Many equine skin diseases appear similar to the eye. Some horses will develop small swollen areas on the skin with hair loss. There are many different possible causes for this, one of which is ringworm. Ringworm is a fungal infection that causes inflammation and hair loss with itching and rubbing on the skin. Ringworm is contagious to people so it is important to determine if ringworm is involved, especially if your horse is around children as they are quite susceptible. Horses can be exposed to ringworm from other horses, cats, and even the soil. Generally, ringworm has circular lesions and the horses do not itch a lot but I have seen horses with it that itch quite a lot.
Ringworm lesions can also look similar to other skin infections, such as Staph, and it is important to determine which infection is involved as ringworm is a fungus and Staph is a bacteria and they require different treatments. There are lots of antifungal products over the counter that claim to be effective but most are not proven to be effective so it is important to know the correct disease you are treating. Diagnosing fungal infection requires hairs and flakes taken off the edge of the lesion be sent to the lab for culture which may require up to 2 weeks. There is a new PCR fungal test that only requires a few days but the test is not fully tested in horses. Most of these Staph infections respond to antibiotics but resistant staph infections like MRSA do develop in horses, so any skin lesions that don't respond to antibiotic treatment should be cultured to determine if the horse has resistant infection.
Flying insect allergies can also appear similar to infections as allergic horses develop raised bumps with hair loss and this can lead to secondary infections. The most common flying insect allergy is to a tiny midge called culicoides, and the horse is allergic to the proteins in the insect's saliva and causes an allergic reaction.
Horses can also develop skin lesions from environmental allergies like dust, mold, and pollen as well as a contact allergy to certain chemicals. These allergies can also cause lumps on the skin, and after the horse rubs and scratches those lumps, infections develop. So there are lots of causes of lumps and bumps on the horse's skin and many of them require different treatments. Before buying an over-the-counter skin treatment, have your vet take a look and make the diagnosis before starting treatment.
Bacterial infections are also common on horse’s skin especially secondary to fly bites or other types of allergy that causes trauma. Although many of these infections are related to staph infection some are not and so it is recommended to culture infected areas to know which antibiotic to use.
Going into the winter, one problem is caused by lice. Lice are fairly large parasites that can actually be seen with the naked eye if you have really good vision. However, if there are only a few lice on the horse, using a lice comb is a good idea. If your horse is itching, use a lice comb and add a drop of oil on the teeth which aids in capturing the parasites. You can use a low power microscope or magnifying glass to visualize the lice and it has been reported that it is better to examine the horse after being worked as lice move to the tips of the hairs and are easier to find after working.