In 1967 in the town of Marburg in Germany some laboratory workers became ill with a haemorrhagic fever (a terrible disease characterized by diarrhoea and vomiting, as well as by haemorrhaging and fever). These people had been in contact with blood, organs and cell cultures from African green monkeys caught in Uganda. Seven of those affected died, and there were five cases of the illness in hospital staff that had been in contact with patients' blood. Investigations revealed that the monkeys had been infected with a virus that had been transmitted to the laboratory staff and from them to the hospital staff.
This virus was of a type never previously encountered, with elongated virions, some of which were straight and some were curved. It was named Marburg virus and is now classified in the family Filoviri-dae, named from the Latin filum, meaning a thread (Figure 21.8).
In 1976 there were outbreaks of a similar disease in Africa near the River Ebola in the Democratic Republic of Congo (then Zaire) and in Sudan. A virus
Genome: ssRNA (minus strand) Capsid symmetry: helical Enveloped virion
Members are Marburg and Ebola viruses
similar to Marburg virus was isolated from patients and was named Ebola virus. Since then there have been a number of outbreaks of disease caused by Ebola and Marburg viruses across central Africa, from Cote d'Ivoire in the west to Kenya in the east and to Angola in the south.
The way in which the outbreaks start has long been a mystery. There is increasing evidence that some outbreaks start when a human becomes infected as a result of contact with the blood of an infected non-human primate. It was known from the original outbreak in Germany that African green monkeys can be infected with Marburg virus. It has since been found that gorillas, chimpanzees and duikers can be infected with Ebola virus, which may be responsible for significant mortality of these species. Ebola and Marburg viruses are present in the blood of infected hosts and transmission to humans can occur through contact with the flesh of infected animals after they have been hunted and killed. Human-to-human transmission readily occurs through contact with the blood of infected individuals.
There are several species of Ebola virus in Africa. A further species has appeared in the US and Italy in monkeys imported from the Philippines; there is serological evidence that animal handlers have been infected, but there have been no reports of this virus causing disease.
It is thought likely that additional animal species act as 'reservoirs' of Marburg and Ebola viruses, and that infection in these species is likely to cause few or no signs of disease. There have been many expeditions to search for reservoir hosts, but very few produced positive results; Ebola virus RNA has been detected in some small mammals including fruit bats, and anti-Ebola virus antibodies have been detected in some fruit bats. There remain unanswered questions concerning reservoir species and host ranges of these viruses.
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