Prevalence of Rift Valley Fever Infection in Ruminants in Madagascar After the 2008 Outbreak

Abstract

A Rift Valley fever (RVF) outbreak occurred in Madagascar from January to May 2008. The objectives of this study were (1) to assess the current and past circulation of RVF virus (RVFV) in livestock in Madagascar and (2) to evaluate the extent and magnitude of the 2008 RVF outbreak in livestock. The results of a country-wide serosurvey conducted in August 2008 on small and large ruminants are reported here. The study included 3437 cattle and 989 small ruminants (227 sheep and 762 goats) sampled in 30 of the 111 Malagasy districts, selected to be representative of the different ecozones and livestock density areas. Sera of animals were tested for the detection of immunoglobulins M (IgM) and G (IgG) against RVFV using commercial enzyme-linked immunosorbent assays kits. Recent infections (presence of IgM against RVFV) were detected in only 9 cattle (0.3% [0.1-0.4]) and 33 small ruminant (3.3% [2.2-4.5]) samples. Past infections (presence of IgG and absence of IgM against RVFV) were detected in 887 cattle (25.8% [24.3-27.3]) and 244 small ruminant (24.7% [22.0-27.4]) samples. Past infections were detected in all sampled sites. All ecozones were affected. In the southern and northwestern areas, the prevalence of cattle showing evidence of past infection with RVFV increased with the age of the animals. Our results suggest that there has been country-wide circulation of RVFV in 2008 in Madagascar, including in parts of the country where no clinical illness, either in animals or in humans, was reported. The data also suggest that the southern and northwestern areas may be endemic for RVFV, and that the virus may spread when ecological conditions are favorable for its amplification.

A Review of Mosquitoes Associated with Rift Valley Fever Virus in Madagascar

“…12 These differences might explain regional differences in RVFV prevalence and outbreaks. 4,6 RVF circulation and occurrences generally happen during the wet and warm season, 4 which correlates with the period of highest mosquito density. 9,15,12 This increase in mosquito vector density is caused by the creation and maintenance of different breeding sites.…”

“…However, the majority of involved Aedes subgenera is present in Madagascar. 9,12,78 Additional detection of RVF is needed (especially in the western domain, where high RVFV prevalence has been reported, and the southwestern domain, where endemic foci areas have been suggested to occur 6 ), particularly in the context of viral maintenance through a possible vertical transmission. Field studies on vector biology and RVF entomological surveys need to be further advanced to determine if endemic cycles occur.…”

“…4 The highest RVFV prevalence rates were observed in livestock in the western and northern domains. 6 The suggested RVFV candidates vectors (An. squamosus and An.…”

“…1,79,80 In Madagascar, this technique was used on a local scale of one domain during a malaria study 81 and could provide interesting insights associated with RVF entomological surveys, particularly in the southwestern domain, where RVF is considered to be endemic. 6 Variation in monthly and annual precipitations (http://iridl.ldeo.columbia.edu/) and patterns of variation in larval development are important factors that vary between biogeographical domains 69 ; hence, this technology should be used for the identification RVF risk areas. It could be very useful to estimate the relationship between abundance of breeding sites and density of adult vectors for additional vector surveillance and control.…”

“…4,5 Another study suggested that this outbreak most likely originated from an endemic cycle localized in southern Madagascar, where virus circulation may occur annually. 6 Although not quantified, it is possible that these two mechanisms co-occur in Madagascar, with a recurrent introduction that helps RVFV maintenance and recirculation in the endemic cycle. 7 The role of mosquitoes in RVF transmission on the island was considered to be associated with the first RVFV isolation and the 2008-2009 outbreak.…”