The tsetse fly survey well corresponds to the general classification of tsetse flies in terms of their habitat and requirements, particularly to the ecology of tsetse species found in the Ghibe valley, southwest of Ethiopia, [9]. The results obtained from this trial show that, tsetse overall apparent density significantly reduced from a mean catch of 17.3 ± 8.6 flies per trap per day of pre-control to 3.7 ± 2.6 flies per trap per day at last monitoring. This resulted in a 78.6% overall mean catch reduction. According to t-test statistics for equality of overall mean catch of flies pre- and post-intervention, the mean catch difference, 13.6%±1.2 SE, was found statistically significant; p=0.000.
The quantitative and qualitative variation in tsetse population, population dynamics, depend directly on biotic and abiotic factors in the environment, seasonal variation in the population size or more precisely in the apparent density is linked essentially to the life span of the imagos, on one hand, and the pupal mortality on the other, [10]. Having in mind the environmental factors contribution on tsetse density, the observed reduction in tsetse density was mainly associated with the pronounced effect of the insecticide based intervention in which it attributed on the adult tsetse shortened life span. This is because, the last monitoring survey was carried out after the short rainy season (April-May) in June 2019 when biotic and abiotic factors in the environment favored tsetse reproduction; during when the life span of the adults increases and the duration of the development of the pupa reaches its optimum, however, the presence of suitable habitat alone is not always in favor of the flies’ reproduction. In this regard, Hargrove, J. W. (2000), [11], stated that as vegetation is vital for providing shade and maintains a suitable microclimate for tsetse as well as habitat for their hosts, and provides flies enough moisture, replenishment of tsetse density rapidly gets better as soon as the first rains have reduced the ambient and soil temperature. Thus, the rapid decline in tsetse density was caused by the pour-on based intervention. This can be evidenced by the drastic decline of tsetse (by 76.7 %) realized three months after nearly 18,311 animals were treated at first time, in November 2017, (Graph 1). Incidentally, with this amount of insecticide used, only 13,104 animals could have been treated if the standard treatment regime was followed, and this signifies a 40% reduction in the amount of insecticide used. The Pearson Chi-Square test statistics too evidenced the presence of strong association between the intervention and tsetse density reduction; X2 0.05= 73.5; p=0.000.
Graph 1. Impact of RAIC technique on tsetse density and its future linear trend to suppress tsetse
Following the restricted application of insecticide at a density of 15 animals per km2 area with nearly three months base, at last monitoring, the relative abundance of G.f.fuscipes was significantly declined from a pre-intervention mean catch by 91.3% followed by G.pallidipes, (76.1%) and G.m.submorsitans, (55.1%), (Table 2), and the relative reduction within the groups pre- and post-intervention was found statistically significant; p=0.000. In this context, two aspects are important for the largest reduction observed in G.f.fuscipes: firstly, this species largely have a preference for feeding on bovid bloodmeal as a result fly-cattle contact was high; secondly, cattle in the present study area graze deep into the valley floor where there is suitable habitat for the specified species in which the fly-cattle contact, once more was very high; thus, these facts permits us to conclude that G.f.fuscipes species in the present area, are the main vectors of trypanosomes; however, a blood meal analysis was not done. Similar conclusion can also be made regarding with G. pallidipes; since it was the second most reduced species. This conclusion is in consistence with the previous studies conducted by [12] who stated that G. pallidipes and G. f. fuscipes feed mainly on livestock and G. pallidipes is the main vector of animal trypanosomosis.
Table2.Relative FTD of tsetse flies pre- and post-intervention time and their relative reduction
Study
Area
|
Fly Species
|
Survey Time
|
No. of Set up Traps
|
M. Catch/
Trap
|
SD
|
MCD
(95% CI)
|
SE
Mean
|
R.FTD
|
RR
(%)
|
Sig. (P-value)
at 0.05 level
|
|
Botor
Tolay
|
G.m.sm
G.pallidipes
|
Pre- Intervention
Post -Intervention
|
75
|
4.9
|
3.9
|
3.3 (2.5, 4.1)
|
0.4
|
2.5
|
68.0
|
0.000
|
56
|
1.6
|
1.3
|
|
|
0.8
|
|
|
Pre- Intervention
Post -Intervention
|
75
|
1.6
|
1.3
|
1.0 (0.6, 1.4)
|
0.2
|
0.8
|
62.5
|
0.000
|
56
|
0.6
|
0.9
|
|
|
0.3
|
|
|
G. f. f
|
Pre- Intervention
Post -Intervention
|
75
56
|
10.7
1.4
|
6.3
1.7
|
9.3 (7.6, 11.0)
|
0.9
|
5.3
|
86.9
|
0.000
|
.i.e. M. Catch/Trap=Mean Catch per Trap; SD=Standard Deviation; MCD= Mean Catch Difference; SE Mean= Standard Error Mean; R.FTD= Relative Fly per Trap per Day; RR=Relative Reduction
Despite a high prevalence of multiple-drug resistant trypanosomes widespread in the target area, the overall mean trypanosome prevalence among the herds dropped from 11.6%±3.2 in November 2017 to the current level of 3.9%±2.3. This resulted in an overall of reduction by 66.4% in which the reduction was found statistically significant; p=0.000. Unlike to pre-intervention period, the dominance of T.congolense being a major cause of infection (63.5%) was reverted, at last monitoring by T.vivax type which accounted for 70.6% of the overall prevalence while the former accounted for a proportion of 23.5%. Significantly higher proportion of infection due to T. congolense, during pre-intervention period, signified cyclical transmission domination than mechanical. The widespread occurrence of T.vivax type infection at last monitoring, on the other, has got its own advantage for the improved general health levels and productivity of livestock, as it is considered being less virulent for cattle than T. congolense, which appears to cause more sever effect than T.vivax, which could be manifested by the mean PCV-value. Additionally, the possible situations of the influence of trypanocidal drugs on trypanosome infection type never be neglected, [14]. They referred as the higher in T.congolense infection rate in tsetse-infested area could be resulted due to ineffective drug treatments against it to the same degree as T.vivax. This is in compliance with the present area pre-intervention time, where the curative and prophylactic capacities of the three trypanocides have been impaired as a result of widespread occurrence of drug resistance by the parasites. Nonetheless, the presence of high proportion of T. congolense infection is mainly associated with its number of antigenic variability.
Above all, the rare detection of T. brucei infection in sampled animals and its absence in dissected tsetse, may be attributed to the longer development cycle and complex pathway that it takes in the fly which hinder its survival and maturation especially when the life span of the fly is shorten,. Besides, in contrast to T.congolense and T.vivax, infection with trypanzoon species is characterized by generally low parasitaemia and a marked invasion of tissue, the parasite has got a lesser probability to be picked up by the fly while ingesting the blood meal, consequentially, infection rate in tsetse is very low [13], and this has been manifested by the lowest infection rate in cattle.
The epidemiology of African animal trypanosomosis (AAT) is almost entirely dependent on tsetse flies. As a result, the trypanosomal infection type in tsetse is of prime importance in determining the type of infection the animals to be contracted with. Fly species differ in their capacity to transmit trypanosomes. Morsitans group flies, except for G.austeni, are good vectors of all trypanosome species. To the contrary, Palpalis group species appear to be poor vectors of most trypanosome species except certain stocks of West African T.vivax, although G.f.fuscipes can be important vector of human infective trypanosomes, [15]. This vectoral capacity difference is associated mainly with the type of lectins found in the gut of different species of flies. It has been established that lectin plays a role in determining refractoriness to infection by killing procyclic trypanosomes, whilst symbiotic bacteria are involved in determining susceptibility to infection by a process that results in inhibition of these midgut lectins and thus reduces their killing effect (blocks the lectin-mediated trypanocidal activity), [16]. Glucosil lectin is found in morsitans and fusca groups whose action can be inhibited by D-(+) glucosamin which is secreted by maternally inherited Rickettsia-like organisms (RLO) while palpalis group species possess the galactosil lectin whose action is not inhibited by D-(+) glucosamin. That’s why, palpalis group tsetse species are less refractory to trypanosomes infection as a result of the effect of the non-cellular factor, galactosyl lectin. It acts as agglutinin, which interferes on trypanosome survival and maturation. Based on these general influences on Glossina infection rates, neither all species nor all individuals within a species are equally efficient, [17].
Provided that G.f.fuscipes outnumbered the sum density of G.m. submorsitans and G.pallidipes before the onset of the intervention, and the former is considered good vectors of T.vivax, the vivax type infection proportion in cattle was lower than congolense type. As shown in the present study, however, both groups of tsetse (riverine and savannah) harbored T.congolense, the highest, (75%), infection rate of T.congolense in the flies was observed in G.pallidipes and G.m.submorsitans which indicates the important role of these species, playing significant role in transmitting T.congolense while they were prevalent with relatively lower number; this is in agreement with many studies which has shown that palpalis group flies are poor vectors of trypanosomes especially, T.congolense. Additionally, the possible situations of the influence of trypanocidal drugs on trypanosome infection type never be neglected, [14]. They referred as the higher in T.congolense infection rate in tsetse-infested area could be resulted due to ineffective drug treatments against it to the same degree as T.vivax. This is in compliance with the present area pre-intervention time, where the curative and prophylactic capacities of the three trypanocides have been impaired as a result of widespread occurrence of drug resistance by the parasites. Nonetheless, the presence of high proportion of T. congolense infection is mainly associated with its number of antigenic variability.
Nonetheless, post intervention period even if these savannah species were more abundant than G.f.fuscipes, congolense type infection in cattle, by far lessen from vivax type. In this context, some possible explanations can be given based on the epidemiology of trypanosomosis: firstly, higher proportions of infection were transmitted mechanically as compared to cyclical transmission, which in turn signifies the importance of nuisance flies in the area, [18]; this is because T.vivax is the species most likely to be transmitted mechanically, other species of trypanosomes pathogenic to cattle can also be transmitted and Glossina spp. may also act as mechanical as well as cyclical vectors [19]; secondly, the proportion of T.vivax infections in tsetse increases under adverse environmental conditions (in our case the progressed tsetse control) when the flees’ shortened life span prevented the completion of cyclical development of the Nannomonas (T.congolense) and Trypanzoon (T.brucei) trypanosome species [20]; consequentially, transmission of these parasites were hindered; thirdly, high T.vivax infection rates in flies arouse for the fact that T.vivax develops in the proboscis, it is far from the action of anti-trypanosomal factor of the flies’ gut, in which beside to shorter development cycle it follows, absence of this trypanosomal action on T.vivax enabled it to be found in higher proportion in tsetse consequentially, in naturally infected cattle. Finally, higher T.vivax type infection in cattle usually encounters when the overall prevalence of trypanosomosis in cattle is low. Off course, the species of animal on which tsetse fed, exerts the greatest influence on the infection rate in flies. Whiteside, E.F (1958), [21] and Jordan, A. M. (1986), [22] identified at least 18 major variables influencing the potential of trypanosomes to develop in tsetse flies (the epidemiology of African animal trypanosomosis), which relate to interactions of tsetse (as endogenous factors associated with tsetse), ecological factors and parasite and host factors, .
One of the major effects of infection with pathogenic trypanosomes is anemia. Measurement of anemia gives a reliable indication of disease status and is correlated with parasitaemia-the higher the parasitaemia, the lower the PCVs. As a result of decline of trypanosome prevalence in the herds, the overall mean PCV-value of animals improved on average by 2.0%. Thus, PCV is a good indicator of the health status of the herd in an endemic area, which negatively correlates to disease incidence. Nonetheless, 44.3% of the animals’ mean PCV values during pre-intervention period and 28.9% of the animals during last monitoring were below the threshold of 25% normal PCV-value.
In this regard, partially or semi-trypanotolerant cattle living in and in close proximity to the target areas were observed. Some animals achieved better control of parasitaemia while they were infected by maintaining PCV in its normal range and this could be an indicative that deferent serodemes of trypanosomes are circulating in the area, against to which individual animals developed resistance, [23]. On the other hand, some animals PCV-value was lower than the threshold level, while they were not infected with trypanosomes, which suggests that anemia is a multi-factorial problem in the area in which management, exercise and heat stress could be attributable [24].However, management factors share the major contributions to anemia, contribution of concurrent infections such as internal, external parasites and other heamoparasites is not negligible. The degree of anemia is directly correlated with the loss of productivity performance and could be a major contributor to death of trypanosome-infected cattle.
Therefore, in areas where multiple drag resistance is widespread, the use of 'pour-on' insecticides, may help as a more sustainable method of tsetse control, consequentially, trypanosomosis, even without integrating the technique with other tsetse control methods. Nonetheless, the effectiveness of this technique works best where livestock are the main host of tsetse. The results of this field trial are in consistent with the findings of Leak, S.J.A., 1995, in which
Table 3. Summary statistics for pre- & post-intervention relative trypanosome prevalence and mean PCV value
Intervention time
|
Infection type
|
No of animals infected
|
Mean PCV
(95% CI)
|
Std.
Dev.
|
Relative prevalence (%)
|
Disease proportion (%)
|
Relative
reduction
(%)
|
Sig. at 0.05 level
|
Pre-intervention
|
T. congolense
|
40
|
19.4 (17.6, 20.5)
|
4.3
|
7.4
|
65.6
|
90
|
P=0.000
|
Post- intervention
|
|
4
|
20.8 (19,0, 22.0)
|
1.3
|
1.2
|
23.5
|
|
|
Pre-intervention
|
T.vivax
|
17
|
24.4 (21.6, 27.0)
|
5.2
|
3.1
|
27.9
|
29.4
|
P=0.000
|
Post- intervention
|
|
12
|
22.3 (19.0, 26,0)
|
2.3
|
3.6
|
70.6
|
|
|
Pre-intervention
|
Mixed
|
4
|
20.0 (13.0, 26.9)
|
4.4
|
0.7
|
6.5
|
75
|
P=0.000
|
Post- intervention
|
|
1
|
25.0
|
|
|
5.9
|
|
|
Pre-intervention
|
Negative
|
479
|
26.4 (18.0, 37.0)
|
3.6
|
|
|
|
|
Post- intervention
|
|
415
|
26.8 (26.4, 17.1)
|
3.3
|
|
|
|
|
95%CI= Confidence interval; Std. Dev = Standard Deviation
ITC control technique used in the present area, with a standard scenario that aimed at reducing G.pallidipes and G. m. suhmorsitans populations at Tolley/Gullele using cypermethrin 2% 'pour-on' formulation [25].
The standard (application of insecticide on the back of cattle) tsetse control technique so far employed, has got a potential to reduce both tsetse population and the disease prevalence within a short time. However, treating cattle with insecticide is an increasingly important means of controlling tsetse flies as livestock keepers in particular and the national economy at
large, it has been hindered by high cost of the insecticide. Use of this control method with lesser amount in more cost-effective manner could be a reliable solution to the problem. In this regard, restricted application of insecticides to cattle has been recognized as a cheap, safe and environment friend farmer-based method to control tsetse and trypanosomosis, [26]. The present findings have assured that the restriction of pyrethroid application to only the belly and legs parts of the body by far reduces not only the amount of insecticide needed per application, according to the present trial by 40% that needed for whole body application. They have also proved its great potential to suppress tsetse population, consequentially, trypanosomosis incidence to prescribed low levels. The technique could be more useful in areas where the creation of fly-free zones is challenging and reinvasion pressure high. The technique provides an extra benefit, compared with targets as nuisance flies and ticks may also be controlled. Since the whole body treatment regime can contaminate the dung sufficiently to affect dung fauna, thereby threatening the important role that such fauna play in dung dispersal, and hence soil fertility and the productivity of pastures and crops [27]. The occurrence of trypanosomal infections in areas apparently free of tsetse promoted that infection can be maintained in nature by the mechanical transmission of trypanosomes by other heamatophagous flies [28, 29]. As other study has shown, since other biting flies concentrate on the lower legs and belly [30] the technique is applicable even for biting flies (e.g. Stomoxys, Tabanus spp. etc.) especially during the late rainy season when these flies are more abundant. Besides, as is proved by [31], Anopheles arabiensis Patton (Diptera: Culicidae) is the most widespread vector of malaria in the Afrotropical Region. Because Anopheles arabiensis feeds readily on cattle as well as humans, the insecticide-treatment of cattle as employed to control tsetse (Diptera: Glossinidae) and ticks (Acari: Ixodidae) might simultaneously affect the malaria vectorial capacity of this mosquito.
Ultimately, application of insecticide in standard or restricted scenario does not have a significant difference in tsetse suppresion except the later system lessens the amount of insecticide used and the impact on non-target organisms. As is shown in the above graph, the technique linear trend to suppress the vector can lead to dramatic decline of tsetse populaiton to close to zero, if repeated treatement of animals is achieved. Tsetse control (eradication if possible) is the most reliable and effective alternative strategy available to date, towards adequately reducing and finally removing tsetse transmitted trypanosomosis risks and losses.