Some individual horses appear more likely to trigger an allergic reaction in susceptible people simply because they release more allergens from their skin, German research findings suggest.
Some horses might be classified as either high or low allergen producers, the researchers concluded.
Allergies to horses are well recognised. In Germany, the prevalence of horse sensitization among adults had been put at 3.5%. In Finland it is 5.4% and in Sweden it has been put at 7.1%.
There is evidence the problem may be increasing.
A Swedish study reported an increase in sensitization to horses from 3% to 10% in 1699 children who were followed from the ages of 4 to 16.
Compounding the problem is the widespread nature of such allergens released in their dander – the tiny flakes shed from their skin. In general, allergens from furred mammals easily become airborne, stick to human clothes and are transported to animal-free public spaces. Horse allergens have been detected in schools, day-care centers and even aircraft.
Four respiratory horse allergens are currently registered in the official list of allergens.
The glycoprotein Equ c 1 is generally accepted as the major allergen. It is a highly predictive marker of an allergy to horses. Up to 76% of patients who are allergic to horses react to Equ c 1.
Equ c 2 is another major allergen, with a sensitization prevalence of 50%.
The serum albumin of horse, Equ c 3, is a minor allergen with sensitization rates of 18-30%.
Another allergen is Equ c 4 — latherin — which is a major component of horse sweat. It is the most likely cause of the frothing seen on sweating horses.
No detection systems are available for the other known horse allergens.
Ruhr-University Bochum researcher Eva Zahradnik and her colleagues, in a study published this week in the open-access journal PLOS ONE, have described their research into the allergens shed from the skin of horses.
Their interest was pricked by the American breed, the Bashkir Curly Horse, which is categorized as hypoallergenic, primarily due to reports of allergic patients experiencing fewer symptoms while handling this breed with its special curly coat. Indeed, the breed has been claimed as the only available hypoallergenic horse breed.
Possible reasons could be lower allergen production and/or reduced allergen release into the air because of increased sebum content in their skin and hair compared to other breeds.
The researchers noted that some recent studies had shown that contact with these horses was possible without significant allergic reactions, based on lung function measurements in allergic patients.
To learn more, the study team set out to compare different horse breeds in relation to allergen content in hair and airborne dust samples.
In total, 224 hair samples from 32 different horse breeds were investigated.
Testing was conducted for the major allergens, including the use of some new tests.
The researchers found there was up to a four-fold difference in antigen and allergen levels between individual animals.
Despite the enormous variability found, levels of the three parameters for which they tested hair — horse dander antigens, Equ c 1 and Equ c 4 — were significantly related to the breed and gender, combined with the castration status of male animals.
Paradoxically, Curly horses had much higher concentrations of all three tested parameters compared to the majority of the investigated breeds.
Tinker horses, Icelandic horses and Shetland ponies were associated with seven-fold reduced levels of horse dander antigen and Equ c 1, and up to 25-fold reduced levels of Equ c 4 compared to Curly horses.
Compared to mares, stallions displayed increased concentrations of horse dander antigens (more than two-fold greater), Equ c 1 (a 3.5-fold increase), and Equ c 4 (a 6.7-fold increase).
No difference was observed between mares and geldings.
In another part of the study, personal nasal filters were used to collect airborne dust during the grooming of 20 Curly horses and 20 Quarter horses. No differences in airborne allergen concentrations were found between the two breeds.
“Breed and castration status had a significant influence on the antigen and allergen levels of horse hair,” the study team concluded.
“However, these differences were smaller than the wide variability observed among individual horses.
“Compared to other breeds, Curly Horses were not associated with lower allergen levels in hair and in air samples collected during grooming.
“Our approach provides no molecular explanation why Curly horses are considered to be hypoallergenic.”
Overall, an enormous variability in antigen and allergen levels was observed between individuals of all breeds, they reported. The horse dander antigen content differed between highest and lowest by about 500-fold, the Equ c 1 content by about 2000-fold, and the Equ c 4 content by about 10,000-fold.
Discussing their findings, the study team said their results suggest that allergen production is strongly influenced by individual factors, and some animals might be classified as either high or low allergen producers.
The repeated collection and investigation of hair samples from some of the horses in the study confirmed this hypothesis.
The results, they said, correlated well and significantly between the two samplings, especially for the single allergens Equ c 1 and Equ c 4. The differences were small (on average 2.5-fold) relative to the variability seen within each breed or among all individuals.
“Despite the high variability among individuals within and among the different breeds, this study showed for the first time that antigen and allergen levels in horse hair are significantly related to breed.
“In our study, the horse antigen and allergen content in hair was also significantly related to the gender combined with the castration status of male animals.
“Whereas mares and geldings did not differ significantly, stallions displayed significantly higher concentrations of all measured parameters.”
Turning to Curly horses, the study team noted that, in the grooming experiment, measurements of airborne concentrations of horse dander antigen, Equ c 1 and Equ c 4, failed to demonstrate a lower release of allergens during the brushing of Curly horses.
All three tested parameters did not differ between Curly horses and Quarter horses, the latter of which are not considered hypoallergenic.
“Taking into account that Curly horses have on average three times higher allergen content in hair compared to Quarter horses, one might assume that the percentage of released allergens into the air is lower in Curly horses.
“Nevertheless, the level of inhaled allergens during grooming of Curly or Quarter horses remains approximately the same.”
They concluded: “These results provide no scientific evidence to suggest that Curly horses have hypoallergenic properties. The reasons why horse allergic patients have fewer or even no symptoms while handling this special breed remain unclear and need further investigation.”
The full study team comprised Eva Zahradnik, Bente Janssen-Weets, Ingrid Sander, Benjamin Kendzia, Caroline May and Monika Raulf, all affiliated with Ruhr-University Bochum; and Wolfgang Mitlehner, who is in private medical practice in Germany.
Zahradnik E, Janssen-Weets B, Sander I, Kendzia B, Mitlehner W, May C, et al. (2018) Lower allergen levels in hypoallergenic Curly Horses? A comparison among breeds by measurements of horse allergens in hair and air samples. PLoS ONE 13(12): e0207871. https://doi.org/10.1371/journal.pone.0207871