Preliminary survey on equine trypanosomosis and its vectors in Asosa and Homosha districts in Benishangul Gumuz Regional State, northwest Ethiopia

R Abebe and A Wolde

Hawassa University, Faculty of Veterinary Medicine, P. O. Box 05, Hawassa, Ethiopia
rahmetoa@yahoo.com

Abstract

A preliminary survey was conducted in January 2009 in four peasant associations (PAs) selected from Asosa and Homosha districts in Benishangul Gumuz Regional State, northwest Ethiopia to estimate the prevalence of trypanosome infection in equine species, identify the trypanosomes species encountered and investigate flies playing a role in the transmission of trypanosomosis. Blood samples were collected from a total of 334 donkeys and 52 mules and examined by dark ground/phase contrast buffy coat technique and Giemsa-stained blood smears.

The over all prevalence of trypanosome infection in donkeys was 6.3% (n = 21). Prevalence was significantly (P<0.05) different between the two districts. None of the mules examined was positive for trypanosome infection. Trypanosoma congolense (52.4%) was the most common trypanosome species encountered during the study period followed by Trypanosoma brucei (28.6%) and Trypanosoma vivax (19.05%). Trypanosome infections were not detected from two of the four PAs surveyed most likely due to the absence of appropriate fly vectors. All the donkeys infected were above two years of age. No significant difference was observed between male and female donkeys regarding to both prevalence of infection and mean PCV (P>0.05 for each factor). The mean PCV (%) of infected donkeys (28.6±1.1) was significantly (P<0.05) lower than that of non-infected donkeys (31.4±0.03). The body condition score of the donkeys was significantly associated with both prevalence of infection and mean PCV (P<0.05).  The fly survey result revealed the existence of Glossina (tsetse fly) and other biting flies belonging to the family Stomoxys, Tabanus, and Haematopota. Glossina morsitans sub morsitans was the only cyclical vector detected in the study area. 

In conclusion, the prevalence of trypanosomosis obtained in the current study is generally low compared to previous studies. As the present study design was a cross-sectional one that only depicts a momentary picture of the infection status in the herd, a further longitudinal study that makes use of serological techniques and entomological survey is recommended.

Key words: Donkeys, Ethiopia, PCV, prevalence, trypanosomes

Introduction

African animal trypanosomosis is one of the major impediments to livestock development and agricultural production in Ethiopia contributing negatively to the overall development in general and to food self-reliance efforts of the nation in particular. While tsetse-borne trypanosomosis is excluding some 180,000–200,000 km² of agriculturally suitable land in the west and southwest of the country; 14 million head of cattle, an equivalent number of small ruminants, nearly 7 million equines and 1.8 million camels are at the risk of contracting trypanosomosis at any one time (Langridge 1976; MoARD 2004).

There are many well documented studies addressing the problem of bovine trypanosomosis in Ethiopia but there is currently very little information about equine trypanosomosis regardless of the fact that these animals play a key role in the agricultural economy of the country where poor infrastructure and very ragged topography in many parts of rural Ethiopia have made transportation by vehicle inaccessible. They are used for pack transportation, riding, carting, and threshing farm cultivation among others. The available scant data suggest that trypanosomosis is among the major health constraints of equine in tsetse infested areas of the country. Yimam (1993) indicated a prevalence of 21% in horses in northern Omo Zone, southern Ethiopia of which 44.1% was due to Trypanosoma vivax, 36.9% to Trypanosoma congolense and 19.0% to Trypanosoma brucei. Kanchula and Abebe (1997) reported donkey trypanosomosis with a prevalence of 21% in the same site. Similarly a 28.5% prevalence of donkey trypanosomosis has been reported by Shelima et al (2006a, b) in a study conducted in Wolayta zone, southern Ethiopia. Further more, Shelima et al (2006b) have stated that trypanosomosis was claimed by farmers to be the leading health constraint of donkeys in the area.

On the other hand, all the available reports are from the southern part of the country and there are no reports from remote States such as Benishangul Gumuz Regional State, where there is large equine population and widespread practice of equine for agricultural activities. The objective of this study was therefore, (i) to estimate the prevalence of equine trypanosomosis, (ii) identify the trypanosomes species involved, and (iii) assess the distribution and density of fly vectors responsible for transmitting the disease in the study area.

Materials and methods 

Study area

The study was conducted in January 2009 in Asosa and Homosha districts selected from Assosa zone, Benishangul Gumuz Regional state. The districts are located at a distance of 675kms north west of Addis Ababa. The criterion for selection of the two districts was their altitudinal variation. Two peasant associations (PAs) were selected from each district based on accessibility. PAs are the smallest administrative units next to district in Ethiopia. The PAs included in the study were Ura, Tsetsa adurno, Ashura, and Bamadone. The first two PAs were selected from Asosa district and the latter two from Homosha district. Geographically Assosa district lies between 8⁰ 30’ and 40⁰ 27’ N latitude and 34⁰ 21’ and 39⁰ 1’ E longitude whereas Homosha district is located between 6⁰ 44` and 6<sup>0</sup> 84` N latitude and 37⁰ 92’ and 38⁰ 6’ E longitude. Altitudes are 1464 meters and 1380 meters above sea level in Asosa and Homosha districts, respectively. The climate is characterized by a long rainy season (June-September accounting for 75% of the total rainfall), a short rainy season (February/March to April/May) and a dry season (October-January). The districts receive an average rainfall of 950-1000 mm annually. The average temperatures are 30⁰c in Assosa and 29⁰c in Homosha district.

Study animals and sampling strategy

The study animals were indigenous breeds of donkeys and mules kept under extensive husbandry which allows free grazing, usually mixed with other livestock in the villages. Mules are used mainly to transport peoples from one place to another. Donkeys are the main type of pack animals in the area used to transport peoples, water, house hold materials, crops, and firewood from one place to another. The equine population in the study area is estimated to be 2875. A total of 386 equines comprising 334 donkeys and 52 mules were selected from the study population by simple random sampling technique according to Thrusfield (1995) with 95% confidence interval, 5% desired absolute precision and 50% expected prevalence. There were no horses in the study area. The age of the selected animals was determined by dentition (Crane 1997) and body condition scoring was based on the criteria of NEWC (2005) as described in Table 1.  

Parasitological examination

Blood samples were collected directly from the ear veins of the study animals into heparinized capillary tubes. The blood samples were examined by the capillary micro-hematocrit centrifugation method to estimate the packed cell volume (PCV) as an indicator of anemia.  After determination of the PCV, the Buffy coat (BC) was examined by dark ground/phase contrast microscope (Murray et al 1983) for the detection of trypanosomes in the blood. For the purpose of species identification, a thin blood smear was prepared from the BC for those samples that were positive on BC examination and stained with Giemsa stain and examined under a microscope using the oil immersion 100x objectives (Murray et al 1983).

Entomological survey

During the study period a total of 60 mono pyramidal traps baited with cow urine, acetone, and octenol were deployed in the four PAs included in the study. The traps were set at approximate intervals of 100-200 meters. All trap positions were geo-referenced by use of hand–held GPS (Garmin 48). The deployment sites include different vegetation types like bush land, wooden grass land, and cultivated land so as to ensure the representation of all habitats that could be related to fly multiplication, behavior, feeding and other related aspects. Traps were allowed to stay at the deployment sites for a period of 48 hours before collection (Dransfield and Brightwell 1992). Caught tsetse flies and other biting flies were counted, identified and sexed but live flies were not dissected for examination of trypanosome infection rate.

Statistical analysis

Data collected from each study animal and laboratory analyses were coded into appropriate variables and entered in Microsoft Excel spread sheet. All statistical analyses were performed using STATA-9 soft ware. The point prevalence was calculated for all data as the number of infected individuals divided by the number of individuals sampled x 100. Categorical data were analyzed by using Chi-square (χ²) test of independence where as Student t-test was used to examine the differences in mean PCV between the study variables. In all the analyses, confidence level was held at 95% and P< 0.05 was required for significance.

Results 

Prevalence of trypanosome infection and species encountered

Results of BC and Giemsa stained blood smear examination in donkeys are given in Table 2.

Out of the total 334 donkeys examined, trypanosomes were detected in 21 (6.3%) animals while none of the 52 mules examined was positive for trypanosomes. Trypanosome infection was found in two of the four PAs surveyed namely Tsetse adurno and Bamadone. Overall, Trypanosoma congolense was the predominant species identified (52.4%) followed by Trypanosoma brucei (28.6%), and Trypanosoma vivax (19.1%).

There was a statistically significant difference in prevalence of trypanosome infection between the two districts and animals with different body condition score (P<0.05) while no significant (P>0.05) variation was seen between male and female donkeys. On the other hand, analysis of the data on the age basis showed that all of the donkeys infected were above 2 years of age (Table 3). 

Hematological findings

The mean (±SD) PCV value of all donkeys tested was 31.2±4.7%. The results of analysis of mean PCV with different factors are given in Table 4.

The mean PCV of infected donkeys (28.6±1.1) was significantly (P<0.05) lower than that of non-infected donkeys (31.4±0.03). Using a PCV value of 30-46% as a normal value (Knottenbelt 2005), 61.9% of the infected and 29.1% of the non-infected animals were found to be anemic. On the other hand a highly significant (P<0.0001) variation was observed between animals with good (32.2±4.4) and poor (29.7±4.8) body condition score while no significant (P>0.05) difference was shown between male and female donkeys.

Entomological survey findings

The results of fly catch are shown in Table 5 and Fig 1.

It was shown that from 60 traps deployed at 4 PAs, a total of 293 flies of different families were caught. Of these 194 (66.2%) belong to the family Stomoxys, 48 (16.4%) Tabanus, 37 (12.6%) Glossina (tsetse flies), and 14 (4.8%) Haematopota. Further more, all Glossina caught were identified to be Glossina morsitans submorsitans.  Stomoxys, Tabanus and Haematopota were caught from all the PAs surveyed whereas; Glossina was captured from Tsetse adurno and Bamadone PAs only where trypanosomes were detected. The overall apparent fly density was 2.44 flies/trap/day. At PA level it was 5.2, 2.17, 1.43, and 1.04 in Tsetse adurno, Bamadone, Ura and Ashura, respectively.

Discussion 

The overall point prevalence (6.30%) of trypanosome infection recorded in donkeys in this study is generally low when compared to previous reports of 18.2 - 28.5% in different districts of southern Ethiopia (Kanchula and Abebe 1997; Assefa and Abebe 2001; Shelima et al 2006a, b). As all these studies used the buffy coat method of diagnosis, the observed differences in prevalence between the present and previous studies cannot be methodological but might be due to reduced tsetse challenge as a result of increased agricultural activities and tsetse control interventions carried out by governmental and non-governmental organizations. Furthermore, investigator competency to detect trypanosomes and variations in geographical and climatic conditions may play a part. Ethiopia is a country where extremes of temperature and rainfall are experienced, altitude being the most important factor (Yilma and Malone 1998). Surprisingly, the prevalence observed in this study was comparable to a 6.2% reported in donkeys in Gambia (Faye et al 2001). The finding that Trypanosoma congolense is the most prevalent trypanosome species in donkeys in this study is in agreement with previous reports of Assefa and Abebe (2001) and Shelima et al (2006a, b). However, it is inconsistent with reports of Kanchula and Abebe (1997) and Yimam (1993) in which Trypanosoma vivax was reported to be the predominant species. The present finding is also in accordance with reports from Kenya (Nudungu et al 1998) and Gambia (Mattioli et al 1994; Faye et al 2001; Dhollander et al 2006).  Glossina morsitans submorsitans was the only cyclical vector detected in the study area and the predominance of Trypanosoma congolense infection in the current study suggests increased contact of donkeys with this tsetse vector. This finding also supports earlier observations of Langridge (1976) who stated that the savanna tsetse flies (Glossina morsitans submorsitans and G. pallidepes) are more efficient transmitters of Trypanosoma congolense than Trypanosoma vivax in the east Africa. Trypanosoma congolense in donkeys causes chronic infection with longer persistence in the blood (Mattioli et al 1994).

The prevalence of trypanosome infection was significantly higher in Asosa than Homosha district. This can be attributed to variation in tsetse fly density between the two districts. The detected apparent fly density (tsetse per trap per day) for Glossina morsitans submorsitans was relatively higher in Asosa (0.43) than in Homosha (0.19). Similar finding has also been reported from a study in Gambia (Snow et al 1996) where trypanosome prevalence was higher in horses and donkeys in areas with high tsetse challenge compared with sites with relatively few flies. Of the four PAs surveyed, trypanosome infection was found only in Tsetsa and Bamadone PAs. The inability to detect trypanosome infection in Ashura and Ura PAs is most likely associated with the absence of tsetse flies in these PAs. This finding confirms earlier study of Sinshaw et al (2006) in three districts bordering Lake Tana in Amhara Regional State where no trypanosomes were detected in all the equines examined due to absence of tsetse flies in the study areas although a large proportion of other biting flies such as Stomoxys and Tabanus were present.

All the mules examined in this study were found to be negative for trypanosome infection. Other than small sample size, this might be attributed to less contact of mules with the tsetse vectors as they are usually kept around villages unless needed for transportation. Unlike mules, donkeys are the major type of pack animals and often travel long distances crossing a high tsetse challenge areas during the day time when fly activity is high and consequently, are more exposed to tsetse flies than mules.

In this study, both the occurrence of trypanosome infection and mean PCV were found to be independent of sex (P>0.05). This finding is consistent with previous reports from Gambia (Dhollander et al 2006) and many other literatures (Urquhart et al 1996; Secka 2003). The body condition score was significantly associated with both prevalence of trypanosome infection (P<0.05) and mean PCV (P<0.0001). It was observed that donkeys in poor body condition had significantly higher prevalence and lower mean PCV than those in good body condition. This finding is in line with what has been stated in other equine trypanosomosis related studies. Seifert (1996) indicated that trypanosome infection causes a progressive loss of condition, weight loss, and the animals become easily exhaustive. The mean PCV of infected donkeys was significantly (P<0.05) lower than non-infected ones. Using the PCV value range 30-46% as a normal (Knottenbelt 2005), 61.9% of the infected and 29.1% of the non-infected animals were found to be anemic. The detection of anemia (lowered PCV) in trypanosome infected donkeys in this study is quite in agreement with many other studies of donkey trypanosomosis (Dhollander et al 2006; Shelima et al 2006a; Pinchbeck et al 2008). However, the observation of anemia in 29.1% of non-infected donkeys and the fact that other diseases of parasitic origin could also produce anemia posed difficulty to associate the low PCV observed in this study with trypanosomosis. Moreover, the study donkeys were not screened for gastrointestinal or hemoparasites during the study period. It is, therefore, essential that other anemia producing parasites are identified and their effects known in order to asses the net effect of trypanosomosis on PCV. Contrary to other studies (Dhollander et al 2006) that found no age specificity, in this study trypanosome infection was observed exclusively in donkeys above 2 years of age. The reason for the absence of trypanosomes in the younger donkeys is not clearly known whether related to natural immunity or some other factors and this require further investigation.
 

Conclusions 

• This is the first study to address the problem of equine trypanosomosis in Benishangul Gumuz Regional State. This study revealed that Trypanosoma congolense is the most important trypanosome species in donkeys in the area and Glossina morsitans submorsitans is the only cyclical vector of it. The detection of horse flies (Tabanus) and stable flies (Stomoxys), which are important mechanical vectors of Trypanosoma evansi may suggest the existence of this species in equine in the area. The prevalence observed in this study is generally low; however, the study design used should be taken into account as a cross-sectional study depicts only a momentary picture of the infection status in the herd.  The diagnostic capability of the buffy coat method is also another factor to be considered because the diagnosis of trypanosomosis by direct parasitological techniques is feasible in the acute state of the illness, when the blood is colonized by a large number of parasites. In the chronic state of the illness, which is characterized by low parasitemia, a good parasitological diagnosis is rather difficult (Rae and Luckins 1984). Pinchbeck et al (2008) reported that the sensitivity of buffy coat method was very low (20%) relative to polymerase chain reaction (PCR)-based diagnosis. Thus, more sensitive techniques such as serology, which detects the anti-Trypanosoma circulating antibodies (Hopkins et al 1998), and PCR should be used for the effective diagnosis of the disease. Therefore, a further longitudinal study that makes use of serological techniques and includes entomological survey needs to be conducted in different seasons and agro-ecological zones so as to generate a complete data on the epidemiology of equine trypanosomosis in the area.

Acknowledgement 

We are grateful to the National Tsetse and Trypanosomosis Investigation and Control Center (NTTICC) for their technical assistance and provision of materials during the field study. We are also grateful to the equine owners for their cooperation during sampling.

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Received 13 October 2009; Accepted 17 November 2009; Published 1 January 2010

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