A key recognition is that some of the benefits of tsetse control in Ghibe accrued to anyone who kept livestock in the control area, whether their animals were treated with pour-ons or not. Other benefits were captured primarily by owners of treated animals— for instance, those arising from control of ticks and other biting flies. Characterising pour-ons as mixed private–public goods, and using data from a 1992 survey of 166 farmers in the control area, Swallow et al. (1995) found that the probability of pour-on adoption was indeed linked to both private and collective factors. Specifically, adoption was positively associated with proportions of cows and oxen in farmers' herds, negatively with distance farmers traversed to reach crushes, and positively with tsetse challenge in the area, which varied seasonally according to the level of rainfall and temperature extant in that part of the country.
The mixed private–public nature of the pour-on technology and the factors that influence its adoption are particularly significant in light of the finding by Leak et al. (1990) of a highly non-linear relationship between tsetse challenge and trypanosomosis prevalence in cattle (Figure 1). Large decreases in disease prevalence can be achieved only with near eradication of tsetse populations. Vector challenge has to fall considerably before prevalence changes much. But then, below a certain level of challenge, the drop in prevalence is precipitous. Given reductions in vector challenge result in greater than proportionate declines in disease prevalence.3 This implies a lumpiness to control efforts that is very much in the nature of a fixed cost or indivisibility. Relatively large-scale, widely dispersed, intense efforts are required; small-scale, spatially confined, incrementally increasing measures likely will not work.4
3. Note that a similar logic applies to changes in the other direction—i.e. given increases in challenge result in greater than proportionate jumps in prevalence.
4. The appropriate basis for comparing alternative technologies thus is also on a large-scale—i.e. thousands of traps and targets vs. tens of thousands of pour-on applications vs. tens of thousands of trypanocidal drug treatments.
Source: Adapted from Leak et al. (1990).
Figure 1. The relationship between tsetse challenge and trypanosome
prevalence.
A 1998 survey in Ghibe suggests that the burden for the near eradication and sustained suppression of tsetse populations in the region has not been shared equally among households (unpublished survey data). Among 126 randomly selected households, only 77 (about 60%) applied pour-ons. And among users, there were significant differences in pour-on use-intensity, measured as the proportion of treated animals in farmers' herds.5 In addition, a strong positive relationship between farmer income and intensity of pour-on use was discerned.6 Most significantly, the data also exposed a very uneven spatial distribution of use-intensity across income groups (Figure 2).7 Not only do high-income farmers exhibit higher use-intensities of pour-ons, they also tend to reside in close proximity to one another. So, too, do poorer households with low use-intensities.
5. The maximum use-intensity was 100 % of the herd, the minimum was 9%, and the mean was 38%. (unpublished survey data).
6. Off-farm income sources are relatively few in the region and only income from agricultural sources was measured. Two-stage least squares regression analysis revealed that in addition to income, the number of oxen as a share of herd size, and the number of men as a share of family size had significant positive effects on use-intensity (unpublished data).
7. Median incomes and use-intensities were used to classify farmers into four groups covering high and low income and use-intensity.
These findings confirm those of Swallow et al. (1995) that externalities (or spillover effects) have been crucial to the outcome of the Ghibe trial. Spillovers arise because cattle owned by several households living in close proximity to one another are brought into collective herds numbering between 30 and 80 and tended by one of the herd owners (Swallow et al. 1995). While there is little overlap in grazing areas used by different herds, these areas adjoin one another. Region-wide reductions in disease prevalence—covering all grazing areas and homesteads—point to efficient and equity-enhancing vector control: the richest households have contributed the most to control, which has been equally consumed by poorer households.
Physical features of the region and associated changes in land use have also contributed to the positive outcome. The control area is located in a relatively gently sloping part of the valley. On one side, it is bordered by a mountain range that reaches an altitude unsuitable for tsetse flies, and on the other by a highland area of intensive cultivation that is also a poor habitat for flies. As a result, once initial suppression of tsetse flies was achieved, the risk of reinvasion was minimal. In addition, the clear economic gains from control induced sustained in-migration from other parts of Ethiopia, major expansions in livestock and human populations, and thus steady removal of tsetse habitats (Swallow et al. 1998b).
The importance of these interacting and mutually reinforcing processes emerges upon consideration of why a similar control initiative has been less successful in another part in the valley (ILRI 1998). This second area is located in a more rugged and lower elevation part of the valley. Animals graze in a few herds, which are spread unevenly across the valley. This leaves large areas of tsetse habitat with inadequate cattle-to-fly contact, most notably the dense thickets along the Ghibe River itself and a number of its tributaries, all of which represent reinvasion fronts. Human and cattle populations within the control area remain small, sparse, and, most critically, with little pressure or incentive to expand. Thus despite regular treatment of cattle with pour-ons, vector challenge and disease prevalence remain high.
Figure 2. Farm income and insecticide pour-on use-intensity in Ghibe Valley, Ethiopia.
