By FRANK DAVIES

  WASHINGTON - A top U.S. official and several public health experts Friday warned of the urgent need to plan for social chaos, severe shortages and a
  possible refugee crisis in a post-Castro Cuba.

  ''There's a real possibility of a complex emergency'' after Castro, including ''a high risk of chaotic migration,'' U.S. Agency for International Development
  administrator Andrew Natsios told a conference on the future of Cuba.

  Two health experts, Richard Garfield and Frederick Burkle, said that Cuba's health-care system, reputed to be one of the best in Latin America, is also
  fragile, short of essential medicines and very vulnerable to political instability.

  Natsios is participating in a commission, chaired by Secretary of State Colin Powell, to study ways to get humanitarian aid to Cuba. The commission's report
  to President Bush is due May 1.

  The conference Friday was sponsored by the Institute for Cuban and Cuban-American Studies at the University of Miami. The institute gets substantial
  funding from the U.S. AID.