Direct or indirect contact with animal allergens frequently causes sensitisation.

Unexpected exposure

Horsehair is encountered in antique furniture, and Horse meat is eaten in some countries in place of Beef. Horsehide is used in making baseballs and certain other leather goods.

Allergen Exposure

Geographical distribution

Except for a few feral populations, and one extremely endangered wild one (Przewalski's Horse), Horses are domesticated. Long used as a means of transportation, pleasure, work, and even war, Horses have been involved in much of human history. Domestic Horse breeds are numerous and highly various. Although little used for work today in developed countries, Horses are widely owned for recreational riding and show activities.

Environment

Horses are found in agricultural and recreational settings.

At least 16 allergens have been isolated from Horse (1-4). Several allergens have been shown to be glycoproteins, including a 27 and a 31 kDa protein (5).

A number of allergens have been characterised:

• Equ c 1, Mw 25 kDa, a lipocalin (6-10)
• Equ c 2, Mw 16 kDa, a lipocalin (6, 8, 10-12)
• Equ c 3 Mw 67 kD, albumin (Ag3) (10, 12)
• Equ c 4, Mw 18.7 kDa (6, 12)
• Equ c 5, Mw 16.7 kDa (6)

Various isoforms of Equ c 1 and Equ c 2 have been identified, including Equ c 2.0101 and Equ c 2.0102 (4) (11) . Both allergens have been cloned from the sublingual salivary glands and have also been found to be expressed in the liver and submaxillary salivary glands (7).

Already in early studies an important 67 kDa allergen has been isolated that was thought to be Horse albumin (13).

In a study assessing the importance of albumin as a cross-reactive allergen in patients sensitised to Cat, Dog and Horse, 117 patients sensitised to Cat were tested for IgE reactivity using skin-specific IgE and RAST assays with Cat, Dog and Horse hair/dander extracts and their purified albumin extracts. Of these patients, 22% exhibited specific IgE to Cat albumin; 41% of patients sensitised to Cat were also sensitised to Dog and Horse. Of these patients, 21% had IgE antibodies to 3 albumins and 17% to 2. Specific IgE binding to Horse extract was inhibited in 30% of samples by its homologous albumin, and IgE binding to Cat and Dog extracts in almost 15% by their respective albumins. The study concluded that albumins from these three animals share some epitopes that account for the cross-reactivity observed in around 1/3 of patients sensitised to Cat, Dog and Horse. Nevertheless, more than 50% of specific IgE that cross-reacts among these 3 animals is directed to allergens other than albumin (14). Similar findings were documented in a second study (15).

IgE-mediated reactions

Horse allergy occurs among people who regularly handle Horses, either professionally or for recreational purposes, resulting in the induction or exacerbation of asthma, allergic rhinitis, allergic conjunctivitis and occupational asthma (16-22). Horse allergy has also been reported to result in angioedema, respiratory distress, and poorly controlled asthma (23).

In a study reviewing children seen for allergy to Horses over a period of 11 years (35 boys and 21 girls, 35 of them were under 10 years of age), the main clinical signs reported were ocular symptoms (36), asthma (30) and rhinopharyngitis (24). All the children had highly positive skin-specific IgE tests and 62% had specific IgE (class 3 and 4) and were polysensitised. In several children, the first symptoms occurred at the time of the first known contact with a Horse or Pony (24). Horse allergy has been reported to decrease with age (25).

Eosinophilic granuloma of the lung with sawdust and Horse protein hypersensitivity has been reported (26).

Other reactions

Contact urticaria from Horse saliva has been reported (27).

A high proportion of workers on a pure-bred Horse farm showed a positive skin response to Saccharopolyspora rectivirgula (51.6%), or showed the presence of precipitins to Acinetobacter calcoaceticus (32.3%). No significant relationship could be found between the presence of symptoms and positive allergy reactions, and the possibility of Organic Dust Toxic Syndrome occurring in a high proportion of the workers was suggested (28).

Farmer's Lung is a rural disease that can be caused by inhalation of airborne Thermophilic actinomycetes. Farmer's Lung occurred in an 11-year-old girl briefly exposed to this mould at a riding school (29).