Lutzomyia longipalpis

The acidity of the sandfly midgut is maintained at a stable homeostatic pH level of ~6, even in the presence of strong buffer solutions. However, the proper digestion of blood requires an alkaline pH of 8.15 in the abdominal midgut for proteases like Trypsin to function properly.[9] Lutzomyia longipalpis females contain the ability to abruptly alter their physiology by switching the internal environment of the abdominal midgut from acidic to alkaline. The presence of undigested proteins serves as the acting stimulus to drive female sand flies to undergo a shift in abdominal midgut pH. Even though the pH of the abdominal midgut is alkaline, carbohydrate digestion is able to function efficiently due to the maintenance of the acidic pH of the thoracic midgut.

Although the feeding of blood is important for the maturation of oocytes, too many blood meals can also negatively impact the lifespan of females. Lutzomyia Longipalpis, like most other blood-sucking organisms, ingest large amounts of blood in a single meal. A female sandfly consumes between three and ten times its body weight in one feeding.[11] Blood is rich in proteins, consisting mainly of hemoglobin (Hb), which accounts for approximately 60% of the blood protein content.[11] The digestion of hemoglobin results in the release of high levels of the prosthetic group heme. Heme acts as a toxic molecule that can generate oxygen-reactive species and bypass membranes due to its high permeability. Elevated levels of heme in female L. longipalpis are suspected to be the cause of increased mortality for females that have ingested multiple blood meals.[11]

The direct bite of an infected sandfly during blood feeding allows for the parasitic transmission of Visceral leishmaniasis from L. longipalpis to the vertebrate host. The sandfly saliva contains potent physiological compounds that cause anticoagulant, vasodilating, and anti-inflammatory activity, which influences the immune response of the host vertebrate.[13] Dogs that have been treated with the LJM17 salivary protein from L. longipalpis presented a more powerful cellular immune response in response to infection. Higher levels of proinflammatory cytokines and chemokines were detected with a specific increase in the production of IFN-γ and IL-10, indicating a stronger and longer-lasting immune response to the disease antigen.[13] The strong immune reaction in response to exposure to the salivary protein indicates the protein's potential use in the field as a treatment to vaccinate dogs who then become immune to the disease.

For transmission of Leishmania to occur, it must first undergo development into an infective promastigote. This crucial step of development occurs in the midgut of Lutzomyia longipalpis. The microbiome of the midgut is a critical factor that influences the growth of the pathogen into its infective state. Sucrose-rich diets result in highly diverse, stable bacterial microbiomes. Meanwhile, blood-feeding diets cause a markable decrease in microbial richness, but this decrease is eventually corrected after a short period of time. Sandflies infected with Leishmania experience a progressive decline in the bacterial diversity of the midgut. Interestingly, the perturbation of the midgut microbiome due to the introduction of antibiotics causes the sand flies to become unable to support the parasitic growth of the pathogen.[9] This highlights the bacterial microbiome of the L. longipalpis midgut as another area of interest that can be explored to control the disease.

One factor possibly contributing to the high L. longipalpis presence near chickens is the feeding of sand flies on chicken blood. Chickens are dormant at night, which leave large areas of exposed skin susceptible to fly feeding. The epidermis of chickens is relatively thin (~0.02 mm), which allows the sandfly to more easily pierce the skin to acquire the blood nutrients.[12]

From an evolutionary standpoint, one theory proposes that male blood-sucking L. longipalpis may have gained a significant mating advantage by staying near chickens and waiting for females to arrive and feed.[12] At any given moment, the number of male flies far outweighs the number of female flies present on a chicken host, which is similar in size to their mating ritual of courtship. Thus, chickens may play a vital role as a hub for the propagation of the fly species.[12]

Another theory is that in general, many of these people who raise chickens in chicken coops tend to also raise other animals, such as pigs and livestock, in their yards. Dogs are commonly used as security to guard the chickens, which presents a convenient group of amplification hosts for the parasite within a concentrated area.[12]

After emergence from pupae, both male and female L. longipalpis initially feed on sugars from plants and aphids, but as development occurs, only adult females are hematophagous. Both genders will consume sugar-rich foods, such as nectar, honey dew, and plant sap, but females will feed on a wide range of vertebrates, including dogs, chickens, horses, and humans.[16] The blood meal serves as an essential step for the maturation of the ovaries. After consuming a fulfilling amount of blood, the female starts its digestion, which requires three to four days.[16] Females are anautogenous insects, so egg development only occurs after a blood meal. Oviposition begins six days after the blood meal and generally lasts for six days.[17] However, a second blood feed is required to start a new cycle of digestion and oviposition.[18]

Males possess glands that secrete pheromones to attract females that can act over a distance of 240 cm. After birth, pheromone biosynthesis occurs after 12 hours, and it takes males 24 hours to become sexually mature. Male courtship behavior involves the aggregation of males who compete with one another by producing sex pheromones.[19] Males encircle females and use the vibrating movement and flapping of their wings to produce audible sounds. These pre-mating signaling frequencies resemble a song that the males use to attract and court the females.[3]

Legume plant.

Because the diversity and quantity of plant wildlife are largely associated with the quality and availability of resources that insects can utilize, acacias have been targeted as a point of interest in their involvement with L. longipalpis and the propagation of its disease.[21] Acacia trees attract the presence of these sand flies due to their ability to provide protection from predators, allowing the flies to proliferate. Sand flies also require sugar from plants as their energy source. Plants from the family Fabaceae are preferentially selected for in the feeding diet of L. longipalpis, while other trees that may be more abundant, such as the Anacardiacea and Meliaceae families are not preferred by the fly species.[2] The Fabaceae family is commonly known as legumes and contain a mixture of carbohydrates ranging from simple sugars to complex heteropolysaccharides. The sandfly species are attracted to the specific carbohydrate composition of this plant species over others.