A parasite recently discovered in a Baltimore reservoir is dangerous to people with compromised immune systems, officials said.

Baltimore City Department of Public Works performed a routine test in Druid Lake Reservoir in late September and found evidence of a microorganism called Cryptosporidium.

According to a news release from the Department of Public Works, Cryptosporidium can cause sickness to people with weakened immune systems. Symptoms can include diarrhea, vomiting, fever and stomach pain.

Howard County officials warned about the finding on Facebook, noting that contracting the parasite is low risk for most residents, and only people with compromised immune systems should take preventative action.

“Those with immunocompromising conditions and other sensitive populations are advised to: drink bottled water, boil water for one minute before consuming or filter tap water,” Howard County officials said in a statement.

How was Cryptosporidium found?

DPW officials said they collected samples from Lake Druid on Sept. 19. A laboratory analysis reported the following week it had found small amounts of Cryptosporidium in the samples DPW noted that monthly testing showed no signs of Cryptosporidium from August samples.

DPW emphasized that Cryptosporidium did not contaminate the source water. DPW will treat the source water before it is sent to finished reservoirs.

What to expect next

DPW will continue to sample for Cryptosporidium until results show it is no longer detected Until then, DPW advises that children, the elderly and immunocompromised individuals take appropriate precautions