One of the most commonly seen problems in guinea pigs is mites. According to Guinea Lynx, pigs can have mite infestations (Selnic mites, a type of mange mite specific to cavies) and not show any symptoms, because in healthy pigs the immune system keeps the infestation in check. But any stressor or illness can over-burden this symptom, leading to a sudden outbreak with no apparent cause. Rabbits don’t have the same predisposition towards endemic infestations, but since they are more often taken outside or allowed to come into contact with dogs and cats, they are more likely than pigs to have fleas.
From personal experience, I would say that about 100% of pet store pigs have mites. EVERY pet store pigs I have ever seen, which has not yet seen a vet, has had mites. This is without exception in nearly eight years of doing pig rescue. I had one poor pig surrendered because he “had a neurological problem”. Ten minutes with the pig, and I could see that he was merely scratching himself into seizures. I advised the owner that she could simply treat for mites and the problem would go away, but she declined because “the vet said pigs don’t get mites.” (This was a vet at the same PetSmart that sold her the pig. We took him in, treated him, and he recovered just fine.)
Due to both the available information and personal experience, I feel that both pigs and rabbits should be on regular preventative parasite treatment regimens, just like we do for dogs and cats. At ACR&S, every new animal is treated on intake. Resident animals are treated monthly (if there are frequent new animals coming in) or twice a year if there are not new animals coming in (like in the Sanctuary).
The standard parasite treatment for pigs and rabbits is Ivermectin. However, the drug acts on both parasites and the mammalian host in the same way: it blocks the GABA receptors. Because it has similar effects on host and parasite, there are numerous reports of toxicity due to overdose . Despite these concerns, owners continue to be partial to Ivermectin because a) it’s available without a vet prescription, from locations such as farm stores, b) it’s very cheap, and c) it can be given orally, topically, or by injection.
At ACR&S, we have had success with using Revolution (generic name selamectin). It’s applied topically (to the back of the head or neck), only needs to be applied once per month, yet can be applied as often as every 2 weeks. It’s not commonly used by the GP and rabbit communities because a) it requires a vet prescription, and b) it’s expensive as heck (like $6-$12 per dose).
As evidenced by the similarity in names, selamectin is related to ivermectin, but its mechanism of action (binding the glutimate gated chloride channels) ONLY affects the parasite, and not the host . Seemingly, this would make Revolution much safer than Ivermectin. Yet Guinea Lynx continues to maintain that Revolution does not have the proven safety record of Ivermectin. 
Selamectin Safety and Efficacy Research
I actually found that selamectin has a very good record of safety, even using much higher doses (18 mg/kg and up) than are recommended by Pfizer and by vets:
1. Veterinary Dermatology 18(1) p.18-22, February 2007. 18 mg/kg used on 42 New Zealand rabbits with psoroptic mange and 37 Angora rabbits with sarcoptic mange. No adverse events and no mortality in either group. PDF
2. Journal of the American Veterinary Medical Association 223(3) p.322-324, August 2003. 48 mixed-breed domestic rabbits with active P cuniculi mite populations and clinical ear lesions. Both 6 mg/kg and 18 mg/kg were tested and deemed effective. No adverse reactions associated with selamectin treatment were observed in any group. PDF (not free).
3. Intern J Appl Res Vet Med Vol(3), p.87-96, 2007. Meta-analysis of other published studies. For Guinea pigs with any of 4 species of chewing lice or with the mange miter Trixacarus caviae: “Beck recommends 15 mg selamectin for animals weighing less than 800 g body weight and 30 mg for animals heavier than 800 g body weight.” For the fur mite Chirodiscoides caviae: “Good results in the treatment of this parasite have been achieved with 2 applications
of 12 mg/kg body weight selamectin with a 2-week interval. In a clinical case, a 3-year-old guinea pig with a body weight of 600 g was treated with a single topical application of 30 mg selamectin. Clinical signs improved within 1 week of treatment, and pruritus had ceased within a few days. There were no adverse effects following treatment.” PDF; I was not able to find the Beck article itself on Pubmed.
4. Material Data Safety Sheet from Pfizer. Tests in rats showed toxic effects on pregnant females and pups at 60 mg/kg/day. PDF
5. Product Profile for the European formulation, sold as Stronghold. “Selamectin has been classified as very slightly irritant to the rabbit skin and slightly irritant to the rabbit eye.” PDF.
Oddly I was not able to find many S&E studies on smaller doses, although that could be due to my search limitations (I don’t have access to vet-specific article databases). The MDSSs indicate that doses about double what these studies are looking at are problematic, at least in rats.
Since efficacy is clearly demonstrated at the lower dose of 6 mg/kg, and safety has been demonstrated for the higher dose of 18 mg/kg, I think it’s sensible to continue to use and recommend the lower dose of 6 mg/kg.
First the disclaimer: I am not a vet. This information is not an acceptable replacement for a veterinary consultation, and you should never administer ANY medication without the supervision of your vet.
If you have spoken to your vet and you are both willing to use Revolution for parasite treatment in your guinea pig or rabbit, dosing takes two steps.
First, you need to weigh your animal on a very precise scale and convert the weight into metric (grams or kilograms). This is where the vet comes in: most vets have a gram scale, or at least have taken enough math that they can do the conversion.
Second, you need to figure out how much of the tube to apply, because the commercially available doses are too large for most guinea pigs or rabbits.
The recommended dose (from Pfizer and as from my research above) is 6 mg of selamectin per kg of animal body weight. The smallest size tube is the Kitten strength (Mauve package) which contains .25 mL of liquid containing 60 mg/mL of selamectin.
60 mg per 1 mL = 6mg per.1 mL. So, an animal which weighs 1000 grams (1 kg) needs to have a dose of .1 mL. Since the tube contains .25 mL, it contains 2 and a half doses for this animal. To subdivide further, for every 100g of weight, add .01 mL (which is the tiniest marking interval on this syringe). So a 600g animal gets .06 mL, a 2300g animal gets .23 mL, a 985g animal gets .0985 mL (which you’ll probably measure out as either .09 or .1 mL, because you can’t actually get that fine a measurement with a syringe like this).
If you use any other package of Revolution, the strength in the tube might be different, and you will have to recalculate the dosage accordingly.
The most appropriate way to dispense Revolution is to squeeze the whole tube out into a sterile container, then use a 1 mL syringe (above) to suck up the correct dose. If you do this, the container needs to have an air-tight cap, otherwise the unused portion will evaporate.
If you want to be less precise, one drop from the tube is approximately .1 mL. So, you can just squeeze out 1 drop per 100 grams. Use this method at your own risk.
If you’ve used a needle to pick up the Revolution, don’t inject it – just spread the fur on the back of the neck and drip it onto the skin. Taking the needle off to remove the chance of a stabbin’ is a good idea. Choose a spot high enough up the neck that the animal cannot reach it by turning her head around, but low enough that she can’t rub it with her front paws. If your animal lives with a friend, keep them separate for about 10-15 minutes, just long enough for it to dry. After that there’s no risk of them licking it off one another (although I’ve never seen that happen).
The liquid in the tube also contains isopropyl alcohol (rubbing alcohol). This is a known irritant to rabbits skin. In well over 300 uses of Revolution, the only adverse effects I’ve ever seen are that most rabbits will shake their heads and attempt to groom off the Revolution for several minutes after the alcohol has evaporated. I’ve never seen any associated redness, swelling, or other dermatitis. Irritable head-shaking and grooming in rabbits can probably be considered a normal reaction to administration, but if it persists more than an hour or is accompanied by dermatitis at the site, you need to see your vet ASAP.
If you are treating an active infestation, you can apply two doses 14 days apart. If you are treating preventively, one treatment a month is sufficient if you or your animal may come into contact with other animals or anything else that could reinfect your pet; once every six months otherwise.
Anecdotal Efficacy Reports
Although selamectin consistently performs well in clinical trials, there are reports from owners where mite infestations were not totally eradicated by selamectin. In fact – remember Pudge? We took him in for another vet check last week. The vet did another skin scraping, and this time, found a single fur mite. So now he feels that Pudge’s hair loss is due to a mite reaction, despite two recent applications of selamectin, and has prescribed oral ivermectin. This will be the first case in my personal experience where selamectin has not been effective, but it’s not an isolated case. I would still recommend selamectin as a first line attack, but where an symptoms seem to indicate a persistent infestation, a change in pharmacotherapy is certainly indicated.
 Guinea Lynx. Accessed March 15, 2008.
 Ramesh Chandra Gupta. (2007) Veterinary Toxicology: Basic and Clinical Principles. Academic Press: Burlington, MA, p 511. Accessed via Google Book Search, March 15, 2008.
 Guinea Lynx. Accessed March 15, 2008.