Equis ISSN 2398-2977
Equine influenza
Contributor(s): Sarah Binns, Christopher Brown, Melissa Kennedy, Ruth Morgan, Carla Sommardahl, David Rendle
Introduction
- Epizootic disease of the equine upper and lower respiratory tract.
- Cause: equine influenza virus A (H3N8).
- Signs: harsh dry cough, pyrexia, lymphadenopathy, dull demeanor, nasal discharge, inappetence. In vaccinated horses a mild cough and mild serous discharge may be the only signs.
- Diagnosis: PCR from nasopharyngeal swabs in acute cases, rising antibody titers on serology.
- Treatment: supportive - fluid therapy, non-steroidal anti-inflammatory drugs. Treatment of secondary bacterial infection with antimicrobials only where indicated. Antimicrobials are not indicated in uncomplicated cases.
- Prognosis: good for adult tracheobronchitis; fair to poor for foal pneumonia. Occasionally fatal if secondary bacterial pneumonia develops.
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Pathogenesis
Etiology
- Equine influenza A virus Equine influenza virus - an orthomyxovirus of the H3N8 subtype.
- The H7N7 which was recognized in the 1960s is now considered extinct.
- There are two main lineages of H3N8 viruses: American and Eurasian - the Eurasian is no longer identified and inclusion in vaccines is no longer recommended.
- The American lineage diverged into Kentucky, South American and Florida lineages in the 1990s. In the 2000s the Florida lineage was identified as two Clades. Clade 2 had been predominant in the Americas and Clade 1 in Europe, however in 2019 a European epidemic was caused by incursion of Clade 2, and Clade 2 now appears to predominate globally.
- Host-species specific: infects horses, donkeys and mules only.
Predisposing factors
General
- Mixing of large numbers of equids, particularly younger animals.
- Stress, eg transport.
- Compromised local immunity, eg young foals.
Specific
- Failure to vaccinate at the recommended frequency with a vaccine that is effective against circulating strains.
Pathophysiology
- Aerosol spread of the organism → tropism for ciliated respiratory epithelium → tracheobronchitis or primary viral pneumonia.
- Aerosol infection → droplets penetrate into respiratory tract according to their size → tropism of virus for ciliated respiratory epithelium → epithelium sloughs → edema and lymphoid and impaired mucous clearance, infiltration of mucosa → tracheobronchitis in adults or primary viral pneumonia in foals.
- Incomplete separation of lobes of equine lung → spread of infection by direct extension.
- May be complicated by secondary bacterial infection, eg streptococci Streptococcus spp.
- Some cases → virus penetrates respiratory tract basement membrane → viremia → myocarditis or hepatic damage → limb edema.
Timecourse
- Incubation period: 1-3 days.
- Virus shed in nasal discharge and droplets for 7-10 days.
- Seroconversion within 10-14 days of infection.
- Cough lasts 1-3 weeks.
- Recovery of uncomplicated infection in 1-2 weeks following onset of clinical signs.
Epidemiology
- Virus cannot survive outside host.
- Replicates in respiratory epithelium of horses, donkeys and hybrids.
- Transmitted via nasal discharges and droplets.
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
- Durham A (2019) Choosing an equine influenza vaccine. In Pract 41 (2), 84-87 BMJ.
- Paillot R, Pitel P-H, Pronost S, Legrand L et al (2019) Florida clade 1 equine influenza virus in France. Vet Rec 184 (3), 101-101 BMJ.
- Rendle D, Ivens P, Bowen M, Lawrence H et al (2019) Equine influenza: A current reference for vets in practice in the UK. UK-Vet Equine 3 (Suppl 5), 1-13 MAGOnlineLibrary.
- Dilai M, Piro M, Harrak M E, Fougerolle S et al (2018) Impact of mixed equine influenza vaccination on correlate of protection in horses. Nato Adv Sci Inst Se 6 (4), 71 PubMed.
- Paillot R, Garrett D, Lopez-Alvarez M R, Birand I et al (2018) The immunity gap challenge: Protection against a recent Florida Clade 2 equine influenza strain. Nato Adv Sci Inst Se 6 (3), 38 PubMed.
- Fougerolle S, Legrand L, Garrett D, Birand I et al (2016) Influential factors inducing suboptimal humoral response to vector-based influenza immunisation in Thoroughbred foals. Vaccine 34 (33), 3787-3795 PubMed.
- Slater J, Borchers K, Chambers T, Cullinane A et al (2014) Report of the International Equine Influenza Roundtable Expert Meeting at Le Touquet, Normandy, February 2013. Equine Vet J 46, 645-650 WileyOnline (pdf download).
- Cullinane A & Newton J R (2013) Equine influenza-A global perspective. Vet Microbiol 167 (1-2), 205-214 PubMed.
- Gildea S, Arkins S, Walsh C & Cullinane A (2011) A comparison of antibody responses to commercial equine influenza vaccines following primary vaccination of Thoroughbred weanlings--a randomised blind study. Vaccine 29 (49), 9214-9223 PubMed.
- Daly J M, MacRae S, Newton J R, Wattrang E & Elton D M (2010) Equine influenza: A review of an unpredictable virus. Vet J 189 (1), 7-14 PubMed.
- Barquero N, Daly J M & Newton J R (2007) Risk factors for influenza infection in vaccinated racehorses: lessons from an outbreak in Newmarket, UK in 2003. Vaccine 25 (43), 7520-7529 PubMed.
- Newton J R, Daly J M, Spencer L & Mumford J A (2006) Description of the outbreak of equine influenza (H3N8) in the United Kingdom in 2003, during which recently vaccinated horses in Newmarket developed respiratory disease. Vet Rec 158 (6), 185-192 PubMed.
- Paillot R, Hannant D, Kydd J H & Daly J M (2006) Vaccination against equine influenza: quid novi? Vaccine 24 (19), 4047-4061 PubMed.
- Newton J, Texier M & Shepherd M (2005) Modifying likely protection from equine influenza vaccination by varying dosage intervals within the Jockey Club Rules of Racing. Equine Vet Educ 17 (6), 314-318 WileyOnline.
- Daly J M, Newton J R & Mumford J A (2004) Current perspectives on control of equine influenza. Vet Res 35 (4), 411-423 PubMed.
Other sources of information
- Paillot R, Marcillaud-Pitel C, D’Ablon X & Pronost S (2017) Equine Vaccines: How, When and Why? In: Report of the Vaccinology Session, French Equine Veterinarians Association 2016, Reims. p 46.
- OIE World Organization for Animal Health (online) Equine influenza. OIE Expert Surveillance Panel on Equine Influenza Vaccine Composition, OIE Headquarters, 16 April 2020. Conclusions and Recommendations. Website: www.oie.int.