Aspectos éticos

RVF

Hlyalomma truncatum is a tick species of the family Ixodidae, widely distributed within the African tropical geographical region [123], which is found throughout the entire geographic range of RVFV [28]. Ticks require blood meals to survive at each of their four life stages: egg, larvae, nymph and adult. Their hosts include humans, mammals, birds, reptiles and amphibians. However, most ticks have a variant of mammalian hosts at each stage of their life and disease transmission occurs through the process of feeding. As pointed out in the introduction, Hyalomma truncatum is both a two-host and three-host tick depending on the hosts species [121]. Thus, a susceptible larvae, nymph or adult may acquire the disease when feeding on an infected host, drop off and switch to another host while in the same stage, and infect that host. Alternatively, susceptible larvae or nymph may acquire the disease by feeding from an infected host then transmit the disease in a later stage to the new susceptible host [120]. The present research study is motivated by

a review undertaken by Nchu and Rand [26] on possible implications of Hyalomma truncatum on the dynamics of RVF outbreaks and it aims to hypothetically evaluate this phenomenon by means of mathematical modelling. This research builds on important features that underline the biology and the ecology of ticks in particular in Sub-Saharan Africa where RVF is endemic. The features are as follows:

1. Wild animals including rodents and livestock (sheep, cattle and goats) are the same hosts for both ticks and mosquitoes that transmit RVF [11, 124]. In addition, immature stages of ticks prefer feeding on hares and rodents, extending the range of feeding hosts [26].

2. Ticks can be transported over long distances on their vertebrate hosts, hence serving as possible hosts for RVFV [28,29].

3. Ticks are widely distributed in the entire geographic range of RVFV. 4. Ticks can often survive for long time between blood meals [125] and the

virus can persist in ticks for their whole lifespan [126]. Thus, long life for ticks means a long time of persistence for the virus increasing the chances of contact between ticks and hosts [26].

5. When feeding on blood, a tick excretes substances in its saliva which have several effects. One is to modulate the hosts immune system. Viruses can benefit from this mechanism, helping it to infect co-feeding ticks on the same hosts vertebrate [127].

6. Ticks cause direct loss through sucking blood [26], reducing host production and increasing host vulnerability to diseases.

7. Mosquito population peaks generally coincide with availability of pastures for domestic livestock and abundance of adult H. truncatum ticks [26].

Putting together the above ticks, mosquito and host’s epidemiological and ecological features we hypothesize that ticks may be contributing to RVF spreading and endemicity. In spite of all these inherent complexities, mathematical models can provide some very useful informative indicators regarding the potential contribution of ticks in the transmission of RVF. Disease outbreaks in livestock in a particular site are very brief [58] and the peak is likely to pass undetected or under-reported. This may be due to interruption in the rainy events or the duration of the rain in a particular site as well as to ruminant incubation period which is very short [11], and the resulting acquired immunity. In livestock the peak is likely to occur after the second or third week after the onset of the epidemic while in humans it is likely to occur between the fourth and sixth week after the first human case [13,58,128,129]. RVF modelling studies have also suggested that arthropods other than Aedes and Culex species may be contributing to RVF transmission by accelerating the cause of the outbreak [4]. Thus, this study aims to assess factors leading to this accelerated exponential phase of RVF outbreaks. [26] suggested that in addition to mosquitoes, optimum climatic conditions, international trade of livestock and livestock products,

ticks could be implicated in the spread of RVF. This would affect the dynamics of the disease, including the number of infected host, host extrinsic incubation period and size of the epidemic. In recent years RVF disease models have been developed for addressing a variety of questions related to disease transmission, maintenance and propagation across geographical regions [4, 5, 48, 69–72, 74]. However, none of these models has included ticks compartments in order to access the possible implications of these blood feeding arthropods in the spread and endemicity of RVF. Characterization of tick host preference at different life stages by means of attached and detached compartments makes this modelling framework unique for investigating how the above ecological tick features could affect the transmission and maintenance of the disease. To explain this, we extend previous deterministic epidemic models with two modes of disease transmission: horizontal (host-vector) transmission and vertical transmission from a female Aedes to its eggs to include compartments of ticks according to their questing and feeding behaviour. Then, we thoroughly investigate the system analytical and numerically and show that certain model parameters are relevant to the start of an outbreak, exponential phase of an outbreak, the prevalence of RVFV and the epidemic size of an outbreak. Some conclusions may also apply to other vector-borne diseases in which ticks are thought to participate in transmission of the pathogen as additional or secondary vectors. Our analysis provides general qualitative insights on the importance of the time ticks spend attached to a particular host and their host life cycle preference. These results suggest that it is possible to diminish the impact of ticks in the transmission of RVF by either inhibiting ticks to attach to a host or by enhancing the immunity of the host to avoid passage of the infection when a tick feeds on the host.