Impact of transmission cycles and vector competence on global expansion and emergence of arboviruses

Abstract

Arboviruses are transmitted between arthropod vectors and vertebrate host. Arboviral infection in mosquitoes is initiated when a mosquito feeds on a viremic host. Following ingestion of a viremic blood meal by mosquitoes, virus enters midgut along with the blood, infects and replicates in midgut epithelial cells, and then escapes to the hemocoel, from where it disseminates to various secondary organs including salivary glands. Subsequently, when mosquito bites another host, a new transmission cycle is initiated. The midgut and salivary glands act as anatomical barriers to virus infection and escape. These complex interactions between the virus and vector dictate the vector competence. Thus, vector competence reflects the success in overcoming different barriers within the vector. Along with these, other intrinsic factors like midgut microbiota and immune responses, extrinsic factors like temperature and humidity, and genetic factors like vector genotype and viral genotype have been discussed in this review. Recent advancement on novel molecular tools to study vector competence is also included. Different modes of arboviral transmission like horizontal, vertical, and venereal and how these play role in sustenance and emergence of arboviruses in nature are also discussed. These factors can be exploited to reduce the susceptibility of vectors for the viruses, so as to control arboviral diseases to certain extent.

Emerging and Re-emerging Arboviral Diseases as a Global Health Problem

“…Through spillover transmission from enzootic amplification cycles, humans can be infected as incidental and dead-end hosts. By contrast, some arboviruses undergo urban cycle involving humans as amplifying hosts and causing several epidemics in urban areas [1][2][3].…”

“…By definition, arboviruses require an arthropod vector in the transmission cycle, in which they must replicate prior to transmission [1]. Most common arthropods include mosquitoes, flies, and ticks along with others hematophagous arthropods [2,3]. Aedes mosquitoes are the most important arboviral vectors; the two main species, Ae.…”

“…aegypti and Ae. albop ictus, allow the transmission of medically important viruses such as chikungunya virus (CHIKV), dengue virus (DENV), and yellow fever virus (YFV) [2]. Other prevalent vectors are Culex mosquitoes, ticks, sandflies, and Culicoides [4,5].…”

“…Following a viremic bloodmeal, virus enters midgut and disseminates through the alimentary tract in the vector and replicates in the salivary glands. During the following blood feeding, injection of infectious saliva allows the transmission to a new host, initiating a new transmission cycle [1,2,4]. Many arboviruses are also maintained in nature through a secondary cycle that involves vertical transmission from an infected female to the offspring.…”

“…In this case, disseminated virus infects the developing eggs, persisting in larval and pupal stages and subsequently into adults. Moreover, venereal transmission allows the transfer of virus from a vertically infected male directly to a female during copulation [2]. The long-term survival is also enhanced by non-viremic transmission, during which infected and noninfected mosquitoes or ticks co-feed on a non-viremic host and the virus is transmitted directly between them, without necessarily infecting the vertebrate host [2,4].…”