Clinical review

Horse allergy in children

Graham Roberts, research fellow, Gideon Lack, consultant. 

Paediatric Allergy and Clinical Immunology, Department of Paediatrics, Imperial College School of Medicine at St Mary's, London W2 1NY

Correspondence to: G Lack gideon.lack{at}sm.stmarys-tr.nthanes.nhs.uk

Allergy to pets, such as cats and dogs, causes significant morbidity in children.¹ Apart from immediate symptoms of hypersensitivity, allergens from pets are associated with chronic childhood asthma. ^{1 2} Clinicians are less familiar with horse allergy. Our clinical experience suggests that it is an important problem, even in an urban environment. Despite a lack of obvious exposure to horse dander this allergen may be responsible for a child's asthma. We present three of the 28 cases of horse allergy seen recently in our clinic.

View larger version (15K): [in this window] [in a new window]   Peak expiratory flow in nine year old boy exposed to horse dander from his sister's riding gear. Lung function deteriorated each weekend apart from one his sister spent away. Once his sister gave up riding, variability in peak flow disappeared, with no dips below 200/min

	Case reports

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Case 1Within minutes of riding a pony at a fête, an eight year old boy developed angio-oedema and respiratory distress. On initial assessment in the accident and emergency department, he was tachypnoeic with a saturation of 90% in air. He improved after treatment with oxygen, nebulised salbutamol, intravenous hydrocortisone, and intravenous chlorpheniramine. Skin prick testing produced a 6 mm weal to horse dander and a 3 mm weal to house dust mite (positive histamine control, 3 mm). He was advised to avoid horses, to carry chlorpheniramine in case of accidental exposure, and to use inhaled salbutamol and intramuscular adrenaline in the event of respiratory symptoms.

Case 2A nine year old boy was referred because of poorly controlled asthma despite receiving inhaled beclomethasone 400 µg/day. His asthma diary showed dips of more than 50% in his peak expiratory flow on Friday evenings (figure). These dips persisted all weekend and were associated with a persistent cough. One concern was that psychosocial circumstances at home might have accounted for his deterioration at weekends, but this was unfounded. His parents mentioned that he had once had a weekend free of symptoms when his sister had been away (figure). On Friday evenings his sister went riding, and she kept her riding gear in their bedroom. Further questioning showed that on direct exposure to horses he experienced rhinitis, conjunctivitis, and wheeze. Skin prick testing with horse dander produced an 8 mm weal (positive histamine control, 3 mm). His asthma was controlled once his sister stopped riding. He no longer requires inhaled steroids.

Case 3A five year old boy was referred with asthma. For two successive summers he had required treatment in the accident and emergency department for wheeze and sneezing, which developed while walking through fields. On the first occasion he had been eating an ice cream with nuts. The episodes were thought to be due to either nut allergy or hay fever. Skin prick testing, however, produced no reaction to nuts and a small weal (2 mm) to grass pollen (positive control, 4 mm). Closer questioning showed that on both occasions he had come into close contact with a horse. Skin prick testing to horse dander produced a 12 mm weal. He was advised to avoid horses and prescribed chlorpheniramine and a salbutamol inhaler for use if accidentally exposed.

	Discussion

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Asthma is an allergic disease of the airways, and allergens have a key role in the aetiology of the disease as well as being important triggers of exacerbations.³ Household pets are well known to cause allergic asthma. A history of past pet ownership, especially cats, has been found to be related to an increased rate of pet allergy and asthma.¹ Symptoms of asthma and increased bronchial hyperresponsiveness in childhood are significantly associated with skin sensitivity to cat dander.⁴ Cat dander has also been shown to induce late phase bronchial responses, which are thought to be important in the pathophysiology of chronic allergic asthma. ^{5 6} Much less is known about horse allergy. One notable textbook on allergy states that "Allergy to horse is now less of a problem in urban North America than in the early years of this century. . . ."⁷ Given that today we do not rely as much on horses as we did early in the last century, we would expect horse allergy to be a rare problem in urban environments, as allergic sensitisation requires chronic or regular exposure to allergen.⁸ Our experience, however, suggests that horse dander is still a major allergen in childhood despite a lack of regular exposure.

Our cases show how horse dander can play a major part in childhood asthma within an urban environment. Horse dander may represent a "hidden" allergen because exposure either is indirect or does not immediately precede the development of symptoms. A detailed clinical history is therefore essential for determining the causal allergen. Peak flow and symptom diaries may be useful in establishing patterns of disease, which can then be related to indirect exposure to possible allergens (case 2). Horse allergen seems to be as tenacious as cat allergen, enabling it to be transferred on clothing.⁹ Such carriage probably explains the presence of horse allergen in domestic dust samples from urban environments.¹⁰ Determining the cause of case 2's symptoms was difficult because the patient presented with late phase asthmatic symptoms that occurred many hours after exposure to the allergen.⁵ In clinical practice, practitioners may be misled into excluding allergy as a cause of symptoms if they do not immediately follow exposure to an allergen. Case 2 shows how late phase, chronic symptoms may predominate over acute symptoms where exposure to allergen is chronic or regular.

Skin prick testing is a simple but useful investigation, which often clarifies the cause of an allergic problem. In case 3, three potential allergens (nuts, grass pollen, and horse dander) were implicated, and skin prick testing allowed recognition of the clinically important allergen. Skin prick testing produces rapid results (15 minutes), is safe and inexpensive, and has a good sensitivity and reasonable specificity.¹¹ In the context of a detailed allergy history, it is a valuable diagnostic tool.

Avoidance of allergens is critical in the management of clinical allergy and may be sufficient to control symptoms.¹² Although it is simple to avoid direct exposure to horses, indirect exposure can be more difficult (case 2). Appropriate rescue treatment for accidental exposures comprises an antihistamine and inhaled -agonist for bronchospasm (case 1). Where past exposure has resulted in life threatening symptoms, children should also have access to a preloaded adrenaline device (for example, EpiPen, Meridian Medical Technology, St Louis, USA) and should be educated about when and how to use it. For some children, exposure is unavoidable. In our experience, these children usually benefit from a combination of prophylactic treatments: long acting antihistamine and inhaled steroids started one day before exposure; and inhaled -agonists used as needed.

Horse allergy is an important paediatric problem even in a young urban population. It causes a wide range of clinical symptoms from urticaria to respiratory distress, with both early and late phase respiratory symptoms. Carriage on clothing allows the aeroallergen access to the indoor environment, and therefore avoidance is difficult. Horse allergens that are "hidden" within the indoor enviroment can result in allergic sensitisation, bronchial inflammation, and chronic asthma. Successful avoidance may dramatically improve the control of childhood asthma and reduce the need for steroids.

	Acknowledgments

   Contributors: GR and GL participated in writing the paper. GL will act guarantor for the paper.

	Footnotes

Funding: GR is supported by a research fellowship from the special trustees at St Mary's Hospital, London.

Competing interests: None declared.

	References

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(Accepted 13 December 1999)