CHAPTER 65 Equine Multinodular Pulmonary Fibrosis
Interstitial lung diseases (ILDs) in the horse are poorly understood and are often diagnostically and therapeutically problematic for the clinician. For decades a subset of ILD has been recognized in equine medicine that is characterized by substantial accumulations of collagen within the lung.
Fibrotic lung diseases can be clinically subdivided into nonprogressive and progressive fibrotic ILD. Nonprogressive fibrotic ILD results in relatively static collagen accumulation within the alveolar parenchyma and is assumed to represent the resolution of prior alveolar injury; examples of such diseases include postinflammatory scarring following pulmonary bacterial infections and alveolar fibrosis following toxic alveolar injury, as develops following ingestion of various toxic plants. Progressive fibrotic ILDs are not static but instead are characterized by continued collagen accumulation in the lung over time. These diseases are often clinically silent until lung disease is very advanced. Historically, in all species, the cause of progressive fibrotic ILD is often not known, and even causal associations have remained elusive.
Recently a clinically progressive nodular fibrotic ILD has been described in adult horses. Development of the disease has been associated with pulmonary infection with the equine gammaherpesvirus, equine herpesvirus-5 (EHV-5). The cause-and-effect relationship between the virus and disease remains to be elucidated, but there is a strong association between pulmonary EHV-5 infection and presence of the disease. Because of the characteristic nodular accumulations of collagen in the lungs of affected horses, the disease has been called equine multinodular pulmonary fibrosis (EMPF).
EMPF is considered a disease of adult horses; the mean age of onset in a study of 24 horses with the disease was 14.5 years. Males and females are equally affected, and the disease has been reported in multiple breeds. EMPF has a characteristic clinical presentation. Owners of the horses with EMPF report a combination of decreased appetite, weight loss, cough, tachypnea, and respiratory distress.
Clinical signs include persistent fever ranging from 101.5° to 104° F (38.6° – 40° C), high resting respiratory rate and effort, and bilaterally increased bronchovesicular sounds with wheezes on auscultation, especially during deep breathing. Additional clinical signs include bilateral nasal discharge, depressed appetite, cough, and weight loss.
The most prominent clinical diagnostic feature of this disease is its striking radiographic appearance (Figure 65-1). The radiographic lesion is a diffuse bronchointerstitial pattern with multiple coalescing circular nodules throughout the lung field. These nodules may be so numerous that radiographically it appears as if little normal lung remains. In some cases, radiographic studies obtained 1 to 3 weeks apart reveal profound progression of the disease. The radiographic appearance of EMPF may be similar to that of the lungs in horses with fungal pneumonia or neoplasia. Radiographic findings can be in stark contrast to the overall physical appearance of the horse, depending on the chronicity of the condition. Many horses are generally in good body condition, and owners report that they have been riding these horses with little noticeable change in exercise tolerance. However, it is not infrequent for horses to be in poor body condition at initial definitive diagnosis, particularly if treatment for other respiratory conditions—such as heaves or pleuropneumonia—has been undertaken prior to diagnosis. Ultrasound examination of the thorax reveals multiple circular hypoechoic masses involving both the periphery of the lung and the pulmonary parenchyma. The masses are bilateral and are distributed throughout the lung field. Pleural effusion is generally not present.
(Radiograph courtesy of Rodney L. Belgrave, DVM, Mid-Atlantic Equine Medical Center, Ringoes, NJ.)
The most consistent hematologic findings include neutrophilic leukocytosis and hyperfibrinogenemia. Lymphopenia, possibly reflecting acute viral infection, was reported in three of five horses with confirmed EMPF. Arterial hypoxemia, reflecting the degree of collagen deposition within the alveolar interstitium and loss of gas exchange surface area, is a feature of EMPF. Cytologic evaluation of bronchoalveolar lavage (BAL) and transtracheal wash samples reveals primarily neutrophilic and macrophagic inflammation, often admixed with abundant mucus; Curschmann spirals may be found rarely. Bacterial and fungal culture of both BAL and transtracheal wash fluid generally fails to yield growth of clinically significant pathogens, and routine virus isolation is generally unrewarding.
The gross pathologic changes associated with EMPF are recognizable and striking; gross lesions are restricted to the lower portion of respiratory tract and usually involve all lung lobes. There are two major patterns of nodular fibrosis in the lungs of EMPF horses. The most commonly reported form consists of nearly diffuse, coalescing nodules of fibrosis (Figure 65-2