POPULATION ECOLOGY OF GLOSSINA PALPALIS IN RELATION TO ANIMAL TRYPANOSOMIASIS IN THE SOUTHERN GUINEA SAVANNAH, NIGERIA

Abstract

Entomological and parasitological survey was undertaken, November 1999-October 2001, in the agaropastoral area of Manchok in the southern Guinea Savanna zone of Kaduna State, Nigeria which had been previously sprayed with chemical insecticide in the 1960s to control the vector of animal trypanosomiasis. Detailed ecological studies on the vector (Glossina) were conducted in a relict forest of River Kajim, Manchok, while Studies concentrated on the hosts (ruminants) and parasites were undertaken throughout the Kaura LGA.

A total of 1492 tsetse flies were caught by acetone-baited biconical and nitse traps placed 100m apart in the relict forest. Of this number, 928 were from biconical and 564 from nitse traps. Additionally, 690 tsetse were taken in 22 other locations within the Local Government Area (LGA) during 132 trap-days. Although both Glossina palpalis palpalis and G. tachinoides used to occur in the area, only the former species was encountered during the present investigation. The fly population was characterised by a lower abundance in the dry season. Abundance was constantly higher in the larger riverine habitats of Chori, Biniki, Tum, Zakwo, Kukum/Daji, Randiyam and Ashim villages.

The nutritional state of the population was distributed as follows: intermediate (52.6%) and hungry (47.4%). For both sexes, the mean hunger stage (MHS) for all non-tenerals was 3.6. flight activity commenced about 09.00 hr during the wet season and 10.00 hr in the dry season; in both seasons, diurnal activity was less pronounced in the early mornings (06.00-11.00 hr) than mid-afternoon (11.00-15.00 hr) and late afternoon (1500-1800 hr). the difference in feeding activity patterns between sexes was marked but not significant (P>0.05) during both dry and wet seasons.

The reproductive status of the 272 female flies dissected was: 80.2% were at different stages of pregnancy, 11.3 had aborted and 8.5 had empty uteri. Monthly pregnancy rates varied within the range, 57.1%-100.0%. Abortion, insemination failure and ovular blockage were the three reproductive abnormalities encountered in 9.9% of the females. Mean longevity of both sexes was approximately 30 days. Within the 0.03 km2 of riverine vegetation at River Kajim, population estimates using mark-recapture and removal trapping experiments indicated mean absolute populations of 189, 248 and 349 flies during the early dry, mid-dry, early-wet and mid-wet seasons respectively, November 2000-July 2001, with a population range of 6300-11633 flies/km2

The overall trypanosome infection rate of tsetse caught in the relict forest was 18.8% (74/409) while the mean infection rate of flies caught in the other locations was 9.9% (68/690). Infection rate was lowest in the dry season and highest in the mid wet season. For mature infections from both sources, Trypanosoma Vivax predominated (54.1%) over Trypanosoma congolense (16.2%) while the inmature procyclics accounted for 29.7% of the total infection. Female infection rate was 68.9% while the rates for males was 31.1%. the total numbers of tsetse landing on one cow per day (06.00-18.00 hr) were 57 and 77 during dry and wet seasons respectively. The numbers of potentially infective bites during the same periods were approximately 15 and 19 respectively, with each animal receiving an estimated 12.6 and 17.3 T. vivax infective bites and 2.2 and 1.5 T. congolense infective bites. The mean trypanosome infection rate from 432 bovines and ovines was 21.5%. infection rates with different trypanosome species occurred in the following order: T. vivax (13.7%), T. brucei (4.2%) and T. congolense (3.5%). Prevalence rates varied significantly between districts (P<0.05), ranging from 9.7% at Zankan to 43.8% at Manchok.

A total of 15 species of other blood-sucking dipterans, in 4 genera were collected. The genus Tabanus contained 10 of the total species collected, followed by Stomoxys (2), Haematopota (2) and Chrysops (1)