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Newsletter

Tuesday, May 08, 2018

What are Viral Haemorrhagic Fevers?

A group of illnesses that are caused by several distinct families of viruses.
A severe multisystem syndrome (multisystem in that multiple organ systems in the body are affected). 

Characteristically, the overall vascular system is damaged, and the body's ability to regulate itself is impaired.
Often accompanied by hemorrhage however, which is rarely life-threatening.


GROUPING OF HEMORRHAGIC FEVER VIRUSES

Arenaviruses
Lassa fever virus
Junun
Machupo
Guanarito
Sabia
White water arroyo virus
Filoviruses
Marburg 
Ebola
Bunyaviruses
Hanta virus
Crimean congo haemorrhagic fever virus
Rift valley fever virus
Flaviviruses
Dengue
Yellow fever
St louis encephalitis

Each of these families share a number of features:
They are all RNA viruses, which are enveloped, in a fatty (lipid) coating. 
Their survival is dependent on an animal or insect host: the natural reservoir. 
The viruses are geographically restricted to the areas where their host species live.
Humans are not the natural reservoir for any of these viruses. 
Humans are infected when they come into contact with infected hosts. However, with some viruses, after the accidental transmission from the host, humans can transmit the virus to one another. 
Human cases or outbreaks of hemorrhagic fevers caused by these viruses occur sporadically and irregularly. The occurrence of outbreaks cannot be easily predicted. 
With a few noteworthy exceptions, there is no cure or established drug treatment for VHFs. 
In rare cases, other viral and bacterial infections can cause a hemorrhagic fever; scrub typhus is a good example

VECTORS OF THE VIRUS


Viruses associated with most VHFs are zoonotic. They are totally dependent on their hosts for replication and overall survival.
 For the most part, rodents and arthropods are the main reservoirs for viruses causing VHFs. 
The multimammate rat, cotton rat, deer mouse, house mouse, and other field rodents are examples of reservoir hosts.
 Arthropod ticks and mosquitoes serve as vectors for some of the illnesses. 
However, the hosts of some viruses remain unknown: Ebola and Marburg viruses are well-known examples.

WHERE ARE CASES OF VIRAL HEMORRHAGIC FEVER FOUND?

Distributed over much of the globe.
 The virus and the disease it causes are usually seen only where the host species live(s). 
Some hosts, such as the rodent species carrying several of the New World arenaviruses, live in geographically restricted areas. Therefore, the risk of getting VHFs caused by these viruses is restricted to those areas.
 Other hosts range over continents, such as the rodents that carry viruses which cause various forms of hantavirus pulmonary syndrome (HPS) in North and South America, or the different set of rodents that carry viruses which cause hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia. 

While people usually become infected only in areas where the host lives, occasionally people become infected by a host that has been exported from its native habitat. For example, the first outbreaks of Marburg hemorrhagic fever, in Marburg and Frankfurt, Germany, and in Yugoslavia, occurred when laboratory workers handled imported monkeys infected with Marburg virus. 
Occasionally, a person becomes infected in an area where the virus occurs naturally and then travels elsewhere. 
If the virus is a type that can be transmitted further by person-to-person contact, the traveler could infect other people. For instance, in 1996, a medical professional treating patients with Ebola hemorrhagic fever (Ebola HF) in Gabon unknowingly became infected. When he later traveled to South Africa and was treated for Ebola HF in a hospital, the virus was transmitted to a nurse. 
She became ill and died. 
Because more and more people travel each year, outbreaks of these diseases are becoming an increasing threat in places where they rarely, if ever, have been seen before.

TRANSMISSION OF HEMORRHAGIC FEVER VIRUSES

Overlap  of the activities of infected reservoir hosts or vectors and humans 
The viruses carried in rodent reservoirs are transmitted when humans have contact with urine, fecal matter, saliva, or other body excretions from infected rodents. 

Bites of the vector mosquito or tick’ 

crushes a tick. 
However,  some of these vectors may spread virus to animals, livestock, for example. Humans then become infected when they care for or slaughter the animals.

person to another, once an initial person has become infected. Eg Ebola, Marburg, Lassa and Crimean-Congo hemorrhagic fever viruses. This type of secondary transmission of the virus can occur directly, through close contact with infected people or their body fluids. It can also occur indirectly, through contact with objects contaminated with infected body fluids.

SYMPTOMS OF VIRAL HEMORRHAGIC FEVER ILLNESSES?
Initial signs and symptoms often include marked fever, fatigue, dizziness, muscle aches, loss of strength, and exhaustion.
 Patients with severe cases of VHF often show signs of bleeding under the skin, in internal organs, or from body orifices like the mouth, eyes, or ears. shock, nervous system malfunction, coma, delirium, and seizures, renal  failure.

Pathophysiology

The diversity of clinical features seen among the VHF infections probably originates from varying mechanisms of pathogenesis. An immunopathogenic mechanism, for example, has been identified for dengue haemorrhagic fever, which usually occurs among patients previously infected with a heterologous dengue serotype. An influential theory explaining this phenomenon is called “antibody-dependent enhancement .” 
In contrast, DIC is thought to underlie the hemorrhagic features of Rift Valley, Marburg and Ebola fevers.
 In most VHFs, however, the etiology of the coagulopathy is most likely multifactorial (e.g., hepatic damage, consumptive coagulopathy, primary marrow dysfunction, etc).

The reasons for variation among patients infected with the same virus are unknown but stem from a complex system of virus-host interactions.
 Some infected persons develop full-blown VHF while others do not: an unresolved issue. 
Virulence of the infecting agent clearly plays an important role. 
The “VHF syndrome” (capillary leak, bleeding diathesis and hemodynamic compromise leading to shock) occurs in a majority of patients manifesting disease from filoviruses, CCHF, and the South American hemorrhagic fever viruses, while it occurs in a small minority of patients with dengue, RVF and Lassa fever.

Clinical and treatment aspects?

Signs and symptoms of VHFs include (by definition) Fever.
Bleeding diathesis.
Flushing of  face and chest,
Petechiae,
Frank bleeding,
Oedema,
Hypotension,
Shock.

The findings of laboratory investigation

This varies somewhat between the viruses .
In general there is a decrease in the total white cell count particularly the lymphocytes , a decrease in the platelet  count, 
an increase in the serum liver enzymes , both the prothrombin (PT) and activated partial thromboplastin times (PTT). 
The haematocrit may be elevated. 
The serum urea and creatine may be raised but this is dependent on the hydration status of the patient. 
The bleeding time tends to be prolonged.

Medical management of VHF patients

May require intensive supportive care.
 Antiviral therapy with intravenous ribavirin may be useful in Bunyaviridae and Arenaviridae infections (specifically Lassa fever, RVF, and HFRS due to Old World Hantavirus infection) 
Interferon may be effective in Argentine or Bolivian hemorrhagic fevers .
 Experimental vaccines for other VHFs are not readily available.
Prophylactic (preventive) ribavirin may be effective for some Bunyaviridae and Arenaviridae infections.


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