommon, the spike in non-imported cases is the highest in nearly eight decades. This article delves into the factors contributing to this unexpected rise, exploring the dynamics of dengue transmission, the role of climate change, and the challenges in vector control.

Dengue Outbreak in Europe Historical Context

Examining the period between 2015 and 2019 reveals a stark contrast. European countries with established Aedes mosquito populations saw approximately 3,000 cases of travel-related dengue but only nine cases of locally transmitted dengue.

However, in 2022, the number of local cases surpassed the cumulative figures of the past 70 years, reaching 65 cases in France alone. The situation was exacerbated in 2023, with over 110 cases reported, primarily in France and Italy, and a few in Spain.

Dengue Transmission 

Understanding the spread of dengue involves unraveling a complex web of factors. Dengue primarily transmits through the Aedes mosquito, with Aedes albopictus being the most prevalent species in Europe.

For local transmission to occur, these mosquitoes must establish themselves in the community, thriving in temperatures between 15 and 35 degrees Celsius during the warmer months.

How Dengue Spreads

• Mosquito Establishment: The Aedes mosquito population, especially the albopictus species, first detected in Europe in the early 2000s, has since spread across the Mediterranean and Central Europe. As of 2023, it has been established in various European countries, posing a significant threat.
• Climate Dependency: Dengue transmission is dependent on warm temperatures. Higher temperatures, both during the day and night, extend the breeding period for mosquitoes, resulting in a higher abundance of these vectors.
• Virus Introduction: Dengue is not endemic to Europe, making the introduction of the virus a critical factor. Travelers returning from dengue hotspots abroad brought the virus back home, setting the stage for local transmission.

Rise in Local Cases

Experts are grappling with the unprecedented rise in locally transmitted dengue cases. Various factors might contribute to this surge, and while some theories exist, a clear answer remains elusive.

Threshold Phenomenon: A Multifaceted Challenge

• Weather Dynamics: Professor Thomas Jaenisch, a global health expert, suggests a threshold phenomenon. While temperatures have been on the rise, other factors are acting synergistically. Extended periods of high temperatures provide an optimal environment for mosquito breeding.
• Mosquito Population Growth: The Aedes albopictus population has expanded its reach since the early 2000s. However, their limited dispersal range, approximately 100 meters, may explain the delayed surge in cases as they gradually spread to new areas.
• Increased Travel-Related Transmission: The global increase in dengue cases outside Europe amplifies the risk of travelers bringing the virus home. Dengue cases worldwide surged from 500,000 in 2000 to 5.2 million in 2019, with estimates suggesting the actual number may be close to 400 million annually.
• Vector Control Challenges: Efforts to control the Aedes mosquito face challenges, including public awareness and suitable control measures. Instances like the inappropriate use of insecticides, as observed in Paris, highlight the need for informed strategies.

Climate Change and Dengue Dynamics

The impact of climate change on the transmission dynamics of dengue cannot be overstated. Higher temperatures create favorable conditions for mosquito breeding and the survival of the dengue virus.

This shift is not only restricted to daytime temperatures but also includes warmer nights, allowing mosquitoes to remain active for extended periods.

Breeding Grounds for Dengue

• Temperature Extremes: Prolonged periods of high temperatures, characteristic of climate change, contribute to longer mosquito breeding seasons. Extended summers and late falls provide mosquitoes with more time to breed, resulting in increased vector populations.
• Changing Climate Patterns: Altered climate patterns may facilitate the establishment of Aedes mosquito populations in new areas. This expansion, combined with heightened temperatures, creates a conducive environment for the spread of dengue.

Role of Global Dengue Dynamics

The interconnected nature of global dengue dynamics plays a crucial role in the rise of local cases in Europe. The increase in dengue cases worldwide, coupled with heightened travel, intensifies the likelihood of virus introduction into European communities.

Global Dengue Trends and Travel-Related Transmission

• Doubling Dengue Cases: Dengue cases outside Europe have doubled from 2010 to 2022, contributing to a higher risk of travelers acquiring the virus abroad.
• Underreporting Challenges: The World Health Organization (WHO) estimates that reported cases likely represent only a fraction of the actual numbers, emphasizing the need for enhanced global surveillance and reporting mechanisms.
• Challenges in Vector Control and Public Awareness
• Efforts to control the Aedes mosquito face challenges, ranging from limited public awareness to the implementation of effective control measures. As seen in the case of Paris, improper use of insecticides underscores the importance of informed and targeted vector control strategies.

Vector Control Strategies and Public Awareness

• Community preparedness: European communities may lack sufficient knowledge on responding to emerging public health threats. Increased public awareness is crucial for timely and appropriate responses to mitigate the spread of dengue.
• Doctor Awareness: Doctors across Europe must enhance their awareness of dengue symptoms, considering that most cases are mild or asymptomatic. Recognizing these cases becomes challenging without a heightened understanding of the disease.
• Strategic Vector Control: Targeted vector control measures should be implemented based on the behavior and activity patterns of the Aedes mosquito. Fumigation, for instance, should align with the species’ activity hours.

Stochastic Events and Preparedness

As experts grapple with the current dengue outbreak, uncertainties surround the future trajectory of the disease in Europe. Professor Jaenisch acknowledges the possibility of a stochastic event, emphasizing the need for continuous monitoring and preparedness.

Monitoring and Future Preparation

• Stochastic Events: The current surge in cases may represent a stochastic event, necessitating ongoing monitoring to discern patterns and anticipate future outbreaks.
• Adaptive Strategies: Developing adaptive strategies in response to changing climate patterns, global dengue trends, and mosquito population dynamics is imperative for effective preparedness.

Conclusion

The unprecedented rise in locally transmitted dengue cases in Europe demands a comprehensive and collaborative response. Understanding the intricate web of factors contributing to the outbreak, from climate change to global dengue dynamics, is essential for formulating effective preventive measures. As Europe grapples with this public health challenge, the importance of continuous research, vigilant monitoring, and informed public engagement cannot be overstated.

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