The effect of deltamethrin pour-on applied to cattle on the transmission of bovine trypanosomosis

Tsetse flies (Glossina spp.) and trypanosomosis, the disease they transmit, have been successfully controlled by applying insecticide to cattle or to artificial baits, termed targets (1, 7, 10). With both types of application the disease transmission is reduced due to a slow decline of tsetse densities in the surrounding areas, and hence a gradual reduction in challenge. However, there is some evidence that disease transmission can also be reduced more directly and immediately by the inhibition of the flies’ feeding responses on insecticide-treated animals (2, 20). Other evidence contradicts this (3, 6, 17).


s INTRODUCTION
Tsetse flies (Glossina spp.) and trypanosomosis, the disease they transmit, have been successfully controlled by applying insecticide to cattle or to artificial baits, termed targets (1,7,10). With both types of application the disease transmission is reduced due to a slow decline of tsetse densities in the surrounding areas, and hence a gradual reduction in challenge. However, there is some evidence that disease transmission can also be reduced more directly and immediately by the inhibition of the flies' feeding responses on insecticide-treated animals (2,20). Other evidence contradicts this (3,6,17).
It is necessary to clarify the importance of this direct effect of insecticide treatments on the tsetse's feeding response or trypanosomosis transmission because it is the one which could offer an immediate benefit to the farmer who treats his cattle, irrespective of whether the cattle are treated in adjacent areas or irrespective of its effect on the tsetse population density (4). By contrast, the effect that depends on the decline of tsetse density cannot be achieved by one farmer alone. If cattle in nearby areas are untreated or treated only sporadically the flies will persist there, allowing a steady stream of flies to invade the areas where cattle are treated properly. In this case, deltamethrin treatments will have less effect on the incidence of trypanosomosis in treated cattle. A related problem occurs when cattle are kept immediately adjacent to a game reserve from which tsetse can continuously invade.
The present work elucidated the importance of the direct effect by studying the incidence of trypanosomosis in groups of deltamethrin-treated and untreated cattle herded in the same area and subject to a similar and constant tsetse challenge.

s MATERIALS AND METHODS
The trial was conducted between August and December 1992 in the Katete District, Eastern Province, Zambia (31°50' E and 13°05' S). The area is on the border of the eastern plateau at an elevation of approximately 900 m above sea level. It is highly cultivated and carries approximately 5 head of cattle/km 2 together with goats, pigs, dogs and few game animals (mainly small antelopes). The prevalence of bovine trypanosomosis is high and constitutes a serious constraint to rural development. Only Glossina morsitans morsitans Westwood 1850 was present taking 75% of its blood meals from cattle (19).

Summary
A trial to evaluate the effect of monthly deltamethrin pour-on treatments on the tsetse's feeding response and, consequently, the incidence of tsetsetransmitted trypanosomosis in deltamethrin-treated and untreated cattle was conducted in Zambia. During a period of 14 consecutive weeks the twoweekly trypanosomosis incidence and packed cell volume of deltamethrintreated and untreated herds, herded in the same area, were compared. No significant differences were found. It is concluded that the reported effects of deltamethrin treatments on trypanosomosis incidence are a result of reduced tsetse challenge due to a decline in tsetse density in surrounding areas and not of the direct effect of deltamethrin treatments on the tsetse's feeding response.
This control method will, therefore, be most effective in those areas where the tsetse population density can be sufficiently reduced to significantly affect disease transmission.

PATHOLOGIE PARASITAIRE
Thirty randomly selected adult oxen (Ngoni breed), aged between 1.5 and 3 years, were divided into two herds; a control herd and one treated with deltamethrin pour-on (Spoton ® , Coopers) of 15 and 12 animals, respectively. At the start of the trial (week 0), all animals were eartagged and treated with diminazene aceturate (Berenil ® , Hoechst) at 7.0 mg/kg body weight. Deltamethrin pouron (Spoton ® , 1% deltamethrin active ingredient) was applied to all animals of the treated herd, in a line along each side of the animal at a dose of 10 ml/100 kg body weight, using a T-shaped hand applicator. Pour-on treatment was repeated at 4-week intervals (weeks 0, 4, 8, 12 and 16).
To avoid risk of contamination, oxen treated with deltamethrin pour-on were kept as one group and kraaled together. All animals were exposed to the same natural field challenge of tsetse by herding them in the same area (approximately 10 km 2 ). Different herdsmen looked after the treated and the untreated groups and kept the two herds separate.
To allow for a persistent effect of a double dose of diminazene aceturate (7 mg/kg) all animals were considered to be protected during the first four weeks after the initial treatment. Trypanosomosis incidence in both herds was calculated at two weekly intervals from week 5 onwards. On each occasion, ear vein blood of all animals was examined for trypanosomes using the hematocrit centrifugation technique (Woo method) and the packed cell volume (PCV) percentage was measured (12). Since diminazene aceturate resistance had not been reported in the trial area, trypanosomal infections were treated with diminazene aceturate at 3.5 mg/kg body weight. Animals given this dose of diminazene were considered to be protected during the subsequent two weeks and where therefore excluded from the next calculation of incidence.
PCV of treated and untreated cattle were compared using a t-test (5). A one-sided Fisher-exact test (5) was used to test whether the trypanosomosis incidence in the deltamethrin-treated herd was significantly lower compared to the incidence in the untreated herd (Statxact, Cytel Software Co).
The apparent density of tsetse in the herds' grazing area was monitored using five epsilon traps baited with acetone (at a release rate of 200 mg/h) (8). Traps were sited in Combretum-and Brachystegia-woodland. In addition, five epsilon traps (control traps) were deployed 10 km south of the grazing area. Trap cages were emptied daily. Live flies were dissected to determine trypanosome infection rates (9).

s RESULTS
In both areas, the apparent density of tsetse followed the normal seasonal pattern with a low apparent density during the dry season (August-November) and a sudden increase at the beginning of the rainy season (December) (figure 1). Apparent density in the grazing area was similar to the apparent density outside the grazing area. This is not surprising considering the low number of deltamethrin-treated cattle in the trial area.
The tsetse infection rate during the trial period increased gradually from 0.63 to 2.5%. A total of 62.5% of the trypanosomal infections in tsetse were congolense-type, the remaining being vivax-type.
The two-weekly trypanosomosis incidence (weeks [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] in the deltamethrin-treated and untreated herds is shown in figure 2. The first trypanosomal infection was detected seven weeks after the onset of the trial. No trypanosomal infections were detected in week 11 in the control herd and weeks 7 and 17 in the deltamethrin-treated herd. Trypanosomosis incidence varied considerably between herds and between weeks. The average twoweekly trypanosomosis incidence, however, was 8.1 and 7.8% for the control and deltamethrin-treated herds, respectively. A total of 16 trypanosomal infections were detected. Trypanosoma congolense accounted for the majority (87.5%) of the infections. The remaining 12.5% was attributed to T. vivax. The probabilities for the null hypothesis of no difference between the trypanosomosis incidence in the deltamethrin-treated and untreated herds are shown in table I. For none of the weeks the difference between the incidence of trypanosomal infections is significant.     Table I Two weekly incidence of trypanosomal infections in deltamethrin-treated and untreated herds and significance of Fisher-exact test The mean PCV (week 5-19) is presented in figure 3. The PCVs of both herds decreased gradually from an average of 32.8% in week 5 to an average of 26.6% in week 15. From week 17 onwards the average PCV increased, reaching 28.2% in week 19. None of the difference between average PCV of untreated and deltamethrintreated herds was statistically significant (p < 0.05).

s DISCUSSION
During the three-month observation period, the incidence of tsetse-transmitted trypanosomosis was never statistically lower in the deltamethrin-treated herd compared to the untreated herd. This lack of association between deltamethrin treatments and disease incidence and the variations in the incidence of trypanosomosis between herds and between samplings could be due to the low sensitivity of the parasitological diagnostic methods to detect trypanosomal infections (13). Such low sensitivity could lead to the misclassification of non-diseased animals and consequently affect parasitological incidence. This low diagnostic sensitivity is, however, non-differential and cannot affect the degree of association (18). It can nevertheless cause substantial variations in the parasitological incidence of trypanosomosis between herds and between consecutive samplings.
The low sensitivity could partly be compensated for by measuring indirect effects of trypanosomosis in both herds. A major characteristic of bovine trypanosomosis is anemia (11). Reliable indicators of anemia are PCVs (15). Significant differences between herd PCVs could therefore be used as an additional indicator of trypanosomal infections and tsetse challenge. No significant differences were observed between the average two weekly PCVs of the deltamethrin-treated and untreated herds. The gradual decrease in PCV during the first 15 weeks of the trial followed by an increase during the last four weeks was attributed to seasonal changes in the pasture condition (16).
According to the parasitological incidence and PCVs, there is no difference between the incidence of tsetse-transmitted trypanosomosis in deltamethrin-treated and untreated herds.
Any repellent or irritant effect of the deltamethrin pour-on, applied at the dose rate and treatment interval used in this trial, affecting the preference of tsetse for either treated or untreated animals cannot be excluded from the current experimental set-up. Nonetheless, results indicate that even if such effects do occur, they are too small to reduce the trypanosomosis incidence to a level that would be a direct benefit accruing to the owners of treated animals.
Consequently, the effect of deltamethrin-treatment of cattle on the incidence of tsetse-transmitted trypanosomosis observed in other experiments or control campaigns seems to be a result of its effect on the population density of tsetse or tsetse challenge rather than its direct effect on the tsetse's feeding response. Successful control of tsetse-transmitted trypanosomosis using deltamethrin-treated cattle (at a dose rate of 10 ml Spoton ® /100 kg body weight and at monthly treatment intervals) will, therefore, depend on the level of induced tsetse mortality and tsetse invasion pressure. The use of this tsetse control method in areas where, due to whatever reason, the tsetse population density cannot be sufficiently lowered to reduce disease challenge will not result in a decline in trypanosomosis incidence.
Trypanosomosis incidence is determined by various host and vector related parameters (14). Calculated disease transmission thresholds and basic rates of reproduction emphasize the difficulty of controlling trypanosomosis caused by T. vivax or T. congolense by anything other than almost complete control of the vector (14). Consequently, application of pyrethroid-insecticides to hosts or stationary baits will only affect the incidence of trypanosomosis when the tsetse population density has been significantly reduced. in the deltamethrin herd in the control herd 15 17 19 Week Packed cell volume (%) Figure 3: Mean two-weekly PCV-% (± s.e.) in the control and deltamethrin-treated herds.

Resumen
Van den Bossche P., Duchateau L. El efecto de la deltametrina por aplicación de chorreo en ganado, sobre la transmisión de la tripanosomosis bovina Se llevó a cabo, en Zambia, un experimento para evaluar el efecto de tratamientos mensuales por chorreo de deltametrina, sobre las respuestas alimenticias de la tsetse y, consecuentemente, sobre la incidencia en la tripanosomosis transmitida por tsetse en ganado tratado y no tratado con deltametrina. Se compararon cada dos semanas, durante un período de 14 semanas consecutivas, la incidencia de tripanosomosis y el hematocrito en hatos tratados y no tratados con deltametrina, en pastoreo una misma zona. No se encontraron diferencias significativas. Se concluye que los efectos reportados de los tratamientos con deltametrina sobre la incidencia de la tripanosomosis son el resultado de una disminución del ataque de la tsetse, debido a una disminución en la densidad de tsetse en el área y no por un efecto directo de los tratamientos con deltametrina sobre la respuesta alimenticia de la tsetse. Por lo tanto, este método de control tendrá mayor efecto en aquellas áreas en las que la densidad de población de la tsetse puede reducirse suficientemente, como para afectar en forma significativa la transmisión de la enfermedad.