A small outbreak of polio in the Amish population has promted a federal government warning to 21 states, including Maryland and Virginia, that the disease may spread among the Amish and others who come in contact with them.

The federal Center for Disease Control in Atlanta sent polio alerts that last week to states that have significant Amish populations. Most of the 21 states exempt the Amish on religious grounds from comPulsory vaccination requirements.

The center said the risk of polio is almost nonexistent for anyone who has had a complete series of three polio vaccine shots or oral doses. Anyone who has dealings with the Amish and has not had all three doses is advised to complete the immunization series.

The alert urged health officials in the 21 states to try to persuade the Amish, who frequently refuse immunization on grounds that it constitutes official intrusion into their lives.

Dr. Melinda Moore, of the center's viral disease office, said these state efforts have met with sucess. The Amish, she said, are "very reasonable people" and now are accepting vaccinations.

The presiding bishops of the Amish community in Lancaster County, Pa., one of the nation's largest Amish concentration, agreed Friday to urge members of the sect to take vaccinations to avoid a spread of polio.

The states receiving the health alert are Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Michigan, Minnessota, Missoiri, Montana, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Tennessee, Virginia and Wisconsin.

The outbreak that spurred the warning was minute by historical standards. Until 1954, when Dr. Jonas Salk perfected the first effective polio vaccine, there were about 20,000 cases annually in the United States.

In recent years there have been about a dozen confirmed cases annually, according to the center, and most of them are among the Amish.

The center has reports on eight cases this year, seven among the Amish. There have also been a small number in Canada. The non-Amish case, a 36-year-old Miflin County, Pa., woman whose husband has frequent contact with the Amish, was confirmed by the Pennsylvania Health Department May 14. That confirmation led to the general warning.

Larry W. Sparks, a center official in Washington, said he has been asked by several local schools whether they should call off the annual field trip to Pennsylvania Dutch country because of the polio cases. The answer, he says, is that "Therehs no danger as long as they're sure that kids have been vaccinated."

Health officials in two southern Maryland counties, St. Mary's and Charles, reported last winter that 36 members of an Amish community on the counties' borders were carrying polio virus, although none had shown symptoms of the disease.

Dr. William Marek, a St. Mary's County health officer, said yesterday that no cases had developed there. Marek said about 70 percent of the Amish had taken vaccinations, and that the protection would probably spread to the remaining community members through the "herd phenomenon," in which the benefits of vaccination are transferred to non-vaccinated people in close contact with those who have been immunized.