Yellow Fever: Recent Resurgence and Pulmonary Pathology
Yellow fever (YF) is a mosquito-borne viral hemorrhagic fever endemic to tropical regions of South America and Africa. While historically contained, YF has experienced a resurgence in Brazil since 2016, extending beyond the traditionally endemic Amazon Basin to areas like São Paulo state. This article details the clinical presentation, pathology, and recent findings regarding YF, with a particular focus on pulmonary involvement.
Understanding Yellow Fever
Yellow fever is caused by the flavivirus and transmitted by Aedes aegypti mosquitoes. The disease is considered a public health concern, recognized by the World Health Organization (WHO) and the Pan-American Health Organization (PAHO) . Several factors contribute to its spread, including increased mosquito distribution, warmer and wetter climates (like those associated with El Niño events), high population density, inadequate infrastructure, and a lack of widespread vaccination .
Clinical Progression of Yellow Fever
The clinical course of YF is typically divided into five periods:
- Incubation Period: Following the mosquito bite.
- Infection Period: Characterized by viremia and influenza-like symptoms.
- Remission Period: A brief improvement in symptoms, with the appearance of serum YFV IgM neutralizing antibodies.
- Intoxication Period: Recurrence of fever, signs of acute hepatitis, oliguria, and hemorrhagic phenomena.
- Convalescence Period: Complete resolution of symptoms.
Pathological Findings and Recent Research
Historically, severe YF pathology has been characterized by midzonal hepatitis. Although, recent studies, particularly those conducted during the 2017-2019 epidemic in Brazil, have expanded our understanding of the disease. These investigations have revealed new insights into viral characteristics, emerging clinical manifestations (such as seizures, pancreatitis, and late-onset hepatitis recurrence), and the kinetics of viral RNA detection .
Pulmonary Involvement in Severe Yellow Fever
Research focusing on fatal YF cases in São Paulo (2017-2019) has highlighted significant pulmonary injury. Autopsy studies revealed severe pneumonia, often caused by bacteria and filamentous fungi, alongside alveolar edema and hemorrhage in all examined cases. A study of 73 fatal cases showed pulmonary hemorrhage in 38.4% of cases, intraabdominal hemorrhage in 35.6%, and pneumonia or hemorrhage in the respiratory tract in 42.4% of cases .
Microscopic examination of lung tissue revealed:
- Alveolar edema in 99% of cases
- Alveolar hemorrhage in 90% of cases
- Secondary suppurative pneumonia in 84% of cases
Commonly detected pathogens included Gram-negative bacilli, yeasts (Candida spp.), and hyphomycetes (Aspergillus spp.). Septic pulmonary vasculitis was observed in 21.3% of cases. The presence of YFV RNA and antigens in pulmonary tissue confirms direct viral involvement, alongside the secondary infections.
Diagnosis and Molecular Analysis
Diagnosis of YF requires a compatible clinical presentation and laboratory confirmation through methods such as positive serum IgM, detection of YFV RNA via RT-PCR, and histopathological findings with YFV antigen detection in tissues. Molecular analysis of lung tissue samples identified bacterial and fungal infections, with Enhydrobacter, Klebsiella, and Candida being among the most frequently detected genera .
Implications for Management and Prevention
The findings underscore the importance of considering secondary infections in the management of severe YF. Early recognition and treatment of pneumonia and other respiratory complications are crucial. Further research is needed to understand the interplay between YFV-induced immune dysfunction and susceptibility to secondary infections. Continued efforts to promote vaccination and control mosquito populations remain essential for preventing YF outbreaks.