Streptococcal infections - Causes of streptococcal infections

Strep A is a group of bacteria that are a common cause of infection in humans. The bacteria can be spread in several ways.

Most infections are minor and confined to either the throat, the upper airways of the head or the skin.

However, if the bacteria manage to penetrate deeper inside the body, more severe infections can occur.

Minor strep A infections

Throat infections

Throat infections are spread in much the same way as a cold or the flu. If an infected person coughs or sneezes, tiny droplets of fluid that contain the bacteria are launched into the air. If you breathe these in, you may become infected.

Read more about the causes of throat infection.

Impetigo

An impetigo infection can occur when the bacteria invade otherwise healthy skin through a cut, insect bite or other injury.

An infection can also occur when the bacteria invade the skin as a result of the skin barrier being disrupted by another underlying skin condition, such as head lice, scabies or eczema.

Cellulitis

Cellulitis usually occurs when the surface of your skin is damaged. It creates an entry point for the bacteria, allowing them to attack the skin and tissue underneath.

You are more at risk from cellulitis if you:

are obese (excessively overweight)
have a weakened immune system
have poorly controlled diabetes
have circulation problems
have chickenpox or shingles

Read more about the causes of cellulitis.

Inner ear infection

Inner ear infection is caused when bacteria in the nose or throat spread up into a tube that runs between the throat and the ear.

Children are more likely to be affected by otitis media as they have a weaker immune system and the tube that runs from their throat to their ear is shorter, so it is easier for bacteria to reach it.

Read more about the causes of otitis media.

Sinusitis

Most cases of sinusitis are caused by viral infections but occasionally bacteria can spread from the throat or the nose and into the sinuses.

Read more about the causes of sinusitis.

Invasive infection

A more serious invasive strep A infection can usually only occur if someone has a wound, injury or other type of trauma that can allow bacteria into the body, and a weakened immune system.

A healthy immune system will usually prevent bacteria from spreading deeper inside the body.

The following people could have a weakened immune system:

babies aged less than six months
those over 75 years of age
those with a health condition that weakens the immune system, such as HIV, cancer or type 2 diabetes
those who inject drugs such as heroin
those who misuse alcohol

There are a number of medical treatments that can also weaken the immune system, such as:

chemotherapy
steroid tablets (corticosteroids)
a type of medication called an immunosuppressant designed to prevent your body from rejecting a donated organ

Strep B

It is estimated that 1 in 5 pregnant women have strep B bacteria in their vagina and/or digestive system.

Strep B bacteria can sometimes be passed on to the baby through the amniotic fluid. Amniotic fluid is a clear liquid that surrounds and protects the unborn baby in the womb.

It is also possible for a baby to contract a strep B infection as it passes through the birth canal during labour.

In some cases a baby can contract an infection a few months after birth, known as a late-onset group B strep infection.

In most cases there is no obvious reason why this happens but known risk factors include:

being born prematurely
being part of a multiple birth – for example, twins or triplets
having a mother with a history of group B strep infection

Most children and adults have a natural immunity to the strep B bacteria, though infections can sometimes occur in people with weakened immune systems.