14-year-old, thoroughbred, gelding,
horse, (Equus caballus).The patient was admitted for evaluation of chronic colic, and had
a distended abdomen on arrival. The horse was diagnosed with uroperitoneum and
was euthanized after a tear in the ventral portion of the urinary bladder was
confirmed on abdominal ultrasound and cystoscopy. Clinical signs of disease
involving other organ systems (e.g. respiratory tract) were not
The patient was in satisfactory nutritional and good postmortem
condition. The peritoneum contained abundant turbid, light-yellow fluid with a
distinct ammonia odor (i.e. urine), and was hyperemic with adherent
plaques of fibrin. The urinary bladder was distended and a 7 x 5 cm thin,
gray-green and friable (necrotic) focus within the ventral wall was confirmed.
However, examination of the lungs revealed multifocal to coalescing 0.5- 5 cm
diameter firm tan nodules that extended into all fields, replacing the
pulmonary parenchyma and elevating the visceral pleura (figure 1). Intervening
pulmonary parenchyma was pale pink to dark red and mildly firm cranioventrally.
The brain, spinal cord, and remainder of the thoracic and abdominal viscera
were grossly unremarkable.
Approximately 80% of the pulmonary parenchyma was effaced by
multifocal to coalescing nodules composed of proliferating spindle cells
(fibroblasts) embedded within a fibrillar eosinophilic (collagenous) stroma,
and infiltrated by large numbers of lymphocytes and histiocytes, with fewer
neutrophils. Occasionally, alveolar macrophages contained 3-4μm diameter
eosinophilic intranuclear inclusion bodies that peripheralized the chromatin
(Cowdry Type A inclusions; figure 2 (arrow)). Within affected regions, the
majority of alveoli were lined by plump cuboidal epithelial cells
(bronchiolization). Affected alveoli, and occasional bronchi and bronchioles,
were filled with numerous foamy macrophages, neutrophils, and cell debris.
Within unaffected regions of lung, alveoli were often ectatic (hyperinflated),
and there was mild multifocal alveolar hemorrhage.
Lung: severe chronic
multifocalcoalescing sclerosing lymphohistiocytic and neutro-philic
bronchointerstitial pneumonia with intrahistiocytic intranuclear inclusion
bodies (consistent with equine multinodular pulmonary fibrosis).
Equine multinodular pulmonary fibrosis, EHV-5
Equine multinodular pulmonary fibrosis (EMPF), one of the
relatively few differentials for interstitial lung disease in the horse,
results in a unique gross and histologic appearance dominated by a nodular
pattern of marked interstitial fibrosis.10 Clinical signs of
affected horses may include tachypnea, dyspnea, nasal discharge, coughing,
lethargy, inappetence, and poor body condition. Affected horses may have
increased bronchovesicular sounds, fever, and/or hypoxemia on clinical exam.14
Hematology typically reveals a neutrophilia (in some cases with increased
bands), lymphopenia or lymphocytosis, and monocytosis.7,14 A nodular
interstitial pattern is often identified on thoracic radiographs and
ultrasound, resulting in differential diagnoses of EMPF, fungal pneumonia and
pulmonary neoplasia. Concurrent with the interstitial pulmonary disease,
thoracic ultrasonography may also reveal pulmonary artery dilation, consistent
with pulmonary hypertension. Transtracheal wash fluid is predominantly
neutrophilic (degenerate or non-degenerate), with fewer macrophages.14
No breed or sex predilection has been established, and although affected horses
are typically middle-aged or geriatric, cases have been reported in horses as
young as four years-old.12
Two gross patterns of EMPF have been described, both of which are characterized by multifocal moderately firm, tan, bulging nodules throughout the pulmonary parenchyma. In the more common of the two gross forms (the form exhibited by this horse), these nodules are numerous but small (up to 5 cm in diameter), and coalesce, often resulting in scant intervening parenchyma. In the second gross pattern, the nodules are less frequent, larger (up to 10 cm in diameter), and discrete, with the intervening pulmonary parenchyma largely unaffected.12 Histologically, both gross forms are characterized by extensive interstitial deposition of mature collagen, accompanied by moderate mixed inflammatory infiltrates, consisting pre-dominantly of lymphocytes, macrophages and neutrophils. Alveoli in affected regions are typically lined by plump, cuboidal epithelium, and airways contain abundant neutrophils and macrophages. Alveolar macrophages occasionally contain ampho-philic to eosinophilic intranuclear inclusion bodies. Less frequently, the nodules consist of dense sheets of disorganized collagen that completely efface the normal alveolar pattern.12
Equine herpesvirus-5 (EHV-5) is a gamma herpesvirus strongly associated with EMPF, and can be identified by polymerase chain reaction (PCR), immunohistochemistry (IHC), in situ hybridization, virus isolation, and transmission electron microscopy (TEM).2,7,12-14 Although causality has not been proven by meeting all of Kochs postulates, the main histologic and immunohistochemical features have been recapitulated in horses inoculated with EHV-5 isolated from EMPF-affected horses.13 However, because inoculated horses lacked clinical signs associated with EMPF, were PCR-negative for the inoculated EHV-5 viruses (within the lungs), and the virus was unable to be re-isolated, the authors suggest that the immuno-histochemical detection of virus in the inoculated horses having deposition of fibrous connective tissue likely represented a pre-clinical, yet latent, phase of viral infection.13 Further supporting EHV-5 as an important etiologic factor in the development of EMPF, quantitative real-time PCR performed in an affected horse revealed higher viral load within the lungs than in other tissues, with the highest load within the most severely affected/fibrotic regions of lung.4 In addition, gamma herpesviruses have been associated with fibrotic lung diseases in other species, including a variably fibrotic broncho-interstitial pneumonia with syncytial cell formation in donkeys associated with asinine herpesvirus-4 and 5 (AHV-4 and 5),3 and human idiopathic pulmonary fibrosis associated with human herpesvirus-8 (HHV-8) and Epstein-Barr virus.10,11 Moreover, mice that are knockouts for the IFNγ cytokine receptor (an important mediator of the Th1 antiviral response), develop progressive pulmonary interstitial fibrosis after chronic infection with murine gamma herpesvirus-68.5,11 Although the precise mechanisms of gamma herpesvirus-induced fibrosis and immune system evasion in most species are unknown, they are likely related to the specific viral-induced cytokine, chemokine, and/or receptor profile within the host, which either directs the immune system toward a Th2 response (thereby inhibiting a Th1 response and facilitating fibrosis), or at least prevents an effective Th1 antiviral response.6,11 Such mechanisms have already been established in the study of human gamma herpesviruses, including viral CCL-1 (vCCL-1), vCCL-2, and vCCL-3 encoded by HHV-8 that activate Th2 cell chemokine receptors (CCR8, CCR3 and CCR8, and CCR4, respectively), and a viral IL-10 homologue produced by human Epstein-Barr virus that results in inhibition of the Th1 antiviral response.6,11 In spite of supporting evidence of EHV-5 infection inciting EMPF, EHV-5 may not be the sole cause. As with many infectious diseases, concurrent viral infections (such as EHV-2 and AHV-4 or 5), and the hosts immune status, may also be important contributors to the pathogenesis of EMPF.1,5,11,12
Although successful treatment with corticosteroid and antiviral (acyclovir) therapy is reported, overall, response to treatment is variable and the prognosis of this disease is generally considered poor.14 This case was unusual in that there were no reported clinical signs that were clearly attributable to respiratory disease. However, a preclinical, latent phase of infection is considered unlikely given the presence of numerous inclusion bodies. Although the uroperitoneum accounted for the most recent episode of colic, it is possible that the pneumonia caused chronic lethargy and inappetence, vague signs that were interpreted as chronic colic. Regional trans-mural necrosis of the urinary bladder was confirmed histologically, and was attributed to pressure-induced ischemia secondary to distension. As there was no evidence of urethral obstruction, a neurogenic cause was suspected. However, no histologic lesions were identified within the spinal cord.
Lung: Fibrosis, interstitial, nodular, multifocal to coalescing,
severe with lymphohistiocytic interstitial inflammation, alveolar neutrophilic
and histiocytic exudate, type II pneumocyte hyperplasia and histiocytic intranuclear
viral inclusion bodies, thoroughbred, Equus caballus.
thank the contributor for both an excellent example and thorough summary of
equine multinodular pulmonary fibrosis (EMPF) in horses. This entity has also
been extensively reviewed in previous Wednesday Slide Conferences (2011 Conference
1 Case 2
24 Case 3
18 Case 1
, but it was chosen again because of its highly distinctive and
unique histomorphology. Participants identified multifocal discrete lung
nodules with abundant interstitial fibrosis, marked type II pneumocyte
hyperplasia, and irregular alveolus-like spaces filled with an inflammatory
exudate composed of neutro-phils, fibrin, and alveolar macro-phages which
occasionally contain a 2-4 um magenta intranuclear inclusion body. Conference participants also noted especially prominent pleural
arteries in areas adjacent to the nodules of fibrosis with hypertrophic smooth
muscle in the tunica media indicative of pulmonary hypertension associated with
As mentioned by the contributor, gamma herpesviruses have been associated with progressive pulmonary fibrotic disorders in humans, donkeys, horses, and rodents. In dogs, canine idiopathic pulmonary fibrosis is a progressive pulmonary fibrotic disorder predominantly in aged West Highland white terriers (WHWT).9 Recently, investigators have tried to elucidate a relation between this disorder in WHWTs and gamma-herpesvirus infection; however, no evidence a connection was found. Given this conditions predilection for WHWT, it is thought that there is a genetic component to this disease in this breed of dog rather than an infectious etiology.9
In addition to pulmonary fibrosis, conference participants discussed the association of a gamma herpesvirus with retroperitoneal fibromatosis (RF), an aggressive proliferation of highly vascular fibrous tissue subjacent to the peritoneum involving the ileocecal junction and mesenteric lymph nodes in rhesus macaques.10 RF is associated with co-infection of simian retrovirus 2 (SIV) causing simian acquired immunodeficiency syndrome (SAIDS) and RF-associated herpesvirus (RFHV). This condition is closely related to Kaposis sarcoma in humans, caused by co-infection of human herpesvirus 8 (HHV8) and human immuno-deficiency virus (HIV), and is one of the first illnesses associated with the development of AIDS.10 Additionally, rhesus macaque rhadinovirus, another gammaherpesvirus, is also closely related to HHV8 and both have been associated with the development of B-cell lymphoma.8
1. Back H, Kendall A, Grandón R, et al. Equine multinodular pulmonary
fibrosis in association with asinine herpesvirus type 5 and equine herpesvirus
type 5: a case report. Acta Vet Scand. 2012;54(57):1-5.
2. Caswell JL, Williams KJ: Equine multinodular pulmonary fibrosis.
In: Maxie MG ed. Jubb, Kennedy, and Palmers pathology of domestic animals. Vol
2. 6th ed. St. Louis, Missouri: Elsevier; 2016:568-569.
3. Kleiboeker SB, Schommer SK, Johnson PJ, et al. Association of two
newly recognized herpesviruses with interstitial pneumonia in donkeys (Equus
asinus). J Vet Diagn Invest. 2002;14:273-280.
4. Marenzoni ML, Passamonti F, Lepri E, et al. Quantification of Equid
herpesvirus 5 DNA in clinical and necropsy specimens collected from a horse
with equine multinodular pulmonary fibrosis. J Vet Diagn Invest. 2011;23(4):802-806.
5. Mora AL, Woods CR, Garcia A, et al. Lung infection with
γ-herpesvirus induces progressive pulmonary fibrosis in Th2-biased mice. Am
J Physiol Lung Cell Mol Physiol. 2005;289:L711-L721.
6. Nicholas J. Human gamma-herpesvirus cytokines and chemokine
receptors. J Interf Cytok Res. 2005;25:373-383.
7. Niedermaier G, Poth T, Gehlen H. Clinical aspects of multinodular
pulmonary fibrosis in two warmblood horses. Vet Rec. 2010;166:426-430.
8. Orzechowska BU, Powers MF, et al. Rhesus macaque
rhadinovirus-associated non-Hodgkin lymphoma: Animal model for KSHV associated
malignancies. Blood. 2008; 112:4227-4234.
9. Roels E, Dourcy M, et al. No evidence of herpesvirus infection in
West Highland white terriers with canine idiopathic pulmonary fibrosis. Vet
Pathol. 2016; 53(6):1210-1212.
TM, Strand KB, et al. Identification of two homologs of the Kaposis
sarcoma-associated herpesvirus (human herpesvirus 8) in retroperitoneal
fibromatosis in different macaque species. J Virol. 1997; 71:4138-4144.
Y, Johnson JE, Browning PJ, et al. Herpesvirus DNA is consistently detected in
lungs of patients with idiopathic pulmonary fibrosis. J Clin Microbiol.
KJ. Gammaherpesviruses and pulmonary fibrosis: evidence from humans, horses and
rodents. Vet Pathol. 2014;51(2):372-384.
KJ, Maes R, Del Piero F, et al. Equine multinodular pulmonary fibrosis: a newly
recognized herpesvirus-associated fibrotic lung disease. Vet Pathol. 2007;44:849-862.
KJ, Robinson NE, Lim A, et al. Experimental induction of pulmonary fibrosis in
horses with gammaherpesvirus Equine Herpesvirus 5. PLoS ONE. 2013;8(10):e77754,
DM, Belgrave RL, Williams KJ, et al. Multinodular pulmonary fibrosis in five
horses. J Am Vet Med Assoc. 2008;232(6):898-905.