BUENOS AIRES, Apr 21 (Tierramérica) - The population's susceptibility to suffering more severe forms of dengue is worrying health experts, as the epidemic in South America expands in Argentina, Bolivia, Brazil and Paraguay.
Dengue is a viral illness that manifests itself in different ways for each person, and can be much more severe for some than others, depending on risk factors. The biggest danger, say experts, is falling ill with dengue a second time.
According to the Pan-American Health Organisation (PAHO), the total cases of infection reported in the Americas fell 5.5 percent last year with respect to 2007, from 900,754 to 850,769, with most of the cases occurring in Brazil and Bolivia.
However, in the same period, the proportion of serious cases jumped 46 percent, and the number of deaths increased 84 percent - from 317 to 584 -, revealing a greater presence of severe manifestations of the fever caused by one of dengue's four virus strains.
That trend has continued this year. From January to early April, PAHO reported more than 215,000 cases in the Americas, many of them in South America - in Argentina, the epidemic is in full swing - and a new hike in deaths, from 1.2 percent of those infected in 2007 to 2.2 percent so far in 2009.
People are infected with dengue by a virus-carrying Aedes aegypti mosquito, which breed in clean, stagnant water in tropical and subtropical regions around the world.
It is thought that after recovering from an initial infection of dengue, the individual is immune to that strain for life. However, that person becomes even more vulnerable to the other strains of dengue.
An estimated 20 to 50 percent of dengue cases are asymptomatic or go undiagnosed because the symptoms - fever, headache, and muscle and joint pain - are confused with the flu.
But other patients develop slight haemorrhaging, a rash or bleeding gums. The most severe cases, of dengue hemorrhagic fever (DHF) and dengue shock syndrome, are more frequent among those who are suffering a second infection.
In Brazil, the Ministry of Health reported that in the first 10 weeks of this year, there were 114,355 cases, of which 603 were severe and 23 ended in death.
In Bolivia, Juan Carlos Arraya, head of the Health Ministry's vector-transmitted diseases, confirmed for Tierramérica that there have been 56,530 cases, the greatest number recorded in the last 20 years in that country. "Likely dengue hemorrhagic fever" affected 174 people, and 25 people died.
PAHO reported 2,277 cases in Paraguay, five of them severe, although they were not diagnosed as DHF. That country has suffered severe epidemics in recent years, with 28,000 cases, 17 of which were fatal, in 2007.
Paraguay's Ministry of Health says that because those who have already overcome a first infection are vulnerable, there are 600,000 people (10 percent of the population) in danger of contracting DHF.
Health authorities in Argentina had recorded more than 12,500 cases by mid-April, although provincial governments and hospital staff believe the real total is three times higher.
This year's is the worst epidemic since the disease reappeared in Argentina in 1998, with five confirmed deaths so far. There have been multiple complaints of delays in releasing official information.
The general outlook has put epidemiologists and virologists on alert. But experts in Argentina and Brazil consulted by Tierramérica disagree on the factors that make people vulnerable to the most serious type of dengue.
The main concern in Argentina is the threat of second infection, because of the higher risk of contracting DHF, which is difficult to control in children ages two to 14, the elderly, or the chronically ill, especially if they do not receive immediate treatment.
But Brazil's Oswaldo Cruz Foundation (Fiocruz) warns that the most serious cases can also manifest in a first-time infection.
In serious cases, the infected individual requires hospitalisation due to increased vascular permeability, loss of plasma and reduced platelets, which are needed to help blood coagulate.
Although PAHO underscores that studies demonstrate that sequential infection heightens the risk of DHF, infectious disease expert Dr. Rogério Valls de Souza of Fiocruz says "there is no definitive data" about how many people are vulnerable to a severe case of dengue because they already had the disease.
"Dengue is related to the type of virus that is circulating. For example, in the Rio de Janeiro epidemic - in the 2008 southern hemisphere summer - the virus that circulated most was type 3, and the most serious cases, not necessarily hemorrhagic, were manifest in the first infection," he noted in a Tierramérica interview.
He said that even in PAHO, there are attempts at analysing how to avoid differentiating between "classic" and "hemorrhagic" for being a strict categorisation that does not always include the most serious manifestations of the disease.
In Argentina, however, the experts maintain that differentiation.
"In the Americas we have four strains of dengue. Once a person has fallen ill, the next time, no matter how much time has passed, if he or she is infected by another strain there is a full chance of developing the hemorrhagic type," Dr. Jorge Gorodner told Tierramérica. Mortality in that case is 15 to 35 percent, he said.
Gorodner, a professor of infectious medicine at the National University of the Northeast and lead researcher at the Institute of Regional Medicine, says this is the first time an epidemic of hemorrhagic cases has occurred in Argentina.
"Almost 100 percent of the infected are susceptible (to DHF). But the severity of the disease depends on each person," said Gorodner, complaining that the lack of epidemiological oversight means the epidemic causes even more harm.
Dr. Alfredo Seijo, head of the dengue unit at the Muñiz Hospital in Buenos Aires, explained to Tierramérica that "the outbreak in Argentina is important due to the number of people who are left vulnerable to DHF."
According to Seijo, the number of DHF cases is on the rise in all countries where dengue has spread.
Experts say the only effective weapon is to reduce the presence of the Aedes aegypti mosquito, especially by eliminating sites and containers where water collects, which are ideal for larvae to develop. In parallel, community prevention campaigns are essential.
Controlling the disease-spreading vector is the key, until a vaccine has been developed. PAHO and the experts consulted agree that despite advances towards developing a vaccine, it is difficult to find a formula that is effective against all four strains of the dengue virus.
Partial immunity could in fact be dangerous, because infection with another strain in a vaccinated individual could manifest in the most severe form, thus generating an even greater risk.
(*Fabiana Frayssinet in Rio de Janeiro, Natalia Ruiz Díaz in Asunción, and Franz Chávez in La Paz contributed reporting for this article. This story was originally published by Latin American newspapers that are part of the Tierramérica network. Tierramérica is a specialised news service produced by IPS with the backing of the United Nations Development Programme, United Nations Environment Programme and the World Bank.)