Despite recent reports, US companies will not be rushing to change their shareholder rights plans in the wake of the CSX case. The poison pills would punish the wrong parties and could be invalid.

In CSX v TCI Management, a district court found that investors building stakes in companies held beneficial ownership of the underlying shares in contracts for difference (CFD), or total return swap, deals.

If this decision stands, then holders will have to disclose their position if, combined with any equity stake they may hold, the CFDs reach defined levels.