Unless the guinea pig is a “skinny pig” (purposely bred to be mostly hairless) bald patches are bad signs and need the prompt attention of a veterinarian. They could be signs of parasites, the fungal infection ringworm, malnutrition or (in the case of intact females) cancer of the reproductive organs.
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Keep in mind that most guinea pigs will always have a bald patch right behind the ear. When you place a fingertip on the spot, it may feel very warm. Unless the skin is scratched open or crusty, these bald patches are nothing to worry about.
Fleas, mites or lice can cause intense itching, which cause the guinea pig to scratch so much that the hair falls out and does not have a chance to regrow until the guinea pig is scratching away again. This type of baldness tends to begin gradually and then can eventually lead to a bald, crusty and miserable guinea pig.
It’s actually quite rare for guinea pigs to get fleas, but it is possible for them to get fleas if another animal in the house gets them or if a flea hitches a ride on the body or clothes of a human being. Humans also can serve as taxis for guinea pig lice and guinea pigs mites. Always wash your hands after petting a strange pet and before touching your guinea pigs.
It’s thought that pine bedding – once the most popular pet bedding for guinea pigs – encourages the growth of lice and mites that can feed on guinea pigs. However, pine bedding that has been specially dried out in a kiln should be free of attractants for guinea pig parasites.
Once thought to be caused by a worm and now known to be caused by a fungus, this is highly contagious to most animal species, including humans. But this is a treatable condition. The bald patches tend to be circular with a reddish ring at the edge (hence the name). Take to the vet right away. The danger is that besides being contagious, the guinea pig could die as a result from an infection while scratching himself open.
Hair is luxury to an animal body. If food is scarce, then the body may lose the hair in order to help preserve nutrients for vital organs. Bald patches or thinning hair can be a sign of malnutrition in guinea pigs, especially vitamin C deficiency. The guinea pig will also have diarrhea with Vitamin C deficiency.
Guinea Lynx, one of the Internet’s premier websites devoted to guinea pig health, also lists diets too low in protein as causing significant hair loss.
Intact female guinea pigs are prone to getting growths in their ovaries. These can be cysts or tumors. These growths often cause a change in the female guinea pig’s hormones which is thought to cause the hair loss. The guinea pig will act otherwise normally, but she will have two symmetrical bald patches on her hindquarters. Spaying is often the only practical treatment.