Wastewater surveillance may complement other existing human surveillance systems to monitor influenza. Wastewater data cannot determine the source of influenza A viruses. Detections could come from a human or from an animal (like a bird) or an animal product (like milk from an infected cow).
No API but the data can be downloaded as a CSV file: https://www.cdc.gov/e0e53cec-a7e1-4357-a582-dd64f0cc3b1f
There is some data here that can be analyzed: https://www.cdc.gov/wcms/vizdata/NCEZID_DIDRI/FluA/H5N1Map.json
This has a tl;dr: https://www.cdc.gov/wcms/vizdata/NCEZID_DIDRI/FluA/H5N1Databites.json