The Minneapolis Star-Tribune reports that a 5-year-old Wright County boy is fighting for his life after contracting a strain of E. coli that killed his younger sister earlier this week.

Kade and Kallan Maresh were sickened by a shiga toxin-producing bacteria on July 9, eventually sending them into acute kidney failure. State health officials are investigating the source for the E. coli that eventually led to hemolytic uremic syndrome (HUS), a complication of the bacterial infection.

Parents Joseph and Tyffani Maresh said the toxin from the bacteria attacked their 3-year-old daughter’s kidneys and her neurological system. “Her brain and heart were being damaged,” the family said. “Our sweet sweet little girl lost the battle. … Kade is still fighting.” Kallan would have turned 4 next month. “We were able to give Kallan a bath and put her favorite jammies on her,” her parents wrote in a journal entry on Sunday. “We got to hold her free of tubes and snuggle and kiss her. She is the most amazing little girl in the world. Our hearts are aching with the deepest sadness.”

Deaths resulting from HUS, which causes kidneys to stop working and red cells to be destroyed, are very rare, said Joni Scheftel, supervisor of the zoonotic diseases unit at the Minnesota Department of Health. Children and the elderly are most at risk, she said.

In an “abundance of precaution,” the animals at a petting zoo the children recently visited were taken off display, she said.

But the children could have been infected with E. coli from any number of other sources that health officials are investigating, she said.

Health officials may be able to zero in on the source by next week once lab results are in, Scheftel said. So far, no other similar cases have been reported, she said.