Marburg's behavior bewilders scientists
Rising death toll flags unexplained character of killer virus.
The current outbreak of the deadly Marburg virus in Angola is raising difficult questions about this enigmatic pathogen and its origins.
As of 5 April, Angolan health officials had reported 181 cases of Marburg haemorrhagic fever, of which 156 have been fatal. The outbreak of the rare but lethal virus, which causes fever and circulatory collapse, is the worst ever recorded.
Health workers' primary concerns are treating those infected and blocking the further spread of the virus. The World Health Organization and other medical groups have set up five mobile surveillance teams in Uíge province, where the outbreak originated, to identify rumours of cases. "Everyone is focused on the cases in front of them," says WHO spokesman Dick Thompson, who is working in Angola.
Researchers interested in the disease are focusing on some unusual features of the latest outbreak. For one thing, the probability of dying from Marburg disease once you've caught it, currently more than 85%, is higher than in previous events. In the first recorded incidence of the disease, which stemmed from infected monkeys shipped from Uganda to Europe in 1967, some 23% of those infected were killed.
The high death toll parallels that of the only other large outbreak of the disease, in the Democratic Republic of Congo between 1998 and 2000. There, more than 80% of infected patients died, according to analyses carried out by Daniel Bausch, of the Tulane School of Public Health and Tropical Medicine in New Orleans, and his colleagues.
It is not clear why the death rate should differ from one outbreak to the next. Some think the various events involved strains of different ferocity. Marburg's cousin Ebola, for example, is known to have strains with widely varying fatality rates.
It is also possible that patients involved in the different outbreaks received varying doses of the virus, Bausch suggests. Patients may also have been infected by different routes, or those in Africa may have suffered from poorer medical care or general ill-health.
A second puzzle raised in Angola is why an estimated three-quarters of those affected have been children under five years of age: a pattern of infection not seen in earlier epidemics.
Again, this might be explained by the possibility that the latest outbreak is caused by a slightly different strain. But experts favour an alternative explanation: that children have something in common that helped them pick up the infection. They might, for example, have received childhood vaccinations from re-used needles contaminated with the virus.
Bats to blame?
The situation in Angola may also shed light on one of the most baffling questions about Marburg and Ebola: where do these viruses spring from? Because Marburg has triggered only a handful of recognized cases since it was discovered, researchers have had little opportunity to get to grips with the disease.
In their studies of the Congolese outbreak, Bausch and his team traced almost all cases to people who had entered a local gold mine. They suspect that the patients picked up the disease from cave-dwelling animals harbouring the virus, perhaps bats.
In the present outbreak, the children may also have come into close contact with bats, Bausch speculates. They might work or play in local caves, or eat fruit from trees in which bats sleep.
Such questions will only be answered once the current situation is under control, and public-health investigators can begin tracing the virus to its source. Researchers will then be able to determine the viral genetic sequences from those infected, to see if all cases stemmed from one patient.
These types of investigation must wait, however, until the public-health threat has eased, says infectious-disease specialist Bob Swanepoel of the National Institute for Communicable Diseases in Sandringham, South Africa.