Washington | Sexual transmission of the Ebola virus may have a major impact on the dynamics of the deadly disease, potentially reigniting an outbreak that has been contained by public health interventions, a new study has found.
The potential for sexual transmission is high for three to four months after the virus has been cleared from the bloodstream, and possible for an average of seven months. Previous research showed that viable Ebola virus remained in the semen of disease survivors for months after it was no longer detectable in their blood, and at least one instance of sexual transmission of Ebola was reported.
We wanted to find out what role sexual transmission might play in the dynamics of an outbreak, said Andrew Park, associate professor at the University of Georgia (UGA). The researchers developed a mathematical model to test various outbreak scenarios.
They created a model population of 1,000 individuals and introduced Ebola virus to track its spread via regular transmission. They assumed that many actions would be taken, from individual behaviour changes to public health interventions, to control the outbreak. In the parameterised model, this resulted in one in four individuals infected throughout the population.
Next, they set out to determine the impact that sexual transmission could have. We wanted to know what it would mean in terms of the size of an outbreak, how long an outbreak lasts, how likely an outbreak is to occur and the reproductive ratio of the parasite, a measure of how effectively the parasite transmits in populations, John Vinson, a doctoral student at UGA.
There were two components of sexual transmission about which very little is known. The first is what proportion of people who survive Ebola are actually able to transmit the virus through sexual contact; the second is how the rate of sexual transmission compares to that of regular transmission. To overcome this lack of data, they ran the model using values that varied widely for both questions but within plausible limits.
Their results showed a clear impact from sexual transmission. When the values of both parameters – the number of sexually infectious individuals and the rate of transmission – were low, outbreaks were smaller and ended more quickly, but as the values increased, so did the size and duration of outbreaks.
Whenever we had die-outs of the directly transmitted infectious individuals, which would otherwise have spelled the end of the outbreak, we had reignition from the sexually infectious individuals transmitting the virus to the susceptible people left in the population, who then served as a source of direct transmission, said Vinson.
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