The U.S. Supreme Court on Wednesday unanimously ruled that a proposed class action over alleged mislabeling of prescription dog food was appropriately returned to state court, deciding that federal courts lost jurisdiction once the plaintiff removed her federal claims.

The dispute centered on whether the case, filed by Missouri resident Anastasia Wullschleger against Royal Canin USA Inc. and Nestlé Purina PetCare Co., should remain in federal court or be sent back to state court. Wullschleger’s lawsuit alleges the companies misled consumers into believing their prescription dog foods were tested and approved by the U.S. Food and Drug Administration. Initially filed in state court, the case was moved to Missouri federal court by the defendants.

The Eighth Circuit had ruled that elements of Wullschleger’s antitrust and unjust enrichment claims implicated federal law, justifying federal court jurisdiction. However, after Wullschleger amended her complaint to remove federal law elements, the Eighth Circuit determined the revised claims were solely state law issues and sent the case back to state court.

The Supreme Court’s decision affirms that ruling. “Wullschleger had reconfigured her suit to make it only about state law,” Justice Elena Kagan wrote for the court. “And so the suit became one for a state court.”

During October oral arguments, Katherine Wellington, a partner at Hogan Lovells representing the pet food companies, argued that the Eighth Circuit’s decision conflicted with the text and structure of Section 1367, which governs supplemental jurisdiction, as well as long-standing precedents.

“It conflicts with the text and structure of Section 1367 and with more than a century of precedent,” Wellington told the court. “Chief Justice [John] Marshall held in Mollan v. Torrance in 1824 that in a diversity case, a federal court’s jurisdiction, once vested, cannot be divested by subsequent events.”

Justice Ketanji Brown Jackson questioned whether federal jurisdiction would remain if a plaintiff dropped federal claims intentionally, asking, “If a plaintiff said, ‘Oops, I didn’t mean to bring the federal claims; I’m dropping them,’ is there still a basis for federal jurisdiction?” Wellington responded that there was not.

Ashley Keller, a founding partner of Keller Postman LLC representing Wullschleger, argued that federal jurisdiction should align with the current claims in the complaint. “A suit arises under the law that creates the cause of action,” Keller told the court, adding that adhering to this principle avoids constitutional issues and unnecessary litigation over jurisdiction.

The ruling resolves the jurisdictional dispute but does not address the merits of Wullschleger’s allegations that Royal Canin and Nestlé Purina misrepresented their prescription dog foods. The case will now proceed in state court.

Royal Canin and Nestlé are represented by Katherine Wellington of Hogan Lovells.

The consumers are represented by Ashley C. Keller of Keller Postman LLC.

The case is Royal Canin USA Inc. et al. v. Wullschleger et al., case number 23-677, in the Supreme Court of the United States.