Fire ants sting and kill invertebrates as their primary food source and have been known to kill farm animals if no other food is available, but they were only thought to attack humans who disturbed their nests.

However, a report in the Annals of Internal Medicine described how they attacked two elderly patients in their beds at two Mississippi nursing homes. One died within five days, the other after 13 months.

The patients were found covered with ants, with ant trails leading from the floor to their beds.

The researcher behind the report said the attacks were the latest in an increasing number of incidents involving humans who have not provoked the insects.

Varying symptoms

Dr Richard deShazo, an allergy and immunology specialist at the University of Mississippi Medical Centre, led the researchers.

He said the effects of fire ant stings range from mild irritation to death from a generalised allergic reaction, depending on the number of stings and the physical condition of the victim.

Fire ants arrived in the US about 60 years ago from Puerto Rico. They almost wiped out the native black ant population in the South-east, and have recently been spotted in some Western states, he said.

They have been responsible for at least 80 deaths following attacks at or near their mound-shaped nests since their first appearance in the US, but since 1989 there have been a number of attacks on people indoors.

The latest two bring the total to 10, including one on a five-day old baby who was attacked in a cot and went into a shock-induced coma but lived, and one on a two-year old with learning difficulties who suffered damage to his cornea after fire ants attacked his eyes.

Nursing home residents were also the target when two elderly patients died within six days of an attack.

Precautions and action

"Any sighting of a swarm of ants indoors is a warning," Dr deShazo said.

"Residents and caregivers of infants, children and bedridden people, such as patients in health care facilities, should be closely watched until the ants are eliminated."

He said ants observed in a house - like those that swarm around a mound outdoors - will be worker ants, making extermination difficult.

This is because the colony cannot be destroyed unless the queen ant is killed, he said.

He offered advice on how to set about destroying a colony:

• Assume that if one fire ant is seen, infestation is taking place
• Check inside and out for evidence of ants
• Exterminate all ants found indoors with liquid pesticides as soon as possible
• Leave bait-based insecticides in the cellar or in the garden or around the edges of the house, as this is gives most chance of the poison reaching the queen and killing her
• Fire ants may invade electrical equipment, including computers, air conditioners and circuit breakers, so inspect these carefully