Zanzibar remains free of tsetse, 18 years after eradication was declared

Up until the late 1990s, the island of Unguja, Zanzibar (United Republic of Tanzania) was infested with tsetse - Glossina austeni Newstead. Trypanosomosis prevalence among the mostly indigenous cattle averaged around 19 percent and the rural farming communities were unable to maintain livestock as a basis for productive mixed farming.

A successful eradication project that included the systematic aerial release of sterile male tsetse flies was implemented from 1994 to 1997 by the Government of Zanzibar with the support of the FAO/IAEA Joint Division. Following suppression of the tsetse population with insecticides and the subsequent phase of sterile male releases it was demonstrated, by intensive monitoring for tsetse flies and trypanosome infections in livestock, that the Sterile Insect Technique (SIT) campaign had been successful in completely eradicating the Glossina austeni population from the island.

The removal of the tsetse fly vector of animal trypanosomosis resulted in the disappearance of the disease, enabling farmers to rear improved breeds of cattle and integrate livestock keeping with cropping in areas where this had been impossible before. The increased livestock and crop productivity significantly contributed to an increase in the quality of people’s lives.

A recent entomological and parasitological survey (2015), carried out jointly by independent experts and the Department of Veterinary Services of Zanzibar, has confirmed the continued tsetse-free status of the island, 18 years after eradication was declared.

This survey was carried out by first conducting a GIS analysis of land use (based on the most recent GlobCover information from 2009) and vegetation index (based on eMODIS, January 2015) that was combined with entomological data obtained from baseline surveys carried out prior to the eradication campaign. From this analysis, twenty-three 1-kilometer square grids covering the most suitable habitat for Glossina austeni were selected. The Jozani National Park, with its dense gallery forest, high humidity suitable for tsetse breeding sites and with wild pigs and primates to provide host blood meals for tsetse, was the most suitable habitat for tsetse reinvasion and was the main focus of the survey during the dry season baseline data collection. It was, indeed, in these areas where the tsetse population density had reached its highest levels before and during the eradication project.

Two-hundred and eleven sticky panels of two types (described in detail by Vreysen et al., 2001) consisting of blue and white cloth coated with a sticky glue, were deployed and left in site for at least 10 days, before being checked for tsetse catches and removed. The tsetse panels captured many non-target species but not a single tsetse fly was caught during the survey. The island has therefore remained tsetse free since 1996.

In addition, a total of 356 cattle aged between 6 and 12 months from 13 villages in Unguja were sampled. The blood was checked for trypanosome prevalence by microscopic examination of the buffy coat layer after micro-haematocrit centrifugation. In addition, serum samples were collected and sent to the mainland Tanzania Veterinary Laboratory Agency for examination using the more sensitive Pan Tryp LAMP molecular diagnosis technique. Microscopic examination detected no trypanosome infections. The molecular tests identified no T. congolense infections, which can only be transmitted by tsetse, but did identify 9 cases of T. vivax infection that need to be followed up to confirm if they are real cases or false positives and whether the possible infection could reach symptomatic levels as the animals sampled had no clinical symptoms of the trypanosomosis. Livestock are occasionally imported from the mainland of Tanzania into Zanzibar so these animals may have been imported with pre-existing infections. A follow-up of the history of the 9 cases of infected animals could therefore be useful.

References

Vreysen, M.J.B. & Saleh, K.M. (2001) Long-term sampling of gamma sterilised male Glossina austeni (Diptera: Glossinidae) with sticky panels on Unguja Island (Zanzibar). Acta Tropica 80: 29–37.

FELDMANN, U., F. MRAMBA, A.G. PARKER, V.A. DYCK, M.J.B. VREYSEN, et al. (2013). Application of the sterile insect technique in Zanzibar to eradicate tsetse flies, the vectors of trypanosomosis. pp 125-132. In Ruane, J., J.D. Dargie, C. Mba, P. Boettcher, H.P.S. Makkar, D.M. Bartley and A. Sonnino (eds.). Biotechnologies at Work for Smallholders: Case Studies from Developing Countries in Crops, Livestock and Fish. FAO, Rome, Italy.

VREYSEN, M.J.B., K. SALEH, F. MRAMBA, A. PARKER, U. FELDMANN, et al. (2014). Sterile insects to enhance agricultural development: the case of sustainable tsetse eradication on Unguja Island, Zanzibar, using an area-wide integrated pest management approach. PLoS Neglected Tropical Diseases, 8(5): e2857.

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