Fly larvae appear quite fast (8-12 hours), and will eat away tissue on your pet if the conditions are present - vomit, diarrhea, skin infection (blood, pus), poor skin/matted hair attract these pests.
This condition can occur in all animals, including humans. Maggots are also used medicinally to clean up (debride) wounds, but only under very controlled conditions.
The larvae only eat dead tissue, but they can be extremely painful and irritating to the pet. These conditions can appear and get worse within hours. Additionally, this condition can be fatal if left untreated. Please contact your veterinarian immediately, rather than wait.
It is important to find the cause of the initial maggot attraction (is your pet vomiting? have diarrhea? a wound?) and to prevent further maggot infestation. Very old, young, or compromised animals will be more at risk.
Treatment involves physical removal of the maggots, wound flushing and cleansing, removal of remaining dead tissue or fecal matter, clipping hair from the area to aid in drying of the affected areas, and often an antibiotic to speed healing. Also, the inciting cause of the myiasis (i.e. diarrhea) must be addressed and treated to prevent reinfection of the maggots.