Personal protective strategies for dengue disease: Simulations in two coexisting virus serotypes scenarios

Abstract

Dengue fever is a common mosquito-borne viral infectious disease in the world and is widely spread, especially in tropical and subtropical regions. At this moment, one of the best ways to fight the disease is to prevent mosquito bites. In this study, we present a mathematical model that carefully considers personal protection for humans. It is an epidemiological model that translates the dengue disease through a system of differential ordinary equations which takes in consideration the dynamics of the disease between human and mosquito populations. This model incorporates a parameter that simulates personal protection measures, namely insect repellent, special clothes, or bed nets, and a parameter that asserts the effectiveness of public awareness to the importance of using personal protective equipment.

In 2012 there was a dengue disease outbreak in Madeira Island, in Portugal, and this study not only tries to predict what could happen if a second outbreak occurs, where it is considered that there are two serotypes of Dengue disease, but also tries to predict the effects and the importance of taking personal protection measures.

The results show that the level of personal measures and the time that people are compelled to use them have a significant impact to prevent dengue disease.

Introduction

Dengue is a mosquito-borne disease and is a major public health issue in the tropics and subtropics namely in the Madeira Island. Dengue is a viral infection transmitted primarily by the Aedes aegypti mosquito, which is found mostly in tropical regions. The mosquito lives in urban habitats and breeds mostly in man-made containers. Ae. aegypti is a day-time feeder and its peak biting periods are early in the morning and in the evening before sunset. Female Aedes aegypti frequently feed multiple times between each egg-laying period. Once a female has laid her eggs, these eggs can remain viable for several months, and will hatch when they are in contact with water [34].

Dengue Fever is transmitted by dengue viruses that are members of the genus Flavivirus and family Flaviviridae. Four immunologically distinct but antigenically similar DENV-1, DENV-2, DENV-3, and DENV-4 serotypes cause dengue [33], [35].

Infection with a dengue serotype results in life-long immunity to that type. However, in subsequent infections, it may have a higher chance to catch the more dangerous forms of dengue, Dengue Hemorrhagic Fever (DHF) and Dengue Shock Syndrome (DSS). The fatality rate of patients with these symptoms is larger and there is not yet a proper treatment for Dengue [34]. The reason for this disease escalation is due to the effects of antibody-dependent enhancement (ADE) [30]. In that way, dengue strategies should be effective against all dengue serotypes.

There is no particular treatment for dengue disease and clinical management depends on supportive therapy, mainly cautious monitoring of intravascular volume replacement. The recently licensed dengue vaccine, Dengvaxia (CYD–TDV) made by Sanofi Pasteur, has been approved, but still needs more improvements [29]. Until the results are not completely satisfactory to the population, dengue prevention and control rely on interventions targeting the vector. Basic control strategies intent to keep out mosquitoes from egg-laying habitats through the application of suitable insecticides or predators to outdoor water storage containers; other control strategies are the use of personal and household protections such as open space spray of insecticide during dengue outbreak [24].

Several mathematical models have been formulated to investigate the effects of dengue on population [2], [7], [10], [11], [12], [21], [25], [30], [32], [35]. Mathematical epidemiology studies about interaction models between host-vector and human populations for dengue disease transmission were proposed in [1], [5], [36]. The focus was on the study of the basic reproductive rate from the stability analysis of equilibrium points using systems of ordinary differential equations (ODEs). There is a large number of studies using mathematical models and computer simulations that discuss the vector control methods [4], [6] and inefficient vaccines [15], [26] as human protection tools. Some of these models combine the use of larvicide, adulticide and vaccination strategies to combat the host-vector and the virus in humans. They use two control strategies: one for mosquito population reduction and the other for human immunization. Hence, modeling dengue disease is of great importance to help us understand the disease’s dynamics and, therefore, interfering with its spreading though control methods verified mathematically.

The protection against mosquito bites could also be analyzed as a control of the disease. Aedes mosquitoes have diurnal biting activities in both indoor and outdoor environments. Therefore, personal protection measures should be applied all day long and especially during the hours of highest mosquito activity (mid-morning, late afternoon to twilight).

In this work, the main focus goes to the personal protective measures, to reduce/eliminate mosquito bites with the final aim of prevent dengue disease. Protective measures adopted by individuals not only help in protecting themselves against mosquito bites, but also help in reducing the mosquito population by denying the blood meal essential for nourishment of the mosquito eggs of the female anopheles mosquito.

Some of these personal protective measures are clothes that minimizes skin exposure during daylight hours , when mosquitoes are most active and, therefore, afford some protection from the bites of dengue vectors. Besides, there are repellents, ideally the ones that contain DEET, that could be applied to the exposed skin and/or clothing [23], [33]. Also, insecticide-treated mosquito nets can afford good protection for those who sleep during the day (e.g. infants, the bedridden and night-shift workers) and can be another way to prevent bites.

The application of these measures could contribute to the decrease the burden cost of this disease [18]. According to these authors, the economic effect of dengue on households, including lost workdays, is substantial.

When studying the time where individuals use personal protective equipment, one has to consider two prominent factors: the duration of the protection (e.g. a can of repellent spray had a short term, when compared with the use of a bed net) and the willingness to use these measures through awareness and publicity campaigns. Both aspects are contemplated in our model.

The paper is composed of five sections. In Section 2, the mathematical model is proposed, including the variables, parameters and the set of differential equations. The numerical results are shown in Section 3, where a series of simulations using distinct measures of personal protection are carried out. Finally, in Section 4, the main conclusions and some future directions are presented.