Arboviruses: Dengue, Chikungunya and zika Flashcards
What are arboviruses?
- Arthropod-borne virus
- Viruses maintained in nature through biological transmission between susceptible vertibrae hosts by blood feeding arthropods (mostly mosquitoes)
- Over 130 arboviruses known to cause disease in humans
- Three virus families:
- Togaviridae
- Flaviviridae
- Bunyaviridae
- There is animal to mosquito to man transmission
- There is man to man transmission
Describe some of the transmission cycles that pertain to arboviruses
- Arthropod borne viruses- virus of vertebrates
- Transmitted by insects vectors
- Cause infections in animals and birds
- Transmitted to man by bite of an infected mosquito, tick and sand fly
- World-wide but more in tropical areas than temperate areas
Transmission cycle
- Man->Arthropod->Man (within human populations)
- Animal-> arthropod-> man (zoonosis)
What is zoonosis
- A zoonosis is an infectious disease that has jumped from a non-human animal to humans. Zoonotic pathogens may be bacterial, viral or parasitic, or may involve unconventional agents and can spread to humans through direct contact or through food, water or the environment.
- represent a major public health problem around the world due to our close relationship with animals in agriculture, as companions and in the natural environment.
- Zoonoses can also cause disruptions in the production and trade of animal products for food and other uses.
- There are over 200 known types of zoonoses
- Zoonoses comprise a large percentage of new and existing diseases in humans
- Some zoonoses, such as rabies, are 100% preventable through vaccination and other methods
Name some diseases that undergo a zoonosis
Zoonoses comprise a large percentage of all newly identified infectious diseases as well as many existing ones. Some diseases, such as HIV, begin as a zoonosis but later mutate into human-only strains. Other zoonoses can cause recurring disease outbreaks, such as Ebola virus disease and salmonellosis. Still others, such as the novel coronavirus that causes COVID-19, have the potential to cause global pandemics.
Arthropod borne viruses
Who is at risk ofaquiring a zoonotic infection?
Zoonotic pathogens can spread to humans through any contact point with domestic, agricultural or wild animals.
Markets selling the meat or by-products of wild animals are particularly high risk due to the large number of new or undocumented pathogens known to exist in some wild animal populations.
Agricultural workers in areas with a high use of antibiotics for farm animals may be at increased risk of pathogens resistant to current antimicrobial drugs.
People living adjacent to wilderness areas or in semi-urban areas with higher numbers of wild animals are at risk of disease from animals such as rats, foxes or raccoons.
Urbanization and the destruction of natural habitats increase the risk of zoonotic diseases by increasing contact between humans and wild animals.
What are the WHO guidelines against a zoonotic infection
WHO works with national governments, academia, non-governmental and philanthropic organizations, and regional and international partners to prevent and manage zoonotic threats and their public health, social and economic impacts.
These efforts include: fostering cross-sectoral collaboration at the human-animal-environment interface among the different relevant sectors at regional national and international levels.
WHO also works to develop capacity and promote practical, evidence-based and cost-effective tools and mechanisms for zoonoses prevention, surveillance and detection through reporting, epidemiological and laboratory investigation, risk assessment and control, assisting countries in their implementation.
What does the WHO One Health approach relate to zoonotic infections?
As part of the One Health approach, the World Health Organization collaborates with the Food and Agriculture Organization of the United Nations (FAO) and the World Organisation for Animal Health (OIE) on the Global Early Warning System for Major Animal Diseases (GLEWS).
This joint system builds on the added value of combining and coordinating alert mechanisms of the three agencies to assist in early warning, prevention and control of animal disease threats, including zoonoses, through data sharing and risk assessment.
Describe the man->arthropod-> man cycle
Reservoir may be in either man or arthropod vector (e.g. dengue, urban yellow fever.
This is a cycle within the human population
In the latter (reservoir in the arthropod vector), transovarial transmission may occur (transmission from the mother to eggs)
Describe the animal->arthropod->man cycle
The reservoir is in an animal (mammalian host)
The virus us maintained in nature ina transmisison cycle involving the arthropod vector and animal. Man becomes infected incidentally
E.G. Japanese encephalitis, EEE, WEE, yellow fever
For yellow fever, infection occurs in forest areas (like when someone goies hunting), then it spreads to urban populations through an arthropod man cycle.
There is a fear in latin america due to the forests
Name examples of important arboviruses in the Americas
Generally there is one serotyp, however dengue has 4 serotypoes
Chikungunya is an alphavirus
(The rest are flaviviruses (RNA virises))
What arthropod are dengue, chikungunya and zika transmitted by?
- Aedes mosquito
- Both A.aegypti & a.albopictus
- A.aegypti has a lyre shaped marking, whereas A.albopictus has a single white line on the thorax
- Typically, aegypti has a lighter colour than albopictus
Describe the geographic distribution of Aedes
- Restricted by tropics
- Different isotherms
- When looking at A.aegypti, it has a 10 degree celcius isootherm. Below the 10 degree isotherm, it cannot survive or proliferate
- Present in the northern hemisphere isotherm
- When looking at A.albopictus, is can survive and proliferate above the 10 degree isotherm, therefore there is more of a widespread albopictus proliferation
Describe the distribution of dengue infection
Largely restricted by isotherms, bwteen the Jan 10 degree to the July 10 degree isotherms. You get good DENV replication in this isotherm.
Outbreak in southern India in the 1940s. Outbreak of different dengue serotypes but they may not occur at the same time.
Describe temporal trends in Dengue serotypes (Thailand)
- Each line represents a dengue serotype
- In this interendemic period, you get outbreaks of other serotypes
- Sometimes there are all 4 serotypes occuring at the same time
What are the features of a classic dengue infection?
- Incubation 2-7 days
- Classic
- Shirt-lived
- High-fever 40oC (saddleback fever, which means that the high fever goes away and comes back)
- Severe muscle pains (break bone fever)
- Erythematous rash followed by morbiliform rash starting on extremities (measles type rash)
- Generalised lymphadenopathy
- Moderately enlarged liver
- Profound leucopenia (dropped white cell count)]May have protracted convalesence
- Means takes people a long time to recover
Describe the initial viraemia process in dengue
- During the initial fever, you have an onset of viraemia
- Immune response kicks in
- During viraemia, you get the greatest risk of severe dengue, which is associated with shock (when plasma goes into the periphery)
- You get a lower circulatory volume as the plasma leaves the circulatory tissue
- Then the concentration of blood cells in blood increases (increased haematocrit concentration)
- Then thrombocytopaenia occurs
Describe how haematocrit relates to dengue
- Haematocrit concentration goes down with anaemia
- However, in dengue when the plasma leaves the circulation and goes into the periphery, you get an increase in the concentration of red blood cells
- Therefore, haematocrit concentration increases with dengue
What type of rashes do you get in dengue fever?
Erythematous (blanching) rash
Milliborm rash (measles type rash)
What is dengue haemorrhagic fever/dengue shock syndrome
- Presents on the 3rd-6th day of classic dengue
- Associated with second ot later infections
- You get more severe dengue with a severe infection
- Increased capillary permeability–>shock
- When the plasma leaves the circulation into peripheral tissues, you get hypovolaemic shock
- Diagnosis:
- Positive tourniquet test
- Spontaeneous haemorrhages (in the eyes & skin)
- Thrombocytopaenia (loss of platelets)
- Increase in haematocrit
What are petechial haemorrhages and other haemorrhages that may occur in DHF?
Mild hemorrhagic manifestations such as petechiae, purpura, epistaxis, and gingival bleeding are relatively common even in nonsevere dengue cases; they occur in as many as one third of these cases. While the etiology of hemorrhage in patients with dengue is poorly understood, mild hemorrhagic manifestations might be due to increased capillary fragility as a result of thrombocytopenia or platelet dysfunction
Bleeding spots in the skin
Small haemorrhages in the skin
Describe the classification of dengue fever
Describe the dengue pathogenesis & disease
- You get bitten by an infected Aedes vector
- Vector transmits the DENV virus
- DENV is taken up by dendritic cells with the DC-Sign receptor
- DR by macrophages with the mannose receptor
- Immune response occurs
- depending on the type of immune response triggered, depends on the risk of getting severe dengye
Also, remember this occurs early in viraemia. Proinflammatory cytokines increase capillary permeability. Increased plasma leakage occurs then shock.
Describe the graph showing dengue seroprevalence in population sample in coastal ecuador
Risk of dengue increases with age
Describe Zika in the americas
What are the clinical symptoms of dengue, chikungunya and zika?
Describe Clinical-microcephaly that may occur in a zika virus infection
- Small head for gestational size (<2nd centile)
- Increase first noted in Brazil and retrospectively in Polynesia
- Highest risk during first trimester
- 20% have neurological abnormality
- Risk of microcephaly ~3%
