Page 1062 - Veterinary Immunology, 10th Edition

intradermal inoculation of actinomycete and fungal extracts (such
VetBooks.ir as Rhizopus nigricans, Candida albicans, S. rectivirgula, Aspergillus

fumigatus, or Geotrichum deliquescens). Affected horses may respond
to aerosol challenge with extracts of these organisms by developing

respiratory distress. Clinical signs may resolve on removal of the
moldy hay and reappear on re-exposure. There is no evidence of
IgE involvement in RAO and little correlation between skin test
results and severity of disease. There is, however, evidence for a

genetic predisposition perhaps involving the gene for the IL-4
receptor. The prevalence of RAO is increased three-fold when one
parent is affected and five-fold when both are affected.
Affected animals usually have large numbers of neutrophils or

eosinophils in their small bronchioles and high titers of antibodies
to equine influenza in their bronchial secretions. The significance of
the latter is unclear. It has been suggested that continuous
prolonged activation of bronchoalveolar epithelial cells by dust

particles and air-borne endotoxins leads to excessive production of
neutrophil-attracting chemokines. These neutrophils then cause
damage by producing proteases, peroxidases, and oxidants.
Removal of clinically affected horses to air-conditioned stalls results

in improvement of the disease, but this is reversed if the horses are
returned to dusty stables. In some cases, RAO may persist even
when horses are moved to low dust environments, probably as a
result of airway remodeling.

Inflammatory airway disease.

IAD is a milder disease in which no labored breathing is seen at
rest. It has minimal clinical signs such as a nasal discharge, cough,
and decreased performance. It affects up to 30% of young horses (<5

years old) in training. Although commonly linked to bacterial or
viral infections, in many cases no infectious agent can be isolated.
Horses with IAD show poor performance, exercise intolerance, and
coughing. Excessive airway mucus is apparent. There is evidence of
inflammation detected by cytological evaluation of bronchiolar

lavage fluid. Cytokine transcripts show elevated IL-1β, -5, -6, -8, -10,
-17, and -23, as well as TNF-α and IFN-γ in affected horses
compared to controls. However, these differences are influenced by

the presence of mast cells, eosinophils, or neutrophils. Thus in

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