TRANSMISSION PATTERNS OF TRYPANOSOMES IN FLY VECTOR POPULATIONS AND BOVINE HOST IN SOUTHWESTERN NIGERIA

Abstract

Bovine trypanosomosis is still a major health problem causing severe morbidity and mortality, and the control of the disease is centered on Glossina, the main vector. Information on other vectors (Tabanus and stomoxyine) transmitting trypanosomes in southwestern Nigeria are lacking. This study was designed to investigate the transmission patterns of trypanosome in vectors and cattle to enable an effective control strategy. Fifteen cattle settlements and institutional farms in the six southwestern states were randomly selected and Nzi traps set to capture transmitting vectors between April 2016 and March 2017. The vectors were identified using morphology, 28S rRNA PCR and Sanger sequencing. Fly abundance with environmental variables, distribution, endosymbionts, bloodmeal sources and feeding behaviour were investigated using standard methods. Insecticidal effects of cypermethrin against fly vectors were assessed using standard methods. Blood (125 µL) was randomly collected from 745 cattle during the same period in some of the locations. The vectors and cattle blood were screened for trypanosomes using 18S rRNA PCR, while Trypanozoon DNA positives were screened for RoTat 1.2 VSG and TgsGP genes. Livestock owners (n = 209) were interviewed using structured questionnaire to determine control methods. Relevant publications in databases from 1960-2017 were screened using PRIMSA checklist to analyse the southwest Nigeria estimates of trypanosome prevalence in livestock and Glossina. Data were analysed using descriptive statistics and one way ANOVA at α0.05. The identified transmitting vectors belong to the genera Glossina, Tabanus and Stomoxys. The total apparent density of trapped flies [Glossina (0.6 flies/trap), Tabanus (0.2 flies/trap), Stomoxyine (36.2 flies/trap)] was highest at temperature of 26-28°C, humidity >80.0% and rainfall of 150-220 mm/month. Distribution of flies increased with vegetation density and decreased in areas with high human population density (>100/km2 ). Sodalis glossinidus (31.3%) was the only endosymbiont in Glossina. Sequenced cytochrome-B mitochondrial DNA segments revealed eight different host species as bloodmeal sources. On the average 78.0%, 19.6% and 10.2% Glossina, Tabanus and Stomoxyine, respectively were fully engorged. Cypermethrin was effective on Stomoxyine using restricted insecticidal application protocol, but ineffective using conventional pour-on protocol. Overall, 71.1%, 33.3% and 22.2% Glossina, Tabanus and Stomoxyine, respectively were positive for trypanosomes. Trypanosomes were identified in Glossina (T. vivax 36.4%, T. brucei 53.5%, T. congolense viii 5.4% and T. simiae 4.7%), Tabanus (T. vivax 43.5%, T. evansi 39.1%, T. simiae 8.7% and T. godfreyi 8.7%), and Stomoxyine (T. vivax 69.2%, T. evansi 20.5%, T. congolense 5.1% and T. simiae 5.1%). Trypanosome DNA was detected in 23.8% of cattle examined with significant increase in Osun State compared to other states. The species identified were T. vivax (43.4%), T. congolense (31.8%), T. brucei (18.2%) and T. evansi (6.6%). Livestock owners (93.9%) used trypanocides, while 60.5% used insecticides. Published articles revealed overall prevalence of bovine trypanosomosis at 21.3% and trypanosome prevalence of 42.6% from 4,808 Glossina species. The mechanical vectors (Tabanus and Stomoxyine) are as important as biological vector (Glossina) in the transmission of bovine trypanosomosis in southwest Nigeria. The distribution and vectorial capacity of captured vector species warrants a new effective control strategy against trypanosomes.