2024-02-14 17:20:51
by Giovanni Rezza *
What causes the increase in cases of tropical diseases spread by “our” mosquitoes? The globalization of carriers and men plays a fundamental role. As well as climate change
In the two-year period 1927-1928, Athens was shocked by a large dengue epidemic, transmitted by Aedes aegypti, a tropical mosquito also widespread in the eastern Mediterranean at the time. After a few years, Aedes aegypti disappeared from temperate climate areas and for decades no more dengue epidemics were observed. In the 1990s, however, another exotic mosquito, Aedes albopictus, the so-called Asian “tiger mosquito”, was introduced into Italy and began to establish itself in much of the peninsula. The first alarm bell rang in 2007, when a tropical virus called chikungunya, transmitted by the tiger mosquito, caused an epidemic outbreak of an acute disease – characterized by fever and severe joint pain – in Romagna. Exactly ten years later, in 2017, a few hundred cases of chikungunya were identified in Lazio, with a secondary outbreak on the Ionian coast of Calabria.
Cases with autochthonous transmission
Meanwhile, sporadic cases and small outbreaks of locally transmitted dengue reappeared in the Mediterranean, this time in southern France and the Croatian coast. In the summer of 2020, when all attention was focused on the Covid-19 pandemic, a small indigenous dengue outbreak was identified in Veneto. But the real change of pace occurred last summer, when a few dozen cases of dengue with autochthonous transmission (i.e. transmitted by Aedes albopictus mosquitoes present in our territory, and not simply “imported”) were diagnosed in Lombardy and in Lazio. What is happening? What causes this increase in cases of tropical diseases spread by “our” mosquitoes?
The role of the tiger mosquito
The answer is not easy, but the globalization of vectors (in this case Aedes albopictus) and humans (in particular fast transport which allows an infected person to reach distant locations in a matter of hours) plays a fundamental role. Therefore, if an epidemic breaks out in Asia rather than in Latin America, the risk that cases will arrive sooner or later varies depending on the number of travellers. But then, once it arrives here, the virus finds a competent vector, that is, in this case the tiger mosquito (different from the Culex mosquito, the one which, to be clear, annoys us at night but which in any case in some areas of Italy can transmit cat fever West Nile), ready to transmit viruses such as chikungunya and, to a lesser extent, dengue.
Climate change
Another factor that could come into play, even if so far it has not been particularly relevant for us (after all, Italy has always been a “part-time” tropical country) is climate change. In fact, Aedes albopictus is slowly advancing towards Northern Europe. At the same time, it is no longer so rare to find the presence of mosquitoes in our homes even in the winter period. Therefore, if until now only the early introduction of a virus at the beginning of summer could lead to a large-scale epidemic, with the lengthening of the hot season the risk that viral circulation persists for a long time is increasingly greater. Finally, the tropical mosquito Aedes aegypti, which transmits the dengue virus much better, has already established itself in Cyprus, appearing again in the Mediterranean.
The measures in Italy
This is why the dengue epidemic that is currently affecting Brazil has meant that, in our country, it was decided to raise the level of attention and controls, as well as considering the opportunity to keep doses of the new Vaccine. Because one thing is clear: reducing the risk of importing competent carriers and promptly identifying cases is more important today than in the past.
* Professor of Hygiene, Vita-Salute San Raffaele University, Milan
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February 14, 2024 (modified February 14, 2024 | 6:18 pm)
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