This is a piece I wrote several days back. Please note that I am not a virologist nor an infectious disease expert (not an MD, actually) -- just an amateur who is reading everything I can about the topic, with a longtime (perhaps perverse) interest in infectious diseases.
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Ebola epidemic backgrounder
September 16, 2014 at 12:57pm
What is Ebola?
Ebolavirus is one of a number of viruses that cause hemorrhagic disease (a disease in which hemorrhaging, or bleeding, is a symptom/characteristic). It is found primarily in Africa, and is thought to reside normally (and non-fatally) in animals such as fruit bats. In human beings, it incubates for 2-21 days, after which symptoms start to show. The disease Ebola causes is known, simply, as Ebola Virus Disease (EVD). EVD, in many ways, sets the standard for fatal, horrible, communicable diseases you do not want to catch.
What are the symptoms?
Initial symptoms mimic the flu. As the disease progresses, bleeding, vomiting, diarrhea, high fever and other symptoms present themselves. Typically, within 1-3 weeks, the patient either starts to recover or succumbs to the disease. Death is due to dehydration/bleeding, as well as organ destruction and failure.
Due to the flulike initial symptoms, a patient may not recognize the illness nor take proper precautions, even though he or she is capable of spreading the virus at this point.
What is the fatality rate?
It depends upon the strain. Typical overall case fatality rates (# of people killed by the virus vs. number of diagnosed cases overall) range from 40-90%, which is the highest of any known hemorrhagic disease, and among the highest of all human diseases.
In the strain currently causing the epidemic in West Africa, the estimated case fatality rate is 70-80% without extraordinary medical intervention.
http://healthmap.org/...
Is there a cure or treatment?
There is no cure for EVD. Extraordinary support treatment (IV fluid and nutrition support, pain management, treatment for secondary conditions) can substantially improve the chances of survival; however, the death rate is still high. People who have survived infection by an ebolavirus strain are immune to re-infection by that strain.
How is EVD spread?
The primary means of spreading EVD is through contact with bodily fluids -- the virus is present in most of them, including blood, stool, urine, semen, sweat, and saliva. Corpses of people killed by EVD remain significant sources of infection. In some ways, communication of the virus resembles that of the common cold (though the lack of respiratory symptoms makes it more difficult to catch in general).
One measure of the infectiousness of a disease is the infectious dose -- the number of virus particles necessary to cause disease. For diseases like influenza, that number is around 1000 virus particles. For highly infectious diseases like norovirus (24 hour stomach flu), it's around 20. For hemorrhagic diseases like Ebola, the infectious dose is estimated to be 1-10 virus particles.
Although studies have not determined whether ebolavirus can be transmitted through intact skin, contact of the virus with wounds or mucous membranes (e.g. the eyes, nose, mouth) can result in disease. Given the highly infectious nature of the virus, this danger is enhanced. Thus, infection control procedures for Ebola specify no contact with virus-carrying material whatsoever (in other words, the constant use of gloves and other barriers such as gowns is essential).
In situations other than epidemics, ebolavirus is normally handled in level 4 biosafety lab conditions - the highest level of biological security precautions and procedures.
Is EVD spread through the air?
Airborne spread of EVD has become a hot topic, and is the source of some confusion as airborne can mean several things.
The protein coat of ebolavirus is not well adapted to survival outside of a host, as it does not resist drying. Mutations necessary to develop this capability would substantially alter the protein casing, which could seriously impact ebolavirus' ability to infect human beings in the first place. Because of this, some researchers have questioned whether there would be any evolutionary advantage to such a mutation and doubt it would become dominant.
Because ebolavirus is potentially present in saliva, however, coughing or sneezing by victims of EVD may present a route of transmission. Droplets from sneezing/coughing can travel several meters. So, while standing downwind of a person up the road with EVD does not present a danger, having a victim cough or sneeze in one's face could be problematic. However, since EVD does not generally cause respiratory symptoms, this is not a likely route of transmission.
It should be reiterated that EVD is highly contagious, regardless of whether airborne transmission ever became a route of infection.
What about health care workers catching the virus despite protection? Isn't that evidence of airborne transmission?
It depends upon the workers. Substantial numbers of local doctors and other health care providers have fallen ill with EVD; it is hypothesized that most such cases are due either to lapses in precautions due to heat/exhaustion, or catching the disease outside the health care facility itself.
Doctors Without Borders, by contrast, has been running 530 beds in 5 ebola care stations in the affected areas since the start of the outbreak, with more than 2000 personnel, and only one international staff member has been diagnosed with EVD. Their protocols--long-standardized procedures for dealing with ebolavirus--have clearly been very effective.
What's the status of the epidemic?
This epidemic started in mid-March of this year, in a region that had never seen EVD before. Due to combination of factors, the disease took hold and is now a very serious problem in three countries (Liberia, Sierra Leone, and Guinea).
At this point, things are very bad. Transmission of the disease has gone to an "exponential spread" situation in Liberia and Sierra Leone, with additional serious transmission taking place in Guinea. This epidemic is the largest of its kind for EVD, by more than a factor of 10. Without serious intervention, it is estimated a majority of people in Liberia and Sierra Leone will catch the disease, and complete destabilization of these and other nations in West Africa is possible.
Many commentators have noted that the current estimate of approximately 5000 cases is likely quite low, with actual numbers of cases perhaps threefold higher. The number of cases is said to be doubling every 3-4 weeks. This rapid expansion of the epidemic is the reason why so many experts are sounding alarms -- and why quite a few have warned that the window to control the epidemic is closing (with some saying it may already be too late).
Predictions of numbers of cases have ranged to several million within the next year. This represents a potential humanitarian catastrophe.
Is there a danger to the U.S.?
The epidemic presents several threats to the United States. The first, of course, is the potential chaos in Africa that EVD could cause if left unchecked. The second is direct import of cases into the United States -- though any such importing would likely--hopefully--be controlled very quickly. The third is the danger of mutation of the virus.
What sort of mutations?
Ebolavirus is part of a class of viruses that encode their genetic information on RNA (the other type is DNA). RNA viruses tend to be sloppy in how they copy their genetic code when they reproduce, meaning they mutate and evolve quickly. Most mutations make no difference; some make the virus less dangerous, and some potentially make the virus more dangerous. Bad mutations can allow a virus to better evade the immune system, re-infect those who have survived previous bouts of the disease, avoid the protection afforded by vaccines or anti-viral drugs, or spread more easily.
Examples of easier disease spread would be changes that allow the virus to cause disease via fewer virus particles, survive longer outside the body, cause a victim to be infectious longer, infect new species, or use new routes of transmission (many experts have noted, however, that such a change in transmission methodology has never before been seen in a virus and is unlikely). The current epidemic strain is mutating rapidly.
At least one group of researchers, commenting upon how the current epidemic does not match previous models of EVD spread, has theorized that virus mutations might be a factor. The nature of the change, if any, isn't clear.
http://www.sciencedaily.com/...