Viruses & Standard Infection Control Precautions

Viruses are intracellular parasites that cannot replicate themselves outside a host cell.

Viruses and Standard Infection Control Precautions

The virus structure comprise an inner nuclear core of either single or double stranded DNA or RNA that forms the viral genome. The genome is encapsulated in an outer protein matrix shell.

Some viruses are enveloped and surrounded by a lipid membrane. Inserted in the outer protein shell is a range of glycoproteins that act to bind the virus to the host‘s cell and then facilitate entry of the virus into the cell.

Some of the viral glycoproteins are antigenic and are recognised by the human immune system. Their antigenic properties are exploited by scientist as target molecules for protective vaccines such as the neuraminidase and haemagglutinins used in the seasonal flu vaccine, or as specific targets for antiviral drug.

Antigenic Drift

Viruses show much higher levels of mutation in the proteins and glycoproteins than human cells. They are considered to be promiscuous as they are able to swap and share their genetic material with other strains of viruses.

A good example of this process in action is illustrated by the influenza virus. Most years, the circulating strains of flu virus show minor mutations in the neuraminidases and haemagglutinins.

This is called antigenic drift.

Every year a small number of epidemic flu virus strains dominate and circulate around the world. To take into account both the dominant strains and the mutations changing the outer glycoproteins, a new flu vaccine has to be manufactured every year to combat the latest infections which is why flu vaccination is given annually.

Antigenic shift

Several times a century pandemic flu occurs. In the recent swine flu pandemic in 2011 a major rearrangement of viral genes occurred that combined genes from birds, pigs and humans to create a novel virus to which the majority of people in the population had no immunity.

This is known as antigenic shift.

Antigenic shift is responsible for pandemic influenza, which due to the lack of natural immunity is associated with high death rates. 

Enveloped Viruses and Disinfection

Once a virus has replicated inside the host cell it is released either by killing the cell or alternatively the virus can bud through the cell membranes picking up a lipid membrane (envelope) as it goes.

The lipid envelope makes the virus susceptible to inactivation by disinfectants and alcohols, a property that is exploited by manufacturers of surgery surface disinfectants and alcohol hand rubs. Examples of enveloped viruses include herpes simplex virus, HIV and HBV.

Latent, Persistent Infection and the Carrier State

Viruses can cause three types of infection:

Acute Infections

Short lived disease and a person’s immune system is usually able to eradicate the virus.

Latent Infections

In which the virus lays dormant within the host cells but the virus cannot be demonstrated on laboratory testing, except when reactivation occurs such as Herpes simplex virus, which is responsible for primary gingival stomatitis and the reactivation infection herpes labialis (cold sore). The virus remains latent in a sensory nerve ganglion. Only during the primary infection and when the virus reactivates can the infection can be controlled by aciclovir. Aciclovir is only effective when the virus is actively replicating as it inhibits the enzymes involved in DNA synthesis. It cannot eradicate the dormant, latent form of the virus.

Persistent or Chronic Infections

In this type of infection the immune system is unable to clear the virus, for example infections caused by hepatitis B (HBV), Hepatitis C (HCV) and HIV. The virus is continually present in the blood stream and other body fluids such as saliva or semen, so the virus can be transmitted to others via sex, sharing needles, contaminated instruments, sharps injuries, and via the placenta or breast feeding. These chronically infected people are referred to as a” carriers”. There may be no apparent symptoms of the disease until months or years later. The carrier maybe completely unaware they are infected. In the UK there are an estimated 215,000 carriers of HCV.  

In order to manage the risk of transmission from undiagnosed carriers of HIV, HCV and HBV, the concept of Standard Infection Control Precautions were developed.

Using this approach all body fluids, secretion and excretions (except sweat) from all patients are treated as if they are infectious. This concept is deployed worldwide and is proven to be highly effective in controlling cross infection in healthcare.