Tropical and Vectors-Borne Diseases: Insights into Biology, Transmission, and Control Measures

2.1 Vectors definition and types

Vectors are organisms responsible for transferring infectious agents from one host to another [1]. These vectors fall into two broad categories: arthropods and vertebrates. Arthropods, which are the most associated with medical importance, include various invertebrates such as mosquitoes, fleas, flies, ticks, mites, and crustaceans like cyclops [2, 3]. On the other hand, vertebrates, although less commonly, can also act as vectors. Examples include rodents spreading hantavirus and animals such as bats and dogs that transmit rabies [4].

Vectors play an essential role in the spread and transmission dynamics of infectious diseases, particularly in regions with tropical and subtropical climates (Figure 1).

2.2 Vectors life cycle

To design effective prevention and control strategies for vector-borne diseases, it is crucial to understand the biology and life cycles of medically significant vectors. The arthropod life cycle consists of several stages, with each stage’s involvement in disease transmission differing based on the species. Typically, the life cycle (Figure 2) progresses through the following stages:

• Egg: Laid by the adult female arthropod in environments like vegetation or stagnant water.

• Larva or Nymph: This stage emerges after hatching from the egg and feeds on organic materials. It undergoes several molts before advancing to the next stage.

• Pupae: A dormant phase during which metamorphosis occurs.

• Adult: The mature, reproductive phase that plays the primary role in spreading diseases, depending on the species, through actions such as biting or defecating [7, 8].

Different developmental stages offer specific opportunities for targeted control and intervention measures [9].

2.3 Vectors characteristics

Several factors influence how effectively a vector can transmit infectious agents. These include the vector’s anatomy, reproductive processes, behavior, competence, and environmental influences. Anatomical adaptations often equip vectors with specialized tools for disease transmission. For example, the mosquito’s mouth parts are designed with sharp blades that pierce the skin of hosts, allowing them to feed on blood and spread pathogens [10]. The concept of vector competence refers to a vector’s inherent ability to acquire, harbor, and transfer a disease-causing agent [11]. This competence can differ significantly between species [12]. Additionally, many vectors have high reproductive rates and short life cycles, enabling the rapid spread of infections. Behavioral patterns, such as feeding habits, preferred environments, and mating behaviors, also influence the likelihood of disease transmission. Environmental factors such as rainfall, temperature, altitude, and humidity shape where vectors thrive, determining the areas where vector-borne diseases are likely to become endemic and cause substantial health impacts [8, 12].

3.1 Medically significant vectors

The role vectors play in transmitting infectious agents significantly contributes to the global burden of morbidity and mortality, particularly in tropical and subtropical regions. Understanding the characteristics of medically important vectors is crucial for developing effective disease prevention and control strategies. Examples of these arthropod vectors include mosquitoes, ticks, fleas, and flies [1].

3.2 Mosquitoes

Mosquitoes belong to the kingdom Animalia, phylum Arthropoda, class Insecta, and order Diptera. They are scientifically classified under the family Culicidae [13]. There are over 3000 species of mosquitoes, but only a small fraction act as vectors for pathogens. Despite their small numbers, mosquitoes are considered the deadliest animals on Earth, responsible for over one million deaths annually due to the infections they transmit [14]. The most significant mosquito species from a public health perspective belong to the genera Anopheles, Aedes, and Culex. These genera are major contributors to the spread of viral and parasitic diseases globally (Table 1).

Mosquitoes breed in various water surfaces depending on the species. Anopheles mosquitoes breed in shallow streams and creeks, Aedes prefer artificial water containers, and Culex lay their eggs on stagnant or freshwater surfaces [8].

Anopheles mosquitoes consist of over 400 species, with only 40 responsible for transmitting the malaria-causing Plasmodium parasites [16]. These mosquitoes prefer to feed on humans, typically biting at dawn, dusk, and night. Anopheles species vary in competence and geographical distribution, with An. gambiae (Figure 3) being the most competent malaria vector in sub-Saharan Africa [16]. In Latin America, An. darlingi is the primary vector, while in Asia, An. dirus, An. minimus, and An. punctulatus play significant roles making it more complex to control malaria in endemic Asian regions [16].

Aedes species transmit viruses such as dengue, Zika, yellow fever, and chikungunya. The two most medically significant species are Aedes aegypti (Figure 4) and Aedes albopictus (Figure 5). These mosquitoes are aggressive daytime feeders and are distributed across tropical and subtropical regions, favoring urban environments. Aedes mosquitoes have spread worldwide through commercial trade, such as tire shipments [9, 20].

Culex mosquitoes are found globally, except in extreme temperature regions. Culex quinquefasciatus is common in tropical and subtropical regions, including North America (Figure 6). They transmit West Nile virus, St. Louis encephalitis, Japanese encephalitis, Rift Valley fever, and filariasis. Culex mosquitoes feed on humans, animals, and birds, crucial in transmitting diseases like the West Nile virus [22].

3.3 Ticks

Ticks are second only to mosquitoes in public health significance, serving as vectors for various pathogens, including bacteria, viruses, and protozoa. Controlling ticks is critical for global health. There are over 900 tick species, with 28 known to transmit pathogens to humans. Ticks are divided into soft (family Argasidae) and hard ticks (family Ixodidae), with hard ticks being the most significant to human health. Notable hard tick species include the black-legged tick (Ixodes scapularis) are responsible for Lyme disease spread; the American dog tick (Dermacentor variabilis) transmits Rocky Mountain spotted fever and tularemia; and the lone star tick (Amblyomma americanum) transmits ehrlichiosis [23].

Similarly, soft ticks play a significant role in transmitting multiple infections. Ornithodoros transmits tick-borne relapsing fever, and the fowl tick (Argas reflexus) affects both humans and poultry. Ticks thrive in warmer climates, and their distribution has expanded due to climate change, urbanization, and international travel [11].

3.4 Flies

There are over 150,000 species of flies worldwide. Key species of medical significance include the house fly (Musca domestica), the tsetse fly (Glossina spp.), and the sandfly. House flies, common in urban areas, spread bacterial and viral pathogens, leading to gastroenteritis [7]. Furthermore, flies larvae (maggots) can cause myiasis over open wounds and vulnerable populations as children. Tsetse flies are vectors of the protozoan parasite Trypanosoma brucei, which causes African sleeping sickness, a disease that can result in severe neurological damage [10]. Sandflies transmit Leishmania parasites. In the Old World (Africa, Asia, and Europe), sandflies belong to the Phlebotomus genus, while the Lutzomyia species are found in the Americas as a new world (Figure 7) [25].

3.5 Fleas

Fleas are insects that inhabit various hosts, including domestic animals like cats and dogs, as well as wildlife. They prefer warm, humid environments. Fleas can transmit multiple pathogens, most notably the bacterium Yersinia pestis, which causes bubonic plague [26]. Fleas also transmit Rickettsia typhi from rats, causing murine typhus, and serve as intermediate hosts for the parasitic tapeworm Dipylidium caninum [27]. Furthermore, fleas can act as ectoparasites, causing tungiasis as it borrows through the patient’s skin, especially around the feet [28].

4.1 Mechanical transmission

In most cases, vectors transfer pathogens externally from one host to another without being involved in the pathogens’ life cycle. House flies are notorious as mechanical vectors for transmitting bacteria, viruses, and protozoa. Mechanical vectors are insects with body specifications that facilitate the transmission, including the internal and external anatomical structures of the vector’s body. Mechanical transmission also relies on the biological characteristics of different phases of the vector emerging during its lifecycle and its feeding habits, allowing direct and indirect contact with the pathogen and host. Mechanical transmission mode is suitable for pathogens that can survive temporarily on the surface of the vector’s body. For example, house flies can carry pathogens such as Amoeba, Escherichia coli, and Salmonella when they land on contaminated matter and food, causing foodborne diseases [29].

4.2 Biological transmission

It occurs when the vector acts as an essential host for the pathogen’s life cycle, which involves a more complex interaction between the vector and the pathogen. In biological transmission, pathogens require the vector to complete its life cycle; in the vector’s absence, the pathogen would die or be incapable of development. Pathogens undergo several sexual and asexual developmental and reproductive changes toward completing their lifecycle and becoming infectious agents before being transmitted to a new host [29, 30]. Biological transmission includes inoculating the pathogen by vector directly into the host’s bloodstream; malaria, dengue, and Zika are excellent examples. In some infections, vectors regurgitate the pathogen during feeding, like in plague disease, where fleas regurgitate Yersinia pestis into the host’s bloodstream. Biological transmission also can occur when the pathogen directly deposits on the host’s skin or mucous membranes. For example, in Chagas disease, the Trypanosoma cruzi parasite is excreted by kissing bugs in the feces and then enters the host through broken skin or mucous membranes [29].

4.3 Historical background of vectors-borne diseases

In 1878, Patrick Manson observed that mosquitoes that fed on blood from infected people with elephantiasis carried the same unsheathed microfilariae he had seen in the blood of infected people. Later, he observed that those mosquitoes were intermediate hosts for the filarial parasite Wuchereria bancrofti. Later, in 1899, Thomas Bancroft confirmed that Wuchereria bancrofti transmitted filariasis to humans through their bites [31].

Mosquitoes transmitting malaria were discovered in 1884 by Ronald Ross and other investigators under the direction of Giovanni Grassi. In 1880 and 1900, respectively, Carlose Finally and Walter Reed proved that the mosquito Aedes aegypti was responsible for yellow fever transmission [31].

In 1881, while investigating an outbreak of yellow fever in Havana, Cuba, Dr. King observed similarities between the symptoms of yellow fever and dengue and their transmission. Later, in 1906, the observation was confirmed after a dengue outbreak in the Philippines [32].

4.4 Vector-borne diseases overview

Mosquitoes are considered one of the world’s most widespread and deadly disease vectors. They develop in various aquatic habitats and in all climates, from the Arctic to the Tropics, and they are responsible for transmitting a wide range of debilitating and deadly tropical diseases, especially in poor and limited healthcare-resource countries [31]. Malaria, dengue fever, Zika, Chikungunya, Yellow fever, Rift Valley fever, and West Nile viral infection are mosquito-borne illnesses that pose a severe threat to human health worldwide [33, 34, 35].

Malaria is the most significant tropical disease transmitted by mosquitoes. Anopheles mosquitoes, particularly the female anopheline species, are the definitive host of the causative agent, the protozoan malaria parasite that belongs to the Plasmodium genus. Among the Plasmodium genus, P. falciparum and P. vivax are responsible for more than 95% of infections worldwide. Species of P. vivax are the widest-spreading species among the tropics, subtropics, and temperate regions, while P. falciparum is widely distributed in the tropics [33, 34].

The mosquito becomes infected when feeding on the blood of an infected human, where the sexual development of the parasite takes place in the mosquito’s gut before migrating to the salivary glands. In the next mosquito bite, malaria sporozoites are regurgitated from the salivary gland into another human’s bloodstream, initiating a new infection [35]. Malaria symptoms are rhythmic fever, chills, fatigue, headache, and, in severe cases, organ failure, coma, and death. Malaria is an endemic disease in many tropical and subtropical regions, particularly in sub-Saharan Africa, Southeast Asia, and parts of South America [33, 34, 35, 36].

Dengue fever is another significant global health concern carried by mosquitoes and is a widely spreading disease. Incidences of dengue have grown dramatically in recent decades, with an estimated 390 million infections occurring annually [37]. In 2015, Brazil recorded over 1.5 million dengue cases and about 500 deaths. Aedes mosquitoes are the common dengue fever vector, mainly Aedes aegypti and Aedes albopictus. These mosquitoes prosper in tropical and subtropical regions usually breeding in sluggish water sources and artificial water containers [37, 38].

There are four serotypes of dengue virus, and individuals infected with one serotype have a lifelong immunity to that specific serotype but not to the others [37]. The infection occurs through the bite of infected Aedes mosquitoes, leading to a range of non-fatal symptoms such as high fever, severe headaches, muscle and joint pain, and skin rash, also known as ‘break-bone fever.’ Subsequent infections with multilabel dengue serotypes elevate the risk of life-threatening dengue hemorrhagic fever. In severe cases, infection leads to plasma leakage, hemorrhage, and organ impairment [37].

Zika is a reemerging flavivirus infection carried by Aedes mosquitoes. Zika virus raised a significant public health concern due to its association with congenital Zika syndrome, fetal neurological complications, and severe impact on pregnant women’s well-being. Like dengue, the Zika virus transmits to humans through subsequent bites of an infected mosquito [39]. Most Zika virus infections in immunocompetent individuals are asymptomatic or mild, characterized by rash and joint pain. For several decades, Zika virus infection remained in Africa and Asia. Later, between 1969 and 1983, the geographical distribution expanded to equatorial Asia, with the virus being detected in mosquitoes and sporadic human cases reported in other countries like India, Indonesia, Malaysia, and Pakistan as a result of international travel mainly [39].

Chikungunya is a mosquito-borne viral infection, like dengue and Zika; it transmits through Aedes mosquitoes (Aedes aegypti and Aedes albopictus). Chikungunya is a tropical and subtropical disease, particularly in parts of Africa, Asia, and the Indian subcontinent. In the twenty-first century, chikungunya invaded the Americas and Europe due to outbreaks and spread of the Aedes mosquitoes. Chikungunya Infection is mild; symptoms include rash, sudden fever, headache, muscle pain, and severe joint pain [31].

Yellow fever is one of the most severe and widespread viral infections transmitted by bites of Aedes mosquitoes. Naturally, the yellow fever virus infects animals, particularly monkeys, and the viral infection is maintained in the monkey-to-monkey sylvatic cycle by forest-dwelling mosquitoes. When the sylvatic cycle is interfered with by human actions such as forest demolition and destruction, the natural cycle of forest mosquitoes is disturbed, and humans get bitten by one of the monkeys’ feeding mosquitoes [31]. Afterwhich, infected humans carry the infection to urban areas Ae. aegypti initiates the person-to-person urban cycle. Yellow fever causes severe hemorrhagic conditions symptomatized by high fever, jaundice, and prostration. The disease is geographically distributed through Africa and sub-Saharan Africa regions, where about 90% of worldwide cases were reported. Yellow fever was also found in Brazil, Peru, Colombia, Bolivia, Venezuela, and South America [31].

Rift Valley fever is a viral zoonotic disease that mainly affects livestock but also can infect humans. Viruses transmit via bites of infected mosquitoes. Aedes mosquito species are the primary vectors, but Culex species can transmit the virus too [40]. Infection is found particularly in Africa and parts of the Middle East, and the transmission to humans occurs during or shortly after periods of heavy rainfall and flooding, which create favorable breeding conditions for mosquitoes and, therefore, their abundance. In humans, the disease causes flu-like symptoms, and in severe cases, it may lead to hemorrhagic fever, encephalitis, or blindness [41].

West Nile viral infection is primarily associated with birds. It is transmitted through the bite of infected Culex mosquitoes, carrying the virus when feeding on infected birds. In the human population, this infection is asymptomatic or mild, flu-like symptoms. However, in rare cases, it can lead to serious neurological complications such as encephalitis or meningitis. West Nile virus spread throughout Africa, the Americas, and Europe [31].

Japanese encephalitis is a mosquito-borne viral disease in Asia and Western Pacific. It is a serious infection transmitted to humans via the bite of infected Culex mosquitoes, particularly Cx. tritaeniorhynchus. In most cases, the infection is asymptomatic or presents mild symptoms. In a few cases, encephalitis develops, and the patient may have paralysis, seizures, or, in worst cases, death [31, 42].

4.5 Other tropical diseases of medical important

Chagas disease, known as American trypanosomiasis, is a potentially life-threatening disease caused by the parasite Trypanosoma cruzi. Chagas disease is primarily transmitted to humans through contact with the feces of infected triatomine bugs, also known as “kissing bugs,” due to their tendency to bite around the mouth. The disease is most prevalent in rural areas of Latin America, where insects reside in the cracks and holes of poorly constructed houses. Chagas disease can also be transmitted through organ transplantation, blood transfusions, consumption of contaminated food, and congenitally during pregnancy. If the disease is left untreated, it could lead to serious heart and digestive complications [43].

Leishmaniasis is a parasitic illness caused by various species of the protozoan parasite called Leishmania. Sandfly acquire the parasite by feeding on infected animals or humans, Phlebotomus in tropical and subtropical of the Old World and Lutzomyia in the New World [30]. Most sandflies are active during warm, humid nights. Their nocturnal feeding habits pose a greater threat to people sleeping outdoors. During the day, they rest in various hiding places such as cracks in walls, rodent burrows, or inside homes. Sandflies tend to rest on house walls before feeding, and that increases the risk of human infection. Leishmaniasis can be seen in different forms: cutaneous leishmaniasis, mucocutaneous leishmaniasis, and visceral leishmaniasis. Disease manifestations are various from self-healing disease to fatal one. Cutaneous leishmaniasis mainly affects the skin, and symptoms usually are self-healing but can leave disfiguring scars, while visceral leishmaniasis affects internal organs like the spleen, liver, and bone marrow and becomes fatal if left untreated [44]. Leishmaniasis is considered a significant public health concern in many tropical and subtropical regions worldwide [30].

7.1 Future research in vector control

Novel vector control interventions have been developed from extensive research in the recent decade. This section will highlight a few innovative measures developed for vector control over the years.

Gene drive is one of the novel control approaches. It is a genetic modification method that spreads favorable traits through interbreeding populations of malaria mosquitoes. This method can be used for virulent population replacement or suppression, by reducing mosquitoes’ ability to transmit a pathogen, reducing female fertility, or biasing the sex ratio toward males (Figure 9) [56].

Another innovative method in vector-borne disease control is introducing Wolbachia bacteria into Aedes mosquitoes, as part of the ‘World Mosquito Program’ that is conducting a randomized controlled trial and programmatic evaluations in several countries worldwide. The method is based on releasing mosquitoes carrying Wolbachia into the field to mate with the wild mosquito population, and over time, the population of mosquitoes carrying Wolbachia increase resulting in transmission reduction of dengue and other arboviruses. Wolbachia infections can reduce the transmission of arboviruses by several mechanisms, including inhibition virus replication within the mosquito, shorting mosquitoes’ lifespan, and creation of cytoplasmic incompatibility phenomena which lead to produce proportion of mosquitoes that are less likely to transmit dengue [57, 58].

Furthermore, eave tubes were developed as a new, promising household protective measure. They are small plastic tubes with insecticide-laden electrostatic netting inserted into the house wall directly below the roof. Mosquitoes are attracted to the house by host odors released through the eave tubes, and then mosquitoes are killed after contacting the insecticide-treated netting. The method is being tested in a large randomized controlled trial in Côte d’Ivoire alongside other house measures such as eave closure and window screening [59, 60].

Lastly, human and livestock treatments such as vaccination and chemoprophylaxis as oral Ivermectin, are promising strategies for controlling and preventing vector-borne diseases. However, further studies are still needed to overcome biological and environmental challenges and problems [31].