Prevalence of Rift Valley Fever Immunoglobulin G Antibody in Various Occupational Groups Before the 2007 Outbreak in Tanzania

Introduction

R ift Valley Fever (RVF), caused by RVF virus, is a mosquito-borne viral disease that is a significant global threat to humans and livestock. Transmission in humans is via direct contact through infected animal products or contaminated foods or aborted fetuses and from the bites of infected mosquitoes, most commonly the Aedes species (Corsol et al. 2008). Humans infected with RVF virus typically develop a mild, self-limited febrile illness, but retinal degeneration, severe encephalitis, fatal hepatitis, and hemorrhagic fever may also occur (Swanepoel and Coetzer 2004). Although few studies have been conducted to assess the economic impact attributable to human RVF, it is thought to be substantial (Meegan 1981, FAO 2003, Davies 2006).

Previous history of RVF outbreaks in Tanzania, which were confined to mainly livestock and mostly affecting northern parts of Tanzania, were recorded in 1956, 1978–79, and 1997–98 (Kondela et al. 1985, Woods et al. 2002). The recent reemergence (early 2007) of the disease among humans and livestocks, covering nine different geographical regions, and the fact that RVF virus replicates in a wide range of competent mosquito vectors (Turell et al. 2008) have raised concern that the virus might spread further into nonendemic (Tanga, Coast, Mtwara, Ruvuma, Rukwa, Kigoma, Shinyanga) regions of Tanzania. These threats emphasize the need for capable surveillance tools and a sound disease control strategy in place. Unpublished, hospital-based reports from this recent outbreak in Tanzania indicate that RVF claimed 144 deaths and the corresponding case fatality rate was 46.6% (WHO 2007).

RVF is known to be endemic in most of the sub-Saharan countries and in some regions in Tanzania (FAO 2002, FAO/OAU/IBAR/UNDP 2001). Between epidemic waves, RVF virus circulates at very low incidence without noticeable clinical manifestation, in both humans and animals (FAO 2003). Much less is known of the prevalence in man and the effect on human health in this region of the world. Such information is important to envisage when designing appropriate strategies that would help reduce its prevalence and effects. This inadequacy of data, consistent with sera bank collected in 2004, prompted the initiation of this study with a view of establishing past exposure to RVF virus before the 2007 outbreak. The objectives of the present study were to estimate the RVF antibody prevalence in apparent healthy, various occupational risk groups of inhabitants in Tanga Municipality, Tanzania. The overall purpose was to collect baseline data to enhance understanding of the epidemiology of the disease.

Materials and Methods

Results

Two thirds of the sampled people were men and one third were women (Table 1). The majority of the sampled people were between 20 and 50 years old. The overall prevalence of anti RVF virus IgG antibody was 4.02%, and none had IgM antibodies. The seroprevalence of anti-RVF virus immunoglobulin (IgG) antibody was higher in men than in women; however, no significant statistical difference was present (p = 0.34). Seroprevalence increased markedly in men aged between 20 and 40 years (data not shown), and antibodies were not detected in the young population (up to 20 years). Among the occupational groups examined, the seroprevalence was 7.3%, 1.5%, and 9.5% in the abattoir workers, livestock keepers, and others categories, respectively (Table 1), although no statistical difference was found among various occupational groups (p = 0.086). None of the study participants perceived RVF to be a zoonotic disease.

Discussion

There is a paucity of virological and epidemiological investigations pertaining to RVF seroprevalence status in general population and various occupational groups in Tanzania. Overall, 60% of the investigated sera were from people who had contact with cattle, either through cattle-keeping at home or through their occupation like abattoir and veterinary staff, and for 40% of people who had no close contact with cattle, like crop farmers and others. This last group might still have contact with cattle products, like raw meat and milk when preparing or consuming food. Interestingly, the detected seropositive status of the general population before 2007 outbreaks support the endemicity of this disease in Tanzania (Kondela et al. 1985, FAO/OAU/IBAR/UNDP 2001, Woods et al. 2002).

The low reactor rate for women in this study could probably be due to the small number of women (n = 67) studied compared to men (n = 132). Recent RVF outbreak in Tanzania showed that the ratio of male:female mortalities to be1.6:1, and patient aged 16–60 years died more than other age group (MoHSS 2007). The higher proportionate infection rate in men than women agrees with the findings of other workers (Nabeth et al. 2001). Traditionally, the majorities of abattoir and cattle-keeping activities in Tanzania are predominantly carried out by men. This may in part explain the high prevalence rate detected in men and in abattoir workers.

The overall detected prevalence of infection in the present study was lower than many other previous reports in Africa, which reported prevalence of 14.8% (Olaleye et al. 1996) in Nigeria, 24.4% (Nabeth et al. 2001) in Mauritania, and 22.3% in Senegal (Wilson et al. 1994). On the other hand, our prevalence rate was higher than reports made in other places (e.g., 2% reported in pregnant women in Mozambique [Niklasson et al. 1987]). The variability in the prevalence of RVF antibodies between reports could be attributed to difference in sampling techniques, investigator technical know-how, climate-agro-ecological factors, as well as diagnostic methods used.

Tanga Municipality is characterized by hot, humid, and wet climate variables. This climate is thought to permit many and high densities of mosquitoes of veterinary and medical importance (James 1979) and may similarly promote high densities of other biting mosquito species that are potential vectors of RVF virus (i.e., Aedes and Culex [Turell et al. 2007, 2008]). Laboratory-based vector competence and field isolation studies carried out in selected African mosquitoes revealed and suggested Culex zombaensis, Culex poicilipes, Culex pipiens, and Aedes caspius to be possible potential vectors for RVF virus transmission (Turell et al. 2007, 2008, Diallo et al. 2005, Meegan et al. 1980). Aedes and Culex species are known to be prevalent in Tanga; however, the role played by these mosquitoes warrants further investigation (Mboera et al. 2000, Magesa and Kamugisha 2006). As found in other studies, (Wilson et al. 1994, Elfadil et al. 2006, Turell et al. 2007, 2008), presence or exposure to a wide range of mosquito vectors (i.e., Aedes spp.) was associated with the increased likelihood human seroconversion to RVF virus. This would be consistent with RVF virus being transmitted by mosquitoes vectors as well with contact with infected animal parts (tissue, blood, urine, uterine fluid, etc.) during slaughtering (Chevalier et al. 2004, Gerdes 2004) or when assisting the delivery of newborn animals and mechanically by contaminated needles during mass vaccination (FAO 2002).

The finding that none of the participants were aware of RVF as a zoonotic disease is striking, and this is a reflection of the poor knowledge of zoonoses by livestock keepers, veterinary field staff, and staff in the health facilities. Limited knowledge level recorded in this study may be due to the general lack of data on RVF and inadequate communication between veterinary and human health care professionals. This low awareness is likely to expose people to increased risk of contracting RVF, as they might not take proper precaution (i.e., use insecticide-treated bed nets or use protective clothing when dealing with abortion cases and during on-farm activities like slaughtering cattle). For instance, a majority of the abattoir staff were found to have no protective clothing during the current study period. Likewise, the habit of consuming raw milk, raw blood, or raw or undercooked meat, however, is still common practice in some communities in Tanzania (Shirima et al. 2003).This shows that the emergency preparedness for RVF epidemic is low. Consistent with low knowledge on RVF, possibly due to the interepizootic nature of the disease occurrence, the role of vectors and RVF virus transmission in Tanzania is not clearly understood. Further research is required. Consistent with research initiatives, targeted information and education of the population (vet, health staff, community) to increase awareness could help increase early case detection and limit further spread of infection.

Conclusions

From findings of this study, it is concluded that there is a clear evidence of human exposure to RVF virus before 2007 epidemics, with all studied occupational groups being equally at risk of contracting RVF virus.

Considering the paucity of epidemiological reports on RVF in the northeast of Tanzania and the absence of any data concerning factors related to either prevention or the spread of the disease, our results could make a useful contribution toward the prevention of human RVF epidemic in Tanzania.