The affix tasi comes optionally after a word denoting a disease ending in the Genitive 6th-Case when the healing of the same is denoted.,
The word रोग means 'disease', and अपनयन means 'healing', 'curing'. Thus प्रवाहिकायाः प्रतीकारं कुरु = प्रवाहिकातः कुरु; so also छर्दिकातः कुरु ॥
Why do we say 'when healing is meant'? Observe प्रवाहिकायाः प्रकोपनं कुरु ॥,