The best information I have found on skin problems came from Debbie Ducommun, the Rat Lady of the Rat Fan Club, so I’ll let her explain:

The most common skin problem in rats is caused by itching. The rat scratches herself which creates scabs, most often on the neck and shoulders, but sometimes also on the face, chin, or forehead. These scabs are sometimes mistaken for injuries caused by other rats. The causes of itching are fur mites, an allergy, excessive dietary protein (such as fish or cat food), eczema, and a fungus infection. A bacterial infection of the skin may also be involved, but it is rarely the primary cause. Itching also seems to be related to stress, common for eczema. It’s also possible that some rats infested with mites can keep them under control until their immune system is suppressed by stress. It’s also possible that some rats with mites eventually develop an allergy to them. If you have more than one rat with scabs, it’s probable that mites are the cause. Regardless of the cause of the itching, when the rat scratches, her back toenails break the skin which can allow bacteria to enter. As the scratches heal, scabs form and can themselves itch which causes more scratching, resulting in a vicious cycle. The first step to breaking the itching cycle is to clip the rat’s back toenails. Trimming off the sharp tips reduces the amount of damage the nails do to the skin. You might need to clip the nails as often as once or twice a week. You can also put a brick or concrete block in your rat’s cage to help her wear down her toenails. You might also apply a vitamin E cream or an antibiotic ointment to the scabs once or twice a day. Neosporin Plus contains a topical anesthetic which can be especially helpful. This treatment may be sufficient to clear up the scabs. If they recur, you need to look for another cause.

Parasite Treatment

If your rat is still itching, the next step should be to treat your rat for fur mites. Although you can have your vet do a skin scraping on your rat to test for mites, this often results in a false negative, so I recommend progressing right to the treatment. The most effective treatment is with ivermectin which paralyzes arthropods by over stimulating a neurotransmitter (GABA). Ivermectin is very safe for use in mammals because we use GABA only in the central nervous system, which is relatively impenetrable by ivermectin. In tests, calves showed signs of ivermectin toxicity only after receiving 20 times the normal dose.

[a little insert from Sara: A very nice fellow named Dennis sent me a tip about ivermectin–he says: “I would like to suggest that you find a better way to measure and control the dose of horse paste you give rats. The paste is very strong, and it is hard to reproducibly measure tiny quantities of strong stuff. The method that I use is to dilute the Ivermectin with vegetable oil. The resulting mixture is much less concentrated, and the larger volumes used can be more easily measured.” If you are considering using ivermectin, check out his website. You can also check out this guinea pig website, which recommends the same type of thing.]

You can get a prescription for oral ivermectin from your vet at a dose of 100-200 mcg/lb (some rats are sensitive to it by injection and can die), or you can buy a horse paste wormer containing ivermectin at a feed store or through a mail order catalog (one brand is Zimecterin). Squeeze out a tiny bit of the paste the size of half an uncooked grain of white rice and put it on a tiny piece of bread for your rat to eat. This dose should be repeated in a week. If necessary, a third dose can be given after another week. You can also use dog or cat ivermectin heart worm tablets, cutting them up into the proper dose. Improvement is usually seen within the first week. You should treat all of your rats, since if one rat has the mites they probably all do, even if they don’t all have scabs. Be sure to continue the toenail trimming and ointment until the scabs are gone. Warning: Although lots of rats have been successfully treated with the ivermectin horse paste without problem, I recently heard of one rat who became paralyzed the afternoon of her second dose (the size of a whole grain of rice–I’ve since cut it in half) and died 3 days later. Paralysis would be the result of an overdose. Perhaps the dose of paste she received was abnormally concentrated, or perhaps she was abnormally sensitive or developed an allergy to the paste. It’s probably a good idea to mix up all the paste first before taking out doses to make sure it is evenly mixed. Use this method at your own risk.

Allergies

The most common foods for a rat to be allergic to are peanuts and dairy products, including yogurt drops. Eliminate these items for at least two weeks to see if this solves the problem. If the problem is another allergy, or eczema, treatment with a steroid will stop the itching. You can try a hydrocortisone cream (be sure to rub it into the skin well), or ask your vet for oral predisone an antibiotic should be given with it because steroids depress the immune system). Sometimes the steroid treatment alone will clear up the problem, but if the itching returns after the treatment, you must try to identify what the rat is allergic to. It is rare for a rat to be allergic or sensitive to most litters, other than pine or cedar shavings, but you might want to try changing your rat’s litter or bedding. Because I think a rat can develop an allergy to fur mites, I recommend treating for mites if you can’t identify another allergen. If you know your rat’s problem is an allergy, the next step is to test for further food allergies. A good testing diet is a mixture of cooked brown rice and raw millet, plus 1 teaspoon of Nutri-Cal per day. You can buy Nutri-Cal at any vet hospital. If you see an improvement in 7-10 days you then add foods one at a time to see if they cause itching. If you identify the food your rat is allergic to, then you can put her back on a normal diet, minus the offending food. It is also possible for a rat to have eczema, which causes itching with no known cause. The treatment in this case is a topical steroid cream or shampoo, and you often have to continue the treatment for the rest of the rat’s life.

Fungus

If your rat hasn’t responded to the ivermectin or the predisone, the only thing left is to have your vet treat your rat for a fungal infection. Like the skin scraping for mites, biopsies or skin scrapings for fungus often yield a false negative. Therefore, you must try the treatment. If the infection isn’t too advanced, try an over-the-counter cream such as Lotrimin. Antifungal shampoos don’t seem to work. For severe infections, you may need to use an oral fungicide such as Nizoral (ketoconazole). Treatment can take several weeks up to 3 months. Because fungus thrives on sugar, a rat with a fungal infection should receive only limited amount of sugar (including fruit) in his diet.

Dry Skin

While oily dandruff is normal in intact males, dry skin and dandruff can be symptom of a poor diet. If your rat is getting an adequate diet, try giving a supplement containing essential fatty acids. Dry dandruff can also occur in rats with hindquarter paralysis since they can’t groom themselves normally. If the humidity in the air is too low, it usually affects the tail rather than the skin. This can prevent the dead skin cells on the tail from shedding properly resulting in patches of scaly skin and discoloration. The solution is to run a humidifier in the room. Ringtail is a skin problem caused by dehydration that is occasionally seen in baby or hairless rats, and rarely in haired adults. Dehydration can occur if baby or hairless rats are kept on litter that is too absorbent (commonly corn cob litter) or in any rat if the water bottle malfunctions. In babies ringtail causes a constriction at the base of the tail. In adults it can cause a moist oozing sore at the base of the tail. The problem usually goes away when the rats are rehydrated, although if the problem is bad enough a baby may lose part of her tail.

Hair Loss

There are two main causes of bald spots in rats. The most common is barbering, a behavior where a rat obsessively grooms itself or another rat to the point of nibbling off the hair. The result is bald patches or areas where the hair looks like it’s had a bad haircut. Usually there is no damage to the skin, but sometimes there can be scabs. The most common areas for self-barbering are the front legs and stomach. The most common areas for barbering another rat are on the head, face, neck, and shoulders. These bald spots are not usually symmetrical. Because this behavior doesn’t usually cause any health problems, there is no reason to separate a barber from her roommates, unless you are showing your rats. Another cause of bald spots is fungus (see above.) Another type of hair loss is a general thinning of the hair. This can occur in a rat infested with lice or tropical rat mites. Although in these cases the rat usually doesn’t self-inflict scabs, constant scratching can cause general hair loss, most commonly on the back. Rex rats may tend to have thinning hair as they grow older or if they are stressed due to disease.

In some other animals, such as dogs and cats, a hormone problem can cause hair thinning, although I haven’t seen this in rats. This type of hair loss is usually seen on the flanks, hindquarters and sometimes the stomach and is usually symmetrical.