11 January 2013

Gambia: New WHO Plan Targets the Demise of Sleeping Sickness

A meeting of experts and African health officials sets in motion a campaign spearheaded by WHO to free Africa from the grip of sleeping sickness. John Maurice reports.

17 so-called neglected tropical diseases have been sitting for decades on the to-do list of WHO.

Together they bring physical, social, and economic suffering to more than 1 billion people, according to WHO. A decade or so ago, WHO and its partners decided to pull these diseases out of the closet and to find the resources needed to finish what has too long been unfinished business. "These diseases are now being brought to their knees with stunning speed", WHO Director-General Margaret Chan told an international meeting in London last year.

Bringing diseases "to their knees" can mean, in health-speak, either eradication (incidence is permanently reduced to zero cases worldwide and no further action is needed); or elimination (incidence is reduced to zero cases worldwide or in a defined geographical area but action might be needed to keep it that way); or elimination as a public health problem (incidence is reduced to a defined level). Until now, of the 17 neglected tropical diseases only two--guinea-worm disease and yaws--have been slated for eradication. Only three, blinding trachoma, leprosy, and lymphatic filariasis, are headed for worldwide elimination. Plans to eliminate a fourth disease, sleeping sickness (human African trypanosomiasis), have recently been agreed on by experts at a WHO meeting held in December, 2012.

Theoretically, sleeping sickness could be eradicated but the parasite that causes the disease (a subspecies of Trypanosoma brucei) is transmitted by a fly, the tsetse fly: eradicating the disease would call for eradication of the fly, a formidable task. The fact, too, that animals can be infected by the parasite and might constitute a reservoir of infection also casts doubt on eradicability of the disease.

Sleeping sickness, however, is certainly a strong candidate for elimination. "What we are aiming for", says Pere Simarro, head of WHO's human African trypanosomiasis programme, "is to bring the disease down to zero cases, to stop transmission, and to keep it that way indefinitely. It means using all the tools at our disposal to find infected people, to rid them of the infection, to stop the infection spreading, and to prevent it from resurging". There are, he believes, good reasons for making these efforts: "For one thing, it's a lethal disease. If you get infected and you don't get treatment, death is almost certain. For another, it's a disease that hits people at the most productive period of their lives and thus represents a serious economic burden on many African countries."

Achieving the zero-cases-zero-transmission target, the WHO meeting participants agreed, will not be easy. Hunting for cases is difficult in the remote jungle villages where most patients--and most tsetse flies--live. Screening for cases and diagnosis of the disease are hampered by the absence of specific signs or symptoms in the early months or years of the infection. During this period, the parasite is still in the patient's blood and lymph and hasn't yet passed into the central nervous system where it will produce a host of visible symptoms--behaviour changes, drowsiness, and other neurological changes.

The available drugs are effective but notoriously toxic and difficult to administer. There is a lack of field-friendly diagnostic tests. Efforts to stop transmission by killing the fly vector by spraying its forest haunts and animal hosts with insecticide or by trapping methods have, up to now, been too costly to deploy on a large scale. A further complication is the fact that there are two distinct forms of sleeping sickness, a West African (gambiense) form, which accounts for about 96% of sleeping sickness cases, and an east African (rhodesiense) form, which is essentially a zoonotic disease involving cattle and wildlife. The two forms differ biologically, clinically, and epidemiologically and call for quite different control strategies.

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