An outbreak of hives on a shiny coat of a performance horse might seem like an emergency because of the effect it has on plans to show or compete, but in reality it's a potential emergency for another good reason: it signals an ongoing hypersensitivity reaction, one that could escalate to full-blown allergic or anaphylactic shock.
Does this mean you you should push the panic button every time your horse gets a case of so-called alfalfa bumps? Yes and no. Most cases of equine hives resolve as quickly as they appear, usually within 24 to 48 hours, and the cause is never figured out. But to shrug them off without a second thought is to invite disaster, because the horse is a practiced and infamous over-reactor to a variety of stimuli, and in most cases of fatal hypersensitivity, the groundwork for the reaction was laid months, maybe even years earlier. Which means that every outbreak of "the bumps" could be a dress rehearsal for the big one.
The first bumps usually show up on the side of the neck, followed by the face, the chest, and the upper front legs. They may or may not be itchy. The bumps are initially distinct and steep-walled, and they retain a depression for several seconds when you press on them with your finger (this is called pitting edema). As they grow in size and number, they may coalesce into large plaques of swollen skin. If the outbreak also involves the tissues that line the respiratoty and digestive tracts, there may also be respiratory distress (like a severe asthma attack, with wheezing and an increasingly anxious struggle to get air) and colic pain that leads to diarrhea.
The skin condition itself is not dangerous. But it's only a sign of an allergic reaction going on inside the body, and if that allergic reaction is widespread enough to involve the major organ systems such as respiratory and digestive tracts, and it's escalating, the horse could be on a fast track to collapse and death.
The biggest mistake is complacency. Because the horse's body tends to be so overreactive, outbreaks of hives are seen pretty often, and as people become acclimated to the condition, they begin to see it as just a nuisance rather than a potential harbinger to a disaster, and it gets "diagnosed" as "alfalfa bumps," spider bites, beestings, creosote allergy, and a number of other unsubstantiated "conditions" that satisfy people's need to pin a reason on things. As a result, the gravity of the situation is completely missed, and the chance to get help early in the process is completely passed by.
TREATMENT Medical treatment is aimed at interrupting the allergic response, employing such agents as antihistamines and corticosteroids, but it should be remembered that this is only treating signs. Another thing to bear in mind is that corticosteroids can cause founder.
To truly resolve the problem, the underlying allergen must be identified and eliminated. To that effect, some diagnostic tests must be run, including blood tests and skin biopsy.
If you notice that your horse is woking harder than usual to get his air, making any wheezing sounds when he breathes, then he is developing a respiratory distress. Loose manure, belly pain or exceptionally gurgly gut sounds are all evidence that your horse might be in serious trouble. Get help. Meanwhile, give him a cool bath and hold ice packs on his swollen eyelids and sooth and help calm his irritated tissues. Do not give him and medications before diagnostic tests.
References:
1. Emergency! by Karen Hayes, D.V.M.