Arboviruses and Vector Borne Viruses

Arbovirus, arthropod-borne virus, is an epidemiological rather than taxonomic category, spanning multiple unrelated virus families united only by their shared transmission mechanism: replication within an arthropod vector followed by inoculation into a vertebrate host during a subsequent blood meal. Geographic distribution, outbreak risk, and diagnostic strategy are determined mainly by vector and reservoir biology.

An arbovirus's transmission cycle requires both a competent vector and a reservoir.

• The vector is an arthropod species, almost always a mosquito or tick among the viruses in this group. It must be capable of becoming infected after feeding on a viraemic host. It then transmits infectious virus to a new host after an incubation period within the vector itself, termed the extrinsic incubation period.
• The reservoir is the vertebrate host population that maintains ongoing virus circulation in nature, sustaining the transmission cycle between epidemic human outbreaks.

Humans are, for most arboviruses in this group, incidental, dead-end hosts. Human viraemia is generally insufficient in magnitude or duration to permit efficient onward transmission back to a vector, meaning human infection represents a spillover from the true maintenance cycle rather than a sustained, self-perpetuating human transmission chain.

Dengue is the principal, clinically important exception within this group, given a human-mosquito-human urban transmission cycle sufficient to sustain transmission without requiring an animal reservoir at all, directly relevant to its capacity for large, sustained urban outbreaks.

Arbovirus geographic distribution is fundamentally determined by vector distribution, itself determined by climate, altitude, and other ecological factors. A given virus's geographic range can therefore be predicted directly from its vector's known range, and can shift over time as vector range shifts.

This is a phenomenon of increasing relevance given climate change's documented effect on expanding the geographic range of several key mosquito vectors.

Aedes mosquito-borne viruses, dengue, yellow fever, Zika, chikungunya, share a broadly overlapping tropical and subtropical geographic distribution, reflecting their shared vector.

Culex mosquito-borne viruses, West Nile virus, Japanese encephalitis virus, have their own distinct, broader temperate-to-tropical distribution pattern, reflecting this different vector's own ecological range.

Tick-borne arboviruses, such as the tick-borne encephalitis virus group found across parts of Europe and Asia, follow a distinct geographic pattern tied to tick habitat and seasonal activity.

The magnitude and duration of human viraemia directly determines two clinically important properties.

• Transmissibility from an infected human back to a vector, relevant to a virus's capacity to sustain urban transmission as with dengue.
• The diagnostic window during which PCR-based direct detection remains sensitive.

Viruses with brief, low-magnitude human viraemia, West Nile virus and Japanese encephalitis virus being the clearest examples, are diagnosed predominantly by serology. PCR sensitivity falls rapidly as viraemia resolves, often before the patient presents with the neuroinvasive disease that prompts testing in the first place.

Viruses with more prolonged or higher-magnitude viraemia, dengue and Zika being relevant examples, retain useful PCR sensitivity for a longer window after symptom onset, informing the specific test timing recommendations that differ between these organisms.

Early in illness, during viraemia, PCR and direct viral detection is most reliable within the first several days of symptom onset while viraemia remains detectable, with the exact reliable window varying by organism.

Later in illness or after viraemia has resolved, serology is the mainstay for organisms with brief viraemia. It is the necessary fallback for any arbovirus once the early PCR window has passed. Use IgM for recent infection, and paired acute and convalescent IgG where seroconversion needs to be demonstrated.

Serological cross-reactivity between related flaviviruses, dengue, Zika, yellow fever, West Nile virus, Japanese encephalitis virus, is a well-recognised source of diagnostic difficulty, particularly relevant in regions or patients with possible exposure to more than one flavivirus. Confirmatory neutralisation testing is sometimes required to resolve ambiguous results in this scenario.

A detailed travel and exposure history, destination, timing, specific activities such as rural or forested exposure or livestock contact, is often the single most useful piece of diagnostic information. It correctly narrows the differential before laboratory testing even begins.

The same clinical syndrome, fever, headache, myalgia, with or without rash, is shared across many of the organisms in this group. It cannot reliably be distinguished on clinical grounds alone.