Effect of tsetse control on trypanosome prevalence in livestock: problems of experimental design and statistical interpretation--a case study in northern Côte d'Ivoire

Use of deltamethrin 'pour-on' insecticide for the control of cattle trypanosomosis in the presence of high tsetse invasion

A deltamethrin 'pour-on' insecticide was applied monthly to over 2000 cattle exposed to a high challenge of drug-resistant trypanosomes and high tsetse re-invasion pressure in the Ghibe valley, south-west Ethiopia. Blood samples were taken monthly from an average of 760 cattle for determination of PCV and presence of trypanosomes. The area of the valley is approximately 350 km2 and the cattle grazed in roughly four locations covering about a quarter to half of the area. Two years before the trial commenced, Glossina morsitans submorsitans Newstead (Diptera: Glossinidae) began to invade the valley. Despite the use of the pour-on the mean apparent density of G. m. submorsitans continued to rise, and, during the 4 years of tsetse control, was more than three-fold higher than that recorded during the previous 18 months. Over the same period there was little change in the apparent density of Glossina pallidipes Austen (Diptera: Glossinidae). By contrast, the mean monthly prevalence of trypanosome infections in cattle over 36 months of age decreased from 38.3 to 29.0%, the incidence of new infections decreased from 26.6 to 16.0% (a reduction of 40%), and packed cell volume in cattle increased from 21.7 to 24.1%. Evidence of a change in apparent parasite transmission rate was demonstrated by regression of infection incidence in cattle on the logarithm of apparent density of G. m. submorsitans. Before the trial started the regression coefficient was 45.8 +/- 6.3 and this reduced to 9.2 +/- 2.5% incidence per log(e) (flies/trap/day) during the period of tsetse control. It was concluded that this indicated reductions in tsetse numbers in the immediate vicinities of cattle in a way that was not reflected in overall tsetse catches. Nevertheless, the comparatively high levels of trypanosome prevalence that persisted in the cattle demonstrates that, where invasion prevalence is high, treatment of small pockets of cattle will not eradicate tsetse. To achieve more significant reduction in trypanosome prevalence in cattle, integrated methods of control utilizing target barriers in the major routes of invasion will be needed.

Feeding patterns of three sympatric tsetse species (Glossina spp.) (Diptera: Glossinidae) in the preforest zone of Côte d'Ivoire

The feeding patterns of Glossina longipalpis Wiedemann 1830, G. medicorum Austen 1911, G. palpalis gambiensis Vanderplank 1949 and G. p. palpalis Robineau-Desvoidy 1830 are described from natural habitats in central Côte d'Ivoire where these tsetse species occurred sympatrically. Blood-meal identification of tsetse flies revealed that in natural habitats wild ruminants were by far the most frequent source of food for each Glossina species, but there were significant differences between the nutritional spectra of single fly species. G. p. gambiensis fed significantly less often on bushbuck and significantly more often on monitor lizard (Varamus niloticus) than both, G. longipalpis and G. medicorum. In G. p. gambiensis the blood of wild ruminant species was significantly more often found than in G. p. palpalis, whereas the latter fed significantly more often on domestic animals. Peridomestic populations of G. longipalpis and G. p. palpalis fed mostly on domestic pig and therefore had significantly reduced host spectra in comparison to natural populations. The significant differences in feeding patterns among the investigated species, subspecies and populations seem not to depend on species specific feeding preferences. Rather, they can be attributed to microhabitat specialization of the various tsetse groups and hence mainly to the different availability of hosts. This implies that environmental factors should be taken more into account when analysing and comparing the feeding patterns of tsetse.

Susceptibility of N'Dama cattle to experimental challenge and cross-species superchallenges with bloodstream forms of Trypanosoma congolense and T. vivax

Susceptibility to Trypanosoma congolense, T. vivax challenge and cross species-superchallenges, and related effects on health and productivity were assessed in N'Dama cattle. Twenty-five N'Dama bulls aged 3-4 years and previously primed with trypanosome infections through natural tsetse exposure over more than one year were used. The experimental herd was divided in five groups each composed of five randomly selected animals. Group 1 was challenged with T. congolense, Group 2 with T. vivax, Group 3 was inoculated with T. congolense followed by a cross-superchallenge with T. vivax, Group 4 was inoculated with T. vivax followed by T. congolense cross-superchallenge. Animals in Group 5 were used as controls. Both T. vivax and T. congolense cross-superchallenges were carried out on Day 14 subsequent to respective initial T. congolense and T. vivax inoculations. All challenges were performed by intradermal needle inoculation of stocks of trypanosome bloodstream forms. In challenged animals (Group 1 to 4), parasitaemia profiles and packed red cell volumes (PCV) were measured for four months. Weight changes were recorded monthly and daily weight gain (DWG) computed. All cattle challenged with T. congolense became parasitaemic. Conversely, one animal in Group 2 and two in Group 3 never displayed patent T. vivax parasitaemia. Both in single (Group 1), initial (Group 3) and cross-superchallenged (Group 4) cattle higher percentage of positive blood samples and higher parasitaemia level were obtained following T. congolense than T. vivax inocula (Group 2, 3 and 4) (P<0.04 or greater). Overall the pre-challenge period, PCV values and DWGs were nearly identical in the five groups. Conversely, over the post-challenge period, cattle singly, initially and cross-superinoculated with T. congolense (Group 1, 3 and 4) displayed lower PCV values and DWGs in comparison with both control animals (Group 5) and with singly T. vivax challenged cattle (Group 2) (P<0.05 or greater). No difference in mean PCV levels and DWGs was found between animals in Group 2 and cattle in Group 5. It was concluded that trypanotolerant N'Dama cattle suffered more from T. congolense and mixed T. congolensel T. vivax infections, while pure T. vivax infection did not produce appreciable negative effects on their health and productivity. Therefore, considering that tsetse and trypanosomosis control campaigns are costly and are justified only when derived economic benefits exceed those of control, and also that an ample mosaic of farming systems exists in West Africa, species-specific trypanosome prevalence and relative impact should be assessed in various cattle populations and breeds differing in trypanosome susceptibility before advising any intervention. Moreover, virulence and related effects of T. congolense and T. vivax endemic stocks on health and productivity in local cattle populations should also be estimated in order to counsel appropriate economic protection measures against trypanosmosis, i.e. vector control and/or strategic use of trypanocidal drugs.

Use of insecticide-impregnated targets for the control of tsetse flies (Glossina spp.) and trypanosomiasis occurring in cattle in an area of south-west Ethiopia with a high prevalence of drug-resistant trypanosomes

In the Ghibe valley, south-west Ethiopia, a tsetse control trial using deltamethrin-impregnated targets was started in May 1990. The mean relative density of the main vector, Glossina pallidipes, fell from 2.1 flies per trap per day in the 12 months prior to introduction of tsetse control to 0.41 flies per trap per day in the 12 months after tsetse control was initiated. The annual mean prevalence of Trypanosoma congolense infections in cattle fell from 32% in the 2 years before tsetse control to 13% in the 2 years following deployment of targets. The largest reduction occurred in the first quarter of 1991 when the mean monthly trypanosome prevalence was 5%. There was also a decrease of approximately 75% in the estimated rate of relapse of trypanosome infections in cattle after treatment with diminazene aceturate. However, in June 1991, socio-political disturbances occurred in Ethiopia and were associated with substantial thefts of targets. Following these thefts, the mean relative density of G. pallidipes rose from 0.41 flies per trap per day in the period May 1990-April 1991, to 1.29 flies per trap per day in the period April 1992-March 1993. Associated with this rise, the mean trypanosome prevalence in cattle increased from 13% in the period May 1991-April 1992 to 28% in the period May 1992-April 1993. This was similar to the mean trypanosome prevalence in cattle during the pre-control period. Thus, while effective tsetse control methods can be used to reduce the transmission of trypanosomes their long-term impact is dependent on their sustainability.