Draft Case Definition for New World & Old World 

Screwworm Myiasis

New World & Old World 

Screwworm Myiasis

Case Definition

(Notifiable)

1. Clinical Signs:

1.1 Clinical Signs: Screwworm myiasis is caused by either of two species of fly larvae (both in the 

family Calliphoridae): Chrysomya bezziana (Old World screwworm) and Cochliomyia hominivorax 

(New World screwworm). Screwworm myiasis is often associated with pre-existing wounds, 

though infestation can also occur on mucous membranes, such as nostrils, eye orbits, ears, 

mouth, and genitalia. Nearly any wound is susceptible to screwworm infestation. This includes 

natural wounds received from arthropod bites, fighting, and disease, or wounds resulting from 

management procedures such as barbed wire fencing, dehorning, castration, and ear tagging. 

Navels of newborn mammals are also common sites for screwworm infestation. Characteristics 

of infested wounds include drainage, suppuration, discharge of blood and serum, and distinctive 

odor. Secondary bacterial infections are also likely to occur. Upon close examination, eggs are 

deposited “shingle-like” in masses. Larvae are visible by the third day of infestation and are 

positioned with their posterior ends at the surface of the wound. In cases where the wound is 

deep, pocket-like, and the opening small, minor movement within the wound may be the only 

indicator of infestation. Animals with screwworm infestations often display discomfort, 

lethargy, and depression, and they may separate from the herd. Anorexia and decreased milk 

production may also be observed. 

2. Laboratory criteria 

2.1 Agent identification: Larvae collected for diagnosis should be removed from the deepest part of 

the wound using forceps to reduce the possibility of collecting non-screwworm species. Living 

specimens should be examined for pigmentation of the posterodorsal tracheal trunks, then 

preserved in 70 percent alcohol (ethyl or isopropyl) for further examination. Young specimens may 

also be placed in 80-95% alcohol. Do not place larvae into other preservative solutions because the 

larvae may contract and darken. Larvae are best preserved when they are killed in boiling water 

prior to placing in alcohol or ethanol, though this preservation method is not necessary. Killing 

larvae by boiling in water has no negative effect on potential subsequent extraction of 

mitochondrial DNA, amplified by polymerase chain reaction (PCR), but it might affect other 

molecular techniques. 

2.2 Serology: At this time, there are no applicable serological tests, nor are they indicated in the 

identification of this disease. 

Animal and Plant 

Health Inspection 

Service

Veterinary 

Services

August 2018
Draft Case Definition for New World & Old World 

Screwworm Myiasis

3. Case Definition and Reporting Criteria

3.1 Suspect case: An animal with clinical signs consistent with blow fly or screwworm myiasis. 

3.2 Presumptive positive case: 

3.2.1 Presumptive positive imported case: A suspect case that has travel history outside the 

United States to any screwworm-infested country within the previous 10 days. 

3.2.2 Presumptive positive autochthonous case: A suspect case that has no travel history outside 

the United States within the previous 10 days, and was 

• Located near a previous confirmed positive, or 

• Identified as screwworm by a lab other than the NVSL, or 

• Identified as screwworm by a collector with screwworm experience. 

3.3 Confirmed positive case: 

3.3.1 Confirmed positive imported case: The National Veterinary Services Laboratories (NVSL) 

confirms presence of screwworm from a presumptive positive imported case by visual 

identification of the egg mass, larvae (first, second, or third instars), or adult fly. 

3.3.2 Confirmed positive autochthonous case: The NVSL confirms presence of screwworm from a 

presumptive positive autochthonous case by visual identification of the egg mass, larvae (first, 

second, or third instars), or adult fly.