Ebolaviruses contain single-stranded, non-infectious RNA genomes. These viral particles measure 80 nanometers in width and can stretch up to 14,000 nanometers in length. They appear as filamentous shapes resembling a shepherd's crook or the number six. The life cycle begins when a virion attaches to specific cell-surface receptors like C-type lectins or DC-SIGN. This attachment allows the viral envelope to fuse with cellular membranes. Inside the cell, the virus travels to acidic endosomes where its glycoprotein GP is cleaved. This processing enables the virus to bind to internal proteins and release its nucleocapsid. The viral RNA polymerase transcribes genes into positive-strand mRNAs which are then translated into structural and nonstructural proteins. Newly synthesized proteins and genomes self-assemble near the inside of the cell membrane before budding off to infect other cells.
The virus replicates very efficiently in many cells producing large amounts of virus in monocytes, macrophages, dendritic cells, liver cells, fibroblasts, and adrenal gland cells. Viral replication triggers high levels of inflammatory chemical signals leading to a septic state. EBOV is thought to infect humans through contact with mucous membranes or skin breaks. After infection endothelial cells lining blood vessels become main targets of attack. Following infection immune cells carry the virus to nearby lymph nodes where further reproduction takes place. From there the virus can enter the bloodstream and lymphatic system spreading throughout the body. Macrophages are the first cells infected resulting in programmed cell death. Other types of white blood cells such as lymphocytes also undergo programmed cell death leading to an abnormally low concentration of lymphocytes in the blood.
Transmission Dynamics And Reservoirs
Fruit bats are believed to be the natural host of the viruses without being affected by it. Three types of fruit bats including Hypsignathus monstrosus, Epomops franqueti and Myonycteris torquata were found to possibly carry the virus without getting sick. Evidence indicates that both domestic dogs and pigs can also be infected with EBOV. Dogs do not appear to develop symptoms when they carry the virus while pigs appear to be able to transmit the virus to at least some primates. Areas undergoing deforestation are among the most likely places for outbreaks due to changes in the landscape bringing wildlife into closer contact with humans. Index cases of Ebola disease have often been close to recently deforested lands.
It is believed that between people Ebola spreads only by direct contact with the blood or other body fluids of a person who has developed symptoms. Body fluids that may contain Ebola viruses include saliva mucus vomit feces sweat tears breast milk urine and semen. The World Health Organization states that only people who are very sick are able to spread Ebola disease in saliva. Most people spread the virus through blood feces and vomit. Entry points for the virus include the nose mouth eyes open wounds cuts and abrasions. Dead bodies remain infectious thus people handling human remains in practices such as traditional burial rituals are at risk. Of the cases of Ebola infections in Guinea during the 2014 outbreak 69% are believed to have been contracted via unprotected contact with infected corpses.