Epidemiology of Rift Valley Fever in Sudan

Abstract

This study was designed to evaluate the present situation of RVF among animal population in different regions of Sudan. The study included serosurveillance and diagnosis of suspected RVF cases. Two serological tests were used to detect anti-RVFV antibodies: Enzyme linked immunosorbent assay (ELISA) and Indirect fluorescent antibody test (IFA). Sera were collected and tested in the year 2001 a total of 1351 (491 bovine, 423 ovine and 437 caprine). Using Sandwich ELISA for the detection of anti-RVFV IgG antibody, the overall prevalence rate was ranging from 3.4% to 10% in animals from Buram (Western Sudan) and Shandi (Northern Sudan) respectively. All positive reactions were detected in sheep (15 samples) and goats (1 sample). 1543 serum samples (612 bovine, 313 caprine and 618 ovine) were tested by Capture IgM ELISA during 2001. No anti- RVFV IgM antibody was detected in all these tested sera. In August 2002 a febrile disease spreaded among cattle in every locality in Khartoum and River Nile States manifested by high abortion rates reaching as high as 70% and deaths of aborted foeti. This raised the suspicion of RVF outbreak. Capture IgM ELISA test has been developed with an inactivated viral antigen. A total of 107 bovine serum samples collected from Khartoum and River Nile State as follows: 55 samples from East Nile Province, 35 samples from Alshajara and 17 samples from the River Nile State, were tested for anti-RVF IgM antibodies using Capture IgM ELISA. 14 Thirty one out of these samples were found positive. The percentage positivity was 25.5%, 14.2% 70.5% respectively. In the year 2003 no clinical disease was reported. Moreover all tested positive in the year 2002 were culled or removed from the herds. In the year 2005, about 410 bovine serum samples from El damazine (60), Elgadarif (31), Kassala (269) and El showak (50) and 257 ovine samples from Kosti and Elshowak were tested for anti-RVF IgM antibodies using IgM ELISA. The percentage positive was 6.6% and 1.8% in Eldamazine and Kassala respectively. No anti-RVFV IgM antibody was detected in the ovine sera. In this study Indirect fluorescent antibody test (IFA) was used for serological diagnosis of RVF using inactivated antigen, 100 bovine serum samples were tested for anti-RVF antibodies,23 out of these were positive. The degree of agreement between ELISA and IFA tests were very high i.e. IFA test can be used in the diagnosis of RVF. For early and rapid detection of RVF virus and an efficient surveillance system of a single tube reverse transcriptase polymerase chain reaction (RT-PCR) method focusing on the NSs coding region of the S segment was developed and used RVF virus genome, resulting in the synthesis of 363 bp DNA amplifiers were detected in RNAs extracted from RVFV vaccine strain (Smithburn) and tissues (spleen and liver) from aborted feti. The assay was specific for RVFV and did not amplify any other hemorrhagic virus. When serial dilutions of RVFV were mixed with DDW, the minimal 15 detection limit was 0.5 plaque forming units. The RT-PCR was efficient for the detection of RVFV RNA. Entomological surveillance data about mosquitoes in Khartoum was recorded during the period of the study.