Rift Valley fever (RVF) epizootics generally appear following an association of favourable conditions, including the combination of extreme climatic events such as El Niño, above average rainfall and changes in hydrological conditions (e.g. the construction of dams or changes in irrigation scheme). During the 1998 epizootic in West Africa , the disease was detected in the livestock population late, when fatal human cases were being reported and the disease had already reached epidemic proportions.
Improving the capacity of veterinary services To control the disease and prevent its repeating occurrence in the future, in 2000 FAO funded the implementation of a regional disease surveillance system for RVF in Mali , Mauritania and Senegal through a regional Technical Cooperation Programme project (TCP/RAF/8931). The main objective of the project was to improve the capacity of national veterinary services to detect the disease early and take appropriate control measures.
RVF surveillance using sentinel animals has been, and must continue to be, active in high-risk areas of West Africa |
Four years after the project's close, surveillance of RVF is still operational in the region. Activities were strengthened in 2004 in the Gambia , Mali , Mauritania and Senegal . These countries participated actively in the RVF surveillance programme through the monitoring of sentinel herds and the active search for the disease in high-risk areas. These activities were financially supported by FAO in Mali , Mauritania and Senegal through letters of agreement established with national veterinary laboratories.
Gambia
The International Trypanotolerance Centre of Banjul , the Gambia , monitored small ruminants (sheep and goats) in the region of Kenieba in September 2004.
142 animals were sampled, and sera were tested for the detection of RVF antibodies, using a seroneutralization (SN) test and enzyme-linked immunosorbent assay (ELISA) immunoglobulin M (IgM) test. Thirty-seven animals were found positive (N=142, X=37) by SN. However, no IgM antibodies were detected.
Serological results for RVF antibodies in small |
||
| Number tested |
Results (positive SN) |
|
| Ovine | 69 |
21 |
| Caprine | 73 |
16 |
| Total | 142 |
37 |
Guinea
In November and December 2004, sera were taken and seropositivity was observed in 80 small ruminants in Guinea . This result confirms the presence of RVF virus in the region of Dubreka.
Mali
Several field missions were organized in Mali in March and April 2004 (Kangaba, Mopti, Nara , Sélingué and Yanfolila), and 926 sera were tested for RVF. Only one serum sample was found to be IgM positive, while 17 were immunoglobulin G (IgG) positive (Mopti, Sélingué and Yanfolila). As in the case of Mauritania (see below), these findings demonstrate a low-level viral circulation in areas at risk.
Serological results for RVF IgG and IgM in small |
||||
Village |
Geographical coordinates |
Number of serum samples tested |
Number IgG positive |
Number IgM positive |
Nara |
N15.18094; |
270 |
0 |
0 |
Mopti |
N14.18094; |
319 |
5 |
1 |
Kangaba |
N11.93932; |
93 |
0 |
0 |
Sélingué |
N11.61910; |
136 |
7 |
0 |
Yanfolila |
N11.170121; |
108 |
5 |
0 |
Total |
|
926 |
17 |
1 |
Mauritania
Two missions were organized between August and November 2004 in the Senegal River valley of Mauritania and in the southwest regions of the country. During the first mission (August 2004–September 2004), IgM antibodies were observed in 5 out of 298 sera tested (1.67 percent). The localities where IgM antibodies were found were: Kankossa (one IgM positive), Kiffa (one IgM positive) and Tidjikdja (three IgM positive).
A follow-up mission (September 2004–November 2004) confirmed the presence of IgM antibodies in Tidjikdja, where two serum samples were found positive.
It is important to note that no clinical signs such as abortions or stillbirths were observed in the herd where IgM antibodies were detected (July 2004–November 2004). These results indicate low-level viral circulation, as observed every year.
Serological results for RVF IgM in small |
|||
Region |
Locality (sentinel herd location) |
Samples taken |
Number IgM positive |
Hodh Ech Chargui
|
Néma (pond) |
Not sampled |
|
Diguenni (pond) |
Not sampled |
|
|
Hodh El Gharbi
|
Kobenni |
30 |
0 |
Tintane |
30 |
0 |
|
Assaba
|
Kiffa (Oumchagar) |
Not sampled |
|
Kankossa (pond) |
Not sampled |
|
|
Tagant |
Tidjikdja (pond) |
30 |
2 |
Guidimaka |
Selibaby ( Senegal River valley) |
Not sampled |
|
Gorgol |
Mbout (pond) |
Not sampled |
|
Brakna |
Boghe ( Senegal River valley) |
30 |
0 |
Trarza |
Keur Macène ( Senegal River valley) |
30 |
0 |
Total |
5 sentinel herds visited out of 11 |
150 |
2 |
Senegal
In Senegal , four field missions were organized in March, April, July and October 2004, and 789 samples were collected and tested for the presence of RVF antibodies. No viral circulation was detected.
In November 2004, a farmer reported abortions and dystocia in a herd of small ruminants. Veterinary services visited the farm and investigated the case. An absence of young animals (less than six months old) in the herd was also noticed. On 18 November 2004 , 30 serum samples were tested at the Laboratoire National de Recherches Vétérinaires in Dakar , Senegal , and RVF antibodies were detected. Sixteen animals were identified as IgG positives, and three as IgM positives. The diagnosis was confirmed by a second field investigation mission: serum was taken for a second time in the same herd, and the presence of IgM antibodies was confirmed (2 positive animals out of 30). This outbreak of RVF was the only one reported and confirmed in 2004 in Senegal .
Serological results in small ruminants presenting clinical signs compatible with RVF in the low valley area of Ross-Bethio, Senegal, November 2004 |
|||||
Number |
Clinical signs |
ELISA 1 IgM |
ELISA IgG |
SN 3 |
Conclusions |
1 |
Weakness |
1.93 |
136.48 |
160 |
IgG positive |
2 |
Weakness, scabies |
0 |
100.18 |
160 |
IgG positive |
3 |
Weakness |
-0.77 |
2.54 |
40 |
Negative |
4 |
Rough hair coat |
-1.16 |
4.36 |
< 40 |
Negative |
5 |
Weakness |
8.50 |
200 |
160 |
IgM and IgG positive |
6 |
Weakness, rough hair coat |
1.16 |
11.62 |
40 |
IgG positive |
7 |
Abortion |
2.71 |
113.61 |
160 |
IgG positive |
8 |
Abortion |
10.05 |
117.60 |
160 |
IgM and IgG positive |
9 |
Abortion |
2.71 |
187.66 |
160 |
IgG positive |
10 |
Rough hair coat, dystocia |
0.77 |
5.08 |
40 |
Negative |
11 |
Weakness |
2.32 |
0.73 |
40 |
Negative |
12 |
Weakness |
0.77 |
-0.36 |
< 40 |
Negative |
13 |
|
3.09 |
214.88 |
160 |
IgG positive |
14 |
|
1.16 |
11.62 |
< 40 |
IgG positive |
15 |
|
1.55 |
1.09 |
< 40 |
Negative |
16 |
|
1.55 |
145.55 |
160 |
IgG positive |
17 |
Recent abortion |
8.89 |
198.91 |
160 |
IgM and IgG positive |
18 |
Recent and frequent abortion |
3.86 |
164.07 |
160 |
IgG positive |
19 |
|
2.71 |
27.23 |
< 40 |
IgG positive |
20 |
|
1.93 |
178.22 |
160 |
IgG positive |
21 |
Recent and frequent abortion |
3.09 |
3.27 |
< 40 |
Negative |
22 |
|
0 |
2.54 |
< 40 |
Negative |
23 |
|
1.55 |
13.07 |
160 |
IgG positive |
24 |
|
-0.77 |
0.73 |
< 40 |
Negative |
25 |
|
2.32 |
0.73 |
40 |
Negative |
26 |
|
0.77 |
2.18 |
< 40 |
Negative |
28 |
|
-2.32 |
-0.73 |
< 40 |
Negative |
29 |
|
3.48 |
137.21 |
160 |
IgG positive |
30 |
|
-2.32 |
0.73 |
< 40 |
Negative |
Total |
|
3 IgM+ |
16 IgG+ |
13 SN+ |
|
1 ELISA: enzyme-linked immunosorbent assay | |||||
Conclusion In 2004, there was only one isolated outbreak of RVF detected, in Senegal; a low level of RVF viral circulation was observed in neighbouring countries. Active surveillance must continue in this region, during the rainy season (June to October), to improve the understanding of the epidemiology of the disease and allow early detection in the event of an epidemic.
