A secondary infection and crusted scabies are two possible complications of scabies.

Secondary infection

Repeatedly scratching itchy skin caused by scabies may break the skin's surface. This will make you more vulnerable to developing a bacterial skin infection, such as impetigo.

Antibiotics may be recommended to control a secondary infection.

Scabies has been known to make some pre-existing skin conditions, such as eczema, worse. However, other skin conditions should settle down after the scabies infection has been successfully treated.

Crusted scabies

Crusted scabies, also known as Norwegian scabies, is a more severe form of scabies where thousands or even millions of scabies mites are present.

Normal scabies can develop into crusted scabies after a skin reaction. The condition affects all parts of the body, including your head, neck, nails and scalp. However, unlike normal scabies, the rash associated with crusted scabies usually doesn't itch.

In crusted scabies, the increase in the number of mites causes thick, warty crusts to develop on the skin. It's often mistaken for psoriasis (a skin condition that causes red, flaky, crusty patches of skin covered with silvery scales).

Crusted scabies affects people with a weakened immune system (the body's natural defence against infection and illness). This includes:

• the very young
• people with brain disorders (neurological disorders, such as Parkinson's disease)
• people with Down's syndrome 
• pregnant women
• elderly people
• people with a condition that affects their immune system, such as HIV or AIDs
• people who are taking steroids to treat other medical conditions
• people who are having chemotherapy treatment

Research has found a healthy immune system appears to interrupt the reproductive cycle of the scabies mite. For example, most people with scabies will only have 5 to 15 mites on their body at any one time.

However, if you have a weakened immune system, the number of scabies mites can increase significantly. People with crusted scabies can have thousands or millions of scabies mites on their body at any one time.

Because of the high number of scabies mites, crusted scabies is highly contagious. Even minimal physical contact with a person with crusted scabies, or with their bed linen or clothes, can lead to infection.

However, contact with someone with crusted scabies will only lead to the normal type of scabies in people with a healthy immune system.

Crusted scabies can be treated using insecticide creams or a medicine called ivermectin, which is taken by swallowing a tablet. Ivermectin kills the mites by stopping their nervous system working.