On a global scale, the public health sector is built on prioritizing diseases that majorly contribute to the leading causes of morbidity and mortality. In the spectrum of accountable diseases, infectious diseases caused by Aedes Aegypti, which is the principal vector among the species family, has the widest ever recorded distribution and is notably responsible for Yellow Fever, Dengue, Chikungunya and Zika virus. Infectious diseases caused by Aedes Aegypti range from how they progress clinically, presenting with severe flu- like illnesses that affect all age groups, to complications that can result in end-organ failure and ultimately death. This literature review will support the urgency for critical assessment of vector control and preventative measures with respect to the discussed diseases and highlighting the causative factors that are leading to the expansion of these diseases internationally through lack of effective mosquito control, globalization and urbanization. Future control of outbreaks is greatly dependent on preparedness and education on preventative measures that needs active participation from both the government and the public.
Introduction
Aedes Aegypti for centuries was known as the “most dangerous animal in the world” as it has been hypothesized that out of the approximate 3500 mosquito species, this particular vector has contributed immensely to human suffering and the death toll. The historical evolutionary course of Ae. Aegypti started from its ancestral root in West Africa and was introduced into the New World about 550 years ago through the European slave trade (Powell, Gloria-Soria & Kotsakiozi, 2018). The timeline and dissemination of this vector continued to the Mediterranean region, when ships from the New World were returning to their European ports of origin between 1800 and 1950. Subsequently, in 1869 when the Suez Canal opened, Aedes Aegypti made its way into Asia in the 1870s, then Australia in 1887 and the South Pacific in 1904 (Powell, Gloria-Soria & Kotsakiozi, 2018).
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Although strong evidence has established that Africa is the Ancestral home of the vector species, there is still no detailed understanding or timeline established as to when this wild species became domesticated. The wild population of Ae. Aegypti uses tree holes and natural pockets of water as breeding sites for larvae and non-human mammals as a blood source, whereas the domesticated form began using human generated water containers and humans as blood source (Powell, Gloria-Soria & Kotsakiozi, 2018). More than transition, this overlap in co-existing environments was a major hallmark in the spread into the tropical and subtropical world and the source of the transmitted infectious diseases it carries globally. To differentiate the two forms of the species, a subspecies classification was implicated, for the ancestral African type named Ae. aegypti formosus and outside Africa, the domesticated form named Ae. aegypti aegypti (Powell, Gloria-Soria & Kotsakiozi, 2018). The infectious diseases caused by the Ae. Aegypti vector has many different routes of distribution due to varying causative and transmission factors, that will be further discussed in detail.
Epidemiology & Transmission
Yellow Fever
Yellow fever virus, a flavivirus, is an acute viral hemorrhagic disease that continues to be prevalent in Africa, Central and South America, even after the introduction of vaccination. This mosquito-borne illness is responsible for an estimated 200,000 cases annually and 30,000 deaths per year, with Africa accountable for 90% of these cases (Mutebi & Barrett, 2002). Africa, along with Central and South America have similar factors, which contributes to the incidence rate, however, there is a high density of A.Aegypti mosquitoes in close proximity of unvaccinated individuals yet Central and South America has a lower rate of transmission. (Mutebi & Barrett, 2002). South America has a higher level of immunity amongst its residents compared to Africa because they have implemented campaigns for vaccination programs during outbreaks, leading them to have better control of their transmission (Barnett, 2007). Another contributing factor as to why South America shows lower transmission rate is due to limited contact between the infected monkeys, that are the host of the virus, in the forest canopy and the human population. By taking the precautious steps towards limiting and preventing exposure, South America has better outcomes (Barnett, 2007).
Dengue
Dengue fever, along with its more severe forms, dengue haemorrhagic fever and dengue shock syndrome are caused by 4 serotypes of the dengue virus (DENV-1, DENV-2, DENV-3 and DENV-4) ('Epidemiology | Dengue | CDC', 2019). Dengue virus, a flavivirus, is transmitted between humans mainly by the Aedes Aegypti mosquito but in some cases it was found to be transmitted from an infected mother to her fetus or through blood transfusions. This highly infectious disease is said to have originated 800 years ago in Africa and was passed down to humans by monkeys. Many factors are said to have played a role in the dissemination of the virus, one of the main ones being transportation of army forces and cargo between almost all continents during the second world war ('Epidemiology | Dengue | CDC', 2019). Over the years, there has been uncertainty in the pattern of incidence, which accounts for being one of the most common arboviral diseases with almost 40% of the population living in an area with a high risk of dengue virus transmission. There are about 100 million infections recorded every year. Of this, there are 500,000 haemorrhagic fever cases recorded and almost up to 22,000 deaths each year ('Epidemiology | Dengue | CDC', 2019).
The disadvantage of the dengue virus from the beginning of its discovery was the ability to captivate and control the disease in a given area, starting from 1970, when the virus was recorded in just 9 countries. However, it is now an endemic in 100 countries around the globe and it is highly prevalent in Central and South America, Africa and Southeast Asia. The World Health Organization has reported an increase of almost a million infected individuals in three regions between 2010 to 2015, with reported outbreaks in Europe specifically in France and Croatia in 2010 and in the Madeira Islands of Portugal in 2012 (Bhatt, et al., 2013). Several risk factors contribute to the development of the dengue fever including living in tropical and subtropical regions such as Mexico, Brazil, Philippines, Thailand and many other countries of Latin America and Southeast Asia (Bhatt, et al., 2013). Studies has indicated that young children are more susceptible to developing the disease due to incomplete development of their immune system, implying that age is also a risk factor. Males were also found to be more commonly affected than females, however the reason for this gender difference has not been thoroughly investigated (Bhatt, et al., 2013).
Chikungunya
The Chikungunya virus (CHIKV) is dominant in continents such as Africa, Asia and the Indian subcontinent, with an estimated 3 million infections occurring each year. Chikungunya was first identified in 1952, in West Africa with low level activity associated with heavy rainfalls contributing to the increasing mosquito population. There have been periodic outbreaks documented in Asia and Africa beginning in 1960, which was followed by several decades of inactivity (Zeller, Bortel, & Sudre, 2016). However, in recent years, there has been re-emergence occurring in areas where there has been no previously transmitted activity such as Europe, Caribbean and South America, as well as re-appearance in India following an absence of viral activity for 32 years. The largest documented outbreak occurred in 2005 on the island of Reunion located in the Indian Ocean, with an estimated 266,000 cases out of a total population of approximately 770,000 (Borgherini, et al., 2007). The next year, India encountered approximately 1.25 suspected cases. The irregularity of these epidemic outbreaks caused by the disease has made it difficult to predict a pattern of behaviour, however it has been suggested that chikungunya follows a similar pattern as dengue, especially in the United States subtropical regions, which helps navigate investigations during outbreaks.
Zika Virus
Zika Virus, is a less commonly discussed disease, amongst the group of diseases caused by the Aedes Aegypti, such as dengue and yellow fever. The name “Zika” comes from the Zika forest in Uganda where the virus was first isolated back in 1947. From the 1960s to 1980s, rare sporadic cases of human infections were found across Africa and Asia, typically accompanied by a mild illness (or illnesses) (Plourde & Bloch, 2016). Very few cases of Zika Virus were brought forward up until, the first reported outbreak of Zika Virus in 2007. Subsequently, in 2013 and 2014, epidemics of Zika virus infection occurred in French Polynesia, New Caledonia, Cook Islands, and Easter Islands. Up until this time, the virus was only found in areas close to equatorial belt in Africa and Asia, however, this belief changed in 2015, when the first recorded outbreak in the Americas and more currently in Brazil occurred, indicating the dissemination of the disease to more areas (Petersen, et al., 2016). The Brazil Ministry of Health estimated around 440,000-1,300,000 suspected cases of Zika virus infection in December 2015. To date, a total of 86 countries and territories have reported evidence of mosquito-transmitted Zika infection. (Petersen, et al., 2016).
Zika virus is mainly transmitted by the bite of the female Aedes Aegypti mosquito, however it can also be transmitted vertically from a mother to her fetus, as well as sexual transmission, from infected men to their partners. It was found that when the zika virus was isolated in semen samples, there was a significant increase in the virus compared to blood and urine samples. The CDC recommends that men that travelled to areas where the virus is endemic should avoid sexual intercourse and wait a minimum of six months before attempting conception. Until now, there has been no recorded case of an infected female transmitting the virus to her sexual partner.
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