Tick bite hypersensitivity.
Tick bite hypersensitivity reactions are caused by a local immune response to the tick bite. The most common presentation
is a focal nodular reaction at the site of the tick bite. A focal area of necrosis is common, but I have seen a range of tick
bite reactions. Pruritus, ulceration, and erythema are variable due to differences in the host's immune response to the bite
and the stage of the reaction.
Mosquito and insect bite hypersensitivity.
The most well-recognized allergic reaction caused by insect bites is mosquito bite hypersensitivity in cats, which is characterized
by a severely pruritic, papular, crusted eruption on the face, ears, head, and footpads.2 I have also seen mosquito bite hypersensitivity in dogs characterized by eosinophilic furunculosis of the face and head
(Figure 1). Owners often report seeing the dog swarmed by small insects or mosquitoes. Depending on the geographic region, clinical
signs can be seasonal or nonseasonal. The lesions tend to be localized to thinly haired areas, and a thorough history usually
reveals that the clinical signs coincide with insect hatches. Owners often report that the lesions resolve if the animal is
Figure 1. A dog with insect bite hypersensitivity.
Ear mite hypersensitivity.
Otodectes species hypersensitivity reactions are most common in cats. With routine ear mite infestations, pruritus is usually limited
to the face, head, and ears, and all stages of mites are easily found. The pruritus associated with ear mite hypersensitivity
may also be limited to the head or it may be generalized. Diagnosis of this disease can be frustrating because, contrary to
routine infestations, the mites are rarely found. A diagnosis is usually made after observing a positive response to miticidal
Intestinal parasite hypersensitivity.
This is a poorly documented and somewhat controversial hypersensitivity reaction. Various intestinal parasites (ascarids,
coccidia, hookworms, tapeworms, or whipworms) may cause pruritus, urticaria, seborrhea, or generalized papular crusted dermatitis.1 Hookworm larvae can penetrate the skin and cause inflammation and pruritus as they migrate. But the pathophysiology of the
pruritus caused by internal parasitism is unclear. It could be due to a parasite byproduct, an immune reaction on the part
of the patient, or another mechanism. Diagnosis is confirmed by ruling out other causes of pruritus, finding parasites on
fecal examination, and observing resolution of clinical signs after parasiticidal therapy.
This is another poorly documented and somewhat controversial hypersensitivity reaction believed to be caused by reactions
to sex hormones. It is unclear if it truly exists in dogs or cats. The clinical signs mimic those of other hypersensitivity
reactions and tend to coincide in females with the onset of estrus.1 In males, seasonality does not occur. Surgical neutering is curative.
Bacterial and Malassezia hypersensitivity.
Many animals with secondary staphylococcal infection or Malassezia overgrowth are pruritic. The pruritus may be caused by the underlying disease as well as by the allergic response to antigens
secreted by these organisms.4,5 Antimicrobial therapy and identification of the trigger that predisposes the animal to overgrowth are the keys to successful
Atopic dermatitis is a hypersensitivity reaction to inhaled or dermally absorbed environmental allergens and is a common cause
of allergic skin disease of small animals. Practitioners must rule out all of the previously discussed diseases before they
can diagnose atopic dermatitis.