I am being deliberately dramatic about this for there are precautions that can be taken to avoid this terrible death. The eggs of the blowfly can hatch within twelve hours of being laid in the animal and they immediately start to burrow deeper under the skin or anus, nostrils, mouth and ears. If they are not immediately eliminated using anti parasitic drugs or dips, the animal will die within twenty four to forty eight hours. Elderly guinea pigs are more prone to this problem than the younger ones for they are more likely to be slightly incontinent and thus attract the fly. Long haired guinea pigs who's rearend hair has not been trimmed back, and consequently can become soiled are also more vulnerable.
Blowfly strike is relatively rare with guinea pigs that are housed indoors.
In hot summer weather the following precautions should always be taken. Never use conventional fly killers or repellents in spray form for they can be very hazardous to guinea pigs. The best answer is to D.I.Y. using essential oils. A mixture of citronella, lemon balm, lavender and geranium is the one I use but there are several other essential oils that repel or kill insects. However, they get used to it and quickly settle down.