Yellow fever, commonly found in the tropical regions of South America and Africa, has two epidemiological cycles: jungle and urban. In the jungle cycle, the virus circulates in the primate population and humans are infected when they enter the jungle and are bitten by infected mosquitoes. Urban yellow fever was eradicated from the Americas (the last cases occurred in 1942 in Brazil). In mid-January of this year, Paraguayan health authorities detected an outbreak of jungle yellow fever.
The clinical manifestations of yellow fever virus infection are acute fever and jaundice (yellowing of the skin and eyes), followed by hemorrhage in 15% to 25% of infected patients. Case fatality can reach 50%.
Since the 1970s jungle yellow fever cases were limited to the northern region of South America. Between 1985 and December 2007, a total of 3,837 cases of jungle yellow fever and 2,229 deaths were reported.
In 2007 and at the beginning of 2008, six states in Brazil (Goias, Federal District, Mato Grosso do Sul, Minas Gerais, Tocantins, and São Paulo) recorded an intense and extensive increase in epizootic yellow fever. These outbreaks of the disease in animal populations were confirmed by laboratory and/or by clinical-epidemiological criteria through the State Health Secretariats.
In January and February 2008, human cases were reported in three states (Goias, Mato Grosso do Sul, and the Federal District); of the 26 confirmed cases, there were 13 deaths. While the affected areas have high vaccination coverage, health authorities intensified vaccination campaigns for individuals over six months of age who had not been immunized previously and/or reside in/or frequent affected areas.
As of 21 February 2008, seven cases of yellow fever had been confirmed in Paraguay, all in the rural area of San Pedro Department, about 300 km northeast of Asunción, the capital. Five other suspected cases were reported in Central Department, which neighbors Asunción; four of these cases died with signs and symptoms highly suggestive of yellow fever. If these cases are confirmed, it would validate the circulation of the virus in urban areas.
In response to these outbreaks, the authorities have expanded epidemiologic surveillance for the detection and testing of suspected cases. Yellow fever vaccination has been intensified in Asunción, Central and San Pedro departments, and in border areas in order to protect the population living in high-risk areas.
To achieve this level of coverage, approximately 3 million doses of vaccine are needed. Thanks to the support of PAHO/WHO, 850,000 doses are being sent from Brazil, 144,000 from Peru, and 2 million doses from the WHO Global Fund. With the support of the Spanish Agency for International Cooperation (AECID), the U.S. Office of Foreign Disaster Assistance (OFDA/USAID), and the Government of Italy, PAHO/WHO has been able to implement emergency projects to intensify epidemiologic surveillance and vector control, increase laboratory diagnosis, expand communications about the risk, and carry out vaccinations in at-risk areas.
In Argentina, authorities reported that on 17 January 2008 dead monkeys were found in Piñalito park, in the department of San Pedro, province of Misiones. In February, yellow fever was confirmed in one of the primates through molecular techniques. Despite the high vaccination coverage in the area, authorities have intensified vaccination activities for previously unimmunized individuals who reside in or travel to the area.
Currently, the recommended yellow fever vaccination strategy is to direct efforts at the population that lives in, or is traveling to the areas where epizootics or human cases were recently registered and where risk of transmission exists. This approach avoids indiscriminate mass vaccination or revaccination.