Zoetis says Simparica Trio beat NexGard Plus on resistant heartworm, in groups of eight dogs

Zoetis said on October 6th, 2026 that Simparica Trio protected 3x more dogs than NexGard Plus from a resistant heartworm isolate in a lab study. The 3x comes from 2 of 8 Simparica Trio dogs with adult heartworms against 6 of 8 on NexGard Plus. That gap is also the sample size.

The isolate, ZoeLA, is resistant to macrocyclic lactones. That is the drug class that includes moxidectin, an ingredient in both products. NexGard Plus is made by Boehringer Ingelheim, and the release carries no response from the company.

The 3x headline rests on eight dogs per arm

The study, published in Parasites & Vectors, challenged 24 dogs with the isolate. Dogs received Simparica Trio, NexGard Plus or no treatment monthly for six months at the labeled dose, with necropsy on day 237. The paper is Zoetis-funded. The release quotes Zoetis staff and Dr. Lindsay Starkey of Oklahoma State University, who sits on the American Heartworm Society board.

Adult worms turned up in 2 of 8 Simparica Trio dogs (25%) and 6 of 8 NexGard Plus dogs (75%). At day 237, 1 of 8 Simparica Trio dogs and 4 of 8 NexGard Plus dogs tested antigen-positive. One NexGard Plus dog had microfilariae, the larval stage, and none on Simparica Trio did. All of it comes from the Zoetis-funded study.

By our math, a Fisher exact test on the 2-of-8 versus 6-of-8 split gives a two-sided p-value of about 0.13, above the usual 0.05 cutoff. The release reports a P of 0.0316 for lower average worm counts, a different measure that compares worms rather than dogs, and it does not say which test produced it. Both can be true, but only one of them is a claim about how many dogs got infected.

The efficacy gap is 1.4 points, not 3x

The release puts efficacy against untreated controls at 99.5% for Simparica Trio and 98.1% for NexGard Plus, which is 1.4 percentage points apart by our math. Both numbers measure worm burden. The "3x" counts dogs instead, 6 of 8 protected against 2 of 8, so the two figures describe different things.

Zoetis points to moxidectin dose, at a minimum of 24 µg/kg per dose for Simparica Trio against 12 µg/kg for NexGard Plus. Dr. Jessica Rodriguez, who leads companion animal parasiticides at Zoetis, said that "not all moxidectin dosages are equal." Zoetis calls this the second controlled head-to-head of the two products against a resistant isolate. The first, funded by Boehringer Ingelheim, used a different isolate, JYD-34, and found no significant difference, at 99.5% efficacy for NexGard Plus and 99.8% for Simparica Trio.

The study shows a lab difference, not field results

The ZoeLA isolate was collected from a naturally infected dog in Louisiana in 2013 and maintained in the lab. A separate, also Zoetis-funded earlier ZoeLA study used it to test Simparica Trio, Heartgard Plus and Interceptor Plus. The release gives no figure for how common resistant heartworm is in dogs outside the lab.

Heartworm is a live business issue. Heartworm diagnosis claims rose 201% between 2020 and 2025 in Embrace's insurance data, according to our coverage. Zoetis also holds an FDA emergency authorization for screwworm on the same product, granted August 13th, 2026.

If we had to guess, this release is aimed at veterinarians choosing a default prevention product.

A small study, and the dog-level gap is not statistically distinguishable at this size.

Source: Business Wire release from Zoetis