Fly larvae as an alternative sample for the detection of highly pathogenic avian influenza A(H5N1) virus

Altered carcasses present a diagnostic challenge to veterinary pathologists. Autolysis and putrefaction may render organs unrecognizable, and scavenging by vertebrates and insects may remove organs entirely. Sampling for viral or bacterial testing is greatly hindered and determination of an infectious cause of death is nearly impossible. An outbreak of highly pathogenic avian influenza (HPAI) in California condors in the spring of 2023 offered an opportunity to explore the utility of blow fly larvae (maggots) as an alternative test sample. An 18-y-old male condor that died during the outbreak could not be accessed and sent for necropsy for ~16 d. At the time of retrieval, numerous third-instar fly larvae were associated with the body. HPAI A(H5N1) virus was detected in the spleen of the condor, with a Ct value of 15.7. A group of fly larvae collected from the bird at postmortem examination and stored frozen was also HPAI A(H5N1) virus-positive, with a Ct of 32. Blow fly larvae may be an acceptable alternative sample if ideal tissues are not available. Caution should be exercised in interpreting negative values, however, because the virus may be transient in larvae and other fly life stages.