Mosquito-Borne Hemorrhagic Fevers

Arboviruses continue to be a significant source of disease, especially in regions where their insect hosts are endemic. This article highlights these diseases, with particular focus on dengue, yellow fever, and viral hemorrhagic fever. A general background is provided, as well information concerning diagnosis and treatment.

Arbovirus is short for arthropod-borne virus. Arboviruses are a large group of viruses that are spread by invertebrate animals (arthropods), most often by biting flies (such as mosquitoes) or ticks. Birds are often the reservoir of infection for mosquitoes, which can then transmit the infection to horses, other mammals, and humans that are not an essential part of the life cycle for most arboviruses ( Table 1 ). This article only discusses the mosquito-borne hemorrhagic fevers in this article.

Table 1

Mosquito-borne infections related to arboviruses

Bunyaviridae	Arbovirus encephalitis: La Crosse encephalitis, California encephalitis Rift Valley fever ^b
Flaviviridae	Arbovirus encephalitis: Japanese encephalitis, Australian encephalitis, St Louis encephalitis, West Nile fever ^a Dengue fever ^b Yellow fever, ^bZika fever ^a
Togaviridae	Arbovirus encephalitis: eastern equine encephalomyelitis, western equine encephalomyelitis, Venezuelan equine encephalomyelitis Chikungunya ^aO’Nyong-nyong fever, ^aRoss River fever ^a

a With skin or mucosal lesions.

b Hemorrhagic manifestations.

Dengue fever

At the present time, dengue is considered the most important viral infection transmitted by an arthropod vector in the world terms of morbidity and mortality. Dengue is the most severe and prevalent disease caused by flavivirus transmitted by insects. The incidence of dengue worldwide has increased dramatically in recent decades. At the present, the disease is endemic in more than 100 countries in the Americas, Africa, southeast Asia, the western Pacific, and the eastern Mediterranean. With global climate change, additional countries, such as Australia may soon have endemic dengue. More than 40% of the population of the world (2.5 billion of people) is at risk for the disease.

The estimate incidence of cases of dengue fever (DF) in the world is 50 to 100 million cases annually. Between 100 to 200 suspected cases of dengue are introduced in the United States by travelers each year. At present, the evolution of the disease in the Americas is similar to southeast Asia during the 1950s and 1960s. The mortality rate of DF is less than 1%. The estimated cases of dengue hemorrhagic fever (DHF) are also high (several hundred thousand cases per year). The fatality rate of DHF is approximately 5%. When patients develop dengue shock syndrome, the mortality rate can be as high as 40%. At the present, dengue is considered the most important viral infection transmitted by an arthropod vector in terms of mortality and morbidity.

In order for a dengue epidemic to occur, it is necessary to have the viral pathogen, a competent vector, and a high number of susceptible human hosts. The outbreaks can have a gradual or explosive onset, depending on the strain and serotype of the dengue virus, the number of susceptible people in the population, the amount of contact between the vector and people, the population density of the vector, and the efficiency by which the vector acquires the virus. In the United States, the Aedes aegypti , the main vector, is seasonably abundant in some southern states including Arizona, New Mexico, Texas, Louisiana, Mississippi, Alabama, Georgia, and parts of Florida. In addition, A aegypti is sporadically reported from elsewhere in the southeast and along the Atlantic seaboard (eg, North Carolina, Tennessee, South Carolina, Arkansas, New Jersey, and Maryland). Isolated dengue outbreaks have occurred in the continental United States, and sustained dengue transmission appears to be established in southern Florida.

Individuals of all ages can be affected. However, elderly individuals are more likely to be at risk because of coexisting medical conditions. Ninety percent of patients with DHF are younger than 15 years. All races can be affected by the disease.

DF can be caused by any of the four serotypes of dengue virus: DEN-1, DEN-2, DEN-3, and DEN-4. After a bout of dengue caused by any particular serotypye, people acquire immunity to that serotype. They can, however, be infected by any (or all) of the other three serotypes, and each successive infection may have more dire clinical consequences. The only vertebrate hosts of the dengue virus are people. There is a jungle cycle between arthropod vectors and monkeys that plays no role in human disease.

Clinical Presentation

People with dengue can have a spectrum of clinical illness ranging from a nonspecific viral syndrome to severe and fatal hemorrhagic disease. The dengue virus infection can lead to one of the four different consequences: classical dengue, hemorrhagic dengue, dengue shock syndrome, asymptomatic infection, and less common manifestations such as hepatitis.

Classical dengue, also called DF, is characterized by fever, headache, myalgia, and rash. DHF is a separate clinical entity and consists in a more virulent form of dengue virus infection. In the past, dengue used to be called breakbone, fever because the disease sometimes cause severe joint and muscle pain that feels like bones are breaking.

Mild hemorrhagic symptoms (including gingival bleeding, petechiae, and a positive tourniquet test) can be seen in approximately one-third of the patients with classical dengue.

Classical dengue

The most frequent form of disease is when the host has primary infection with classical dengue. Abrupt fever is the initial manifestation and can be associated or followed by headache, severe myalgia, retro-orbital pain, anorexia, vomiting, nausea, arthralgia, and fatigue. In 3 to 4 days, a morbilliform eruption ( Fig. 1 ) or a scarlatiniform rash can appear. This rash is centrifugal, and it is common to detect palmo-plantar edema associated with pruritus ( Fig. 2 ). In children, abdominal pain is more frequent. The headache of classical dengue occurs predominantly in the frontal region and can be the chief complaint because of its intensity. Although the rash is usually composed of macules and slightly elevated papules, it can also be purely macular or even urticarial; such cases are not easy to differentiate from a benign drug eruption, perhaps even to those medications used to control the symptoms.