The Ebola virus causes an acute, serious illness – Ebola virus disease (EVD). It is thought that fruit bats of the Pteropodidae family are natural Ebola virus hosts. Humans are infected through direct contact (through broken skin or mucous membranes) with the blood, secretions, organs or other bodily fluids of infected animals such as chimpanzees, gorillas, fruit bats, and monkeys found ill or dead in the rainforest.
The incubation period is between 2 to 21 days. Humans are not infectious until they develop symptoms. The first symptoms are the sudden onset of fever, fatigue, muscle pain, headache and a sore throat. These early symptoms are followed by vomiting, diarrhoea, bleeding and symptoms of impaired kidney and liver function.
Ebola virus spreads from the initial infection site to the lymph nodes, liver, spleen and adrenal glands. The endothelial cells, liver cells, and several types of immune cells (monocytes, lymphocytes and dendritic cells) are the main targets of viral infection (1). Macrophages are among the first cells infected by the virus, and the infection results in programmed cell death (2). Macrophage infection leads to the production of pro-inflammatory cytokines, which may induce bystander apoptosis in lymphocyte populations, thereby contributing to lymphopenia and immunosuppression. IL-6 and macrophage-derived TNF-α also induces changes in vascular permeability. The production of tissue factor by infected macrophages leads to dysregulation of coagulation. Hepatocyte infection leads to decreased synthesis of liver-derived clotting factors. Infection of adrenal glands results in hypotension and metabolic disorders, which together with immunosuppression and coagulopathy contribute to multiple organ failure and shock (3). Thus the overall systemic body virus spread leads to hypovolemic shock, disseminated intravascular coagulation, and - if untreated - finally can lead to multiple organ dysfunction syndrome and death.
In acute disease, patients are extremely viremic, and there is evidence of messenger ribonucleic acid (mRNA) of multiple cytokine activation. In vitro studies reveal that these cytokines lead to shock and increased vascular permeability, the basic pathophysiological processes most often seen in viral haemorrhagic fever infection. Another prominent pathological feature is pronounced macrophage involvement. Inadequate or delayed immune response to these novel viral antigens may lead to rapid development of overwhelming viremia. Extensive infection and necrosis of affected organs also are described. Haemorrhagic complications are multifactorial and are related to hepatic damage, consumptive coagulopathy.
Phase I can be characterised as the transfer of the Ebola virus from an animal carrying the virus to a human, usually via small cutaneous lesions. Similar principles apply in human-to-human transmission during Ebola outbreaks. Phase II can be characterised as the early symptomatic stage — usually between days four and ten — where symptoms of a viral illness appear and gradually progress toward more advanced manifestations of the disease. Finally, Phase III represents the advanced Ebola virus disease, with haemorrhagic manifestations, impaired immunity, and end-organ failure.