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INFECTIOUS DISEASE EPIDEMIOLOGY
Emerging Vector-borne Diseases
Vector-borne Infection
Cross-cutting factors related to emergence: Increase in and ease of global travel – infected
travelers can spread infection to new geographies Rapid urbanization in major cities in the tropics –
corresponding hyper-densities of urban slums Ubiquity and abundance of non-degradable
containers – has created breading sites for many important mosquito vectors close to large human populations
Vector control programs have largely been abandoned due to failing public health infrastructure and external economic pressures to “privatize” traditionally state-provided services
Vector-borne Infection
Other factors: Building large dams Deforestation and changes in land use in
combination with expanded human habitation
Introduction of new virus-amplifying hosts or efficient vectors into new areas (e.g. Aedes albopictus into the Americas)
Climate change could also amplify endemic transmission
Arboviruses
Viruses which are transmitted by an arthropod-borne vector are called arboviruses
They are important causes of encephalitis and hemorrhagic fever in many parts of the world including the United States
Only specific species of mosquitoes or ticks in specific ecologic systems can transmit these viruses
Because the viruses depend on the vector for transmission, the diseases are seasonal in temperate climates, but remain year-round in tropical climates
How the viruses overwinter in temperate climates is unknown
Arboviruses
Abroviruses are classified into four families: Togaviridae, Flaviviridae, Bunyaviridae, and Reoviridae
All are relevant to human disease, but we will focus on the Flaviviruses, which are of greatest consequence globally and in the United States, and especially with respect to emergent infection
Flaviviridae
Single-stranded, linear, (+) sense, RNA genome
Icosohedral capsid structure Enveloped Replicates in cytoplasm of the host
cell Virus transmitted by bite of infected
arthropod
Dengue
Known as a human disease for hundreds of years
Over the last 20 years dengue fever, dengue hemorrhagic fever (DHF), and dengue shock syndrome (DSS) have emerged as the most important arthropod-born viral infection worldwide
~ 100 million cases of dengue fever per year, with ~ 250,000 cases of DHF per year This is what is reported, actual numbers likely
much higher
Dengue
4 separate serotypes: dengue 1, dengue 2, dengue 3, and
dengue 4 Infection with multiple serotypes is
most relevant to severity of disease
Dengue
Dengue fever: Transmission Virus is transmitted by the bite of the
Aedes aegypti or Aedes albopictus mosquitos
Ae. aegypti is a much more efficient vector because it much prefers humans
Dengue
Reservoir: Humans
Dengue
Dengue fever: signs and symptoms Flu-like illness (but not respiratory) Fever Intense headache Myalgia and arthralgia, also usually
intense…nicknamed break bone fever Maculopapular rash Some minor bleeding is possible, gums
and petechiae, during the febrile phase
Dengue
DHF is much more severe form of dengue: Carries a case-fatality rate of 5% to 15% Associated with a second infection with a new
serotype following a previous infection with a different serotype
The first infection sensitizes Fc receptors on vascular endothelial cells to virus-associated antibodies
The formation of immune complexes associated with a subsequent infection with a new serotype in a sensitized host leads to vascular damage and associated leakage of the vascular beds
This vascular pathology results in hemorrhage and/or fluid accumulation in tissues (DHF/DSS)
Dengue
Because of the large number of dengue infections each year…
Billions of people are at risk for DHF/DSS
Dengue – The vector
General mosquito biology: Purpose of the blood meal? Production of eggs by the female How do they find mammals? They follow CO2 and heat gradients Aedes aegypti is the main vector for dengue virus Small water containing objects Transition from forest to human household
environments Bite during the day Cohabitate with humans
Dengue – the vector
Aedes albopictus Another vector for dengue that has been
introduced into exotic environments across the globe including the United States
How? The tire industry Tires are stacked in large quantities on large
ships and circulated throughout the world The tires catch and retain rainwater These have served as the sites for oviposition
for Ae. albopictus
Mosquito distribution in US
Dengue
Factors favoring dengue emergence: Increases in the use and disposal of
non-degradable containers and peridomestic storage has led to more A. aegypti breeding sites
Encroachment on tropical forest
Dengue
PAHO program to eliminate A. aegypti from the Americas in 1947
…Why? Yellow fever By 1972, A. aegypti had been eliminated from 73%
of the areas previously occupied by the mosquito However, the identification of a jungle cycle of
yellow fever resulted in the end of the campaign By 1998, A.aegypti had colonized more geographic
area in the Americas than it had prior to 1947, as well as a much greater global geography
Dengue
Prevention and control: Very difficult to prevent because mosquitoes
live in and around the home and bite at all times, especially during the day, so residual insecticide spraying is ineffective
Vaccines have not progressed because they need to be effective against all serotypes and immunity must be very long-lived, otherwise the host may be placed at greater risk of DHF/DSS following vaccination
Dengue
Prevention and control: Most effective means of prevention is
destroying larval development sites Must target water filled containers in and
around the home This strategy is remarkably effective at
controlling the mosquito, but is also labor intensive
This was the singular approach used in the 1947 PAHO campaign