Public Health England investigated a cluster of the rare E. coli 055 in Dorset last year
Two children from Dorset are being treated in hospital for serious kidney problems following an E. coli infection as investigations reopen into a previous cluster.
Three other members of the same family are being tested for E. coli 055.
A cluster of cases of the bacterial illness in Dorset was investigated last year but no common source was found.
Public Health England (PHE) said it was reviewing data but admitted it was unlikely to find a single source.
A multi-agency team led by PHE and local authorities has been reconvened to investigate the outbreak.
Noeleen McFarland, from PHE, said the strain seemed to be "particular to Dorset".
"We will continue to investigate thoroughly if there is a potential point source."
One of the two children is being treated for haemolytic uraemic syndrome (HUS) - a complication of an E. coli infection.
Results of tests for the strain of Verocytotoxin-producing Escherichia coli (VTEC) infection are awaited on the second child.
The children were said to be "responding well" to treatment.
Wash hands
It said the bacteria would most likely remain in the general environment.
Parents in the county are being warned to make sure their children wash their hands thoroughly, especially after playing outside or with pets, and to cook meat well.
Ten people in the Blandford area of Dorset were diagnosed with E. coli 055 between July and November 2014.
A further two cases were identified in Portland, four in Bournemouth and Poole as well as three cases outside the county which had links to people from Dorset.
Nursery children were among those infected with the bacterial illness, which can lead to kidney problems in some cases.
More than 500 people were swabbed in an attempt to find a common source.
Clusters of the strain had not been identified in England since records began in 1994.
PHE said any previous cases of the 055 strain had been associated with travel.
Avoiding E. coli infection
Wash hands thoroughly after using the toilet, before and after handling food, and after handling animals
Remove any loose soil before storing vegetables and salads
Wash all vegetables and fruits that will be eaten raw
Store and prepare raw meat and unwashed vegetables away from ready-to-eat foods
Do not prepare raw vegetables with utensils that have also been used for raw meat
Cook all minced meat products, such as burgers and meat balls, thoroughly
People who have been ill should not prepare food for others for at least 48 hours after they have recovered
Source: Public Health England, external
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