Autochthonous infestations by dipteran larvae or myiasis occur both in the USA and Canada. An aetiological classification of myiasis-causing flies has al– ready been given in the section discussing myiasis in Europe. As has already been stated, most myiasis-causing flies belong to one of three major families: Oestridae, Sarcophagidae or Calliphoridae, although representatives of other families, such as Muscidae and Phoridae, also are known to cause human myiasis.

In the USA cases of human myiasis due to the following species have been reported: the secondary screwworm fly (Cochliomyia macellaria), the common cattle grub (Hypoderma lineatum), the green bottle (Lucilia sericata), the sheep botfly (Oestrus ovis), the black blow fly (Phormia regina), the stable fly (Stomoxys calcitrans) and the grey flesh fly (Wohlfahrtia vigil (= W. opaca)). Two cases of myiasis caused by C. macellaria and the little house fly (Fannia canicularis) have been reported from Canada.

The New World screwworm (Cochliomyia hominivorax) was at one time found throughout the southern USA, Central America and tropical regions of South America. As a result of massive state, federal and international eradication programmes, extant populations of C. hominivorax are no longer found in the USA or Mexico; isolated reports are often traced to the importation of infested animals from locations where the screwworm is still prevalent. Screwworm populations are found in Central and South America and in certain Caribbean Islands. Adult females deposit eggs in open wounds or discharging orifices, such as the nose. Larvae invade adjacent living tissue, including cartilage and bone.