The unusual protein-rich sweat produced by horses may help keep them cool, but it can spell a lot of trouble for allergy sufferers.
Horses lather up when they sweat, and it’s all because of a special protein in the sweat appropriately named latherin.
This protein-rich sweat is an unsual adaptation to meet the challenges of keeping cool, with the latherin being a key component. It produces a significant reduction in water surface tension at low concentrations acting as a wetting agent to help evaporative cooling through the waterproof pelt of horses.
Latherin is found in the horse, zebra, onager and ass. It is similar to a family of proteins found previously only in the mouths and associated tissues of mammals.
Latherin has also been detected in the saliva of horses.
Researchers in Germany have delved into respiratory allergens from furred animals, paying quite some attention to latherin and the risk of environmental and occupational exposure for allergy sufferers.
Eva Zahradnik and Monika Raulf, from the Ruhr-Universität Bochum, looked at potential exposure to a range of allergens from furry animals.
The pair, in a review published in the journal Veterinary Sciences, said furry animals − whether kept as pets, farm livestock or laboratory animals − were important allergen sources.
Animal allergens ubiquitous
Animal allergens are commonplace, although their concentrations differed considerably, and can even be found in places where no animals lived.
The prevalence of sensitization to furred mammals appeared to be increasing worldwide, varying between different countries according to cultural differences, environmental factors, and rates of pet ownership.
Several population-based studies from different countries have shown that, over the past decades, the prevalence of sensitization has been increasing in both adults and children.
For example, a study in Sweden showed an increase in sensitization from 1994 to 2009 to cats (rising from 16% to 26%), dogs (13% to 25%), and horses (8% to 10%).
Mammalian allergens are present mainly in the skin, saliva and urine of animals. These allergens stick to fur and are efficiently dispersed into the environment as animals shed hair and dander, and secrete and excrete bodily fluids.
These allergens tend to be carried on small dust particles that remain airborne for long periods after disturbance. They attach to human clothing and hair and can be easily transferred to environments never previously occupied by the animal.
Once indoors, the allergens accumulate primarily on different textiles, such as carpets, upholstered furniture and mattresses where, most likely due to their high stability, they remain detectable for a long time.
Four protein families
All major allergens from mammals can be classified into four protein families: lipocalins, secretoglobins, kallikreins and − you guessed it − latherins.
An allergen is any substance that elicits an immunoglobulin E (IgE) antibody response. Officially, a major allergen is one that is recognized by IgE antibodies in more than 50% of patients allergic to the allergen source.
Zahradnik and Raulf noted that latherin, later named Equ c 4, was a major component of horse sweat. Its three-dimensional molecular structure and proposed method of action was recently reported in a study.
“To date, the IgE binding frequency to Equ c 4 in 77% of horse sensitized subjects has been only reported by one study,” they said. “To evaluate this result, further investigations are needed.”
So, what are the chances of exposure in various environments?
Allergen measurements have been performed in several studies in horse stables and their immediate surroundings.
In general, outdoor horse allergen levels declined rapidly with increasing distance from the stable.
In one study, airborne levels of latherin were more than 500-fold higher inside the stable, at 439,000 units per cubic meter (U/m³) than at the stable entrance (1140 U/m³), and more than 3000-fold higher than at a residential building only 12 meters from the stable (150 U/m³).
Horse allergens could not be detected in air samples collected 40 meters from the stable in one study, a finding supported in a later study.
The authors said horse allergens generally spread in ambient air approximately 50 meters outside the stable and outdoor area where horses were kept, such as pastures or riding grounds.
“Depending on wind speed and direction, low levels of allergen (2–4 U/m³) were occasionally found in open areas between 300 and 500 meters from the stable.”
Horse allergen levels were also found to be influenced by season. In one study, median latherin levels measured at pastures in autumn (20 U/m³) were half those measured in summer (43 U/m³), and winter levels (9 U/m³) were a quarter of those measured in summer.
The median level at the entrance of the stable was 316 U/m³ in summer and 123 U/m³ in winter.
Possible reasons for the reduced spread of allergen could be the increased rainfall in autumn, and the covering of horses in winter.
Horse allergen was also detected in indoor (6 of 45) and outdoor samples (16 out of 26) that were collected using Petri dishes from homes located up to 250 meters from stables.
“To date, there are no studies on horse allergen levels in homes of occupationally-exposed persons or those exposed from leisure activities,” they noted.
The pair pointed to an investigation into the presence of horse allergens in Swedish schools. It revealed that allergen levels were higher in classrooms containing a higher proportion of children who had regular contact with horses in their leisure time.
“This result strongly suggests that transfer of allergens in animal-free environments takes place via contaminated clothing.
“A recent study from Malaysia provides another example of the variability of allergens in different parts of the world. Cat, dog and horse allergens were not detectable in schools, suggesting relatively low levels of these allergens in tropical countries as shown previously for educational facilities in Brazil and Singapore.”
A recent study from Sweden measured cat, dog and horse allergens in the cabin of a commercial aircraft.
Samples were collected from the cabins of aircraft operated by two European airlines, with nine airliners from each company targeted. One airline used textile seats and the other leather seats.
Levels of a specific dog allergen, Can f 1, were found to be 27 times higher in aircraft cabins fitted with textile seats than those fitted with leather. The cat allergen Fel d 1 was found be 50 times higher with the textile seats, and levels of Equ c 4 − that’s latherin from horses − were 75 times higher.
“Cabins are usually cleaned between flights at the airport; however, only the floors are cleaned, which enables allergens to accumulate in the seats, especially in the textile material.”
The authors said allergen avoidance, which is the first line of prevention against developing allergic symptoms, may be difficult for those who are sensitized.
“Allergen levels can be influenced by pets in and around the environment, the number of pet owners, geographic location, building characteristics, season, humidity and the presence of furniture and carpeting.
“The relationship between allergen exposure to animal allergens (especially cats and dogs) and IgE sensitization or the development of allergic symptoms has been investigated in a number of studies. So far, the results are inconsistent.
“While many studies found a positive correlation between allergen exposure and incidence of allergies, this association could not be confirmed in other studies. Some studies even showed that high exposure to allergens can have a protective effect against IgE sensitization and allergies.”
They continued: “A possible reason for these contradictory findings could be that nearly all studies examined the allergen exposure in one particular environment, mostly at home.
“Levels of animal allergens that additionally occur in other environments contribute substantially to the total exposure and should be further considered in epidemiological studies.”
Respiratory Allergens from Furred Mammals: Environmental and Occupational Exposure
Eva Zahradnik and Monika Raulf.
Veterinary Sciences 2017, 4(3), 38; doi:10.3390/vetsci4030038