Ebola: a deadly virus able to hijack the host immune response

February 13, 2017

2014 was marked by the most alarming Ebola epidemic ever seen. To date, this pathology has affected more than 20000 people in West Africa and has brought more than 8000 casualties (World Health Organisation). This tragedy has occurred in three major countries: Sierra Leone, Liberia and Guinea. It is in this state that the epidemic started with the infection of an infant who is thought to have contracted the disease through a fruit bat (Baize et al, 2014). Indeed, Ebola haemorrhagic fever is a zoonotic disease in which bats are believed to forward the virus to human (Baize et al, 2014). In addition, primates could also be affected (Feldmann and Geisberg, 2012). Hopefully, the rate of infection is decreasing currently. However, threat is still present, as no treatment is available at this time. This report will help us to know more about this dramatic but unknown pathology.

What is Ebola?
Despite all the damages it causes, Ebola remains a poorly known pathogen. First of all, it is a virus from the family of the Filoviridae which are characterized by an enveloped single-stranded negative-sense RNA (Feldmann et al, 2012). It has been discovered for the first time in 1976, in Republic of Democratic Congo (before known as Zaire), in a river called Ebola, hence its name. So far, five strains have been described: Zaire, the most known, Sudan, Bundibugyo, Tai Forest and Reston (Tseng and Chan, 2014).

Epidemiology of Ebola

Since 1976, various Ebola epidemics are been out broken, with more or less gravity, few people affected and more control. The Zaire strain was responsible for the majority of Ebola cases. The mortality rate varies between 25 and 90% (Tseng and Chan, 2014). Nevertheless, the last outbreak occurred in December 2013 in Guinea, spread really fast, over three countries (Guinea, Sierra Leone and Liberia) in West Africa. At a time point that the epidemic was out of control. To characterize more Ebola disease, it is necessary to pinpoint the mode of contamination. The virus is spread from individual to individual through fluid like blood, saliva or sweat (Tseng and Chan, 2014). Its time of incubation varies from 2 to 21 days with deaths occurring between day 6 and 16 generally. Virus can survive a couple of days even after the death of its host, contributing to the difficulty to control the epidemic. Indeed, African people are making several days-funerals to bury their beloved ones. Therefore, they are not free from an eventual infection. Nobody is protected from this lethal pathogen. So far, nearly 8000 people died from this infection (World Health Organization).

Symptoms of the disease

Usually, the infection begins with symptoms like flu, characterized by high fever and chills, which could delay the diagnostic of the disease, decrease the probability of survival of the patient and impair the control of the infection. Later, the pathology manifests itself by a combination of pains such as gastrointestinal pains (anorexia, nausea, vomiting or diarrhoea), respiratory (cough, chest pain), vascular (hypotension or oedema) or neurologic (headache, coma) (Tseng and Chan, 2014).

Diagnostic of the pathology

Regarding this disease, one of the difficulty resides in the fact this infection shares commune symptoms with others pathologies like the malaria, flu or meningitis. Therefore, assays like ELISA, or RT-PCR are necessary to confirm the positivity of the patient (WHO 2014).

Biology of the infection
To resume briefly, the infection is characterized by a poor immune response and high inflammation which contributes to a septic shock and multi-organ failure responsible for the death of the patient. Basically, the strategy of the virus reposed mainly on the capacity to evade the host immune response. Moreover, it targets and replicates in most of the cells like the lymphocytes, the macrophages, the epithelial cells, the endothelial cells or the hepatocytes, after its entry through the mucosal surfaces (Tseng and Chan, 2014). This crucial fact allows it to rapidly spread in the vital organs of the host, resulting in the multi-organ failure (Grupta et al 2007) and the massive elimination of immune cells by apoptosis (Busico et al, 1999). This explains why no antibody anti-Ebola is found in dead people (Busico et al, 1999). In the other hand, a delay of the appearance of antibodies targeting Ebola has been observed in the survivors of the infection (Busico et al, 1999). This shows that this pathogen is perfectly controlling the host immune system.

Treatment

Currently, there is no prophylaxis or curative treatments towards Ebola. As a consequence, Ebola is the only known pathogen ranked in a category 4, which means that careful manipulations of the virus in laboratories are crucial in order to avoid infections (World Health Organization). The only way to prevent to be in contact with this pathogen is to stay away from infected people or wear protective equipments if needed to take care of them. Moreover, incineration and cleaning with at least 1/10 bleach help to fight the spreading of the disease (Center for Disease Control and Prevention USA). Finally, infected patients need to be maintained in quarantine to first prevent any future infections but also to rest, to get theirs oxygen saturation and blood pressure at their exact level in hope to be cured of the infection (Tseng and Chan, 2014). An important fact to pinpoint is that even if they recover, survivors can be still prompt to transmit the virus up to seven weeks (World Health Organization).

To come back to the treatment, the main reason why no treatment is available to this day, is that there are no efficient antiviral drugs. Indeed, either the consequences of these medications were no survival or too much secondary effects which were at the end lethal for the.Two vaccines are currently in study: among them, Zmapp which consists in a combination of 3 humanized murine antibodies generated by mice who recovered from the virus patient (Tseng and Chan, 2014) .

To conclude, the Ebola haemorrhagic fever is a serious pathology occurring currently in West Africa. So far, it has been responsible for nearly 8000 deaths and no treatment is available. From what we know, this virus is smart enough to avoid the immune system and preventing the host from fight back. This leads us to think that Ebola could hijack the whole immune system.