Fur Mites
Two different mites can occasionally be found on guinea pigs, one harmless and another that bites our guinea pigs and can even bite us:
- Chirodiscoides Caviae is a relatively harmless fur mite that only lives on guinea pigs.
- Cheyletiella Parasitivorax is an extremely irritating fur mite that can infest dogs, cats, and other animals and cause dermatitis in humans.
Chirodiscoides caviae --- also known as "Static Lice" Cheyletiella parasitivorax --- also known as "Walking Dandruff"
Chirodiscoides caviae may cause little discomfort and no signs even though occurring in great numbers. Harkness and Wagner describe it as a "small harmless fur mite" and claim adult mites, though very small (perhaps .5 mm) may be seen moving in the fur. Eva and Peter Gurney call these insects "Static Lice" and describe finding them firmly attached to the base of the hairs, usually located on the hind quarters (what they describe as the lice are actually the eggs). By stroking the fur, you may feel the eggs as a bumpiness of the coat and may resemble the eggs in the picture at the right. They generally do not cause irritation to the guinea pig's skin but in some cases may cause some itching. Adults can with difficulty be seen moving in the fur and juveniles may be seen paired with more mature females.
Cheyletiella parasitivorax, rabbit fur mites, live on the karatin layer of the skin. These large, non-burrowing mites can be found on cats, dogs, rabbits, guinea pigs, and other animals and can bite, but do not live on humans (cause small reddened, itchy spots). About .3 mm long, they are a pale yellow to white mite. Supposedly these mites are difficult to see and get their name from the fact that they look like tiny bits of dandruff crawling in the fur. Mette the Vet says "Cheyletiella live off the skin. To dissolve the skin they excrete digesting enzymes on the skin surface which digests a small part of it, hereafter the mite eats the dissolved skin." This is what causes the extreme discomfort and scratching, hair loss and scabs. According to her, pet owners can experience itching and red skin eruptions from these digestive enzymes after being in contact with their guinea pigs. 
Find images of ectoparasites at www.radil.missouri.edu.
In rabbits: "Cheyletiella parasitivorax might not cause signs in rabbits or it can result in loose hair that can be pulled out in clumps. Affected skin may be slightly reddened, oily, hairless, and scaly in patches over the back, shoulders, and head. Rabbits with Cheyletiella mite infestations appear to have increased "dandruff" on the skin and deep in the fur." (Harkness and Wagner, The Biology and Medicine of Rabbits and Rodents) In contrast, sarcoptid mites are supposedly rarely found on the face and limbs in rabbits and are microscopic. In most animals there will be itching with varying amounts of dandruff and may be scabs or crusts of dried exudate.
Treatment: Thoroughly clean the living quarters and treat your guinea pigs topically with ivermectin at recommended doses (0.5mg/kg). Place 1/3 of the dose behind each ear and 1/3 in the rump area. Cheyletiella parasitivorax may be particularly difficult to get rid of in the home.
Avoid flea and tick powders and shampoos -- many contain pesticides that are dangerous for guinea pigs.
More on Parasites: