Natural occurrence of Cochliomyia hominivorax myiasis in surgically treated cattle under different macrocyclic lactone regimens: implications for animal welfare and surveillance

Introduction The re-emergence of the New World screwworm ( Cochliomyia hominivorax ) in the Americas represents a renewed challenge for livestock production, animal welfare, and regional surveillance systems. Myiasis caused by this obligate parasitic fly produces rapid tissue destruction, severe inflammation, and intense pain in affected animals, making its prevention a critical component of welfare-oriented livestock management. Despite the widespread use of macrocyclic lactones within livestock health programs, field-based epidemiological evidence describing the occurrence of screwworm infestation under natural exposure conditions remains limited. This study aimed to document the natural occurrence and characteristics of Cochliomyia hominivorax myiasis in surgically treated cattle exposed under endemic conditions while receiving different macrocyclic lactone regimens. Methods An observational field study was conducted in southern Mexico using Brahman × Simbrah crossbred cattle (12–16 months old). Animals were evaluated in two observation periods (November–December 2025 and January 2026) and allocated into three groups: untreated control, ivermectin (3.5%), and doramectin (1%). A standardized full-thickness surgical wound was created in each animal to allow natural oviposition by screwworm flies. Cattle were monitored for 21 days post-surgery for the occurrence of myiasis, larval burden severity, larval developmental stages, and wound progression. Recovered larvae were morphologically and morphometrically examined to confirm species identity and developmental stage. Results During the first observation period, myiasis occurred predominantly in ivermectin-treated cattle, with infestations detected as early as three days post-surgery and larval burdens frequently classified as severe. Two animals in the untreated control group developed myiasis on Day eight, whereas no infestations were observed in the doramectin-treated group. In the second observation period, infestation frequency decreased, with one case detected in both the control and ivermectin groups and none in the doramectin group. All recovered larvae corresponded to late second (L2) and third (L3) instars of Cochliomyia hominivorax and exhibited preserved morphology and high motility. Discussion These findings highlight the multifactorial nature of screwworm infestation under natural field conditions, including environmental variables and pharmacological context. The study provides epidemiologically relevant evidence that can contribute to risk assessment of myiasis occurrence, improvement of livestock surveillance systems, and evidence-based decision-making in integrated screwworm control policies aimed at protecting livestock health and welfare in regions facing the re-emergence of Cochliomyia hominivorax .