New Study Confirms Rapid Action of PROTAZIL® (1.56% diclazuril) Antiprotozoal Pellets for the Treatment of Equine Protozoal Myeloencephalitis (EPM)

RAHWAY, N.J., September 25, 2025 – Merck Animal Health, known as MSD Animal Health outside of the United States and Canada, a division of Merck & Co., Inc., Rahway, N.J., USA (NYSE:MRK), today announced the results of a new study on PROTAZIL, its pelleted treatment for equine protozoal myeloencephalitis (EPM), which is a neurological disease in horses. The study shows the active ingredient, diclazuril, reaches effective levels, known as the minimum inhibitory concentration (MIC), against Sarcocystis neurona, the primary protozoa to cause EPM, in the cerebrospinal fluid (CSF) within just 12 hours of one oral dose.

         “With a progressive neurological disease like EPM, time is of the essence,” says Phil van Harreveld, DVM, MS, DACVS-LA, senior equine professional services veterinarian for Merck Animal Health. “Prior to this study, we didn’t have clear data on how quickly diclazuril reaches a concentration level in the cerebrospinal fluid known to inhibit this parasite. We can now definitively state that PROTAZIL rapidly reaches therapeutic levels in the CSF after just one dose, offering veterinarians and horse owners reassurance in their treatment of this devastating disease.”

         EPM is a debilitating neurological disease caused by protozoal infection of the central nervous system, leading to symptoms such as incoordination, muscle weakness, and gait abnormalities in affected horses. Up to 90% of the U.S. horse population has been exposed to S. neurona;[1] fortunately, not all infected horses will develop clinical disease. For those that progress to disease, 60 to 70% of horses show clinical improvement with early treatment.¹

Key study findings

         The North Carolina State University study involved six healthy adult horses that were neurologically normal and negative for EPM. Diclazuril levels in both plasma (blood) and CSF were measured at several time points following a single oral dose administered at the labeled dose of 1 mg/kg top-dressed on sweet feed. Highlights include:

• Diclazuril concentrations in the CSF surpassed the MIC for S. neurona (1 ng/mL) in all horses by 12 hours post-administration.
• The CSF concentrations peaked at 24 hours, while plasma concentrations peaked later, at 48 hours. A horse with active EPM carries the parasite in its nervous system, which is why it’s helpful to know CSF concentrations and not just blood levels of the drug.
• Not only did diclazuril reach the MIC quickly, but it also remained well above the MIC for eight days, suggesting a sustained therapeutic effect.

---

¹Reed SM, et al. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment and Prevention. J Vet Intern Med 2016;30:491-502.

          “Our most significant finding from the research is the speed at which diclazuril reached the MIC for S. neurona in the horse’s CSF,” says Kate Hepworth-Warren, DVM, DACVIM-LAIM, lead researcher and assistant professor of equine internal medicine at the NC State College of Veterinary Medicine. “This is especially critical in acute cases where the clock is ticking. Initiating diclazuril therapy early can give the horse the best possible head start against aggressive disease progression.”

         This research was presented at the 2025 International Veterinary Emergency & Critical Care Society Meeting.

         Horse owners are encouraged to speak with their veterinarian to determine whether PROTAZIL is the right treatment for their horse. Learn more about EPM here.

Please see Prescribing Information for PROTAZIL HERE.

IMPORTANT SAFETY INFORMATION:

         PROTAZIL^® is contraindicated in horses with known hypersensitivity to diclazuril. The safety of PROTAZIL^® in horses used for breeding purposes, during pregnancy, or in lactating mares, and use with concomitant therapies in horses has not been evaluated. Do not use in horses intended for human consumption. Not for human use. For complete safety information, refer to the product label.

About Merck Animal Health

         At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than a century, we’ve been at the forefront of research, bringing forward medicines, vaccines and innovative health solutions for the world’s most challenging diseases. Merck Animal Health, a division of Merck & Co., Inc., Rahway, N.J., USA, is the global animal health business of Merck. Through its commitment to The Science of Healthier Animals^®, Merck Animal Health offers veterinarians, farmers, producers, pet owners and governments one of the widest ranges of veterinary pharmaceuticals, vaccines and health management solutions and services as well as an extensive suite of connected technology that includes identification, traceability and monitoring products. Merck Animal Health is dedicated to preserving and improving the health, well-being and performance of animals and the people who care for them. It invests extensively in dynamic and comprehensive R&D resources and a modern, global supply chain. Merck Animal Health is present in more than 50 countries, while its products are available in some 150 markets. For more information, visit www.merck-animal-health.com and connect with us on LinkedIn, Facebook, X (formerly Twitter) and Instagram.

Forward-Looking Statement of Merck & Co., Inc., Rahway, N.J., USA

         This news release of Merck & Co., Inc., Rahway, N.J., USA (the “company”) includes “forward-looking statements” within the meaning of the safe harbor provisions of the U.S. Private Securities Litigation Reform Act of 1995. These statements are based upon the current beliefs and expectations of the company’s management and are subject to significant risks and uncertainties. There can be no guarantees with respect to pipeline candidates that the candidates will receive the necessary regulatory approvals or that they will prove to be commercially successful. If underlying assumptions prove inaccurate or risks or uncertainties materialize, actual results may differ materially from those set forth in the forward-looking statements.

         Risks and uncertainties include but are not limited to, general industry conditions and competition; general economic factors, including interest rate and currency exchange rate fluctuations; the impact of pharmaceutical industry regulation and health care legislation in the United States and internationally; global trends toward health care cost containment; technological advances, new products and patents attained by competitors; challenges inherent in new product development, including obtaining regulatory approval; the company’s ability to accurately predict future market conditions; manufacturing difficulties or delays; financial instability of international economies and sovereign risk; dependence on the effectiveness of the company’s patents and other protections for innovative products; and the exposure to litigation, including patent litigation, and/or regulatory actions.

         The company undertakes no obligation to publicly update any forward-looking statement, whether as a result of new information, future events or otherwise. Additional factors that could cause results to differ materially from those described in the forward-looking statements can be found in the company’s Annual Report on Form 10-K for the year ended December 31, 2023 and the company’s other filings with the Securities and Exchange Commission (SEC) available at the SEC’s Internet site (www.sec.gov).