Three people have been hospitalised for paratyphoid over the past week and the cases are being investigated by the Hawke's Bay District Health Board.
Paratyphoid is a similar illness to typhoid fever and can cause serious illness.
Medical Officer of Health Oz Mansoor said one of the cases reported eating shellfish gathered at the Napier Marina.
There were signs warning against consuming shellfish gathered from this area because of risks that the water may be contaminated, he said.
"We are investigating this further as it is a known risk factor, and people should avoid gathering and consuming shellfish from this area," he said.
There is no sewerage discharge into the Napier Marina. Contamination is likely to be from boats or storm water runoff.
Dr Mansoor said anyone who had consumed shellfish from water that may be contaminated and was unwell should contact their family doctor.
Most cases of paratyphoid fever in New Zealand are associated with overseas travel. It is caused by a bacteria that usually spreads through food or water that has been contaminated with the faeces of an infected person.
In New Zealand, food vehicles have included shellfish taken from sewerage-contaminated beds.
In other countries, shellfish, raw fruits and vegetables, contaminated milk and milk products have been to spread the disease.
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Paratyphoid fever is a similar illness to typhoid fever but the clinical manifestations
tend to be milder, the duration is shorter and the fatality rate is much lower.
It often manifests as acute gastroenteritis.
Typhoid typically presents with a sudden fever, headache, malaise, dry cough, relative bradycardia and hepatosplenomegaly (when the liver and spleen swell).
Less commonly there can be rose spots on the torso, abdominal pain, constipation, diarrhoea, and cerebral dysfunction.
If untreated, the illness may last for three to four weeks and be complicated by intestinal perforation or haemorrhage, death or relapse.
More information on paratyphoid can be found at health.govt.nz/system/files/documents/publications/cd-manual-typhoid-may2012.pdf