New type of host defence against Zika, dengue infections revealed Premium

Research explains why some viruses present in bodily fluids don't spread through those fluids, revealing a novel defense mechanism against viral infection.

The mere presence of a virus in a bodily fluid doesn’t mean it is transmitted via that route. The Zika, dengue, and chikungunya viruses are present in fluids like saliva and semen but don’t spread orally or sexually. This fact has puzzled scientists for years, and now a research team has finally explained why.

On March 29, 2020, the World Health Organisation (WHO) posted a tweet that sparked concerns among scientists and prompted many to contact the WHO to say there was enough evidence to discredit the tweet’s contents, and suggest it steer clear of such assertions. The tweet was later proven to be factually incorrect but the WHO remained unconvinced for almost two years.

It read “FACT: #COVID19 is NOT airborne”.

Transmission is a crucial event in a virus’s life cycle, yet studying it has been very tricky. A virus that can’t transmit is of no consequence to anyone. Successful viruses have extraordinary adaptations selected through years of evolution to ensure they can jump to new hosts.

Most human viruses achieve this by ensuring they are present in bodily fluids that contact the outer environment, and subsequently a new host.

Once inside the new host, a virus must be present at the correct location to infect new target cells. Viruses are usually highly selective in the cells they infect. This phenomenon, called tropism, occurs because most viruses have special proteins on their outer surface that make contact with a receptor on the host cell. Any cell-type that makes the receptor can be infected by the virus.

For example, the receptor for HIV is a protein called CD4. Only cells that make the CD4 protein, such as T-cells and macrophages, can be infected by HIV. Similarly, SARS-CoV-2 uses a receptor called ACE2. Cells of the respiratory tract and some cardiovascular cells all express ACE2, and are target cells for SARS-CoV-2. HIV can’t infect respiratory cells and SARS-CoV-2 can’t infect T-cells or macrophages.