Retrospecitve and Cross-Sectional Investigation of Rift Valley Fever in Small Ruminants in Pastoral Areas of Ethiopia
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Date
2004-06
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Addis Ababauniversity
Abstract
Rift Valley Fever Disease is a peracute or acute, febrile, mosquito-borne, zoonotic disease
characterized by high rates of abortion and neonatal mortality. It occurs primarily in sheep,
goats and cattle. It is a zoonotic disease causing an unexpectedly very high numbers of cases
and deaths. It is caused by Rift Valley Fever Virus of the Bunyaviridae family and genus
Phlebovirus.
Due to the epidemics in Saudi Arabia and Yemen, in September, 2000 export trade of live
animals from east African countries, including Ethiopia, was banned by the Gulf countries
due to fear of importing risk of Rift Valley Fever. In some of the countries the disease was
identified but in others like Ethiopia there was no evidence of the disease, except suspicion
and inability of the country to prepare scientific evidence on the situation of the disease. In
response to this and to safe guard its population a National surveillance strategy had been
launched in 2001 and is being implemented, though it hadn’t generated an official report. The
ban affected the country as a whole but there were parts of the community affected more, the
pastoralists and investors in the sector. The pastoralists provide about90% of the export
animals particularly small ruminants. Therefore this survey was proposed in September 2003,
with the aim to support the surveillance programme by generating a scientific report from the
pastoral areas through determining the sero-prevalence of RVF in the small ruminants and
detecting the risk factors associated with.
Across-sectional and retrospective survey was conducted on 1176 sheep and goats sera to
detect anti-Rift Valley Fever Virus IgG and IgM antibodies using indirect ELISA. All the
samples were collected in 1999 and the year 2003/2004 from Afar regional state and Borana
zone. Of the total 900 sera tested, for cross-sectional survey, 29 (3.22%) were found positive
to IgG antibodies; but 30 samples including the 29 IgG positives, tested for the detection of
IgM were with no any positive. Because the serum samples used for the retrospective survey
were tested in the year 2000 with an ELISA kit that have many limitations that resulted in
high false positives; therefore, a retest of 246 sera including 142 previously positives resulted
in only 32 (22%) positives reducing the previous prevalences of the total sera from 17.9%
(142/820) to 3. 9% (32/820). The recalculated respective prevalence of Afar and Borana was
2.6% (13/500) and 5.9% (19/320). The respective prevalence by the previous kits was 12.8%
and 24.4%.
XI
The cross-sectional survey prevalence is 3.2%, 4% and 2% for the total sero, Afar region and
Borana zone, respectively. Prevalence showed increase in Afar while there was a decrease in
Borana. The risk factors found significantly with sero-positivity were species of animal, living
in irrigated land and areas found near permanent water sources and with flooding problems by
univariate analysis of risk factors but only animal species, distance from permanent water
sources and flooding. The age-specific sero-prevalences analyzed based on positives
predictive value of the test revealed that there was no any indication of viral activites within
the last four years. The geographic distribution of the sero-positives was mapped using a
georeferenced data and the sero-prevalence data.
The predicative value of the test was less than 50%, ranging form 33.4% to 50%, therefore, it
will be erroneous to give any conclusion on the situation of the infection or disease in the
country based on this serology with out any supportive confirmatory diagnosis of the causal
agent either from vector or vertebrate host
.
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Keywords
Pastoral areas