Rotavirus

Rotavirus is a major cause of diarrhoeal disease in infants and young children. It belongs to a genus of double-stranded RNA viruses within the Reoviridae family.

This virus is highly contagious and spreads through the faecal–oral route. This means it moves from waste to the mouth via contaminated hands, surfaces, or objects. Most children worldwide will be infected at least once by the age of five. While the infection is often manageable, it can lead to severe gastroenteritis. This condition causes nausea, vomiting, and watery diarrhoea. Dehydration is the most common cause of death related to these infections.

The virus has a complex, organized structure. It is a non-enveloped particle measuring up to 76.5 nanometers in diameter.

At its center, the genome consists of 11 unique double-helix molecules of RNA. These segments are numbered 1 to 11 based on their size. These RNA strands are protected by a three-layered icosahedral protein capsid. The virus uses six structural proteins, known as VP1, VP2, VP3, VP4, VP6, and VP7, to build its shape. It also produces six nonstructural proteins, or NSPs, once it has infected a cell. These proteins help the virus replicate and manage the host cell's resources.

Different species of rotavirus affect different hosts. There are 11 known species, labeled RVA through RVL. Species RVA is the most common and causes over 90% of human infections. Other species target specific animals. For example, RVH infects pigs, while RVI infects cats. Species RVD, RVF, and RVG are found in birds, and RVJ affects bats.

Within the Group A rotaviruses, scientists use a dual classification system. They look at two surface proteins to define G-types and P-types. The glycoprotein VP7 determines the G-type, while the protein VP4 determines the P-type. Many combinations exist, but only a few predominate in human infections, such as G1P[8] or G2P[4].

The infection process follows a very specific sequence of steps. First, the VP4 protein spike on the virus surface attaches to glycans on a host cell.

The virus enters the cell through a process called receptor-mediated endocytosis. This creates a small bubble called an endosome. The VP7 and VP4 proteins then disrupt the endosome membrane. This disruption changes the calcium concentration inside the cell. A drop in calcium causes the VP7 proteins to break into single subunits. This leaves the VP2 and VP6 proteins surrounding the RNA, forming a double-layered particle (DLP).

Once the double-layered particle is formed, the virus begins to replicate. The 11 RNA strands stay protected inside the protein shells. This allows the virus to use its VP1 enzyme to create mRNA transcripts. By staying inside the core, the virus can evade the host's innate immune responses. Most viral proteins then gather in a specialized area called a viroplasm. This viroplasm forms around the cell nucleus about two hours after infection. Two nonstructural proteins, NSP2 and NSP5, are thought to help build these viral factories.

Finally, the new viruses migrate to the endoplasmic reticulum to gain their outer layer before being released through cell lysis.

Specific proteins play vital roles in making the disease more severe. For instance, NSP1 blocks the interferon response, which is part of the body's immune system. NSP3 helps the virus by shutting down the host cell's own protein synthesis. Most notably, NSP4 acts as a viral enterotoxin. This was the first viral enterotoxin ever discovered. It induces diarrhoea by elevating calcium levels in mammalian cells.

Because the virus is so efficient at hijacking cells, it can be very difficult to control through standard sanitary measures alone.

Understanding rotavirus has led to significant public health improvements. Ruth Bishop and her colleagues discovered the virus in 1973 using electron micrographs.

Before widespread vaccination, the impact was massive. In the United States, rotavirus caused about 2.7 million severe cases and 37 deaths every year. In 2019, rotavirus caused an estimated 151,714 deaths globally among children under five.

However, since the introduction of vaccination programmes in the 2000s, hospitalisation rates have fallen significantly. Today, routine childhood immunisation is a key tool in reducing the incidence and severity of the disease.