Ebolaviruses
GENERAL CONCEPTS:
- The Ebolaviruses are composed of one genus and 5 species: Zaire, Bundibugyo, Sudan, Reston and Taï Forest.
- Disease caused by Ebolavirus was first recognized in 1976 in the Sudan and in the Democratic Republic of Congo.
- The current (2013-present) outbreak in west Africa is caused by the Zaire species and is the largest and deadliest to date.
- It estimated that a total of 26,079 suspected cases and 10,823 deaths have occurred as of April 19, 2015.
DISTINCTIVE PROPERTIES:
- Ebolaviruses belong to the family Filoviridae which includes Cuevavirus, Marburgvirus, and Ebolavirus.
- The viruses contain a single stranded, negative RNA genome and have a lipid bilayer envelope.
- The Ebolavirus genome is about 19 kb and it encodes 7 proteins: NP, VP35, VP40, GP, VP30, VP24, and L. Protein L is the RNA-dependant RNA polymerase, VP35 is a polymerase cofactor, VP30 is a transcription activator, and GP is a surface glycoprotein involved in receptor binding and fusion. The other proteins are involved in assembly and release of the filamentous viral particles.
- Fruit bats are thought to be the natural reservoir for the Ebolaviruses.
- Transmission of the virus occurs through close contact with infected blood or other bodily fluids.
PATHOGENESIS:
- Human to human transmission results from direct contact with bodily fluids of infected individuals or by contaminated materials.
- The incubation period for Ebolavirus disease is variable from 2 to 21 days.
- Infected people are not infectious until they develop symptoms, which initially include fever, fatigue, muscle pain, headache and sore throat.
- Later symptoms include vomiting and diarrhea followed by internal and external hemorrhaging.
- Mortality from the disease ranges from 25% to 90%, with an average of about 50%.
- Patients who recover from the disease may remain infectious for up to 2 months.
HOST DEFENSES:
- Host defenses against Ebolavirus have not been well characterized.
- Disease survivors have been shown to possess high antibody titers directed against the GP protein while non-survivors had undetectable antibody levels.
- Some reports suggest that these antibodies are not protective but that they may delay the onset of disease when given to exposed individuals. Animal tests demonstrate protection using relatively high doses of antisera.
- Since the cytotoxic effects of GP protein include dysfunction of the inflammatory response and loss of integrity of vascular integrity, GP may serve as a potential vaccine target.
EPIDEMIOLOGY:
- The filovirus family was initially described in 1967 following an outbreak of severe disease in vaccine plant workers who contacted infected monkeys from Uganda. The causative agent was named Marburg virus, after the town in which the outbreak occurred.
- Ebolavirus outbreaks have occurred since 1976 and until recently, were confined to Sub-Saharan Africa. The total number of cases have been less than 2500.
- The 2014 outbreak in West Africa has surpassed the total of all others ten-fold and has included infections in the United States and Europe.
DIAGNOSIS:
- Clinical: Ebolavirus disease is difficult to diagnose based on clinical symptoms, which may resemble multiple other diseases.
- Laboratory: In the laboratory, numerous tests including ELISA and RT-PCR can be used to ascertain the presence of viral proteins or RNAs. Samples used for these tests are extremely biohazardous and should only be performed using maximum biosafety procedures.
CONTROL:
- Sanitary: The most effective control measures include:
- Outbreak surveillance, monitoring and containment,
- Reduction of human to human transmission,
- Reduction of wildlife to human transmission.
Protective clothing is essential when treating patients or any materials they may have contacted.
- Immunological: Vaccines for Ebolavirus are under investigation.
- Chemotherapeutic: There are no chemotherapeutic methods for treating the disease. Supportive care and rehydration are important.