Flea and fly bite hypersensitivity: what every nurse needs to know

Allergens are antigens that cause type 1 hypersensitivity reactions, and an allergy is an exaggerated response to this stimulus. Over-reaction to antigen stimulation can manifest itself in an animal's skin. All pets can be affected to some extent by flea and fly bites, but an allergic pet will react with disproportionate severity, causing just a few bites to significantly harm the skin. The hyper-sensitivity is a physiological aberration where the host's immune system erroneously recognises a nontoxic foreign substance that has entered the body as harmful. In an effort to combat the substance, called an allergen, the animal's immune system releases a chemical compound called histamine. The eagerness to scratch characteristically suggests the presence of a flea or fly bite allergy, and is often accompanied by dermatitis. The most common skin allergy encountered in small animal veterinary medicine is flea bite hypersensitivity, also known as flea allergy dermatitis (FAD). FAD can often coexist with other allergic conditions such as atopic dermatitis. This disease exhibits pruritic dermatitis in animals that have become sensitised to antigenic material in flea saliva. However, dogs exposed to flea burdens on a continuous basis may become partially or completely immuno-tolerant and this condition may be broken when dogs are exposed in an intermittent manner (Wilkerson et al, 2004). Flea infestations also lead to human irritation and flea bite reactions, as well as eroding the human–animal bond. As a result, flea control should be considered part of a ‘one health’ approach, improving human wellbeing with flea control, as well as the well-being of the pet. Desire and medical necessity to control fleas has led to more than $1 billion in the USA, and €1.1 billion in Europe, being spent on companion animal flea control annually (Rust, 2005). Similarly, fly bite sensitivity in horses caused by Culicoides spp. is a common cause of dermatitis, unofficial recording by organisations such as the national sweet itch society suggest 3–5% of horses in the UK may be affected but there is no official figure and this may be an underestimate.