Health authorities in US have warned that the number of meningitis infections, which has already claimed 14 lives, could rise further.
The Centers for Disease Control and Prevention said it had contacted 12,000 of the nearly 14,000 patients who may have been exposed to the contaminated steroid injection believed to have caused the fungal meningitis outbreak.
"CDC and public health officials are referring any patients who have symptoms (of) possible meningitis or possible joint infection to their physician," the CDC's Dr. J. Todd Weber said in a conference call Thursday.
"However, we know we are not out of the woods yet."
Weber said the onset of symptoms typically comes one to four weeks after the injection, which is usually made in the spine or joints.
"However, we want to emphasize that we know fungal infections can be slow to develop, and that there are indeed reports of longer periods of time between the injection and the onset of symptoms," he said.
"Patients and their doctors will need to be vigilant for at least several months following the injection."
Health officials on Thursday said the total number of cases had risen to 172 in 11 states.
The rare fungal infection -- which inflames the protective membranes covering the brain and spinal cord -- often goes undetected until it is too late because its flu-like symptoms can be mild at first.
Treatment requires a hospital stay and intravenous anti-fungal medications, but meningitis is not contagious in this form.
US health officials launched an investigation after the first case was discovered in September and found fungal contaminants in steroids produced by the New England Compounding Center.
The Massachusetts-based company subsequently issued a voluntary recall of all of its products and shut down all operations.
The outbreak has led to calls for tighter regulation of the loosely controlled pharmaceutical compounding industry.
Critics said drug manufacturers have found a way to sidestep costly and strict oversight by classifying themselves as pharmacies, which are given freer rein to mix drug compounds for patients.