Transmission of South American Trypanosoma vivax by the neotropical horsefly Tabanus nebulosus

A study was carried out to assess whether transmission of Colombian T. vivax could be achieved by interrupting tabanids feeding on a parasitaemic host and transferring them to a 2nd host to resume their blood meal. In April 1988, two 3-day-old male Friesian calves (No.1 and No.2) were bought in Bogota where natural infections with T. vivax have so far not been recorded. In June, the calves were transferred to the tropical north coast where they were kept in a fly-proof stable. The calves were examined for infection by blood smear and the Haematocrit Centrifuge Technique (HCT) twice a week. On 2 and 3 December, a highly parasitaemic steer (approximately 1-5 X 105 parasite/ml), was tied to a tree and tabanids alighting on the steer were caught and removed with the aid of a drinking glass after partial engorgement. The tabanids were taken to the isolation pen and placed on the back of calf No.1 which had been tranquillized. Tabanids were released outside the pen after they had been given the opportunity to terminate their meal. Approximately 3 weeks later, the experiment was repeated with calf No.2 as receptor, using calf No.1, which had become infected, as donor. No haemoparasite infections could be detected by direct parasitological methods in either of the calves prior to the transmission experiment. Three tabanid species were caught while feeding on the infected steer (Tabanus pungens, T. claripennis and T. nebulosus). On more than 20 occasions, T. pungens and T. claripennis did not resume a blood meal after having been captured, whereas T. nebulosus quite readily resumed an interrupted meal. The time between the 2 blood meals ranged from 1-10 min. Trypanosoma vivax was detected in the bloodstream of calf No.1 on 12 December, i.e. 10 days after the first tabanids had fed on the calf. Clinical signs began to show approximately 2 weeks after the onset of parasitaemia and consisted of progressive debilitation up to a point where the calf was unable to rise. The calf died 12 days after treatment with Berenil. In the 2nd transmission attempt with calf No.2, the data were similar to those seen in calf No.1, but the calf recovered after treatment. The experiment demonstrates that Tabanus nebulosus is capable of transmitting local T. vivax from an animal with high parasitaemia to a susceptible host when an interrupted blood meal is resumed within 10 min or less.