The potentially fatal but rare infection which causes brain swelling

This is different from meningitis, where the delicate tissues that cover the brain and spinal cord are affected.

A relatively rare condition, it affects between four to six thousand individuals every year in the UK, though the estimated worldwide incidence is potentially hundreds of thousands.

The most common cause of encephalitis is a viral infection, such as herpes simplex (that which causes cold sores), measles, varicella (colloquially known as chicken pox) and rubella (German measles).

Viruses not common to the UK include rabies (found in infected animals), Japanese and West Nile as well as tick and mosquito borne viruses.

Although many will acquire viruses such as chicken pox and herpes simplex, thankfully only a tiny proportion will suffer encephalitis.

A delayed immune response to a viral infection, sometimes days to weeks after exposure, may result in encephalitis, due to a fault in the immune system.

Sometimes autoimmune encephalitis is triggered without prior exposure to a virus.

Bacteria and fungi are rarer causes of encephalitis, however in half of cases no definite culprit is identified.

Inflammation causes the brain to swell. The brain is a soft structure within a fixed container, the skull, which cannot expand to cope with the swelling.

This results in a range of symptoms. In the early stages fever, irritability and generalised aches and pains may be reported, especially if the cause is viral.

Later, it may progress to significant behavioural changes including agitation, impaired cognition, seizures and indeed coma.

Although autoimmune encephalitis may have a more protracted and perhaps less obvious presentation, it is vital that anyone with any worrying neurological symptom is promptly brought for urgent medical attention, as timely intervention has a significant impact on outcome including survival.

While history and examination are vital in assessing any patient, they cannot alone diagnose encephalitis.

Imaging, either computed tomography (CT) or magnetic resonance imaging (MRI) will look for inflammation or other reasons for the person’s unwell, namely a cancer (also known as a Space Occupying Lesion or SOL for short), or a bleed on the brain.

A lumbar puncture will be performed to look for any organisms in the cerebrospinal fluid (CSF).

While for some encephalitis may be managed in a ward setting with reduced stimulation to promote recovery, for others it may be severe enough that admission to an intensive care unit (ICU) is warranted.

Those looking after a patient with encephalitis can include anaesthetists, microbiologists, physicians, and even neurosurgeons, if a decompressive craniotomy (removal of part of the skull) is needed to relieve the pressure inside the skull as a result of the increased pressure.

Antivirals will be used if the cause is viral, antibiotics if it is due to a bacterium, and antifungals if this is the reason.

Autoimmune encephalitis may require medications to dampen down the immune response including steroids, intravenous immunoglobulins and sometimes plasma exchange.

Antiepileptics are needed if seizures are an issue.

The outcome for encephalitis is very varied. A shocking fact is that mortality can be between 10-30 per cent.

For those who survive, they and their family will have to live with an acquired brain injury (ABI). For some, disability will be mild, with full or almost near complete recovery. Unfortunately for others there may be significant issues that affect both physical and psychological wellbeing.

The brain recovers at a much slower rate than other body tissues and part of rehabilitation focusses on working within these new limitations.

Like any significant illness, it has the potential to affect not just the individual but also their loved ones.

Prevention or reducing your chances of suffering encephalitis starts with something as easy as robust hand hygiene.

It is recommended that you have the Measles Mumps and Rubella (MMR) vaccination.

If you are travelling abroad, please make sure you are appropriately immunised. There are jabs available for rabies, Japanese and tick-borne encephalitis.

To reduce your chances of being bitten by a tick, use an insect repellent containing N, N- Diethyl-meta-toluamide (DEET), wear clothing that reduces the amount of flesh exposed and sleep under insect nets.

If you are bitten by a tick, remove it immediately.

Tweezers designed specifically for this are readily available relatively cheaply.

In the unfortunate event of being bitten by an animal which potentially has rabies, seek immediate medical help.

The same goes for if you or anyone you know displays any symptoms that might suggest encephalitis.

World Encephalitis Day is on Tuesday, February 22.

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