Encephalitis

Encephalitis - inflammation of the brain - is most commonly caused by a viral infection, which can be contracted through insect bites, food, drink or skin contact. Once the virus is in the bloodstream it localises in the brain, which is swamped with white blood cells attempting to fight of the infection. This causes the swelling and can lead to nerve and brain damage, and bleeding within the brain.

St Louis Encephalitis

Named after the town where it was first classified in 1933, St Louis encephalitis (SLE) is the most common viral encephalitis in the US.

It is thought to occur naturally in birds and is spread by mosquitoes.

It produces flu-like symptoms such as fever, headaches and lethargy, and in severe cases high fever and convulsions.

The elderly are most at risk from the virus, with 30% of those infected dying as a result.

However, most people who pick it up show no symptoms.

West Nile Fever

West Nile Fever too is a viral disease of birds and is transmitted to humans by mosquito bites. It is most common in Africa, and Asia, although it has been seen in Europe around the northern Mediterranean area.

However, it has now been identified in New York and doctors at the US Centers for Disease Control fear it may spread southwards and gain a grip on the continent.

Symptoms include headache, weakness, fever and sometimes nausea and vomiting. In addition, skin rashes are common.

Australian Encephalitis

Symptoms of Australian Encephalitis - also known as Murray Valley Encephalitis - vary from mild to severe with permanent brain damage or sometimes death.

First reported in 1917, it has been found across the Australian mainland and is passed from water birds to humans via mosquitoes.

Only one person in 500 infected becomes noticeably ill, although if they do the common symptoms are fever, anorexia and headache sometimes along with vomiting, nausea, diarrhoea and dizziness.

In severe cases, brain function may be impaired after a few days and lethargy, irritability, drowsiness and confusion may set in. Convulsions and fits come next leading to the possibility of coma and death.

California Encephalitis

Covers a group of viruses carried by mosquitoes and some mammals, but primarily the La Crosse Encephalitis which accounts for 8% to 30% of all cases of encephalitis in the US.

It is most likely to affect children and many cases produce no symptoms. In those cases where they become apparent, symptoms are likely to be mild.

They start with a slight fever or headache but fast develop into severe headache, high fever, muscle aches, stiffness in the back of the neck and disorientation.

There is a risk of permanent brain damage in severe cases.

As with other insect-borne disease, outbreaks are most likely to occur during mosquito breeding season.

Colorado Tick Fever

The Colorado Tick Fever (CTF) virus is transmitted exclusively by female wood ticks and causes fever, chills, headaches along with nausea and vomiting.

A small minority of patients suffer a more severe form of the disease, including anorexia and fatigue which takes several weeks to overcome, while children sometimes suffer from a pronounced rash and internal bleeding.

The disease is limited to the western US and is most prevalent from March to August.

Eastern Equine Encephalitis

A rare but frequently deadly virus, Eastern Equine Encephalitis too is spread from birds to humans and horses via mosquitoes.

It is found mainly on the east coast of the US and the eastern Gulf coast, and while most people show no symptoms, those who do will suffer one of two fates.

Some get symptoms similar to a mild case of flu - fever, headache, and sore throat - while others will suffer infection of the central nervous system, which leads to severe headache, seizures and coma.

About half of those in the second category will die, while those who survive may have permanent brain damage.

Symptoms usually appear within four to 10 days of infection, and there is no cure.

Young children and people over 55 are most at risk.

Japanese Encephalitis

Mosquitoes transfer the virus that causes the disease from infected animals - usually pigs and wading birds - to humans.

It occurs mainly in the rural areas of China and Korea, but is also found in most countries of South and East Asia.

Most people who are infected show only mild symptoms or no symptoms at all, although, at advanced stages, the disease may be fatal.

It begins like flu with headache, fever, and weakness, and vomiting, confusion and delirium may also be present.

In about one of every 200 cases, the illness progresses to inflammation of the brain, with more than half of those cases ending in permanent disability or death.

Western Equine Encephalitis

Western Equine Encephalitis is fatal less often than its eastern namesake and is found in the western and central US.

The effects of infection range from no symptoms to fatal disease. Milder effects consist of a headache with, perhaps, a fever while the severe form causes sudden high fever, headache, drowsiness, irritability, nausea, and vomiting.

This is followed by confusion, weakness, and coma or, in young infants, seizures.

Symptoms usually appear within five to 10 days of infection, there is no cure and people of all ages are at equal risk.