6 - Trypanosome prevalence in cattle herds exposed to a range of tsetse challenge levels in northern Cote d'Ivoire

G. NANKODABA, L. COULIBALY, P. HECKER S.G.A. LEAK and A. SCHUETTERLE

Introduction
Description of the area
Methods
Results
Discussion
References

Introduction

The previous paper by Leak et al. described the across-site relationship observed between tsetse challenge and trypanosome prevalence in trypanotolerant livestock. This presentation will describe within-site relationships between the two parameters observed in Cote d'Ivoire.

The prefectures of Boundiali and Tengrela in the north of Cote d'Ivoire form an important entry point for cattle going into Cote d'Ivoire from neighbouring Sahelian countries. These cattle, together with those kept by the local Senoufo, Peuhl and Dioula communities in the region, supply meat to the urban populations of Cote d'Ivoire. Trypanosomiasis, transmitted by tsetse flies, is one of the constraints to livestock production in the region. As one possible solution to the problem lies in the rearing of trypanotolerant breeds, cattle herds from this area were selected by the ATLN in order to determine the comparative productivity of trypanotolerant and susceptible breeds under quantified levels of tsetse challenge. The relationship between estimates of tsetse challenge and trypanosome prevalence in sixteen cattle herds is reported here.

Description of the area

The prefectures of Tengrela and Boundiali are in the north of Cote d'Ivoire. The climate is tropical sub-humid with a dry season from November to April in which the Harmattan wind affects the area. The rainy season lasts from May to October with peak rainfall in July, August and September. Tengrela, 135 km north of Boundiali, is drier, with an average annual rainfall of 1013 mm between 1973 and 1984 compared with 1204 mm for Boundiali over the same period.

Vegetation in Boundiali District is sub-Sudanian, characterized by dry woodland and savanna, while Tengrela is in the Sudanian vegetation zone with islands of dense, dry forest occurring in the savanna. The Bagoue River flows through the region from south to north, and its gallery forests form the major habitat for tsetse of the palpalis group.

Cattle kept in the region include Bos taurus (N'Dama, Baoule), Bos indicus (West African Zebu), and crosses between the two.

Methods

Estimates of tsetse challenge and trypanosome prevalence have been made at villages on a monthly basis following the protocol of the ATLN (ILCA, 1986a, 1986b), as described in the previous paper.

Trapping of tsetse was carried out for five days per month using an average of ten traps at each village. Traps were sited close to grazing or watering areas of cattle. As some cattle in the area are transhumant and were not present for blood examination for several months each year, only data for herds which were continuously recorded are used here.

As numbers of tsetse captured were low in many villages, particularly in Tengrela prefecture, insufficient tsetse were dissected to give monthly estimates of trypanosome infection rates for tsetse at all villages. For this reason analyses of mean annual estimates are reported. The relationship between tsetse challenge and trypanosome prevalence was examined using linear regression analysis. An arcsin transformation of trypanosome prevalence estimates was used in order to stabilize the variance, and these estimates were regressed on a log₁₀+1 transformation of tsetse challenge.

Results

Tsetse species

Three tsetse species were detected in the region, G. palpalis gambiensis (Vanderplank) and G. tachinoides (West.) were found throughout the area and a small number (5) of G. morsitans submorsitans (Newst.) were captured in Boundiali prefecture. Efforts to trap greater numbers of the latter species with box traps failed. It thus appears that this species occurs at very low densities at this northern limit of its range in Cote d'Ivoire, and it is not considered further. G. palpalis and G. tachinoides both inhabit gallery forests of the Bagoue River to which they are largely confined in the dry season, while in the rainy season G. tachinoides may also be detected in forest islands at some distance from watercourses.

Relative densities of tsetse

Tables 1 and 2 show the estimates of relative densities of G. tachinoides and G. palpalis at each village. There was an increase in relative density of G. palpalis in Boundiali prefecture from 1984-1986. In Tengrela prefecture the relative density of G. tachinoides increased slightly while that of G. palpalis changed very little. Rainfall data were only available for the towns of Boundiali and Tengrela, and no statistically significant relationship between tsetse density and rainfall could be observed. There were indications, however, that the relative density of tsetse increased during the rainy season. One factor which affected the measurement of relative density was the extensive flooding of the Bagoue River during the rainy season which necessitated removal of traps from certain areas along the river banks during the months of August and September. This resulted in an apparent drop in relative density during a period of peak rainfall. It is possible that if rainfall data were available for each village study site separately, significant relationships between rainfall and tsetse density at those villages could have been detected.

Table 1. Relative density (flies/trap/day) and trypanosome infection rate (%) of G. palpalis and G. tachinoides estimated at villages with cattle herds in the north of Cote d'Ivoire.

Village		G. palpalis		G. tachinoides
		Relative density	Infection rate	Relative density	Infection rate
Boundiali:
	Bagoue est	3.0	13.9	0.9	16.3
	Nondara	2.3	7.5	0.1	15.0
	Kantara	1.6	8.2	0.2	9.4
	Kouto	0.5	16.6	0.3	24.4
	Koulikoro	0.3	6.9	0.2	12.2
	Kolia	>0.1	*	-	-
Tengrela
	Zanasso	1.8	4.5	0.1	*
	Daragnani	1.3	2.7	0.1	2.4
	Tchalaga	0.5	3.5	0.1	*
	San	0.3	0.7	>0.1	*

* Insufficient number of tsetse dissected.

Table 2. Relative density of tsetse (flies/trap/day) in the prefectures of Boundiali and Tengrela. (From villages at which data were collected throughout 1984-1986).

Year	Boundiali		Tengrela
	G. palpalis	G. tachinoides	G. palpalis	G. tachinoides
1984	0.6	0.3	0.9	0.6
1985	1.1	0.3	0.9	0.1
1986	3.7	0.7	0.9	0.1

Trypanosome infection rates and infection type in tsetse

The mean Trypanosome infection rates in G. tachinoides and G. palpalis are shown in Table 3 for the period 1984-1986. In both Boundiali and Tengrela prefectures Trypanosome infection rates declined in each tsetse species from 1984-1985. This decline continued in Boundiali prefecture while infection rates remained at a low level in tsetse in Tengrela district.

Table 3. Trypanosome infection rates in tsetse at Boundiali and Tengrela (%) from 1984-1986.

	1984		1985		1986
	G. palpalis	G. tachinoides	G. palpalis	G. tachinoides	G. palpalis	G. tachinoides
Boundiali	20.6	18.2	8.8	12.6	10.1	17.7
Tengrela	5.1	9.8	1.1	0.0	0.2	1.6

Table 4 shows that in G. palpalis 75 and 88% of infections were of the "vivax-type" in Tengrela and Boundiali, respectively, while in G. tachinoides there were 81 and 92% "vivax-type" infections. These results indicate that tsetse of the palpalis group are poor vectors of T. congolense as has been previously reported.

Table 4. Trypanosome infection type in tsetse in the prefectures of Boundiali and Tengrela (percentage of the total number of infections).

Species		Number dissected	Number infected	"vivax- type"	"congolense type"
Boundiali	G. palpalis	4,458	480	88	12
	G. tachinoides	2,214	261	92	8
Tengrela	G. palpalis	4,581	89	75	25
	G. tachinoides	453	34	81	19

Tsetse challenge and Trypanosome prevalence in cattle

Estimates of tsetse challenge and respective estimates of Trypanosome prevalence in village herds are shown in Tables 5 and 6. Tsetse challenge decreased in Boundiali prefecture from 1984 to 1985 and increased from 1985 to 1986. In Tengrela prefecture tsetse challenge decreased each year. The decrease in tsetse challenge in Tengrela prefecture was due to the decline in trypanosome infection rates in both tsetse species from 1984-1985 as already mentioned. Infection rates also decreased in Boundiali prefecture, but overall tsetse challenge increased from 1985-1986 due to an increase in relative density of G. palpalis. The relative density of G. tachinoides did not change significantly.

Table 5. Estimates of tsetse challenge and trypanosome prevalence in cattle in the villages of the Network in the north of Cote d'Ivoire.

	Herd number	Village	Tsetse challenge	Trypanosome prevalence
			Mean	Mean
Boundiali	39	Bagoue Est	24.7	10.9
	Peuhl-37	Kantara	18.1	27.9
	35	Kantara	18.1	13.4
	41	Nondara	18.1	10.9
	42	Nondara	18.1	10.8
	43	Kouto-Boyo	13.8	11.0
	Peuhl-40	Koulikoro	4.3	18.1
	38	Kolia	0.0	6.7
Tengrela	15	Zanasso	8.9	2.3
	21	Daragnani	5.8	6.7
	16	Tchalaga	4.4	7.7
	17	Tenderime	1.1	2.2
	14	San	0.9	1.7
	13	San	0.9	1.2
	19	Tengrela ville	0.0	0.5

Table 6. Estimates of tsetse challenge and trypanosome prevalence in cattle at Boundiali and Tengrela, 1984-1986.

	1984		1985		1986
	Tsetse challenge	Trypanosome prevalence	Tsetse challenge	Trypanosome prevalence	Tsetse challenge	Trypanosome prevalence
Boundiali	17.6	16.8	13.5	9.4	49.7	17.4
Tengrela	4.8	3.1	1.4	1.2	0.4	0.2

Trypanosome prevalence was higher in Boundiali prefecture than in Tengrela where it was at a relatively low level throughout the period. There was an increase in trypanosome prevalence in cattle following peak rainfall in October and November each year.

Regression analyses of transformed estimates of trypanosome prevalence and tsetse challenge show a highly significant relationship between the two parameters across years (P<0.001) (Figure 1). To avoid auto-correlation leading to an artificially significant "p" value when using data from the same villages in consecutive years, regression analyses were carried out separately for each year of study. Results still showed a significant relationship between the two parameters for 1984 (P<0.01); 1985, (P<0.05); and 1986, (P<0.05) (Figures 2, 3 and 4). These values indicate that an apparently simple linear relationship exists between trypanosome prevalence and log₁₀+1 tsetse challenge.

Figure 1. Relationship between tsetse challenge estimates and trypansome prevalence in cattle in Northern Cote d'Ivoire - 1986.

Figure 2. Relationship between tsetse challenge estimates and trypansome prevalence in cattle in Northern Cote d'Ivoire - 1985.

Figure 3. Relationship between tsetse challenge estimates and trypansome prevalence in cattle in Northern Cote d'Ivoire - 1984.

Figure 4. Relationship between tsetse challenge estimates and trypansome prevalence in cattle in Northern Cote d'Ivoire - 1984-1986.

Discussion

Despite the relationship between the two parameters, there were some anomalous results, such as those for herd 40 at Koulikoro, which had a much higher trypanosome prevalence than would have been anticipated from estimates of tsetse challenge in 1984 and 1985. Herd 37 at Kantara also had a trypanosome prevalence twice as high as that of herd 30, also at Kantara. Both herds 37 and 40 are owned by Peuhl villagers who tend to take their cattle greater distances for grazing than herdsman of other communities. It is thus likely that they are exposed to a higher tsetse challenge elsewhere. Herd 30 is owned by Senoufo. In addition to such anomalous results, differences were observed between estimates of tsetse challenge at villages within a few kilometers of each other and between trypanosome prevalence estimates for cattle herds from the same village or from villages in close proximity. This highlights the need to accurately determine the grazing and watering areas of cattle. In northern Cote d'Ivoire, where the same tsetse species occur throughout the area, cattle and management systems are similar, and environmental factors show a gradation rather than a sharp contrast. The most important factor in obtaining accurate estimates of tsetse challenge may be to determine the areas in which cattle actually are exposed to tsetse challenge and to estimate challenge in those areas.

In the second phase of the ATLN in Boundiali prefecture, a tsetse-control trial will be carried out using insecticide-impregnated traps and screens in the gallery forest of the Bagoue River and its tributaries. In addition to the baseline data obtained for tsetse challenge, trypanosome prevalence and productivity of cattle in the area, data to be collected from new village herds both in the experimental area and outside, will enable a more complete evaluation of this tsetse control trial than has been possible with similar tsetse-control schemes previously.

References

ILCA. 1986a. The ILCA/ILRAD Trypanotolerance Network: Situation Report, December, 1985. Proceedings of a Network Meeting held at ILCA, Nairobi. ILCA, Addis Ababa, Ethiopia.

ILCA. 1986b. The African Trypanotolerant Livestock Network, Indications from Results, 1983-1985, December 1986, ILCA, Addis Ababa, Ethiopia.