Epidemiological and Clinico-pathological Study of Maedi-visna Virus Infectio in Sheep in Selected Districts of the Amhara National Regional State, Ethiopia
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Date
2004-06
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AAU
Abstract
An epidemiological clinical and pathological stud of aedi-Visna virus (MVV) infection in
sheep \vas carried out in highland areas of the Amhara ational Regional State (ANRS). The
purpose of the stud \ as to estimate the pre alence and distribution of the disease and
identi associated risk factors \vith the occurrence of the disease. A total of 2417 serum
samples \vere collected from smallholder farms (n = 1491 and sheep breeding ranches (n =
926) from ~oyember 2003 to February 200 . Indirect-Enzyme Linked Immunosorbent Assa
i-ELISA) and Agar Gel Immunodiffusion (AGID) tests were employed to determine the
presence of antibodies against infection. All sheep sampled \vere clinically investigated
\,here as fe\\ seropositive sheep '\ ere slaughtered and examined for gross lesions and
histopathological pictures.
The apparent seropre alence of infection was found to be different between the
management types; 6.6% (95% CI = 5.4 - 7.9) in smallholder farms and 30% (950/0 CI = 27.1
- 33.1) in sheep ranches. The difference in pre alence between the two sheep management
types among ranche~ and eredas was statistically significant (P < 0.001). The prevalence of
infection increased 'th increasing flock size and found to be higher in ranches (82.6%
than in the smallholder farms 18.8% . The breed related seropre alence aried from 16% in
indigenous sheep to .6% in Awassi sheep. The age-specific seropre alence increased with
the age of sheep from 2.8% in yearlings to 6.5% at 7 years old. F10ck size, breed, age and
contact v. ith ranch sheep were significant (p < 0.0001) risk factors associated with the
o curren e and transmission of infection.
In clinical disease in estigatio~ 31 out of 3 8.3%) seroposi ti e sheep were found to
express the clinical disease. Among the seropositi es with 0 ert clinical disease, 96.8% \ ere
from sheep ranches \"hile 3 .2% from smallholder farms. There \\ as statistically significant ¥
8.3' d.f = l ' P < 0.001 association bemeen the clinical disease and the
seropositivi . The clinical disease increased \vith age of animals. The large proportion of the
clinical disease in ranches and older animals \\ as associated ,vith the flock management and
high Ie el of infection.
The clinical disease ,vas significantl different among sheep breeds (P<0.05), higher in
Indigenous enz breeds than the \yassi and their crosses. C inical manifestations with
respiratory symptoms ( 5.2% \\ ere folIo ed b progressi e emaciation (16%). ecrops)
VIII
examination revealed focal and diffused consolidation of lungs, broncho-interestitial
pneumonia. The histopathological investigations with peribronchiolar and perivascular
mononuclear cell infiltration, smooth muscle and lymphoid hyperplasia and thickening of the
alveolar septa indicated the progressive interstitial pneumonia characteristic of MVV
infection.
The serological, clinical and pathological findings suggested that MVV infection is a major
health problem in highland sheep of the ANRS. The ranch management and husbandry
practice consisting o~ large flocks and high proportion of older animals has contributed for the
occurrence and spread of MVV in a higher rate in ranches than in smallholder farms. The
sheep ranches are considered major sources for the spread of MVV infection to the
smallholder farms. The findings of MVV antibodies in areas hypothetically considered free is
suggestive for poor knowledge and misdiagnosing of the disease. The serological test
agreement between ELISA and AGID found to be almost perfect and this would encourage
the use of the two tests in combination for accurate serological diagnostics in the control of
MVV infection.
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Keywords
Lentivirus, Maedi-Visna Virus, Prevalence, Smallholder Fanns, Sheep Breeding Ranches, ELISA, AGID, Pathology, Amhara National Regional State.