Parasite [1982 - USA] sci fi horror

Set in the year 1992, ten years after a limited nuclear war as destroyed most of the USA, scientist Paul Dean has developed a new form of leach-like parasite which can grow to large portions every time it eats. Then, evil government agents storm his lab and he's forced to flee deep in the nearby desert and pursued by a "merchant" (government agent) intent on reclaiming the last two parasites Paul has. The first is in a silver container, the other is in Paul's abdomen. Paul seeks shelter in a small desert town populated by scavengers which threaten his work, while one local girl, Patrica Wells, forms an alliance with Paul to help him find a way to kill the parasite within him without killing himself, as well as find a way to kill the other one which gets loose and will soon multiply into many thousands unless killed soon.