A. Jouan

Rift valley fever among domestic animals in the recent west African outbreak

Severe haemorrhagic disease among the human population of the Senegal River Basin brought the Rift Valley fever virus (RVFV) outbreak of 1987 to the attention of science. As in previous RVFV outbreaks, local herdsmen reported a high incidence of abortion and disease in their livestock. Serum samples were obtained from domestic animal populations from areas near Rosso, the best studied focus of human infection, as well as other areas distant from known human disease. Among animals from the area of high incidence of human disease, antibody prevalence was as high as 85%, with approximately 80% of the sera positive for both RVFV IgG- and viral-specific IgM antibodies. In contrast, human populations in the same area had lower RVFV antibody prevalences, 40% or less, with 90% also being IgM-positive. Sera from livestock in coastal areas 280 km south of the epidemic area were negative for RVFV antibodies. Thus, the detection of RVFV specific IgG and IgM antibodies provided evidence of recent disease activity without the requirement to establish pre-disease antibody levels in populations or individuals and without viral isolation. Subsequently, detection of modest levels of IgG and IgM in the Ferlo region, 130 km south of the Senegal River flood plain, established that RVFV transmission also occurred in another area of the basin. Similar serological testing of domestic ungulates in The Gambia, 340 km south of Rosso, demonstrated antibody prevalence consistent with a lower level of recent transmission of RVFV, i.e., 24% IgG-positive with 6% of the positive sera also having RVFV-specific IgM.

Rapid diagnosis of Rift Valley fever: a comparison of methods for the direct detection of viral antigen in human sera.

Human sera collected during the 1987 Rift Valley fever (RVF) epidemic in the Senegal River basin were analysed using three enzyme immunoassays to establish the best method for rapid diagnosis of RVF. A biotin-avidin-enhanced antigen detection method utilizing monoclonal antibodies proved most sensitive. Eighty-two viremic human sera were tested, and this assay detected antigen in 29.3% of the samples.

Isolation of the rift valley fever virus by inoculation into Aedes pseudoscutellaris cells: Comparison with other diagnostic methods*

The Rift Valley fever epidemic, which arose in the south of Mauritania beginning on October 15, 1987, enabled a comparative study of different diagnostic methods among humans. During the first two weeks of the epidemic, four parallel methods were used: inoculation into Aedes pseudoscutellaris cells, inoculation intracerebrally into suckling mice, tests by immunocapture of the circulating antigen and detection of type IgM gammaglobulins. Of 370 examined sera, 181 showed at least one marker of recent infection. Inoculation into A. pseudoscutellaris cells was by far the most sensitive and easiest method to use. Detection of the antigen by immunocapture was also a useful technique, since it allowed quick aetiological diagnosis or examination of sera conserved under poor conditions. However, its sensitivity was weak, as it could only detect 26% of positive cases. Vero cells used on a limited scale, in this particular case seemed less sensitive than A. pseudoscutellaris cells. Of a total of 991 sera, 221 diagnoses were reported by discovery of the virus and 271 by detection of specific IgM. In every case, A. pseudoscutellaris cells seemed most appropriate as the system of reference.

Assessment of an rDNA probe filter hybridization assay for the detection of rift valley fever virus RNA in human serum samples from the mauritanian epidemic

The Rift Valley fever virus (RVFV) epidemic that occurred in southern Mauritania during the 1987 rainy season provided a unique opportunity to test and evaluate a recently developed, M-segment-specific, nucleic acid filter hybridization assay on a large collection of infected human serum samples. It afforded the opportunity to compare the procedure with two other methods for detecting virus: virus isolation and antigen detection by ELISA. The filter hybridization procedure employed a polyethylene-glycol-precipitation and proteinase-K-digestion sample treatment step developed specifically for preparing serum samples for hybridization. The procedure was less sensitive for detecting RVFV in the Mauritanian human viremic samples than in sera from experimentally infected monkeys used to evaluate this procedure. It was also less sensitive than an antigen detection procedure used to test the Mauritanian samples. However, we were able to detect virus RNA in a significant proportion of the virus-isolation-positive samples. Advances in sample preparation, labelling and detection procedures, and hybridization methods will improve the sensitivity, precision and ease of use of this assay and increase its value as a diagnostic tool.

HIV2 is responsible for AIDS cases in Senegal

Both human immunodeficiency virus type 2 (HIV2) and human T-lymphotropic virus type IV (HTLV-IV) have been recently isolated in West Africa. Although previous serological surveys have revealed a high prevalence of seropositivity to HTLV-IV in healthy populations in Senegal there have been no reported cases of HTLV-IV-related acquired immunodeficiency syndrome (AIDS). There have however been 2 AIDS cases in Senegal involving individuals with HIV2 infection. In addition 4 Senegalese patients tested positive for the HIV2 virus in hospitals in the country in the 1 month period of March 15-April 15 1987. One of these patients is a 50-year-old man with Kaposis sarcoma; a 2nd is a 33-year-old man with Kaposis sarcoma. The remaining 2 patients show symptoms of viral encephalitis. Blood samples were 1st rested by ELISA with a negative antigen as a control for specificity against HIV1 and HIV2. The difference if any between anti-HIV2 and anti-HTLV-4 seropositivity needs to be clarified.

Isolation of Rift Valley fever virus from human peripheral blood mononuclear cells: Mauritanian epidemic.

During the Mauritanian Rift Valley fever (RVF) epidemic of 1987, peripheral blood mononuclear cells (PBMC) were studied from 78 sick patients. RVF virus (RVFV) was isolated in 5 cases, on Aedes pseudoscutellaris AP61, from both PBMC and serum. Among the 78 cases studied, RVF was proven in 19 cases (24.3%) by specific IgM detection, and in 12 cases (15.3%) by virus isolation from serum, of which 3 also exhibited anti-RVF IgM. Among the 5 PBMC-positive RVFV cases, 2 strains were isolated in the presence of specific IgM from patients presenting with neurologic signs. These observations raised the question as to the role of specific IgM in cellular infection, and suggest that, in certain cases, mononuclear cells may promote RVFV dissemination into brain cells. Further investigations need to be undertaken to determine the RVFV receptor expressed on PBMC membranes.