Why do flea bites itch?

Summary

Flea bites itch from saliva being introduced into the wound. Flea saliva contains allergenic materials which lead to immune responses. The reaction’s severity depends upon the affected individual’s sensitivity.

Details

Sensitization to Flea Bites

Flea bites react differently depending upon the individual’s previous exposure to the insect. There are five stages of sensitization.

1. Without previous exposure, there’s no sensitization. A bite won’t react.
2. Subsequent bites lead to delayed reactions, occurring after 24 hours.
3. With continued biting, both immediate reactions and delayed reactions occur.
4. More encounters diminish the delayed reaction’s intensity. Eventually only immediate reactions appear.
5. With more exposure, complete desensitization (immunity) is reached.

Animals

Sensitization in guinea pigs closely follows the five stages. They gain immunity after 180 days of flea exposure. In dogs, the process is more unpredictable. Full immunity hasn’t been observed in domestic pets, even at 44 weeks.

Humans

In humans, there’s almost always a delayed reaction. However, one source state only 63% of humans develop late responses. Young children, who are building immunity, experience more intense delayed reactions than adults. At least 18 months of continued biting is needed to gain full immunity. Even then it may not develop. Immunity has mainly been observed in indigenous people of flea-infested areas.

Immediate & Delayed Reactions

Soon after a flea bites, skin in the area rises. This is called whealing. Wheals are slightly pruritic (itchy). However, they don’t bother people much because they fade away quickly. 24 hours later, extremely itchy welts (papules) appear. These lesions cause many complaints in health clinics. The irritation peaks in severity within 12-24 hours.

Flea Bites & Saliva

How Fleas Bite

Fleas bite using their fascicle, which is composed of three slender stylets. The outer two are sharp, serrated structures called maxillary lacinae. They puncture the skin, but don’t enter the blood vessel. The center stylet, called the median epipharynx, enters the capillary to draw blood.

Introduction of Saliva to the Wound

Four pear-shaped salivary glands lie within a flea’s abdomen. They connect to the mouth with long ducts, and excrete large droplets of saliva. When biting, the lacinae press close together to form a salivary canal. As blood is imbibed, saliva is introduced to the wound through this channel. Drops of saliva fall near the puncture, but not into the blood vessel itself.

Flea saliva contains antigenic materials which induce various immune responses in the host, including immediate and delayed reactions.

Flea Probing

Prior to biting, fleas will probe skin to find a good feeding location. This probing actions alone is sufficient in creating a bite reaction at each site. Saliva is introduced at each probe site. It has anticoagulants which help fleas locate a blood vessel.

Feeding Interruptions

If fleas are disturbed while feeding, they may relocate a safer nearby location. This results in multiple bites in close proximity.

Allergenic Materials in Flea Saliva

Haptens

Delayed reactions are caused by small molecules known as haptens. A hapten is an incomplete antigen and won’t trigger an immune response by itself. It becomes a complete antigen once it attaches to a larger carrier, usually a protein. With flea saliva, the hapten binds to collagen within skin, triggering an immune response and inducing hypersensitivity.

Two specific haptens have been characterized in flea saliva. One has a molecular weight between 4 and 10 kDa. The other is fluorescent blue, aromatic, and less than 1 kDa.

Antigens

Haptens can create a delayed (cell-mediated) response. However, they don’t explain the immediate (IgE-mediated) whealing. Wheals occur when mast cells release histamines, and this response can’t be triggered by low weight haptens.

IgE responses are T-cell dependent. Thus, some flea allergens are higher weight proteins. This lead researchers to look for larger IgE-binding antigens. And 15 were found, ranging from 14 to 150 kDa. Prominent components had molecular weights of 25, 40, and 58 kDa. The presence of many distinct allergens cause the reaction, not one dominant component.

Subsequent studies characterized more possible antigens. Using flea salivary gland extracts on mice, protein band peaks occurred at 56, 54, 42, and 40 kDa. These antigens may play a minor role in hypersensitivity, though none was observed. Again using mice and salivary gland extracts, it was found that proteins with weights of 40 kDa and 8-12 kDa play an important role in flea bite hypersensitivity. The major flea allergens for cats and dogs have weights of 14–40 kDa.

Anticoagulants & Apyrase

Flea saliva creates an enzymatic reaction with skin. It dissolves, softens, and spreads the tissue, which makes penetrating the skin easier. The saliva also contains anticoagulants that assist in the uptake of blood. And the anticoagulant properties help fleas locate blood during exploratory probing. Digestion and liquefaction of skin tissue introduces foreign substances to the body. As a result, local inflammatory reactions occur in the surrounding tissue.

Apyrase is an antiplatelet enzyme found in cat flea saliva. It’s unknown if apyrase is an allergen within the mixture of salivary secretions, but it’s likely.

Basophil

Basophil, a type of white blood cell, plays a role in the allergic reaction to flea bites. Basophils constitute a major part of the cells found at delayed reaction sites. As an individual continues to be bitten, it’s likely that more antibodies bind to the basophils, which increases histamine release and results in immediate whealing.

Flea Debris Allergens

Certain antigens are found in whole fleas, though most are limited to the saliva. Flea debris (dead fleas, eggs, larvae, etc) contains allergens which are distinct from salivary antigens. They’re similar to those in cockroaches and other insects. Flea allergens in house dust may become airborne, evoking allergic symptoms in certain individuals (e.g. those with asthma). This is especially true in heavy infestations. Flea debris allergens were characterized at molecular weights of 34, 35, 39, 54 and 60 kDa. The 34 kDa protein was particularly IgE reactive.

The Purpose of Itchy Bites

After hatching, flea larvae must feed on adult flea feces (flea dirt). They’ll die without it. Thus, it’s crucial for eggs and feces to fall from the host together. Luckily, for the fleas, it does. The eggs and blood feces share similar spatial distributions, providing larvae with food immediately upon hatching.

Females fleas must consume a blood meal before every egg deposition. This partly explains how the feces and eggs fall together. However, the bites’ itchiness plays a much larger role. The irritation results in animals scratching themselves. And as animals groom, they dislodge both eggs and flea dirt. The eggs aren’t sticky and easily fall even without grooming. However, the fecal blood is irregular-shaped and often gets lodged in hair. It’s primarily dislodged through scratching.

References