BY: SAI MANOGNA (MSIWM013)

Vector-borne infections, diseases caused by insects- and tick-borne pathogens, have long affected human affairs. The bite of a tiny mosquito carrying malaria parasites in the marshes of what is now called Iraq defeated Alexander the Great, the conqueror of many nations. To this day, vector-borne diseases remain influential, flooding the hospitals of sub-Saharan Africa with malaria victims, suppressing nations’ economies, and destroying industrial operations where they remain endemic. Specific, less common agents cause blindness and terrible disfigurement. Together, they form an alarming set of possible health and livelihood risks to those who travel and work or live in the tropics where they have the most significant effect.

Vectors :

Arthropod (insect) vector transmitted pathogens are some of the most harmful and volatile on earth. They are perhaps the hardest to stop or manage because they are so immune to intrusion and so deeply rooted in the habitats and environments of the regions they infest. In this equation, vectors make all the difference since they increase the range and transmissibility of pathogens exponentially over those that rely on transmission through direct human touch. Vectors help pathogens cross the gap between humans and various host animals (mice, rodents, monkeys, birds, prairie dogs, pigs). Over cycles less favorable to transmission (winters, dry seasons), some harbor reservoirs of pathogens. Without approaching the vector directly, vectors are facilitators of several harmful disease-causing species, the prevention, and treatment of which can not be successful for long.

For their entire lives, vectors stay infected, which is longer than others. 

Individuals think. For example, a mosquito does not have a set lifespan. In their first week of life, many die, but some will live almost forever. They are restricted by the harm that accumulates on their wings and appendages that are not repairable and do not degrade as soon as they wear out. In water, predation, desiccation, and trapping are likely to kill more mosquitoes than any other source. West Nile vectors that emerge in August of one year on the East Coast of the United States can be active over winter and in May of the following year for a life span of at least nine months.

The bulk of transmissions are due to mosquitoes and ticks. Large vector-borne diseases are involved, including sand flies and black flies. Even within a closely related community, each of these species has specific habitat requirements and feeding habits, which can differ significantly. For instance, around the globe, dozens of species of Anopheles mosquitoes can spread malaria. Some of them bite at night, some only at dawn and at dusk, in bright sunshine, some breed. Others never come out of the forest’s deep shade. Saltwater, for others, is lethal. In water with a saline content approaching that of seawater, some excel. Generalization, generally, fails when it comes to vector biology. The unique vector behaviors provide the keys to managing them and preventing infection from spreading.

The insect-borne disease of industrial significance :

Mosquitoes: Mosquitoes, by far the most important vector of disease, number more than 3,000 species worldwide. Only the female mosquito can spread the disease since only she, and not the male, has the knife-like part of the mouth required to extract blood from her victims. She requires a blood meal to allow protein for egg creation.

Sand Flies: Closely related to mosquitoes, sandflies are blood-feeders and breed in caves, rodent burrows, manure piles, and other dark areas that hold moisture and are rich in organic matter. They are weak fliers, tending to travel from host to host in fast “hopping” flights. Their bodies are so tiny (3 mm) that they are hard to detect before they begin to bite. Their bite has been creating extreme pain for a few days.

Black Flies: Black flies are yet another relative of mosquitoes specializing in flowing water from small trickles to large rivers. Unlike mosquitoes, black flies eat through the skin and never eat indoors. They can attack such large quantities that their salivary fluids alone can cause a person to get sick, causing a disease called “black fly fever.” They can also vector a nematode that can live in the human body for up to 15 years, killing tissue in the internal organs, most notably in the skin, causing blindness in the eyes.

Ticks: Ticks usually have a much longer life span than a mosquito. Hard ticks eat just a few times during their lifetime, limiting their risk of being infected. However, the persistence and host selectivity of hard ticks allow them to be relatively useful vectors. Soft ticks are long-lived nests and burrow dwellers. Like mosquitoes, they will feed several times during their lifetime.

Important Vector-borne Diseases:

A. Malaria : 

Malaria occurs in every tropical and subtropical landscape across the globe, often allowing seasonal excursions to temperate areas. The genomes, metabolisms, and life cycles of protozoan parasites are more complex than almost any other vector-borne threat. This makes it a difficult target for medicines and vaccines because the parasite’s shape-shifting pathways make it possible to avoid chemical and immunological defenses. They are also a moving target, deliberately modifying their outer coating during each step of their life cycle and producing a diverse antigenic and metabolic wardrobe through sexual recombination, a generator of diversity that is not available to simpler microbes such as viruses and bacteria.

1. Four parasite species affect humans, but more than 95 percent of cases include two of them, Plasmodium falciparum and P. vivax. The most dangerous pair, P. falciparum, crosses Africa’s deep tropics towards South America and Asia.

2. P. vivax, which may grow in mosquitoes at colder temperatures, has a broader range, spreading beyond the fringes of P. falciparum’s distribution and coexisting with P. falciparum in many regions.

3. P. falciparum malaria poses the greatest threat to the industry of any vector-borne disease, as it can kill an unprotected person very quickly and can reinfect and repeatedly weaken even those who develop semi-protective immunity. 4. Most deaths in local communities occur in children between 6 months and 2 years of age. 

5. Their weakness stems from a lack of immunological defense. Immune evasiveness of malaria parasites prevents full immunity production, but older children and adults who have undergone multiple infections enjoy some degree of defense against the most extreme manifestations of the disease.

Prevention: Prevention of malaria needs serious care while visiting areas where it is transmitted. While no vaccine is currently available, prophylactic drugs and steps to minimize exposure to night-biting Anopheles mosquitoes, such as bed nets and repellents, could be beneficial. Unlike other diseases, malaria patients also never get a second chance.

B. Dengue Fever :

1. Dengue virus tends to be the epithet of malaria in several respects. While malaria transmission is most prevalent in rural areas, Dengue is a city disease. 

2. Although Anopheles ‘malaria vectors bite mostly at night, Aedes’ dengue vectors bite mostly during the day. While the initial malaria infection usually causes the most severe symptoms, a second dengue infection may be much more severe than the first if it includes a different virus’s different serotype. 

3. Dengue fever can be painful (hence the nickname of “breakbone fever”) and Weakening, but generally not life-threatening when first obtained. 

4. However, there are significant manifestations in places where more than one of the virus coexists’ four significant strains. 

5. Being exposed to second, different strains of the virus may cause a severe immune reaction called Dengue Hemorrhagic Fever (DHF), leading to a significant risk of death, especially in children and younger adults. 

6. Currently, about five percent of the hundreds of thousands of people who receive DHF die, while timely and successful medical attention can dramatically reduce this case’s fatality rate.

Prevention: There are no prophylactic drugs or vaccines available to prevent Dengue, but steps that restrict or prevent biting, such as repellents or elimination of water-bearing containers in which mosquito vectors can be created, are helpful.

C. Arboviruses : 

There is a wide range of often-hazardous viruses almost everywhere on the globe where mosquitoes are found. Each of these individual viruses typically has limited ranges, and many affect relatively small populations; we consider them to be a group because of their mutual importance. 

More infamous pathogens and those currently in the news due to the revival or extension of their borders. In many places, these infections may account for poorly reported: “Fever of Unknown Origin” (FUOs) typically occurring in seasons when mosquitoes are most involved.

i. Chikungunya :

Due to its recent revival, Chikungunya tops this list in places including India, Sri Lanka, Mauritius, and Europe’s countries engaged in regular tourism to these destinations. Concern has recently emerged that it will soon expand its range in Europe due to the spread of Asian tiger mosquitoes. (Aedes albopictus), which can serve as a significant vector for this infection. 

Africa and Southeast Asia are also part of the traditional range of this virus. 

Chikungunya infection may be severe and temporarily crippling but is usually not life-threatening in otherwise healthy people. There is currently no vaccine or curative drug treatment available. Prevention must be focused solely on interventions that reduce mosquito bite exposure.

ii. Yellow Fever :

Yellow Fever is a deadly illness that has a worldwide prevalence. 

Because of the availability of an efficient vaccine, this has been significantly reduced. It is primarily Aedes mosquitoes spread by day-biting and can cycle in both urban and rural areas. Monkeys act as maintenance hosts in its rural incarnation, and tree-hole and bromeliad breeding mosquitoes move it on. No one should fly to an area where, without being vaccinated, Yellow Fever remains endemic. Some nations also need evidence of vaccination for admission.

iii. West Nile Virus (WNV) 

The virus has now spread to the New World, previously confined to Africa, the Middle East, and Southern Europe. It has become an omnipresent fixture of North America’s summer landscape and continues to make incursions into South America. Since most individuals who get infected experience nothing more than flu-like symptoms. The occurrence of a potentially lethal cerebral hemorrhage and irreversible neurological damage is a small percentage. Transmission occurs primarily through urban vectors since the most responsible mosquito (Culex pipiens) for amplifying the virus prefers relatively polluted bird populations environments. There is currently no available vaccine or curative drug treatment available.

iv. Tick-Borne Encephalitis (TBE) 

TBE differs from all the other arboviruses mentioned so far in that ticks rather than mosquitoes pass it on. TBE is found from China to Europe in temperate regions. Although infections can often be mild, in around 10-20 % of patients, permanent or long-lasting neurological damage can occur. Only 1-2% of the cases are fatal. For this infection, rodents are the primary maintenance hosts, and hard ticks in the Ixodes genus act as the primary vectors. No vaccine or curative drug treatment available at present.

Life cycle of TBE Virus : For all three life phases of the Ixodes tick, i.e. the larvae, the nymph and the adult tick, the dog will serve as the host. As with humans, the nymphs and the adults that feed on dogs are rather more numerous. There is also evidence of human dietary infection through TBE virus-contaminated milk. Although this sometimes triggers human infection clusters, we are not aware of such an infection path for dogs.

v. Rift Valley Fever (RVF) :

In North Africa and sub-Saharan, RVF is transmitted to animals and humans by 

Aedes mosquitoes day-biting. Humans also acquire infections through direct contact with contaminated animals’ blood during slaughter. Most RFV cases are relatively mild, but there is a case fatality rate of more than 50 percent in the hemorrhagic type of this disease. The overall case fatality rate due to human infection is probably less than 1 percent. RFV is one of the few viral pathogens transmitted to infected mosquito larvae, causing individual mosquitoes to be infected before they bite a host. Currently, no vaccine or curative drug treatment is available.

D. Lyme Disease :

Lyme Disease is also spread by the same forms of hard ticks that transmit TBE, but Lyme Disease has a broader range in North America. Lyme disease has never killed someone, but if left untreated, it can still be crippling. Upon reaching the synovial (joint) fluid or entering the central nervous system by the spirochete bacteria (Borrelia) that cause it, routine antibiotics can no longer enter it. The pathogen can cause symptoms such as arthritis, memory loss, and other neurological problems. 

Prevention: Prevention includes limiting tick bite contact with repellents, clothing treated with insecticides, and basic knowledge of tick habitats and their presence in the body. It takes at least two days for LD spirochetes to become triggered, and the risk of infection would be negligible if an attached tick can be removed before that time. A vaccine was previously available but, due to incomplete effectiveness, is no longer on the market.

E. Leishmaniasis :

Leishmaniasis involves many protozoan infections that can cause severe organ damage to anything from skin sores (in its mildest form). In almost every part of the tropics, certain types of leishmaniasis can be found. 

Nevertheless, the main areas of concern include North Africa, the Middle East (Iraq is a significant problem), and Southwest Asia. It is difficult to treat infections, and the treatments commonly used can be hazardous to humans and cause many side effects. Currently, no vaccine or curative drug treatment is available. 

The vectors act as Sand Flies, a relative of mosquitoes, which breed in caves, animal burrows, and manure piles. Weak, nocturnal fliers. When the wind is high, they will not be involved. Bed nets and repellents include preventive measures. Not as effective in preventing sand fly bites as they are in defending against mosquitoes, several commercial insecticide-treated bed nets have been identified.

F. African Trypanosomiasis (Sleeping Sickness) :

Like those present in Latin America, African trypanosomes are transmitted by Tsetse flies, which are present only in Africa. African Sleeping Sickness “is induced by this pathogen, which can induce coma by invading the central nervous system.” Especially common in mixed savannah/woodland environments are Tsetse flies. Ranges of Trypanosoma brucei gambiense 

Although Trypanosoma brucei rhodesiense is found in East and Southern Africa, it is mainly found in West and Central Africa. A more rapidly advancing and the Rhodesian type creates acute infection, but both will kill people if left untreated. Currently, no vaccine or curative drug treatment is available.

G. Lymphatic Filariasis (Elephantiasis) :

Generally, filariasis does not kill but can cause severe impairment. Multiple nematode worms cause this mosquito-borne infection that invades the lymphatic system, causing swelling and tissue accumulation in different parts of the body but affecting the legs in particular. This condition causes gross distortion of appendages known as “elephantiasis” in its most extreme manifestation. Major surgery and thorough tissue removal provide the only treatment for infections that reach this degree of severity. It requires several years of prolonged infection to develop more severe symptoms, so it does not present a significant concern to staff in the short term. On-the-job exposure, however, can cause several years of deteriorating health and pain. 

Throughout India, Africa, and parts of Southeast Asia and Oceania, filariasis is found. Wuchereria bancrofti or Brugia malayi are responsible for most cases. In the genus Culex, night-biting mosquitoes act as the primary vectors, so bed nets are an efficient way to reduce exposure to this parasite. Currently, no vaccine or curative drug treatment is available.

H. Onchocerciasis (River Blindness): 

Nematodes also cause onchocerciasis, but it’s vectors are black flies that breed in clean, flowing water and are linked to mosquitoes. The adult worms that cause onchocerciasis cause the body to develop fibrotic tissue under the skin to develop hard lumps or nodules. Clinically, the most significant concern occurs from the millions of microfilariae pre-larval worms shed from these nodules which migrate into the skin, causing pruritus and blindness when they enter the eyes in some instances (hence the word ‘river blindness’). Ivermectin, a well-tolerated medication, can destroy microfilaria and temporarily inhibit adult female worms’ ability to reproduce. Black flies only strike outdoors and during daylight hours, so bed nets are not useful for exposure prevention. However, the repellents that function on mosquitoes are typically successful. They are against black flies.

Onchocerciasis is restricted to Africa and Latin America. River-inhabited black flies. The primary vectors in Africa serve as (Simulium damnosum). The significant vectors in Latin America are black flies called Simulium ochraceum and metallicum. At present, no vaccine is available.

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