Clinical Observations of Equine Influenza in a Naïve Foal Population
ACVIM 2008
J.E. Axon; C.M. Russell; G.A. Tyner; M.E. Burbury; J.B. Carrick
Scone Veterinary Hospital (SVH)
Scone, NSW, Australia

Equine Influenza (EI) was detected on the first Thoroughbred stud in Scone on 2nd of September 2007. Patients in the SVH intensive care unit (ICU) became infected and showed clinical signs 2 weeks later. During the ensuing 6 weeks 22 foals were admitted to the ICU with respiratory signs attributed to EI infection. There were estimated to be 4,000 foals and 1000 late pregnant mares in the region exposed to EI. Three syndromes were associated with EIV infection of foals which resulted in hospitalization and treatment. All dams of the foals had clinical signs of EI.

Acute respiratory distress: Seven foals (2-12 days of age) presented with severe dyspnea. Auscultation of the thorax revealed minimal breath sounds or wheezes and crackles over the entire thorax. Thoracic ultrasound revealed numerous comet-tail artifacts which coalesced to form sheets and long thin superficial areas of consolidation. Thoracic radiographs revealed changes typically associated with interstitial pneumonia. Foals had normal or slightly elevated WBC and fibrinogen concentration, and severe hypoxemia with hypo-, normo- or hypercapnia. Treatment included broad-spectrum antimicrobial and anti-inflammatory therapy, oseltamivir phosphate (Tamiflu), nebulization and INO2 insufflation. All foals that presented with ARDS died. Post-mortem examination confirmed bronchointerstitial pneumonia due to EIV.

Tachypnea without secondary bacterial infection: Fifteen foals (2-12 days) presented with an elevated respiratory rate. The foals were bright, nursing intermittently and had normal or slightly purple discolored mucous membranes. Auscultation of the chest revealed wheezes and crackles over the entire thorax. Thoracic ultrasound and radiographs, clinicopathologic results and treatment regimes were similar to the ARDS foals. All foals responded to treatment and survived.

Bronchopneumonia with secondary bacterial infection: Older foals (> 10 days of age) presented with an increased respiratory rate, fever and elevated WBC count and fibrinogen concentration. Wheezes and crackles were audible on auscultation over the entire thorax. Thoracic ultrasound revealed widespread comet-tail artifacts, large areas of consolidation and fluid bronchograms. Radiographs revealed mixed bronchointerstitial pattern and alveolar pattern. Transtracheal aspirates indicated septic suppurative inflammation and culture grew Streptococcus equi sp. zooepidemicus. Treatment included broad-spectrum antimicrobial therapy, oseltamivir phosphate and INO2 insufflation. All foals responded well to treatment and survived.

Infection of in-hospital patients: Critically-ill neonates hospitalized during the outbreak developed signs of EIV infection. The majority remained bright and continued to nurse, however the disease was more severe in 6 foals. Three foals died due to complications associated with EI.

Foals born subsequent to the outbreak did not show clinical signs of EI. This may be due to partial transfer of colostral immunity or the lack of in-contact infected horses. No foals discharged from hospital have returned with respiratory problems.

Originally presented at the Rossdale & Partners Foal Care Course (, Newmarket, January 2008.

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Jane Axon