Dengue: Cases rocket worldwide and the disease becomes a global issue

Publicação: 2 de October de 2019

Even with the scientific knowledge and technological resources currently available, the elimination of this vector seems unviable today

Pan American Health Organization has triggered epidemiological alert in Latin America after the region reports more than two million cases of dengue this year, the highest number since the 2016 major outbreak

From January to August 24, 2019, Brazil registered 1,439,471 dengue cases, an increase of almost 600% over the same period last year, when the country had confirmed 205,791 cases. On September 9 the Pan American Health Organization (PAHO) activated the epidemiological alert in Latin America after the region reported more than 2 million cases of dengue this year, the highest since the major outbreak of 2016.

For the infectologist and doctor of Tropical Medicine, Pedro Tauil, it would be possible to prevent major dengue epidemics with improved vector control. The expert points out that the Aedes aegypti is, for now, the only vulnerable link in the chain of transmission. Improved control measures depend on population participation, improved housing and sanitation conditions in urban areas. “Slums, “mocambos”, invasions and tenements need to be reduced. Never before has there been such a high concentration of population in urban areas. About 2 billion people in the world today live in risk areas in tropical and subtropical countries. In addition, strategic points, such as vacant lots, cemeteries, tire repair shops, junkyards, should be subject to frequent entomological inspection by health surveillance agencies, Dr. Tauil points out. But he acknowledges that, as a task that has not been effective, innovation in this control is being sought, as the Australian technique being tested in several countries, including Brazil, of Aedes infected with the Wolbachia bacteria. These insects become infected with arboviruses, but do not become infective, disrupting the cycle.

While control measures remain cautious in the country, the disease continues to affect people. Until August 24, 591 deaths have been confirmed and 486 are still under investigation. Although a new dengue vaccine is in its final testing phase, as Health Minister Luiz Henrique Mandetta said, it is expected to end the final phase of developing human tests only by 2020. “We are very hopeful about the vaccine and hope it will arrive next year. Until then, we will have a very hard summer, with great difficulty due to the reintroduction of dengue serotype 2 which has since 2018, found many people susceptible to the disease”, said the minister. Until the new Brazilian vaccine becomes a reality, early diagnosis and treatment are the only ways to reduce the lethality of the disease, as there are no safe and effective vaccines.

If we go back in time, in a not so distant Brazil, we will see that for almost 60 years, from 1923 to 1982, the country did not register dengue cases in its territory. Brazil did not lack ambition to eliminate Aedes aegypti. In the 1950s, the mosquito was eradicated at a time when dichlorodiphenyltrichloroethane (DDT) could be used, a powerful pesticide, now banned for its environmental impact. “Brazil and 17 other countries in the Americas eliminated Aedes from their territories to reduce the risk of urban yellow fever. But later, all were reinfested. International transport of goods and people has greatly increased in density and speed, allowing the transport of infected people and mosquitoes much more frequently today”, emphasizes Dr. Tauil while pointing out the need to seek innovation in vector control measures in order to become more effective.

Dengue is currently one of the major reemerging diseases and has become a global problem making no tropical or subtropical country successful in its control. Southwest Asia, for example, faces a major outbreak, and the Philippines has the largest number of confirmed cases and deaths. This is the worst dengue outbreak in the country since 2012, killing more than a thousand people and leading to the declaration of a national epidemic. Honduras is also facing one of the deadliest epidemics of the past decade, as warned by the Pan American Health Organization (PAHO). According to reports, the country has registered since January more than 70 thousand suspected cases, of which 14.381 were reported as severe dengue, and 135 deaths were confirmed. In Central America, Honduras tops the highest number of deaths from severe dengue, followed by Guatemala (37), Nicaragua (15) and El Salvador (4), according to figures released by the ministries of health of these countries. In addition to these regions, there are epidemics of the disease in Colombia, Bangladesh, Nepal, Pakistan, among others. Dr. Tauil explains that whenever one of the 4 dengue serotypes is reintroduced in a region, it encounters a population not immune to it, the number of cases increases. “Naturally acquired immunity is serotype specific. Thus, the existence of a mass of people already infected with a serotype acts as a protective mass against the spread of the disease”, he adds.

In addition to the Aedes proliferation and dissemination facilities offered by current urban living conditions, insect resistance to the most widely available larvicides and insecticides are other limitations imposed on their control. For Dr. Tauil, the solution is to reduce the infestation density as much as possible and to seek more effective means of vector control, such as new effective insecticides. The deterioration of public health infrastructure, with reduced human and financial resources, which occurs in the vast majority of countries is also a contributing factor. However, considering the scientific knowledge and technological resources currently available in relation to dengue, the elimination of this vector from large and medium-sized cities seems to be impossible today.

According to the expert, dengue control is labor intensive and has a very high cost. It relies on multisectoral actions, as sewage, housing and education, which are not easily accomplished. The current complexity of urban life facilitates the proliferation of vectors and makes their control difficult. New control techniques are needed, as well as the discovery of safe and effective vaccines and antiviral drugs. “In Brazil, Fiocruz has been working on the Wolbachia project and the irradiated mosquitoes project. The Butantã Institute in São Paulo is working on tests for a new vaccine. There are researchers looking for new antiviral drugs to treat patients”, he details.

Aerial spraying of insecticide against the dengue mosquito

In September, the Federal Supreme Court (STF) decided to maintain the validity of aerial insecticide spraying to combat Aedes Aegypti. Upon resuming the analysis of the case, the Supreme Court considered that aerial spraying is valid, but prior authorization by health authorities will be required, as well as a statement from the environmental authority. In the case under judgment, the Attorney General’s Office (PGR) filed a lawsuit in 2016 seeking to declare unconstitutional the provisions of the law that allowed the execution of necessary measures to control the mosquito. The law establishes as one of the ways to disperse chemical substances by aircraft upon approval of health authorities and scientific proof of the effectiveness of the measure. But in the opinion of the specialist in Tropical Medicine, aerial spraying is low in effectiveness and contaminates the environment. For him, it is a resource to be used ultimately in times of major epidemics.