Lymphatic systems help clear the body of extra fluids and infection, but when they don't work properly, deadly excess fluid is retained in the body.
That was the case with a young boy whose family sought help at Children's Hospital of Philadelphia.
By testing various medications using tiny zebrafish, doctors miraculously saved his life.
10-year-old Daniel was a healthy active boy when his body began to swell. A massive overgrowth of his lymphatic system was overwhelming his major organs.
"He had such an overgrowth of the lymphatic vessels that he was leaking fluid into the pericardium, which is the membrane around the heart," said Hakon Hakonarson, MD, Ph.D., Director of Center for Applied Genomics at Children's Hospital of Philadelphia.
His lungs suffered too, severely diminishing his oxygen.
Dr. Hakonarson said, "His lung capacity was 23%. You know, barely compatible with life."
Doctors discovered the genetic culprit with a blood test and a simple DNA cheek swab.
The gene was out of control, meaning Daniel's lymph system was always in the 'on' position, very similar to cancer cells growing out of control.
So, doctors began to test anti-cancer meds on tiny, translucent zebrafish.
Christoph Seiler, Ph.D., Research Core Director at Children's Hospital of Philadelphia said, "The fish develop from a cell to a normal animal in five days."
"We put this cancer med into the fish, you can't give them a pill or an injection, so you basically just put it into the water," stated Dr. Hakonarson.
One melanoma drug called trametinib stopped the mutation in its tracks. Doctors then gave the drug to Daniel.
"His lymphatic system essentially normalized. The child came off oxygen, started walking, started running, started biking and he is essentially with normal daily activities," Dr. Hakonarson told Ivanhoe.
A tiny fish and a very smart team of scientists making a big impact.
Scientists have long used lab rodents or other mammals to help understand human diseases, but 70% of human genes are also found in the zebrafish, making them a good animal for study.
Contributors: Cyndy McGrath, Supervising Producer; Donna Parker, Field Producer; Roque Correa, Videographer and Editor.
Health Watch: How zebrafish helped save the life of a 10-year-old boy