RNA Viruses: Infection, Transmission & Diseases

In RNA viruses, the entire genome consists only of RNA. However, they are not a uniform group of viruses. Their characteristics and replication strategies are different.

What are RNA viruses?

The term RNA virus is a collective name for a variety of viruses whose genetic material consists solely of RNA. Their replication strategies are completely different. What all RNA viruses have in common, in addition to their RNA genome, is that they require a host organism to reproduce. Almost all plant viruses, many animal viruses and some bacteriophages are RNA viruses. In most cases, these have only one RNA strand. However, there are also double-stranded RNA viruses. Single-stranded RNA viruses may contain a minus-stranded RNA genome or a plus-stranded RNA genome. In some cases, they also have a plus minus strand. Minus strands are RNA single strands that are constructed in the opposite direction to translation. The reverse is true for plus strands. Minus strand RNA viruses contain a complementary single strand as genome, which must first generate a plus strand for protein synthesis. For replication, the minus strand is replicated into the plus strand. The plus strand generates a minus strand again. In the case of plus strand RNA viruses, the single strand corresponds to mRNA and can immediately synthesize viral protein. To replicate the virus, the complementary minus strand is first built, which again serves as the basis for synthesizing the next plus strand. Retroviruses are a special form of RNA viruses. They incorporate their RNA genome into the DNA of the host cell with the aid of the enzyme “reverse transcriptase”. However, the ICTV (International Committee on Taxonomy of Viruses) does not count retroviruses as RNA viruses, even though their genome consists of RNA.

Occurrence, distribution, and characteristics

Viruses in general and RNA viruses in particular are ubiquitous. However, they cannot replicate without a host organism and therefore infect it by several routes. RNA viruses are the causative agents of such infectious diseases as influenza, rubella, polio, hepatitis E, SARS, dengue fever, Lassa fever, and Ebola. The rotavirus or the norovirus also belongs to the RNA viruses. The HI virus is probably the best known retrovirus. The transmission routes of the individual viruses are very different. The influenza virus, for example, is transmitted by droplet infection through the air. Many intestinal viruses are passed on by smear infection. The risk of infection can be reduced by hygienic measures. However, viral diseases that are easily transmitted via the air, such as influenza, can lead to epidemics or even worldwide pandemics in crowds of people. Short-term vaccinations help against the type of influenza that is currently present, but this can change. Other diseases such as Ebola are partly present in the tropics and are transmissible through food with infected meat or physical contact. The HI virus, on the other hand, is difficult to transmit. Infection can only occur during the exchange of bodily fluids such as blood or semen.

Meaning and function

A viral infection always represents a health disorder of the body. This is true for both RNA and DNA viruses. Viruses of any kind cannot survive outside a host organism. Thus, they are always dependent on a living organism for their replication. Regardless of whether an infection with viruses, bacteria or fungi occurs, the body reacts by forming antibodies against foreign proteins. Therefore, it often happens that a lifelong immunity occurs after an infection with a certain pathogen. Only if the pathogen changes genetically, it can repeatedly infect the same organism. Bacteria, fungi and DNA viruses have double-stranded DNA in their genome. Due to the double strand, mutations occur relatively rarely, because the DNA in the form of the second strand has a backup copy of the genetic code. Any errors in the replication of DNA are usually eliminated by repair mechanisms. RNA viruses lack this backup copy. In addition, the host organism does not possess an enzyme to repair errors in RNA replication. Mutations are constantly taking place on the RNA virus, allowing it to evade many of the body’s defense mechanisms. Because viral strains of RNA viruses are constantly changing through mutations, lifelong infection can occur. Infection twice with a genetically identical strain is usually not possible.

Diseases and ailments

When infected with RNA viruses, different courses of disease are to be expected. For the course of the disease, it plays a role whether system-relevant organs are affected, which virus strain is currently active, and the general health situation of the affected person. At the same time, it is also not insignificant how severely the infected cells are damaged. The strength of the immune system‘s reaction is also decisive for the course of the disease. A strong immune reaction can even worsen the situation if the body temperature rises too much and healthy cells are destroyed in addition to the infected cells. A temperature that is too high would be given by a fever of over 40°C, which persists for many hours. Only then are the body’s own proteins also affected by denaturation. In general, fever helps the body fight viruses. Normally, the elderly and young children are particularly at risk of dying from complications during influenza because their body’s defenses are lower. However, during the Spanish flu in 1918, a particularly large number of young and middle-aged people died, caused by a particular type of influenza virus. With RNA viruses, there is always a risk of a particularly severe course due to their high mutability. Furthermore, RNA viruses that are insignificant today may mutate into highly infectious virus strains in the future. A preventive development of vaccines has been ruled out so far. Vaccines can only be developed for existing virus strains. The particular persistence of HI viruses is also due to their strong mutability. In the course of HIV infection, there is constant modification of the virus, so that it can constantly resist the immune response of the organism.