Four people who have worked at Greenmeadow Community Farm have tested positive for cryptosporidium, and a further 13 possible cases in staff and volunteers are under investigation.
The one adult and three teenagers who have tested positive had all bottle fed lambs and kid goats that had diarrhoea.
There have been no reported cases of illness among members of the general public who visited the farm.
Dr Lika Nehaul, Consultant in Communicable Disease Control for Public Health Wales, said: “Cryptosporidium is a disease that is very common in young farm animals and can easily be passed to people who come into contact with those animals.
“It causes watery diarrhoea, stomach pain, nausea and vomiting, and can last for up to a month. Healthy people will usually make a full recovery.
“Everyone who has been unwell had direct contact with the bottle fed lambs and kid goats at Greenmeadow Community Farm. As part of our investigations, we are checking on all those whom we believe had contact with these animals.
We are not aware of cases of illness in any visitors to the farm who did not feed these animals. However, as a precaution, the affected animals have been removed from the farm.
“The farm director instigated and has fully co-operated with our investigations.
“Handwashing after coming into contact with farm animals is of the utmost importance in preventing infection with cryptosporidium. There is no reason for anyone to avoid visiting petting farms as long as they ensure that anyone who has touched animals thoroughly washes their hands with hot water and soap immediately afterwards.”
GPs in the Gwent area have been advised of the outbreak and asked to be alert to possible cases of diarrhoea or vomiting in people who have visited the farm.
Anyone who is unwell with gastrointestinal symptoms is advised to contact their GP or NHS Direct on 0845 46 47.
People who are unwell with vomiting or diarrhoea should not return to work or school for 48 hours after their symptoms have stopped.
More information on cryptosporidium is available from the Public Health Wales website at: http://www.wales.nhs.uk/sitesplus/888/page/44044.