Investigation of an unusual morbidity event in lactating dairy cows in the Texas Panhandle and eastern New Mexico.

In March and April 2024, an unusual morbidity event (UME) in the Texas Panhandle and eastern New Mexico was reported to the Texas A&M Veterinary Medical Diagnostic Laboratory. In total, 73 of 158 (46.2%) grade A dairy farms reported decreased milk production, reduced feed intake, and/or abnormal milk consistency. To identify potential etiologies, 22 affected operations submitted samples from 250 clinically affected cows and 9 operations submitted samples from 69 unaffected cows. Initial diagnostic efforts found no consistent causative factors between operations. Nine peridomestic birds and 10 domestic cats from affected premises tested positive for the highly pathogenic avian influenza virus (HPAIV) H5N1 subtype. On March 25, 2024, the United States Department of Agriculture confirmed HPAIV in unpasteurized milk from dairy cows in the region. Feces, serum, and whole blood from affected cows tested negative for H5N1 HPAIV. Milk from affected cows had 6.0 (95% CI [3.4, 10.8]) odds of being positive compared with unaffected cows. Our initial investigation ruled out many common pathogens allowing for the exploration of novel etiologies and facilitated testing of samples from affected premises through the National Animal Health Laboratory Network. When investigating an UME, clinical signs and histories from affected individuals should dictate sample and test selection; however, it is imperative that samples from unaffected individuals be submitted for comparison when determining a potential etiology.