Many insects, including flies, mosquitoes, gnats, ticks, and mites can inhabit or irritate the skin. Some of these insects carry diseases that they transmit to companion animals, causing more illness than the insect bite or sting itself. Very importantly, some of these insects can cause disease in human beings.
The pattern of itch from these insects can be generalized or be highly specific to the feeding patterns of the insect itself. And over time, symptoms that began with specific patterns of itching can generalize as the skin worsens and, especially, if secondary infections develop.
Ectoparasite identification can be easy or it can be challenging. A parasite that resides on the skin may be found in large numbers (Demodex mites) or in very small numbers (Sarcoptes mites). Or they can be difficult to collect in samples for examination (Cheyletiella mites). Dermatitis caused by a parasite that does not reside on the skin, such as mosquitoes or gnats (Culicoides species), can only be diagnosed by accurate history and physical examination. Sometimes the diagnosis is made by treating for a presumptive parasite and noting if symptoms resolve.
It is not uncommon for veterinary dermatologists to be referred cases that have had extensive (and expensive) allergy diagnostics without successful resolution of the itching. Ultimately, for many of these cases, a non-allergy cause of the discomfort is discovered and treated.