Viremia

Viremia (from Latin virus — poison and Greek haima — blood) is a pathological condition characterized by the presence of viable viral particles (virions) in the systemic bloodstream. This is a key stage in the pathogenesis of most systemic viral infections, enabling generalization of the process.

Etiology and pathophysiology

Viremia may be primary or secondary.

Primary viremia occurs when the virus, after replicating at the portal of entry (for example, in the nasal or intestinal mucosa), overcomes local immune barriers and enters the lymphatic vessels for the first time, followed by the bloodstream. This phase is usually brief and asymptomatic.

While circulating in the blood, viruses rapidly reach their target organs (the liver, spleen, and nervous system), enter them, and initiate extensive replication.

When hundreds of thousands of new virions are simultaneously released from destroyed cells in these organs back into the bloodstream, secondary viremia develops.

It is this surge that initiates the systemic inflammatory response: the immune system responds by releasing cytokines, causing fever, chills, and marked intoxication.

Clinical significance

The viremic phase directly correlates with the severity of the clinical presentation and the period of maximum contagiousness (infectivity) of the patient.

In pregnant women, a high viral load in the blood enables the pathogen to cross the placental barrier and infect the fetus (intrauterine infection).

In medical practice, this stage comprises the following aspects:

• in laboratory diagnostics — detection of viral DNA or RNA in blood plasma (using PCR) is evidence of the acute, active stage of the disease;
• from a pharmacological perspective — systemic antiviral drugs (for example, drugs against influenza virus) are most effective specifically during the viremic phase because they inhibit viral replication, reduce its concentration in the blood, and prevent further entry into healthy cells.