Simulation Modelling of Population Dynamics of Mosquito Vectors for Rift Valley Fever Virus in a Disease Epidemic Setting

<div><p>Background</p><p>Rift Valley Fever (RVF) is weather dependent arboviral infection of livestock and humans. Population dynamics of mosquito vectors is associated with disease epidemics. In our study, we use daily temperature and rainfall as model inputs to simulate dynamics of mosquito vectors population in relation to disease epidemics.</p><p>Methods/Findings</p><p>Time-varying distributed delays (TVDD) and multi-way functional response equations were implemented to simulate mosquito vectors and hosts developmental stages and to establish interactions between stages and phases of mosquito vectors in relation to vertebrate hosts for infection introduction in compartmental phases. An open-source modelling platforms, Universal Simulator and Qt integrated development environment were used to develop models in C++ programming language. Developed models include source codes for mosquito fecundity, host fecundity, water level, mosquito infection, host infection, interactions, and egg time. Extensible Markup Language (XML) files were used as recipes to integrate source codes in Qt creator with Universal Simulator plug-in. We observed that Floodwater Aedines and Culicine population continued to fluctuate with temperature and water level over simulation period while controlled by availability of host for blood feeding. Infection in the system was introduced by floodwater Aedines. Culicines pick infection from infected host once to amplify disease epidemic. Simulated mosquito population show sudden unusual increase between December 1997 and January 1998 a similar period when RVF outbreak occurred in Ngorongoro district.</p><p>Conclusion/Significance</p><p>Findings presented here provide new opportunities for weather-driven RVF epidemic simulation modelling. This is an ideal approach for understanding disease transmission dynamics towards epidemics prediction, prevention and control. This approach can be used as an alternative source for generation of calibrated RVF epidemics data in different settings.</p></div

Simulated RVF epidemic.

<p>(A) Compartmental phases after allowing infection to flow from Aedines to Culicines for virus amplification, recovered hosts are not allowed to flow back into the susceptible hosts. (B) Calculated host mortality per developmental stages due to infection with RVF virus. <b>Abbreviations</b>: SAdult = Susceptible adults, SLamb = Susceptible lamb, EAdult = Exposed adult, ELamb = Exposed lamb, IAdult = Infectious adult, ILamb = Infectious lamb, RAdult = Recovered adult, RLamb = Recovered lamb.</p

Simulated infection initiation before virus amplification.

<p>(A) Floodwater Aedines initiate infection. (B) Culicines without infection from infected sheep to prevent virus amplification.</p

Diagrammatic presentation of RVF vector population dynamics simulation model.

<p>Adult mosquitoes lay eggs directly in breeding sites or in soil above water level (the latter remain inactive for many years). Hatching of inactive floodwater Aedes eggs depend on water level in breeding sites which in-turn depends on amount of daily rainfall. Our model considers mosquito growth and mortality in each developmental stage depend on temperature, water level and host availability. Mosquitoes move from susceptible to infectious phase after contact with infectious host. Hosts remain in the susceptible phase until after effective contact with infectious mosquitoes, and then hosts flow from susceptible to exposed, infectious and recovered phases. <b>Abbreviations</b>: b = births, d = natural mortality, d₂ = mortality due to disease, temp = depends on temperature, waterLevel = depends on water in breeding sites, transovarial = transovarial transmission.</p

Parameters description.

<p>Parameters description.</p

Sheep population dynamics controlled at the environmental carrying capacity of 200 sheep per square kilometre.

<p>(A) Growth stages without infection. (B Growth stages after introduction of controlled infection within Aedines mosquitoes only.</p

Daily weather data for Ngorongoro district from 1994–1999 only for the first 1000 days.

<p>(A) Daily temperature in °C. (B) Water level in breeding sites (millimetres) calculated from daily rainfall data.</p

Simulated RVF vector population dynamics showing developmental stages from eggs, larvae, pupae and adults.

<p>(A) Floodwater Aedines depending on water level in breeding sites and host availability. (B) Culicines.</p

Vector population dynamics simulation results indicating unusual increase in mosquito population in late 1997 and early 1998.

<p>Similar period to when large-scale RVF outbreak occurred in Ngorongoro district. Disease outbreak did not occur in Mwanza and Musoma areas during the same period.</p

Predicted risk areas for dengue epidemics in Tanzania for the year 2050 climate scenario.

<p>Colour intensification indicate increased probability of risk for dengue epidemic to occur in the area.</p