MARROW RECIPIENT, 3, BETTER; PARENTS HEARTENED BY SON'S RECOVERY.

Byline: Kevin F. Sherry Daily News Staff Writer

Steven Koss crawls over the furniture at his grandparents' house, a typical 3-year-old playing a gleeful game of tag with his baby sister.

After spending six cramped weeks in a small hospital room, the young bone-marrow transplant recipient is making good use of his hard-won freedom.

``She was chasing him up the couches,'' said Steven's mother, Shannon Smith, as she watched 8-month-old Serena crawl after her brother. ``He was just happy to see her.''

Steven was discharged late last month from UCLA Medical Center, where he underwent a bone marrow transplant Dec. 23. The transplant was an attempt to cure his aplastic anemia, which robbed his body of the ability to make oxygen-carrying red blood cells, infection-fighting white blood cells and blood-clotting platelets.

Steven spent 42 days in the hospital - seemingly a lifetime to a young boy, but actually far less time than his doctors anticipated.

``He's really doing great,'' said Margie Weiman, a clinical nurse specialist for bone-marrow transplants at UCLA. ``He's had no complications since he left the hospital.''

Although Steven must be monitored for at least a year, his family couldn't be happier with his progress.

Doctors ``were surprised how well he's done,'' said the boy's father, Scott Koss. ``They've never seen anyone recover this fast.''

Steven and his family are staying with Scott's parents in Newbury Park while their yard is landscaped so the youngster is not exposed to any dust or particles, Koss said.

Steven still cannot play with other children or go outside without a protective mask because his immune system is impaired. He is especially susceptible to viral and fungal infections and lung problems, and he takes up to eight doses of medication every day.

``He really can't do a lot,'' Scott Koss said. ``He has to stay basically isolated for six months.''

A catheter will remain in Steven's chest for up to a year to aid in the administration of drugs and the drawing of blood, his mother said.

He also must visit the hospital for a check-up once a week for the next six months. Doctors will check his weight - it's going up - and also watch for symptoms of graft-vs.-host disease, a dangerous condition caused when white blood cells produced by donor marrow attack the surroundings inside a body unfamiliar to them.

Steven has been recovering so well that doctors will gradually reduce the amount of steroids he has to take to suppress the graft-vs.-host disease, Smith said.

Steven's own bone marrow was destroyed to make way for the donor's, so his immune system is now similar to that of a newborn, Weiman said. After doctors are confident that Steven has stabilized, he will need to be immunized again against measles, mumps and other common childhood ailments.

``He'll be normal eventually,'' Weiman said.

People in Ventura County have rallied around Steven, Smith said. Countrywide Mortgage in Simi Valley sent a giant get-well sign to Steven's hospital room. Members of the family's church, St. Patrick's Episcopal, donated gift certificates to help the family maintain Steven's special low-bacteria diet.

The family is glad to be back in the Conejo Valley and looks forward to the next year with a combined sense of hope and hesitation.

``It's pretty overwhelming still,'' Smith said. ``We got over the big hurdle, I think.''

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3 Photos

PHOTO (1 -- Valley edition only) Steven Koss

`Doing great'

(2 -- 3 -- color -- Simi and Conejo edition only) Steven Koss, 3, gestures to his `get well' sign, above, at UCLA Medical Center, where he underwent a bone marrow transplant Dec. 23. At left, Steven gives a thumbs up.