Transovarian transmission of dengue virus

Transovarian transmission of dengue virus refers to the passage of the virus from an infected female Aedes mosquito to her offspring through the eggs. This mechanism allows the dengue virus to be maintained and propagated within mosquito populations even in the absence of ongoing human infections.

Key points on Transovarian transmission

1. Mechanism

The virus infects the ovarian tissues of female mosquitoes and is incorporated into developing eggs during fertilization. Infected eggs then hatch into larvae that carry the virus, becoming infectious adult mosquitoes capable of transmitting dengue to humans.

2. Virus Maintenance in nature

The vertical (transovarian) transmission sustains the dengue virus in the environment during interepidemic periods when human cases may be low, acting as a reservoir that facilitates outbreaks when conditions become favorable.

3. Transmission routes within mosquitoes

Transovarian transmission can occur via

1. infected female laying infected eggs after feeding on viremic hosts,
2. Infected males transmitting the virus sexually to uninfected females.
3. direct infection of ovarian tissues enabling virus inheritance.

4. Evidence

Studies have shown high rates of transovarian dengue infection in both Aedes aegypti and Aedes albopictus mosquitoes, including males (who do not blood feed), confirming virus inheritance rather than just horizontal transmission.

5. Implication

Transovarian transmission complicates vector control because infected mosquitoes can emerge independent of recent human infection. It underscores the importance of targeting immature mosquito stages (eggs & larvae) in control programs to break this cycle.

Thus, transovarian transmission is a significant biological mechanism by which the dengue virus persists in mosquito populations over time.

Horizontal & Vertical  transmission of dengue virus

In dengue virus transmission by Aedes mosquitoes, Vertical & horizontal transmission are two distinct pathways

Horizontal  transmission

1. This is the classical and primary mode of dengue virus transmission.
2. It occurs when an uninfected female Aedes mosquito bites and feeds on the blood of a viremic (i.e., virus-carrying) human, acquiring the virus.
3. After an extrinsic incubation period inside the mosquito, the virus reaches the mosquito’s salivary glands
4. The infected mosquito then transmits the virus to another human host during subsequent blood meals through its saliva.
5. This cycle between humans and mosquitoes sustains the spread of dengue in the population.

Vertical  transmission

1. Vertical transmission, also known as transovarian transmission, occurs when an infected female Aedes mosquito passes the dengue virus directly to her offspring via her eggs.
2. The virus infects the mosquito’s ovaries and is transmitted to the eggs, resulting in infected larvae and eventually adult mosquitoes capable of transmitting dengue without needing an infected human blood meal first.
3. Vertical transmission helps maintain dengue virus circulation during periods of low human infection or environmental conditions unfavorable to horizontal transmission.
4. This form of transmission is important for virus persistence and may complicate control efforts because infected mosquitoes can emerge independent of recent human cases.

 Thus, Horizontal transmission means the virus is transmitted between mosquito and human, i.e., the human-mosquito-human cycle, while vertical transmission means the virus is passed from a female Aedes mosquito to progeny, i.e., dengue virus survival and reservoir persistence within mosquito populations.