This outbreak of RVF resulted in nearly 400 cases of significant illnesses with many deaths. However, the outbreak potentially infected more than 180,000 people, based on the proportion of participants in the serosurvey who had IgM antibodies to RVF virus. We recognize that this extrapolation relies on the exposure rate to be fairly evenly distributed across the entirety of each of the districts and in a similar intensity as in the case villages, and therefore it may be an overestimation. Case detection guided the locations selected for the serosurvey. However, because case detection was presumably far less than 100% sensitive, it is likely that there were many other similarly affected villages that were not surveyed within these districts. We are not able to estimate the number of people infected in other areas in Kenya. This gross estimation of RVF virus infections in the three most-affected districts in Kenya remains useful because it indicates the large burden of this disease to Kenya during epidemic periods. The high mortality rate during this outbreak, was likely heavily influenced by the low proportion of patients infected with RVF (many with mild illness or who were asymptomatic) who sought medical care, and is unlikely to represent emergence of a lethal strain of RVF virus.
In addition to the toll on health, the outbreak likely had substantial economic impact. Bans on slaughtering were imposed in each of the affected areas and aggressive attempts were made to stop movement of livestock from affected areas to unaffected areas. In areas like Northeastern Province where the principal source of food is livestock and where a substantial number of people work in the livestock industry, the quarantines and slaughter bans were in effect for more than 2 months. While likely effective at minimizing the severity of the outbreak, these interventions had devastating impact on livelihoods.
This large outbreak of RVF followed heavy rains and flooding within the affected areas by 1 month or more. Although sporadic cases of RVF may occur during interepidemic periods, outbreaks are thought to occur when topographical depressions called dambos suddenly flood causing the eggs of dormant RVFV-infected floodwater Aedes
spp. mosquitoes to hatch.28
Northeastern Province is normally dry (mean annual rainfall = 40 cm or less for most of the province). However, during September through December 2006, 39.4 cms of rainfall was recorded in Garissa with the subsequent formation of many dambos and large temporary lakes in low-lying areas. This flooding displaced entire villages and became breeding grounds for vector species. Infected Aedes
transmit RVF to a variety of livestock and wildlife, which may develop high and sustained viremia,29
and directly to humans; when virus levels in bloodstream are high, RVF may then be transmitted to other animals or to humans by other mosquitoes, and theoretically by any biting or blood-sucking insect. It appears that heavy rains and flooding preceded advent of outbreaks by at least 1 month in each of the affected areas.
The previous RVF outbreak in Kenya in 1997–1998, had a somewhat similar geographic distribution and pattern of spread,9
although the previous outbreak was not known to have significantly affected the Baringo area, a major geographic focus for illnesses during the recent outbreak. Despite flooding throughout Kenya around the time of both outbreaks and widespread availability of livestock, both outbreaks started within Northeastern Province and spared most of the western half of the country. While there are theories that certain areas are receptive for RVF transmission, there are no data that would provide explanation.30
The predilection of specific soil types in RVF affected areas, and not in other areas, would be consistent with the notion that soil type may influence flooding, drainage and potentially the ability for infected Aedes
egg stages (which remain in the soil) to remain infectious in the ground until heavy flooding at which time maturation of egg stages and mass breeding occurs resulting in epizootics and ultimately epidemics. Solonetz soils have a sodium-rich subsource, are low in organic matter, and often contain strata rich in mineral deposits.31
Solonetz is typified by a subsurface richer in clay than the topmost layer and are usually found in areas where annual precipitation is usually low, which is the case in most of the affected areas. Planosols are similar to solonetz and in that they are permeable on the surface, but have a much slower draining substrata—they are coarse textured soils often over a finer textured subsoil, usually clay. Both types of soils tend to retain water near the surface well during rainy seasons.31
During dry seasons, they often support grasslands—in the Northeastern province area, these are semi-sparse grasslands. Solonchaks, found in the Baringo area, are found in marshy, high saline areas and can form solonetz soils upon drying and leaching of surface sodium.32
The solonetz-solonchak transition provides a linkage between the soil types in the Lake Baringo and those of Northeast Province. It is not clear whether these factors contribute to “RVF geographic receptiveness” and, if they do, whether receptiveness occurs by promoting survival and maturation of potentially infected Aedes
egg stages or through more rapid occurrence of flooding given excessive rainfall (or both). Research, combining geologic, entomologic, and virologic components would be useful to examine this possibility and better characterize factors that promote regional RVF outbreaks.
During the peak of the outbreak in Northeastern Province, animals that could not be sold or slaughtered were transported from Northeastern Province to Kilifi, where the outbreak had not yet appeared. RVF viruses may have been transported to Kilifi District by infected animals and spread by competent vectors and transmission-facilitating animal practices rather than through flooding and breeding of infected floodwater Aedes, such as likely occurred within other high incidence areas. This would be consistent with differences in soil and rainfall patterns within Kilifi. If correct, this hypothesis would have important considerations for control of future RVF outbreaks, especially relating to tighter enforcement of restrictions of movement of livestock from affected areas, often strongly driven by hardship resulting from local quarantines and livestock bans. As with avian influenza control, a strategy for compensation of livestock owners for hardships brought on by public health restrictions might be needed to minimize potential for spread of RVF viruses to new areas.
As a zoonosis with a hypothesized mosquito-egg reservoir,28
prevention and control strategies are complicated and expensive. Aggressive application of larvacides into flooded areas known to be receptive for RVF transmission (perhaps where RVF-specific Aedes
are prevalent), livestock immunization, and community education focused on reducing risk exposures (like slaughtering or handling sick animals and drinking raw milk) would all be best applied before an outbreak occurs, based on accurate predictions of RVF outbreaks. Work is ongoing using extensive geographic information from human, veterinary, and entomologic surveillance during the outbreak, along with meteorological compilations and satellite images to develop refined RVF forecasting models. Such models, if highly specific, would be used to trigger large-scale pre-outbreak disease prevention measures. The models will need to be highly specific and sensitive to generate the will to use precious, limited funds in the resource poor areas where RVF is known to occur.
Evidence for transovarial transmission in floodwater Aedes
mosquitoes, the basis for the mosquito-egg reservoir hypothesis, has only been demonstrated once in a study in Kenya in the 1980s. Thus far, we have not found substantially different genetic sequences from viruses isolated from humans in different geographic areas. Existence of such differences would have supported the concept that separate outbreaks of RVF occurred in each area, resulting from separate occurrences of maturation of infected Aedes
egg stages. Genotypic findings included in this report are consistent with recent work showing low diversity of 5% at the nucleotide and 2% at amino acid levels when the full genomes of 33 RVF isolates collected over multiple years from divergent geographic regions were analyzed.23
Given that most regions within the RVF genome are highly conserved, sequencing may not be a useful molecular epidemiologic technique for this virus; however, sequencing the entire genome should be pursued to rule out geographic-specific genetic differences. Thus, we cannot yet determine whether RVF was physically moved from area to area (i.e., by movement of infected animals or mosquitoes) or sprang up separately from maturation of infected Aedes
eggs within recently flooded areas. The 2006–07 Kenya isolates displayed amino acid differences that may be significant with further analyses to localize the viral epitopes affected by mutations. For example, across the 1,055 amino acid span of the M segment analyzed, the Kenya 2006 isolates had amino acid differences at 10 positions when compared with Saudi 2000-10911 and Kenya 9800523 strains (strains from earlier outbreaks had identical amino acids at those positions).
Although transmission of RVF virus to humans can occur by direct exposure to infected animal secretions, and also by infected mosquitoes, it is unclear whether the route of transmission is associated with disease severity. Direct exposure to infected animals may be more likely to result in symptomatic or severe RVF disease, because the inoculum from viremic animals is much greater than that transmitted by mosquitoes. Our finding that CFR was highest in young males supports this hypothesis, because livestock herding and slaughtering is practiced by young males. In contrast, the 128 participants in the serosurvey with evidence of recent mild or asymptomatic infection had a male:female ratio closer to one. In addition, the greatest number of cases of RVF illness during the outbreak was in Northeastern Province, where herding and slaughtering were very common practices; however, the highest seroprevalence was in Baringo, where animal handling is not a predominant occupation, and where mosquito densities during the outbreak were very high.33
This outbreak resulted in substantial loss of human life and suffering. Ongoing studies will attempt to quantitate the short- and long-term economic impact, which are likely severe. Previous outbreaks of RVF have resulted in bans of importation of livestock from outbreak areas, lasting for many years after the outbreaks have concluded. Because outbreaks are intermittent, effective strategies for predicting and preventing them are needed. Recently published models focusing on climatologic and ocean temperature, have indicated a potential for reasonably specific and sensitive forecasting.34
In addition to the direct health impact, this sudden outbreak, as with avian influenza outbreaks in other countries in Africa,35
diverted already stretched public resources from addressing the major endemic public health problems in the region, like acquired immunodeficiency syndrome (AIDS), tuberculosis (TB), malaria, and childhood illnesses including respiratory and diarrheal diseases, all with far greater long term public health burden.36
Very limited veterinary resources were also strained, making it unlikely that long term priorities or other epizootics could be addressed. Strategies for greater diagnostic, epidemiologic, and health systems capacity are needed in sub-Saharan Africa to make it possible to sustain focus on the critical health problems while addressing emerging disease threats when they occur. Furthermore, this outbreak highlights a critical need for new paradigms for how veterinary and human health organizations and ministries, so often functioning entirely independently, must develop combined and synergistic approaches to prepare, detect, and respond to outbreaks of zoonoses,37
especially those with potential to have grave ramifications for human health and livelihoods.