No matter how hard you try chances are your cavy will probably be exposed to one of these horrid parasites in the course of its lifetime. Mites and lice are far more prevalent than we'd like to believe.
Lice are large enough to be seen on the coat, usually at the base of the hairs on the skin. The running form (adults) are viewed as tiny little whitish worms, or the eggs (nits) can be viewed as tiny white, red or black specks usually sticking on the actual tips of the hairs. One of the ways I check for lice on any pigs is to lift up the ear-flap from the face and to push back the hairs directly in front of the ear opening. This is the spot that they seem to be most prevalent. You will often see a thinning of the coat, a dulled appearance to the overall coat, and possibly a very itchy guinea pig.
Microscopic skin scrapings are not necessary to diagnose lice. They are readily visible on the coat. The lice make a small incision in the skin of the animal, yet draw fluids from the skin, not from the blood. For this reason, you cannot use an ingested or injected agent to kill them off. They must be drowned or killed with pyrethrins, topically.
The old method of getting rid of lice involved a dousing massage of oil of sassafrass, which is not readily available these days. Instead, we use a topical application (either bathing or spray) of a pyrethrin product. I personally like Ectosoothe shampoo or Zodiak kitten flea and tick shampoo. Both are safe to use on kittens, which also have a very sensitive respiratory tract. I have used both shampoos effectively and safely. The trick is to leave the shampoo on for the full 10-minute contact time in order for the pyrethrins to work. You must rinse and dry the guinea pig fully, and be sure to repeat the treatment in a week to kill hatching nits.
According to Richardson (2000) you can also use a suspension called Seleen (1% selenium sulfide) as a shampoo. Contact time on the Seleen is for 5 minutes, and again, repeat weekly if necessary. This treatment is also good for seborrhea.
The latest treatment of lice for the larger scale hobbiest or breeder is to use Advantage, Revolution or Frontline Plus on the cavies. You would get the exact same treatment for your dog (for larger herds, buy the largest package you can find, for the most treatments) and place a single drop on an adult animal behind the ear flap. For a youngster, I simply wet a q-tip with the Advantage, Revolution or Frontline, and sparingly wipe down the area behind the ear. This will also need to be repeated in 7-10 days, and in the case of severe infections, again for a third treatment 10 days past the second treatment.
One thing to remember is that these products are licensed by the FDA to only treat sheep, cattle, dogs and horses. They have not been licensed to treat cavies. While many vets use them and have done so with great success, your vet legally cannot recommend them for cavies, as it would be considered 'off label' use. Legally your vet cannot resell these items for a purpose that it was not intended for, so don't be surprised if your vet refuses to sell it to you. If need be, find a friend who uses it on their dogs and ask to swipe a drop or two for your cavies. You may also order it online from many sources.