Costco Dog Food Lawsuit Puts Contested Science to the Test

The useful starting point is not whether grain-free dog food is dangerous. It is the gap between two public records. The FDA has said the dilated cardiomyopathy reports it received “do not supply sufficient data to establish a causal relationship” between certain diets and canine DCM; the new Costco complaint alleges Costco still should have warned consumers about a heart-disease risk tied to its Nature’s Domain grain-free products. That gap is where the litigation risk sits. This is a litigation-risk analysis, not legal advice and not a veterinary conclusion.

The timing can be confusing. The central Costco class action was filed in June 2026, but the contested science includes a 2025 prospective feeding trial and other recently published work that both sides are likely to characterize very differently.

The FDA record gives defendants a sentence plaintiffs must plead around

The FDA opened its investigation in July 2018 into a potential link between certain diets and canine dilated cardiomyopathy. By November 2022, the agency had received 1,382 DCM reports, and its earlier brand table identified 515 canine DCM reports through April 2019. In December 2022, the FDA said the adverse-event reports “do not supply sufficient data to establish a causal relationship” and that it did not intend to release further public updates “until there is meaningful new scientific information to share.” [1]

That language is not a defense verdict. It is also not nothing. In a false-advertising or warning case, a defendant can use it to argue that the federal regulator most directly engaged with the issue stopped short of causation. Plaintiffs can answer that “insufficient data to establish” causation is not the same as “no risk,” especially where the theory is notice, omission, or misleading health marketing rather than a completed agency finding.

Nature’s Domain also appears in the FDA’s reported-brand data. In the agency’s table of 515 canine DCM reports through April 2019, Nature’s Domain ranked seventh, with 29 reports. [1] That number is procedurally useful to plaintiffs because it ties the brand to the FDA record. It is also procedurally useful to defendants because the FDA repeatedly framed those materials as adverse-event reports, not proof that a listed product caused disease.

What the Costco complaint actually has to do

West v. Costco Wholesale Corp., No. 26-cv-2182, was filed on June 23, 2026, in the Western District of Washington. The complaint alleges that Costco knew since at least 2018 about an association between grain-free dog food and DCM, continued to market Nature’s Domain grain-free dog foods as healthy, failed to warn consumers, and did not conduct clinical feeding studies to substantiate safety representations. [2][3]

As of July 27, 2026, Costco has not yet responded. That matters. At this stage, the public record is a complaint and reporting about the complaint, not a judicial finding, discovery record, expert report, or admissibility ruling. The pleading can be important without being proven.

The complaint’s legal work is narrower than the consumer-health framing suggests. It does not need the court to decide, on day one, that Nature’s Domain causes canine DCM. It needs to survive the early argument that the challenged marketing and omissions were not misleading because the science was unsettled and the FDA had not found causation.

The evidentiary split is not just “studies versus no studies”

The plaintiff-side posture has improved since the earliest FDA updates, but not in a way that eliminates the defense problem. Tufts Petfoodology’s June 2026 update says there are now more than 30 peer-reviewed studies on diet-associated DCM and discusses, among other materials, a June 2025 biomarker paper identifying phospholipidosis in the American Journal of Veterinary Research. [4]

That is meaningful for notice and plausibility. A company facing a growing peer-reviewed literature base has a harder time treating the subject as mere internet noise. But the Tufts post is itself a research-summary blog, not a peer-reviewed study. It points to published work; it does not replace expert testimony applying that work to the legal elements in a particular case.

The defense side has more than the FDA’s December 2022 pause. A Hill’s-funded prospective feeding trial published in July 2025, after an 18-month trial completed in August 2021, found no link between grain-free diets and DCM. The delay between completion and publication is itself contested in KetoNatural’s litigation against Hill’s. [5]

Funding source is not an admissibility veto. Nor is a single prospective feeding trial the last word on a scientific dispute. Its litigation value is more precise: it gives defendants a published counterweight to the claim that the DCM-grain-free association has matured into a reliable causal conclusion. If plaintiffs use the literature to prove notice, the showing may be manageable. If they use it to prove general causation, the fight becomes much harder.

Why warning claims can survive longer than causation claims

A failure-to-warn theory can sometimes live in the space between suspicion and proof. Plaintiffs can argue that the FDA investigation, reported brand data, veterinary literature, and internal knowledge allegations made the alleged risk serious enough to disclose. They do not necessarily need an FDA causation finding to plead that a reasonable consumer would have wanted to know about the controversy before buying a product marketed around health.

But a warning theory still has to avoid a common slippage. Association, adverse-event reports, and mechanistic hypotheses are not interchangeable with admissible causation testimony. A court may allow plaintiffs to plead that Costco had notice of a risk signal, yet later exclude an expert who overstates what the same materials can prove.

That distinction is likely to matter more at class certification, summary judgment, and Daubert than at the first complaint-stage motion. A consumer-fraud claim may ask whether the omission was material to purchasers. A product-liability claim involving disease may ultimately require reliable proof that the challenged diet can cause DCM and, depending on the claim, that it did cause injury. Those are different burdens, even when the same studies appear in the briefs.

The companion cases show a litigation wave, not a settled answer

Costco is not the only target. An April 2026 Blue Buffalo class action alleged similar false-advertising claims involving grain-free products and DCM risk. [6] That filing helps show that plaintiffs’ lawyers are testing a category-wide theory. It does not show that courts have accepted the science or the advertising theory.

The more destabilizing counter-case is KetoNatural v. Hill’s Pet Nutrition, filed in February 2024 in the Northern District of Oklahoma. KetoNatural alleges that Hill’s conspired to fabricate the DCM-grain-free link. In July 2026, the Tenth Circuit partially revived the suit and concluded that Hill’s marketing statements about grain-free diets “may constitute false advertising.” [5]

That allegation should be kept in its procedural box. It is not an established finding that the DCM-grain-free link was fraudulent. Still, if KetoNatural’s theory were to gain traction on a developed record, it could attack the factual premise plaintiffs rely on in consumer DCM suits. The unusual posture is that one line of litigation treats the grain-free DCM concern as an omitted risk, while another attacks the same concern as commercially manufactured falsehood.

The open procedural test is Daubert

No federal court has yet supplied the missing organizing decision: a Daubert ruling in these DCM dog-food cases that decides how competing expert causation testimony should be handled against this mix of adverse-event reports, observational work, biomarker research, prospective feeding data, and FDA restraint.

That is why the Costco case may become more important than its current procedural posture suggests. If it remains a pleading dispute, the court may only decide whether plaintiffs alleged enough to proceed despite the FDA’s insufficient-causation language. If the case moves into expert discovery, the harder question becomes whether plaintiffs can convert emerging science into reliable courtroom causation while defendants use the FDA record and contrary published work to argue that the inferential leap is too large.

The case does not need to prove anything about dog food to become useful to product-liability litigators. It may instead force a court to decide how far warning and advertising claims can proceed when plaintiffs have a growing peer-reviewed literature base and defendants have an agency record saying causation remains unproven.

References

1. FDA Investigation into Potential Link between Certain Diets and Canine Dilated Cardiomyopathy — FDA.gov
2. Costco Lawsuit Says Nature's Domain Grain-Free Dog Food Not as Safe or Healthy as Advertised Due to Heart Disease Risk — ClassAction.org, June 26, 2026
3. Kirkland Signature Nature's Domain Grain-Free Dog Foods — TINA.org
4. Update on Diet-Associated Dilated Cardiomyopathy: June 2026 — Tufts Petfoodology, June 22, 2026
5. Appellate Court Rules Hill's Pet Food Lawsuit Will Continue — Truth About Pet Food, July 2026
6. Family sues Blue Buffalo over DCM, grain-free claims — PetfoodIndustry