Probably the most common cause of skin disease seen in the hamster is caused by the Demodex mite. The two species of mite implicated in this disease are D. aurati and D. criceti. Both of these mites may be found on healthy hamsters but they cause a problem if the hamster's immune system is compromised in some way. This is often due to stress, perhaps due to problems with the housing or diet of the hamster or associated with some other problem. Signs of this disease are loss of hair usually on the back and the flanks, with mild scaling of the skin. The hamster is not usually itchy or uncomfortable.
Diagnosis is made by visualizing the mites or their eggs under the microscope after taking a skin scrape from the affected area. Treatment with a mange wash e.g. Derasect is usually effective and fairly safe. This should be done weekly until no further live mites or eggs can be found on the skin scrapes.
A much more serious disease called mycosis fungoides looks like infestation with demodex so if no mites can be found on the skin scrapes it may be necessary to take a skin biopsy. If mycosis fungoides is confirmed the prognosis is unfortunately very poor.
Sarcoptic mange may rarely be seen in hamsters it is usually a result of poor hygiene. Hamsters with this condition are severely itchy and may cause themselves considerable injury through excessive itching. If severely affected it may be kindest to consider euthanasia, however this disease can often be treated with an appropriate wash.
Ringworm can affect hamsters. It is actually a fungal disease not a worm! This tends to be contracted from contaminated bedding, other pets or even humans! Trichophyton mentagrophytes and Microsporum spp are the causes of ringworm. Signs of ringworm are bald patches with some brittle hair, dry skin with some scaling especially around the ears. Isolate infected animals and thoroughly clean the cage, change the bedding and take care to always wash your hands following handling these pets since you can catch ringworm too! Treatment with griseofulvin is often recommended sometimes together with a wash called malaseb. If possible it may be a good idea to shave affected areas but this should be done by your vet since it is easy to cause severe injury to thin hamster skin with scissors and clippers. Treatment often has to last 3 weeks or longer before the hamster is clear.
Hamsters may suffer from bacterial skin diseases often due to bite wounds from fighting. Hamsters may also traumatize their own skin if they have an underlying painful condition. A generalized skin disease can be caused by infection with Streptococcus.
Hamsters may suffer from skin cancer but it is rare. Sometimes people get worried when they see the dark greasy spots on their hamster's back; these are normal glands used for marking the male hamster's territory!