Bluetongue disease in small ruminants in south western Ethiopia: cross-sectional sero-epidemiological study
Abera et al. BMC Res Notes
Bluetongue disease in small ruminants in south western Ethiopia: cross-sectional sero-epidemiological study
Temesgen Abera 2
Molalegne Bitew 0
Debebe Gebre 2
Yosef Mamo 2
Yosef Deneke 2
Sukdeb Nandi 1
0 Ethiopian Biotechnology Institute , P.O. Box: 32853, Addis Ababa , Ethiopia
1 Virus Laboratory, Center for Animal Disease Research and Diagnosis (CADRD), Indian Veterinary Research Institute (IVRI) , Izatnagar, Uttar Pradesh, 243 122 , India
2 Jimma University, College of Agriculture and Veterinary Medicine , Jimma , Ethiopia
Objective: The status of bluetongue disease, vectors for transmission of the disease and the serotypes involved are not clearly known in Ethiopia. This sero-epidemiological study was conducted to determine the seroprevalence and associated risk factors of bluetongue in small ruminants of South Western Ethiopia. Result: 422 serum samples were screened for the presence of bluetongue virus (BTV) specific antibodies using competitive enzyme-linked immunosorbent assay (c-ELISA) and 30.6% (129/422) (confidence interval CI 26.2-35%) of the sheep and goat serum samples were found positive. Multivariate analysis of several risk factors like age, sex, altitude, body condition and species of animals were studied and it was observed that species of animals, age and altitude had significant influence (P < 0.05) on seropositivity to BTV. Goats showed more seropositivity to bluetongue as compared to sheep [AOR = 2.4, 95% CI (1.5-3.9), P = 0.001], adult animals were more seropositive [AOR = 3.1, 95% CI (1.9-5.1), P = 0.001] than other age groups and animals at the lowland [AOR = 3.1, 95% CI (1.5-6.4), P = 0.002] showed more seropositivity to bluetongue than midland and high land. Sex and body condition of the animals had no statistically significant (P > 0.05) effect on seropositivity to bluetongue.
Bluetongue; c-ELISA; Risk factors; South West Ethiopia; Seroprevalence; Small ruminants
Bluetongue (BT) is an infectious and non-contagious
arthropod borne viral disease of domestic and wild
]. Bluetongue virus is a member of the genus
Orbivirus in the family Reoviridae. Its genome consists of
ten double-stranded (ds) RNA segments coding for seven
structural proteins (VP1–VP7) and four non-structural
proteins (NS1–NS3 or NS3A, and NS4). At present 27
serotypes have been reported throughout the world [
Bluetongue is endemic primarily in the tropical and
subtropical regions between the latitudes of 40oN and
35oS where vectors (Culicoides spp.) are present. In
Africa, Bluetongue has been known in South Africa for
over a hundred years [
] and to date 21 of the 27 known
BTV serotypes have been isolated from sheep [
major vector in Africa is considered to be C. imicola. The
incursion of BTV serotype 2 into North Africa (Algeria,
Libya, Morocco and Tunisia) has been confirmed in 2004
]. However, in Eastern Africa the information about the
prevalence of bluetongue is still unclear. Toye et al. [
in western Kenya reported the highest seroprevalence
(94.2%) of bluetongue in cattle and detected nine
different serotypes (BTV 1, 3, 7, 12,15,16, 19, 22 and 24) by real
Ethiopia is endowed with huge livestock population
including 53.99 million cattle, 25.5 million sheep and
24.06 million goats [
]. Albeit with this huge potential,
the status of bluetongue disease and vectors involved in
the transmission in Ethiopia has not been well explored.
This could be due to the fact that bluetongue disease is
misdiagnosed with other highly prevalent ruminant
diseases having similar clinical signs. However, 46.67%
sheep from different agro-climatic areas of central
] and 34.1% sheep of Amhara National Regional
State, Northwestern Ethiopia were found seropositive
to bluetongue based on competition ELISA [
reports focused only on sheep and covering small
agroecological area vis-à-vis huge livestock population and
larger size of the country. Besides, virus isolation and
characterization at serotype and molecular level had
never been attempted. At present there is no vaccination
program for control of bluetongue in small ruminants
has been implemented in the country. Keeping in view
of the above facts, the present study has been carried out
to have a clear picture about the bluetongue disease in
south western Ethiopia.
This study was the part of the bluetongue virus survey
of the whole country and undertaken with the objectives
to determine the seroprevalence of bluetongue in sheep
and goats in selected districts in south western Ethiopia
and identify the potential risk factors associated with the
Study was conducted in selected districts of south
western Ethiopia namely Jimma town, Bonga town and
Bedele district. From each district statistically
representative peasant associations (PA) were selected like Shobe,
Sidisa, Qare and Yabela from Bedele, Ifa Bula, Bore and
Qofe from Jimma and Bonga zuria 01 and 03 from Bonga
were selected (Fig. 1).
The study animals were small ruminants at different
agro-ecological zones found in different peasant
association of selected districts.
Study design and period
A cross-sectional study was conducted to estimate the
seroprevalence of bluetongue virus (BTV) from
December, 2015 to August, 2016 in small ruminants at south
Sample size determination
The sample size was determined according to Thrusfield
] with 95% confidence interval and 5% absolute
precision. There was no previous research work done in the
study area so that the expected prevalence rate was taken
as 50%. Therefore, 384 small ruminants were obtained for
the study using the formula. However, to increase
precision of the result, a total of 422 animals (248 sheep and
174 goats) were included. From each district samples
were taken proportional to the total study population in
the study districts of which (141 from Jimma district, 101
from Bonga district and 180 from Bedele district).
N = 1.962P exp (1 − P exp)/d2
where, N = sample size, P exp = expected prevalence,
and d = desired absolute precision.
Convenience sampling was used to select study districts
from different agro-ecological zones whereas simple
random sampling was used to select specific animals for
sampling frame and PAs. Body condition scores and age
determination were done according to Vatta et al. [
Sample collection and testing
About 5 ml of blood was collected aseptically from the
jugular vein of each sheep and goat using plain serum
vacutainer tubes and needles. The blood was allowed to
clot for 1–2 h at room temperature, stored horizontally
overnight at 4 °C in the refrigerator and then the serum
was separated. All the sera were stored at − 20 °C and
transported on ice and submitted to the National
Veterinary Institute (NVI) to screen for the presence of BTV
group-specific antibodies using the Bluetongue Antibody
Test Kit, c-ELISA (IDvet, 310, rue Louis
Pasteur-GrabelsFrance) which is based on recombinant VP7 protein. The
test was performed as per the manufacturer’s protocol.
Data management and analysis
The data collected has been uploaded in Microsoft Excel
spread-sheet and analyzed using SPSS version 20.0
software. Associations between seroprevalence and its
potential risk factors were tested in a univariable and
multivariable logistic regression analysis. Finally, odds
ratios and its 95% confidence interval (CI) were
calculated and risk factors with a P value < 0.05 were taken as
In the present study, 422 serum samples from sheep (246)
and goats (176) were collected and screened for the
presence of BTV specific antibodies by c-ELISA. Out of 422
sera tested, 129 (30.6%) (95% CI 26.2–35) were found to
be positive for BTV-specific antibodies (Table 1).
On the basis of univariate and multivariate logistic
regression analysis, it was concluded that risk factors
like sex, and body condition had no significant (P > 0.05)
impact (Table 2) whereas species, age, districts and
agroecology had significant (P < 0.05) influence on the
seroprevalence of bluetongue. It was found that goats were
2.3 times more likely to be positive to group specific BT
virus antibody than sheep. Adults were three times
compared to young animals and animals of lowland compared
to highland areas were three times more likely to be
seropositive to BTV respectively. Among the districts, small
ruminants in Bedelle district were 2.3 times more likely
to be seropositive to BTV than Jimma district (Table 2).
From this study it is revealed that antibodies to BTV
is highly prevalent in sheep and goats of south west
Ethiopia. The results of the present study revealed the
first confirmation of BTV antibody in sheep and goats
from South west Ethiopia. Although, seropositivity to
bluetongue virus has been reported earlier in small
ruminants in Ethiopia, it has covered only a small region
without mentioning the association of risk factors [
9, 10, 13
In the present study 30.6% (129/4220 serum samples of
small ruminants were found positive to antibodies to
BTV. This findings is in agreement with the results of
] who reported 34.1% seropositivity to BTV
in indigenous sheep of Northwestern Ethiopia. The
seropositivity to BTV in small ruminants (28.6%) in India
], domestic livestock (23%) in Kazakhstan [
in Iran (35.9%) [
], sheep in Iran (34.93%) [
in Iran (39.47%) [
], small ruminants in Turkey (29.5%)
], sheep in Iran (37.7%) [
] and again sheep in Iran
] has also been reported from other parts of
The present finding is found to be relatively lower than
the previously reported seroprevalence to BTV in small
ruminants by different authors in different countries such
as 41.17% in small ruminants in Southern Ethiopia [
46.67% in sheep in central Ethiopia [
], 78.4% in small
ruminants in Grenada [
] and 45.7% in small ruminant
in India [
]. However, 30.6% seropositivity to BTV in
small ruminants in the present study is comparatively
higher than the reports of other researchers like 6.57% in
sheep in Southeast Iran [
], 6.96% from small ruminants
(13.7% in goats and 5.70% in sheep) in Algeria [
and 5.3% in small ruminants in Kerala and Karnataka
] respectively. In the present study, the
difference in the seropositivity might be due to difference in
animal species, age of sampled animals, immune status of
sampled animals, agroclimatic zones, ecology, and types
of culicoides vectors.
In the current study, seropositivity to BTV in goats
was higher than sheep. Higher seroprevalence among
goats compared to sheep indicating that goats play an
pivotal role in the epidemiology of BTV. Whereas, sheep
which are highly susceptible animals to BT show distinct
clinical signs and die of the disease. It is also an
established fact that goats with minimum clinical
manifestation maintain high titer of BTV and may be the potential
source of infection to other susceptible animals [
It has been recorded that BT virus infection is
increased with increasing age of animals and this is in
agreement with the reports of Yilma and Mekonnen [
Assessing age as a risk factor, there was a statistically
significant association (P < 0.05) with the prevalence of BTV
specific antibody in the study animals. It was shown that
the younger animals started to get infected with BTV
after the age of included at category of adult level. At this
age, the animals are usually released into the pasture for
grazing, where they are likely to be exposed to infection
by vectors and subsequent BTV infection. Young age
groups are usually kept indoors and are well taken care of
by the owners from contracting infectious diseases,
particularly the insect and tick-borne infections [
This study was also assessed the effect of agro ecology
on the seroprevalence of the BTV antibody and found
that animals at low altitude are more prone to BTV
infection than high altitude. This is in agreement with
different authors [
]. The prevalence correlated with the
probable distribution of the Culicoides vector. In addition
environmental changes can influence the incidence,
distribution and evolution of infectious diseases,
particularly those transmitted by arthropod vectors .
This manuscript is part of mega project which comprises
only the serological BTV test report of the south
western Ethiopia. The project is working on virus isolation,
molecular characterization and identification of vectors
responsible for the transmission of the disease in
Ethiopia. As a limitation, this manuscript doesn’t include the
molecular and virus isolation data.
BT: bluetongue; BTV: bluetongue virus; CSA: Central Statistical Authority; NVI:
National Veterinary Institute; OR: odds ratio; CI: confidence interval; c-ELISA:
competitive enzyme-linked immunosorbent assay; AOR: adjusted odds ratio;
OD: optical density; VP: viral protein; PAs: peasant associations; SPSS: Statistical
Package for Social Science; RT-PCR: reverse transcriptase polymerase chain
MB participated in study conception and design, solicit grant, performed data
analysis, and drafted the manuscript. TA, DG and YM collected the data from
the field and National Veterinary Institute (NVI), YD and SN assisted in study
follow up and manuscript revision and also assisted in analysis interpretation
and gave inputs in the final manuscript. All authors read and approved the
The authors are thankful to Jimma University, College of Agriculture and
Veterinary Medicine for the facilities provided to carry out the present work. Wubet
G/Medhin at National Veterinary Institute (NVI), Ethiopia is duly acknowledged
for laboratory analysis.
The authors declare that they have no competing interests.
Availability of data and materials
Manuscript or Additional information is available with the corresponding
author on request.
Consent for publication
Ethics approval and consent to participate
The study was approved by the ethical committees of the College of
Agriculture and Veterinary Medicine, Jimma University. All animal owners were
signed a letter for consent for the agreement on including of their animals in
this study project.
This research work was supported by Jimma University, College of Agriculture
and Veterinary Medicine, Annual Research Grant of the year 2015/16.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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