Healthy. Dengue: explosion of cases in South America. It’s Italy?

More than three million and 500,000 probable cases and just over 1,600 deaths, in just under four months. Are the dengue numbers in Brazilwhich from the beginning of the year to date have already exceeded the total infections of 2023. During a meeting held on April 17 at the headquarters of the Pan American Health Organization (PAHO) – Advances and Perspectives in Tackling Dengue –The Ministry of Health However, he underlined that the country is starting to show a downward trend in cases, with ten states recording a decline in infections and 12 stable ones. Only in the territories of Paraná, Bahia, Maranhão, Ceará and Sergipe is there reportedly still an increasing trend. “We are starting to see a significant reduction in cases – he underlined Ethel Macielsecretary of health and environmental surveillance at the ministry – but the numbers are still high”.

At the moment there are no specific therapies for the treatment of the disease, but two vaccines are available, Dengvaxia and Qdenga. For this reason, in response to the emergency, the State purchased 5.2 million doses of the vaccine tetravalent Qdenga, while 1.32 million were provided free of charge to the country: in this way the world’s first public vaccination campaign against the disease began in February. Some, however, believe that the effort is still too modest to resolve the situation. “To have an impact on rates – declares Ana Lúcia de Oliveira, infectious disease expert at the Federal University of Mato Grosso do Sul in Campo Grande, Brazil -, there is a need for mass vaccination.” And even large-scale vaccination, according to researchers, will not achieve decisive results if basic sanitation problems are not addressed. There are those who also trust in a future other vaccinenot yet approved but under development at the Butantan Institute in São Paulo which, in a clinical trial, demonstrated an overall efficacy of 80% against symptomatic dengue with a single dose.

Not just Brazil

Over the last 20 years the global incidence of dengue has significantly increased: from 2000 to 2019, global cases even increased tenfold, going from 500,000 to 5.2 million people infected by the virus. And the World Health Organization believes it is likely that real cases are underestimated, considering that they are largely asymptomatic and that notification is not mandatory in many countries. In 2023, most infections were reported in the region of the Americas, with a total of 4,565,911 cases, of which 7,653 serious and 2,340 deaths. This year Paho has already confirmed more than 5.2 million cases of dengue in the Americas, driven by Brazil as seen, followed by Argentina, Peru and Paraguay.

The disease is transmitted by infected mosquito bites (mainly Aedes aegypti, but also Aedes albopictus or tiger mosquito) and in the majority of cases it occurs asymptomatic or with mild symptoms. “The dengue virus is a bit particular – observes Claudia Fortuna, virologist at the National Reference Laboratory for arboviruses at the Department of Infectious Diseases of the Istituto Superiore di Sanità –. If it is true that in most cases the disease is asymptomatic or flu-like and mild, when a person is infected for the second time paradoxically they are not protected more efficiently by the immune system, but have a greater risk of developing the disease serious, because our antibodies are unable to neutralize the virus responsible for the second infection. The virus thus replicates more efficiently, with higher titres in the blood, and can give rise to a severe clinical picture, up to the hemorrhagic form of the disease that is so worrying and frightening. The risk, therefore, relates above all to secondary infection, which we must avoid by limiting the spread of this virus.” It is no coincidence that the World Health Organization, considering the growing danger of transmission and the increase in cases and deaths, has assessed the dengue risk as high globally.

Behind the increase in cases

The reasons behind the significant increase in dengue virus infections globally, and in South America in particular, must be sought in multiple directions. “First of all, climate change and the increase in human activities – underlines Cristiano Salata, professor of microbiology and virology at the University of Padua – are interfering with natural ecosystems and are favoring the proliferation of mosquitoes on territories that were previously free from them ”. Precisely the expansion of the distribution of vectors, combined with population growth, is one of the factors associated with the increased risk of spreading dengue virus infection. “There is a close association between humans and animals – underlines the teacher – and this generally applies to all zoonoses”.

A phenomenon like El Nino, which causes strong warming of the surface waters of the Pacific Ocean, may have significant impacts. In 2023, for example, has provoked a severe drought in the northern region of Brazil and this has forced the population to store water for consumption, increasing the number of places with stagnant liquids and potential mosquito breeding sites. In the southern part of the country, however, numerous floods took place, which caused an accumulation of water and therefore, also in this case, suitable sites for the proliferation of insects.

It must also be taken into account that dengue occurs mainly in urban contexts, where the population density is often high. “Cities are growing – observes Marcia Castro, of the Harvard TH Chan School of Public Health in Boston, Massachusetts – and are becoming havens for mosquitoes.” In the face of growing urbanisation, however, sanitation infrastructures end up being inadequate: uncollected waste becomes a breeding ground for insects, as does stagnant water.

According to the WHO also the weakness in surveillance systems of many countries affected by dengue can lead to delays in reporting cases and a failure to identify symptoms, contributing to the increase in severe cases. “When arboviruses are endemic – underlines Claudia Fortuna – it is difficult to apply the same rules and tools that are used when dealing with a single imported case or a small outbreak. In South America there are many arboviruses and control is more difficult. These are areas perhaps close to the Amazon forest where monitoring and control is more difficult. Furthermore, funds are needed. The situation in those countries is clearly more complicated.”

Ssituation in Italy

“In Brazil – claims Cristiano Salata – cases of dengue are very numerous and this poses risks on a global level, also due to the increase in international travel. Infected mosquitoes, for example, could be transported with goods. Or, those traveling in endemic areas can contract the virus and help circulate it once they return to their country, in Italy for example, encouraging its inclusion in the ecosystem”.

In the WHO European region, in recent years there has been no shortage of outbreaks of indigenous dengue in some states including Croatia, France, Portugal, Spain and Italy. In our country, dengue is one of the arboviruses subject to surveillancetogether with West Nile, usutu, chikungunya, zika, tick-borne encephalitis and neuro-invasive Tuscany virus infections, as part of the National Plan for prevention, surveillance and response to Arbovirosis (PNA) 2020-2025″. It must be said that the mosquito Aedes aegyptithe main vehicle for transmitting the disease, is not present on Italian territory, while the Aedes albopictus (the tiger mosquito). From January 1st to April 8th 2024, there were 117 cases of dengue virus infection, all linked to travel to endemic areas. There have been sporadic local cases in the past: the first outbreak of its kind dates back to August 2020 with 11 cases in the Veneto region (northeastern Italy), caused by a DENV-1 viral strain closely related to a previously described strain circulating in Singapore and China.

“In Italy, every case that is imported has been monitored for years – argues Claudia Fortuna – but recently, precisely in the face of this emergency situation in Brazil, several circulars have been issued to update the Plan: in airports and ports, for example, an increase in checks is expected not only against the vector, through the disinfestation of vehicles coming from the areas where the epidemic is occurring, but also of those returning to Italy from endemic areas. And clinicians were alerted, precisely to increase attention towards those particular symptoms which could then be traced back to an infection from an endemic area”.

Fortuna explains that in the event of infection in our country we proceed with a disinfestation operation around the area where the human case is present and this reduces the possibility that a mosquito could bite the infected patient and subsequently transmit the disease to others. Dengue in particular, but also other arboviruses, have a fairly short window of presence in the blood, so if you intervene in the early stages of the infection you can quickly avoid the spread of the disease.

“At this moment – ​​concludes Fortuna – we must be vigilant and citizens have an important role. The hot season is about to begin, mosquitoes in the area will increase and with them the risk of possible epidemic outbreaks.” Small precautions can make a difference: “For example, it is important to avoid leaving stagnant water (such as in saucers), where the mosquito can lay its eggs and where the larvae develop. Travelers returning from endemic areas must be particularly attentive to possible even mild symptoms which may however suggest the development of a possible infection. These are small precautions, but if we are all careful the risk is eliminated.”