Influenza

Outline

Definition
Pathophysiology
Systems Affected
Incidence/Prevalence
Geographic Distribution
Signalment
Signs
Causes
Risk Factors

BASICS

Definition

Infection of the respiratory tract by the EIV
Highly contagious
Most economically important contagious respiratory disease of the horse

Pathophysiology

Inhalation of aerosolized virus allows deposition of viral particles throughout respiratory tract
Virus enters epithelial cells lining respiratory tract, replicates, and is released into the airways
Infected cells undergo apoptosis
Rapid spread of virus causes desquamation and denudation of respiratory epithelial cells and clumping of cilia
Impaired mucociliary clearance results and may persist for 4 weeks post infection
Secondary bacterial infections may occur

Systems Affected

Respiratory
Musculoskeletal (rare)
Cardiac (rare)

Incidence/Prevalence

Outbreaks occur among large groups of susceptible horses, often young racehorses
Age and previous exposure (natural or vaccination) determine prevalence
Morbidity can approach 100% in young naive horses

Geographic Distribution

Endemic in North and South America, Europe, the Middle East, and Asia
First outbreak in Australia in 2007 (currently EIV free)
Not present in New Zealand or Iceland

Signalment

Equidae of all ages are susceptible
Young horses (1–3 years) are more commonly affected

Signs

1–2 day incubation period
Fever, anorexia, depression
Frequent dry cough
Mucoid nasal discharge. May become mucopurulent to purulent if secondary bacterial infection occurs
Limb edema
Conjunctivitis, epiphora
Submandibular lymphadenopathy
Muscle stiffness, soreness
Clinical signs typically less severe in vaccinated horses

Causes

Caused by an influenza A virus, family Orthomyxoviridae, genus Influenza A virus and Influenza B virus
Subtypes identified by surface antigens HA and NA
2 subtypes of the virus exist:
- A/equine/1 (H7N7)—has not been identified since 1979
- A/equine/2 (H3N8)—currently identified virus
Virus undergoes antigenic drift (minor changes of HA and NA surface proteins) and antigenic shift (major change)
2 antigenically distinct but related influenza A viruses currently co-circulate in America and Europe: Florida clade 1 and Florida clade 2

Risk Factors

Where virus has been reported
Age and immunity. Epidemics often occur when young, susceptible horses congregate
Spread throughout the world, possibly facilitated by international transport of horses
Donkeys are more severely affected

Diagnosis ⬆ ⬇

Outline

Differential Diagnosis
CBC/Biochemistry/Urinalysis
Other Laboratory Tests
Imaging
Pathologic Findings

DIAGNOSIS

Differential Diagnosis

Infections with EHV-1, EHV-4, EVA, and Streptococcus equi ssp. equi can look similar
Influenza typically spreads more rapidly
Cough is less commonly observed in animals infected with EHV, EVA, or S. equi
The lymphadenopathy seen in cases of S. equi infection is typically more marked

CBC/Biochemistry/Urinalysis

Early lymphopenia and eosinopenia followed by a monocytosis may be seen, although transient
Leukocytosis and hyperfibrinogenemia may occur if a secondary bacterial pneumonia develops
Increased creatine phosphokinase and aspartate aminotransferase have been rarely reported in horses which develop severe myopathies

Other Laboratory Tests

Diagnosis based on classic clinical signs but can be confirmed by many methods—often needed in vaccinated horses with mild signs
Virus isolation of nasopharyngeal swabs—infected, nonvaccinated horses may shed virus for up to 7–10 days. Take within 24–48 h of the development of clinical signs to maximize chances of isolating virus. Place in cool transport medium
Antigen capture ELISAs (nasopharyngeal swab) used in diagnosis of human influenza are validated in horses. Rapid turnaround time. Useful in an outbreak situation
RT-PCR of nasopharyngeal swabs—rapid, highly sensitive, specific, and widely available; first choice in outbreak control and screening programs for EI
Seroconversion—4-fold increase in antibody titer between acute and convalescent samples obtained 10–14 days apart is considered diagnostic. hemagglutinin inhibition, virus neutralization, and single radial hemolysis tests can be used

Imaging

Ultrasonographic or radiographic evaluation of the thorax can be used to diagnose pneumonia as a sequel.

Pathologic Findings

Foals more likely to die in an outbreak of naive horses than older animals
Pregnant mares may abort or deliver stillborn fetus
Tracheitis
Bronchointerstitial bacterial pneumonia

Treatment ⬆ ⬇

Outline

Appropriate Health Care
Nursing Care
Activity
Diet
Client Education

TREATMENT

Appropriate Health Care

Affected animals should be isolated to prevent spread
Most horses will recover from EI infection within 1–3 weeks

Nursing Care

House in well-ventilated stalls.

Activity

Stall rest or limited exercise until the respiratory epithelium has healed
Early return to exercise can delay recovery. May lead to serious long-term and life-threatening complications (myocarditis)

Diet

Feed palatable feeds with low-dust content.

Client Education

Animals new to a population or potentially exposed horses returning to a group should be isolated for at least several days (ideally 2 weeks). Monitor closely for development of a fever or nasal discharge
If an outbreak of influenza or other respiratory disease is suspected, a quarantine should be established. Arrivals and departures from the premise should be prevented
Owners should be aware that vaccination of horses does not prevent infection

Medications ⬆ ⬇

Outline

Drugs (S) of Choice
Contraindications
Precautions

MEDICATIONS

Drugs (S) of Choice

No antiviral drugs are marketed for the treatment of EI
Antiviral drugs such as amantadine and rimantadine have been investigated but antivirals are typically not used
Treatment is supportive—NSAIDs such as flunixin meglumine (1.1 mg/kg PO, IV every 12 h) or phenylbutazone (2.2–4.4 mg/kg IV, PO every 12 h) can be used to alleviate fever, depression, and muscle soreness
Secondary bacterial pneumonia should be treated with antimicrobials based on sensitivity pattern of organisms identified from transtracheal wash. Pending culture results, broad-spectrum antimicrobial combinations targeted at commonly isolated organisms (Streptococcus spp. and Actinobacillus spp.) can be used (such as penicillin, tetracycline, ceftiofur, and gentamicin; trimethoprim–sulfamethoxazole)

Contraindications

Corticosteroids are immunosuppressive. Recovery from influenza depends largely on an effective immune response. These drugs should not be used.

Precautions

NSAIDs should be used judiciously in horses that are inappetent and dehydrated owing to the potential development of gastrointestinal and renal toxicity.

Follow-up ⬆ ⬇

Outline

Patient Monitoring
Prevention/Avoidance
Possible Complications
Expected Course and Prognosis

FOLLOW-UP

Patient Monitoring

Monitoring of vital signs, appetite, and attitude will allow horses that develop secondary sequelae such as bacterial pneumonia to be identified.

Prevention/Avoidance

Biosecurity measures and vaccination are the basis for prevention and control
Hand-washing, minimizing horse-to-horse contact, and controlling fomites significantly reduce the risk of disease transmission
Several vaccine types are available—all provide short-lived immunity compared with up to 12 months following natural infection
Based on current OIE EI vaccine recommendations vaccines should include viral antigens of the currently circulating virus strains, including representatives of the Florida clade 1 and clade 2 lineages (both H3N8)
Vaccines available (country dependent) include:
- Whole inactivated virus vaccines, immuno-stimulating complex adjuvanted vaccines (ISCOM and ISCOM-Matrix), a live attenuated EIV vaccine, and a recombinant poxvirus-vectored vaccine
Vaccination can be used in the face of an outbreak—most useful in previously vaccinated animals
Foals
- Schedule will depend on type of vaccine. AAEP guidelines:
  - Inactivated—first dose at 6 months, second dose 3–4 weeks later, third dose at 10–12 months of age
  - Modified live vaccine (intranasal)—first dose at 6–7 months; second dose at 11–12 months of age
  - Canary pox vector vaccine—first dose at 6 months; second dose 5 weeks later
  - ISCOM vaccine—first dose at 6 months; second dose 4 weeks later; third dose 5 months after dose 2
- Broodmares—booster vaccination 4–6 weeks before foaling (do not use intranasal vaccine due to limited antibody production, limited transfer of passive immunity)
- Adult horses at low risk—yearly booster
Vaccination rules for competition horses vary depending on regulatory body governing the competition.
- FEI/AAEP:
  - Primary course—2 vaccinations not less than 21 days and not more than 92 days apart
  - Booster vaccination within 7 months of second vaccination
  - Boosters within 6 months + 21 days of previous vaccination
- UK Thoroughbred racing:
  - Primary course—2 vaccinations not less than 21 days and not more than 92 days apart
  - Booster not less than 150 days and not more than 215 days after second vaccination
  - Yearly booster

Possible Complications

Secondary bacterial infections, causing pneumonia and pleuropneumonia, are potential sequelae. The destruction of the mucociliary apparatus by the virus removes 1 of the major defenses of the respiratory tract
Auscultation of the thorax may reveal abnormal lung sounds such as wheezes and crackles in the case of pneumonia, or dull ventral sounds if pleural effusion develops
Myocarditis and immune-mediated myositis—rarely reported as a sequela to influenza infection
Persistent coughing has been anecdotally reported

Expected Course and Prognosis

Most recover within 1–3 weeks
Respiratory tract recovery can take longer, often lost to competition for >3 months
The prognosis is excellent if sufficient rest is provided
Secondary bacterial pneumonia or pleuropneumonia results in worse prognosis, but most will recover with aggressive treatment and prolonged rest

Miscellaneous ⬆ ⬇

Outline