Most Zanzibar residents journey to and from mainland Tanzania via four- to six-hour ferry trips, but records of their origin and destinations are poor, Tatem said. Inspired by recent cell phone-based epidemiological research in Europe, he and five colleagues obtained records of three months of calls from customers of the Zanzibar Telephone Company, which covers all of Zanzibar and Tanzania.
The October-December 2008 records, which contained no names or other identifying information, tracked the movements of a 770,369 customers by showing each customer's calls and where the calls originated or at least the region where the call originated.
So, for example, the researchers could see that a person made calls from Zanzibar's main city of Stone Town Monday, Tuesday and Wednesday, but on Thursday and Friday from Dar es Salaam.
Most callers never left Zanzibar, which means they posed no threat of reintroducing the Malaria parasite, Tatem said. About 12 percent did leave the islands, but most of those only visited relatively safe Dar es Salaam, and usually for just one or two days at a time.
However, a few hundred residents made trips to regions of western and southern parts of Tanzania, where as many as 40 percent of the residents have the malaria parasite.
The cell records did not reveal any information about the threat from non-residents visiting Zanzibar. Many visitors to the historic center of the spice trade are likely to be tourists who either come from countries where malaria is not a problem or are taking anti-malarial drugs, so pose little threat for malaria importation. However, mainland residents visiting for work may carry infections, so future work will need to assess these risks too, Tatem said.
Tatem said the study gives Zanzibar's government several options should it move forward with el
|Contact: Andy Tatem|
University of Florida