Human New World screwworm myiasis
2026 Case Definition

Background

New World screwworm (NWS) myiasis is caused by infestation with larvae of Cochliomyia hominivorax. Unlike other fly species, C. hominivorax larvae feed on living tissue before pupating and can infest all mammals and, rarely, birds. Flies lay hundreds of eggs on preexisting wounds, mucous membranes, or body orifices such as nostrils, eye orbits, ears, mouth, umbilicus, and genitalia. This form of infestation constitutes traumatic myiasis (fly larvae/maggot infestation characterized by tissue destruction and expanding lesions in living tissue). Human cases can occur in regions with active transmission, though far less frequently than animal cases. Risk factors include open wounds or sores, medical conditions that slow healing or cause bleeding, a reduced ability to protect oneself from flies, young or advanced age, sleeping outdoors or in unscreened locations (especially during daytime when flies are active), and extended exposure to livestock in affected regions.

NWS myiasis has re-emerged as a health issue in Central and North America. NWS cases in animals and humans have increased in number and geographic range since 2022. In June 2026, United States Department of Agriculture-Animal and Plant Health Inspection Service (USDA-APHIS) reported the first animal case of NWS in the United States. An introduction of NWS to the U.S. and establishment of the fly could devastate agricultural operations and wildlife populations, and lead to morbidity in infested humans if left untreated. Ensuring cases are promptly identified, investigated, and used to assess the risk of NWS introduction or spread is critical to the U.S. One Health response to NWS.

Clinical Criteria

In the absence of more likely etiology, healthcare provider-observed visible maggots or egg masses in or around an open wound (including the unhealed umbilicus of a neonate), surgical site, mucous membrane, or body orifice.

Laboratory Criteria

Confirmatory Laboratory Evidence:

• Morphologic identification of first, second, or third instar NWS larvae by a qualified laboratory, where qualified laboratories include the CDC, United States Department of Agriculture (USDA), or any laboratory that is designated by CDC or USDA as competent to identify NWS larvae.

Presumptive Laboratory Evidence:

• Morphologic identification of first, second, or third instar NWS larvae by a non-designated laboratory that is certified by Clinical Laboratory Improvement Amendments (CLIA) or another accreditation body as proficient to perform an Ova and Parasite Examination assay, OR
• If a specimen (including eggs) is damaged or otherwise unidentifiable by morphologic examination, detection of C. hominivorax deoxyribonucleic acid (DNA) by molecular testing in a specimen (including eggs).

Note: The categorical labels used here to stratify laboratory evidence are intended to support the standardization of case classifications for public health surveillance. The categorical labels should not be used to interpret the utility or validity of any laboratory test methodology. 

Epidemiologic Linkage

In the two weeks prior to symptom onset, travel to or residence in a geographic area designated by an authoritative source (e.g., USDA, World Organisation for Animal Health (WOAH), CDC, World Health Organization (WHO)) as endemic for NWS or experiencing an active NWS outbreak.

Criteria to Distinguish a New Case from an Existing Case

The following should be enumerated as a new case:

• A person not previously enumerated as a case who meets the confirmed, probable, or suspect case classification criteria, OR
• A person was previously enumerated as a case and infestation resolved or completed successful treatment (defined as removal of all larvae and resolution of the prior infestation) AND newly meets case classification criteria at least 7 days after the resolution of the previous infestation.

Reports should not be enumerated as new cases when they represent ongoing management of the same untreated or incompletely treated infestation, follow‑up specimens from the same episode, or duplicate notifications referring to the same person and the same infestation event.

Suspect

• Meets clinical criteria AND epidemiological linkage criteria.

Probable

• Meets clinical criteria AND presumptive laboratory evidence.

Confirmed

• Meets clinical criteria AND confirmatory laboratory evidence.