Introduction
Rift Valley fever (RVF) is an arthropod-borne viral zoonosis that primarily affects domestic ruminants, humans and some wild animals (OIE
2002,
2007). The RVF virus (RVFV) belongs to the Phlebovirus genus under the Bunyaviridae family. Major RVF epizootics (in livestock) and epidemics (in humans) have occurred in several countries in both Africa and Middle East (Bird et al.
2007). In this paper, the word outbreak is used interchangeably with epidemics or epizootics and it means that reported number of RVF cases in people and livestock is higher than normal. The last two outbreaks in eastern Africa occurred in 1997/1998 and 2006/2007 (Woods et al.
2002; Nguku et al.
2010; Anyangu et al.
2010). The long inter-epidemic/epizootic period (IEP) is attributed to association between the outbreaks and occurrence of El Niño rains. The latter are associated with anomalous warming of sea surface temperatures in the eastern equatorial Pacific and the western equatorial Indian Ocean. The above normal rains that follow the El Nino events cause flooding especially in low-lying areas, favouring the hatching of
Aedes mosquitoes that transmit RVFV (Linthicum et al.
1991; Diallo et al.
2005).
In livestock, Rift Valley fever outbreaks occur after bites from infected mosquitoes (Linthicum et al.
1985; Davies and Highton
1980). A majority of human infections result from contact with blood or organs of infected animals (WHO
2010; LaBeaud et al.
2008; Sang et al.
2010; Nicholas et al.
2014), while few result from bites by infected mosquitoes. Peaks in human RVF incidences coincide with outbreaks (epizootics) in livestock (Woods et al.
2002; Archer et al.
2013). Impacts of RVF outbreaks go beyond livestock producers to affect public health, other livestock value chain actors and connected sectors of the economy (Swanepoel and Coetzer
2004; ILRI
2008; ROK
2009; Pépin et al.
2010; Rich and Wanyoike
2010).
In managing human RVF, governments seek to optimise health gains by reducing number of human cases, severity or duration of disability and deaths. In the process, budgetary constraints introduce difficult decisions on how to allocate limited resources. Health economists support the decisions by providing data on disease burdens (monetary and non-monetary) as well as cost-effectiveness of control options. Monetary costs include control costs and opportunity costs. Disability adjusted live years (DALYs), a non-monetary measure recommended by World Health Organisation (WHO), reflects premature death and reduced quality of human life (disability) in non-fatal Cases (Murray
1994). One DALY is equal to one lost year of “healthy life”.
Cost-effectiveness analysis helps to prioritise public health sector’s investments allowing decision makers to compare financial costs and gains made or likely to arise from different interventions. Expressed as cost of intervention per DALY averted, WHO sets thresholds based on per capita national incomes (World Health Organisation
2014). An intervention that costs less than three times the national annual per capita GDP is considered cost-effective, whereas one that costs less than once the national annual per capita GDP is considered highly cost-effective. For zoonotic problems such as RVF, gains in human health arise from both animal and public health interventions. Therefore, examining costs and benefits at both levels and in particular benefits to public health sectors arising from animal interventions becomes important. Mostly, zoonotic transmission is animal to human and not the reverse making effective interventions to lie outside public health sector. Assessing costs and benefits of control from a multisectoral perspective facilitates identification of strategies that yield the highest benefits to both sectors. Further, knowledge of distribution of benefits would inform animal control cost sharing between animal and public health sectors.
This cost-effectiveness analysis (CEA) examines impacts of four livestock sector level RVF intervention strategies on public health and identifies those that offer highest benefits to the public health sector.
Discussions and Recommendations
This study sought to demonstrate public health sector benefits gained from controlling RVF at animal level. Due to unavailability of animal–human RVF transmission model at the time, we applied two separate models linked through data. The results showed significant public health sector monetary and non-monetary burden (DALYs) associated with the 2006/2007 RVF epidemic in high-risk areas in PAP systems. Considering that the systems carry 53, 66, 73 and 99.7% of the cattle, sheep, goat and camels found in high-risk areas, and that human transmission is mostly through animal contact, the DALYs estimated could constitute a large proportion of the national burden.
For the same epidemic, our estimates of 3974.05 DALYs are lower than higher estimates of 4035.6 reported in Orinde (
2014). The difference lies in the data used. Orinde used a disability weight of 0.652 for all cases, and only considered line listed cases and human population in only three Counties. Nguku et al. (
2010) report that not all line listed cases were due to RVF. Our study considered human population in all RVF high-risk areas in PAP system, and used prevalence-derived incidence to accommodate for under reporting. Both studies imply that the national burden of RVF associated with 2006/2007 RVF outbreak might be higher than the estimates. The animal–human RVF transmission modelling showed that under the animal control base strategy, the magnitude of a next hypothetical epidemic would be nearly similar.
Total DALYs associated with the 2006/2007 and the hypothetical 2014/2015 (under base strategy) translate to 852 annual unweighted, undiscounted DALYs, that represent 7% of the upper limit and more than twice the lower limit of the global RVF burden reported in LaBeaud et al. (
2011a). Inherent in DALY estimation process, it is the YLL from RVF mortality that accounts for the largest proportion of DALYs estimated.
Based on WHO thresholds for cost-effectiveness (WHO
2013) and compared to baseline improved vaccination coverage in camels and cattle from 0% to between 7 and 51% (depending on species, age targeted and strategy), and sheep and goats (1–2 fold) 2 years before an RVF epidemic can be considered to be highly cost-effective from a public health perspective: in terms of reduction in DALYs and direct treatment costs for human cases. Under base practice, only 4–9% small ruminants were annually vaccinated for 7 years before the hypothetical 2014/2015 outbreak. World Health Organisation’s Choosing Interventions that are Cost-Effective (WHO-CHOICE) project indicates that an intervention with a cost per DALY averted that is less than three times the national annual GDP per capita is considered cost-effective, whereas one that costs less than once the national annual GDP per capita is considered highly cost-effective. The strategies can significantly (23–26%) reduce DALYs. The base practice is not cost-effective for the public health sector. Also, the benefit cost ratio that compared allocated costs to saved monetary costs shows that compared to baseline, the two alternate strategies with enhanced vaccination had a BCR of about one, while Alternate 3 had less than 1. Higher or equal monetary benefits over allocated costs reflect additional benefits to the DALYs averted.
Based on our models, enhancing surveillance while keeping vaccination level at base would yield only small benefits to the public health sector. However, effective animal surveillance systems would allow public health sectors to implement early public health communication to minimise contact with infected animals. Such benefits were not captured in our modelling. Enhanced animal surveillance also supports earlier implementation of livestock sanitary bans; and therefore, potential contacts through slaughter and marketing activities.
However, the results must be interpreted from a perspective of that in modelling, animal–human transmission, this study faced challenges as no animal–human transmission model had been developed to support multisectoral analysis. At the same time, few datasets on joint animal and public health outbreak investigations existed. This study, therefore, relied on few data points from five epidemics in four countries. Similar difficulties were reported by other similar studies (LaBeaud et al.
2011a; Randi
2011), to an extent that the latter assumed that, in case of an incursion, human RVF outbreak in Southeast Texas, USA, would acquire spread and infection rates similar to West Nile Virus. On the other hand, LaBeaud et al. (
2011a) presented annual global burden of RVF as a range of 353–11,958. In addition, while animal transmission model estimated the number of animals that would abort, be lactating or infected and slaughtered during the outbreak in Kenya, lack of similar data for the other four outbreaks in other countries denied the authors an opportunity to modifying the transmission based on relative risk. Further, due to the same data and modelling challenges, we combined all animal species data which make it hard to tease apart the relative contribution from different risk factors such as drinking un-boiled milk as fewer people would drink raw milk from sheep, goats and cattle compared to camels.
To overcome these challenges, and particularly to strengthen One Health economic analysis of zoonotic diseases, there is a need for future joint epidemiological investigations to generate data to support animal–human RVF epidemiological modelling. Also, there is need for public health studies that estimate (i) the relative contribution of different public health measures such as surveillance and communication to the outcome of the epidemic and (ii) animal–human contact rates and transmission probabilities. Generating longitudinal data on human and livestock cases during both the epidemics and inter-epidemic and scale of measures applied would support modelling of livestock-human transmission as was the case of brucellosis modelling in Mongolia in Roth et al. (
2003).
Further, in modelling the magnitude of the hypothetical 2014/2015 RVF epidemic, we assumed that changes in human behaviour prior to and during epidemics would not change and therefore, the same force of infection is maintained. This was influenced by observations that the PAP areas are under developed, and receive relatively lower quantity and quality of health services including community-based communication for behaviour change. Consequently, therefore, incidences of zoonosis are higher than in other farming systems as shown in the case of brucellosis (Regassa et al.
2009; Racloz et al.
2013) and anthrax (Nkedianye and Herrero
2007). Some risk factors for human brucellosis and anthrax such as living close proximity to livestock, handling livestock and consumption of raw products are similar to those of RVF. Owange et al. (
2014) highlight pastoralist’ perception where mosquito bites are perceived as the key risk factors compared to contact with infected livestock and livestock products which are contrary to other studies (Woods et al.
2002; Anyangu et al.
2010). Finally, we note that errors could have resulted from the modelling process where two models are used to arrive at this cost-effectiveness analysis. The errors could have made the model less sensitive to changes in some of the key processes being studied.
While the results show that increasing peace time animal vaccination coverage reduces the magnitude of human outbreaks, the baseline practice shows that national governments seem to find it difficult to achieve good coverage levels as the risk is perceived to be low. While new vaccines are being developed including multivalent ones that might be possible to be applied more frequently alongside those of other diseases, vaccination coverage for all diseases is considered to be lower than expected. A better strategy—e.g. routine vaccination at a given coverage and a reactive vaccination when a risk warning is given to shore-up the desired levels of coverage—is required. Despite that enhanced surveillance is expected to reduce potential number of human cases following early implementation of sanitary bans, the modelling failed to capture this dynamics. Therefore, the real value of animal surveillance in terms of reducing human could not be explicitly explained. To better respond to future outbreaks, contingency plans and decision support tools are suggesting more pragmatic efforts of implementing surveillance.
Acknowledgments
This study was funded by the International Development Research Centre (IDRC) Canada through the Agriculture and Research Platform. The collaborating organisations in the implementation of the project included: IFPRI, SWISS TPH, ILRI, Egerton University and CDC-KEMRI. The authors acknowledge all field staff, key informants and livestock keepers who freely provided information. Also acknowledged are Austine Bitek of Kenya and Bouna Diop of FAO-ECTAD, Eastern Africa.