Convicted Soviet spy Svetlana Ogorodnikova, serving an 18-year federal prison sentence in Pleasanton for espionage conspiracy, announced in an "open letter to President Reagan" that she has begun a hunger strike to protest alleged refusal to supply proper medical treatment by prison officials. "Since my imprisonment my health condition has deteriorated greatly," she wrote. "I have become sick with pneumonia, bronchitis, plurisy (sic), my kidney, breast, etc. I have received no medication. Currently I am coughing up blood regularly. The hospital here says I don't need medical treatment, and tells me . . . it's normal to cough up blood." A spokesman for the Northern California prison denied that Ogorodnikova has actually started a hunger strike and said she is receiving medical attention.