Why are only some viruses transmissible by blood and how are they actually spread?

This week we’re running a series in collaboration with the Australian Red Cross Blood Service looking at blood: what it actually does, why we need it, and what happens when something goes wrong with the fluid that gives us life. Read other articles in the series here.

Since the 1980s, most of us have been aware we need to be careful when coming into contact with the blood of others, because some viruses can be transmitted in this way. But why is it only some viruses are transmissible by blood, and how does the virus actually move from person to person?

Viruses are non-living, infectious agents that use our own cells to function and replicate. Unlike bacteria and fungi, they can’t do this on their own, and must find an appropriate host cell they can enter and replicate in.

Blood-borne viruses are those found at levels that can be detected in an infected person’s blood. They can be passed from person to person by blood, and in some cases other bodily fluids, including semen and breast milk. They can be transmitted from an infected person by intravenous drug use, a needle stick injury with contaminated needles, blood transfusions and sexual intercourse. They can also be transmitted from mother to child, before, during or even after birth.

The most important blood borne viruses for human health are the human immunodeficiency virus (HIV), Hepatitis B and Hepatitis C. These viruses persist in the blood long-term or for life.

Many other viruses may be found briefly in blood, but they generally don’t persist and are not considered significant “blood-borne” pathogens. Any infectious agent with a blood-borne, or “viraemic” phase has the potential for blood borne transmission, and so may be important for blood transfusions.

For many infections, this viraemic period persists until the immune system is able to cure the infection by killing all infected cells. For some viruses, including Zika and Dengue, the viraemic phase lasts a matter of days. For other viruses such as Hepatitis B and C, or HIV, this viraemic phase persists.

Treatment of blood-borne viruses aims to stop the virus replicating. If a blood-borne virus is not detectable in blood, then a person is generally considered non-infectious.

How viruses spread through blood

HIV infection can occur when a person is exposed to this virus. Typically, this involves sexual contact or direct blood to blood contact, either via an open wound or penetration of skin by a contaminated needle.

The virus then travels to the lymph nodes, where it enters cells of the immune system called “T cells”. Here, a cat and mouse battle begins. The virus and immune system use complex tactics to outsmart each other.

Perhaps the greatest of these tricks belong to HIV; the virus incorporates itself into the DNA of host T cells, and hides from the immune system in protected sites, such as the central nervous system. This makes it impossible to clear the virus from the body completely, and without treatment HIV remains detectable in blood.

The cells responsible for attacking the virus instead produce new virus particles and attack other, infected, T cells. Without treatment, this leads to destruction of the immune system, unusual infections and a syndrome known as the acquired immune deficiency syndrome (AIDS), which is almost invariably fatal.

In contrast, influenza is spread by respiratory droplets. Virus particles are transmitted when an infected person coughs or sneezes. Droplets are inhaled by a new host, the virus binds to target receptors on the respiratory tract surface, and replicates in cells of the respiratory tract.

Unlike HIV, influenza cannot insert itself into the host’s genes. Instead, the immune system detects infection, turns off viral reproduction and clears infected cells. While medications are available to aid this process, they are often unnecessary, as the immune system is often able to cure influenza independently. Previous influenza infection or vaccination provides some immune memory and provides protection and faster immune clearance of the virus.

Influenza can be found in the blood for a brief period of time, particularly in severe cases, when the person is showing symptoms of flu. Although blood transfusions are not routinely tested for influenza infection, blood donors are asked not to donate if they are unwell.

Transmission via transfusion

Blood transfusions are an important, lifesaving component of modern medicine. But this technology is associated with a small degree of risk, including blood-borne virus transmission.

In Australia, blood transfusions are extremely safe, made secure by legislative, scientific, and practical measures. Legislation precludes certain groups of individuals from donating blood, with a view to minimising the chance of a person with a blood-borne virus donating.

Donated blood is tested for the most important blood-borne viruses, including HIV, Hepatitis B and Hepatitis C. These tests are highly sensitive, and detect close to 100% of these infections.

Blood products destined for recipients with weakened immune systems, like transplant recipients, are tested for additional viruses that are not considered problematic in most people.

Practically, a number of important safeguards are also in place to ensure that the right person gets compatible blood, at the right time.

Read other articles in the series:

Essays on blood: why do we actually have it?

From animal experiments to saving lives: a history of blood transfusions

Explainer: what’s actually in our blood?

Blood groups beyond A, B and O: what are they and do they matter?

What can go wrong in the blood? A brief overview of bleeding, clotting and cancer

Blood tests and diagnosing illness: what can blood tell us about what’s happening in our body?