Horse allergy can manifest as allergic rhinitis, conjunctivitis, asthma, anaphylaxis and food allergy. A variety of expected and unexpected allergic reactions have been reported after exposure to horse allergen. IgE sensitization occurs despite the absence of reported horse exposure and the degree of prevalence in individuals directly exposed to horses is high. Numerous allergens have been characterized, with at least 16 allergens isolated to date and the major horse allergen is Equ c 1, a lipocalin. The horse allergen seems to be as tenacious and problematic as cat allergen, given its potential for passive transfer.

Worldwide distribution

Sensitization rates in occupational settings (e.g. stables) vary between 3.6 and 16.5%, whereas individuals with respiratory allergy reported a sensitization rate of 5.38% (1). Occupational exposure to horse dander is a risk factor for horse sensitization; sensitized individuals can develop asthma or impaired lung function secondary to exposure. One study showed that 30% of patients referred for indoor pet dander allergy also reported clinical allergic symptoms to horses (2). In Spain, horse was positive in 12% of patients who were tested to a battery of aeroallergens (3). It has been suggested that the horse allergen should be regarded as a “hidden allergy” because of the lack of occupational or recreational exposure to horses in most sensitized individuals. Often, sensitization results from unexpected exposure where horse products are used or incorporated (e.g., antique furniture, brushes, instrument bows). Stray animal exposure is also identified as a risk factor for horse sensitization during travel or vacationing abroad (4).

Risk factors

Sensitization to horse dander can vary based on climate, geography, and level of exposure. Surprisingly, individuals riding in the humid coastal region of Sweden had a lower rate of horse sensitization compared to the rate in the colder north region of the Arctic Circle. This finding is intriguing, suggesting that the higher the allergen exposure the higher the tolerance is. It has been shown that while women rode more often than men, men had higher rates of sensitization(5).

Living environment

According to information from FAO (Food and Agriculture Organization) from their last census in 2010, there are almost 58 million horses in the world. In regions like South America, Asia, Africa and Eastern Europe they are mainly an essential form of transport, but also a source of meat, hair and leather. In the rest of the world, they are principally important for sports and leisure activities. (6)

Main

The main route of exposure is inhalation. However, individuals can be exposed to horse allergen not only in stables and in the countryside but also through indirect contact during public transportation, work, or school (1).

Secondary

Horse bite can also be a possible route of exposure leading to anaphylaxis in some rare cases as seen in a case report published by Guida et al. (7) 

Other topics

Mattresses stuffed with horsehair could be a route of exposure for the horse allergic individuals. Egmar et al., (8) studied whether new mattresses, one-third of which were stuffed with horsehair, had horse allergen. The findings were that horse allergen was not detected when vacuum cleaned. This finding was contradictory to the same investigators identifying horse allergen in most used mattresses, in which horse allergen concentration increased over time despite no horse exposure. The authors concluded that allergens from the horse had been deposited from customers and not simply transferred from the interior of the mattresses. It is important to know that allergen concentration correlated to the time period that the mattresses were used by customers.

Another issue of concern is the presence of horse allergen in ambient air and settled dust in the vicinity of stables. Elfman et al. investigated the dispersion of horse allergen around a stable and included wind speed, wind direction and vegetation in their calculations(9). Horse allergen levels were found to be high near stables but dispersed poorly through the air as evidenced by low levels found away from the source. The passive transfer from horse allergen carried by individuals in direct contact with horses is problematic. Dispersion of horse allergen should be avoided as much as possible in order to protect individuals allergic to horse allergen.

The clinical picture of horse allergy is variable and can affect both children and adults. Horse allergy can manifest as allergic rhinitis, conjunctivitis, asthma, anaphylaxis, and food allergy (10).

Allergic rhinitis

A cross-sectional Swedish study identified horse sensitization to be a significant risk factor for rhinitis in 7- to 8-year-old children(10). Thirty-seven percent of patients presenting with rhinitis were sensitized, more common than birch, timothy, mugwort, house dust mite, or mold sensitization. Most cases that report horse exposure as a cause of asthma exacerbation also report concurrent rhinitis symptoms. (11, 12)

Asthma

Since Salter’s original description of “Animal Emanations” causing asthma in 1860, several studies have documented horse-induced respiratory symptoms in horse allergic patients.  Roberts and Lack published a series of case reports suggesting that horse allergy can contribute to persistent asthma (11). Horse allergy cases also occur in heavily urbanized environments. Several cases also link respiratory distress and angioedema in children who are sensitized and recall specific horse exposure. Gallagher et al. found evidence in New Zealand that working with horses also is a risk factor for organic dust toxic syndrome, farmer’s lung, and chronic bronchitis (1, 13).

Urticaria and anaphylaxis

Anaphylaxis and urticaria have been reported in horse allergic individuals (1). A 42-year-old woman who, after being bit, developed large local erythema and edema followed by generalized urticaria, rhinitis, and wheezing. A 35-year-old veterinary employee experienced an episode of sneezing, rhinitis, and urticaria following a period of postoperative care for a horse, which included intense contact with horse saliva. (1)

Other topics

Allergic reactions to mare’s milk have been described but they seem to be rare. The observed high incidence of cow’s milk allergy has prompted the search for alternative milk sources and mare milk is a potential candidate. Godovac-Zimmerman et al., have documented that mare’s milk has different amino acid sequences of milk proteins compared to cow’s milk, implying distinct IgE binding (14). Furthermore, the composition of mare’s milk seems to be closer to human milk than cow’s milk. Businco et al.,  have shown through double-blind placebo-controlled food challenges (DBPCFC) that mare’s milk could safely be substituted for cow’s milk in 96% of children (15). Half of the children who failed the DBPCFC were sensitized to mare’s milk. Restani et al., found weak cross-reactivity between mare´s and cow´s milk (16).

There are several case reports about individuals allergic to horse dander reacting with generalized urticaria, angioedema, and an asthma exacerbation after ingesting mare´s milk. Therefore, sensitization to horse dander can lead to a reaction to mare’s milk allergy (1).

The inclusion of horses into the standard allergen panel for children and adults has been supported by several authors (17, 18). However, current horse extract may contain varying amounts of immunologic allergens and is not standardized (1). Based on these differences, Fjeldsgaard and Paulsen (19), concluded that allergen extracts used for diagnosis and immunotherapy should be prepared from both horse dander and hair and skin scrapings. Franke et al., (20) investigated five-horse epithelial allergen extracts from various qualities of raw material by several immunochemical and biochemical methods. The allergen present in each commercially available extract was variable and depended on the origin of the raw material.

Since the number of important allergens in commercial horse extracts can vary extensively, and as natural preparations may be contaminated with other components, with the potential to cause false-positive skin-specific IgE test results, recombinant Equ c 1 has a role to play in assessing allergy to horse (21).

Allergen immunotherapy

There are few clinical studies published regarding horse immunotherapy. Fernandez-Tavora et al., (3) published a study focusing on horse dander immunotherapy safety and efficacy. Twenty-four horse-allergic patients, 16 of whom had asthma, underwent immunotherapy with extracts containing Equ c 1, Equ c 2, and Equ c 3. Cluster immunotherapy was performed in 21 patients, whereas 3 underwent conventional immunotherapy. Two (0.33%) systemic reactions and three (0.5%) large local reactions occurred in four subjects during the induction phase.

Sensitivity to horse dander as assessed by deliberate exposure with horse riding decreased significantly on maintenance dosing. Conjunctivitis was reduced in all patients, rhinitis in 93%, cutaneous symptoms in 87%, and asthma symptoms in 90% of those studied, 95% of patients were highly satisfied with the therapy. The long-term follow-up found an immunologic cure in 29% of participants after 2 to 5 years of treatment. Despite one horse immunotherapy study showing significant improvement in clinically sensitive patients, the 1997 World Health Organization Position Paper on Immunotherapy lists horse immunotherapy among the areas needing further clinical research (1).

Allergenic molecules

The allergens of Equus caballus listed in the World Health Organization and International Union of Immunological Society (WHO/IUIS) Allergen Nomenclature Database (http://www.allergen.org) are:

Reviewer: Dr. Christian Fischer and Dr. Magnus Borres

Last reviewed: November  2020