Equine Rhinitis Virus Study Guide

Overview and Clinical Importance

Equine Rhinitis Viruses (ERV) are highly prevalent respiratory pathogens belonging to the family Picornaviridae. Two distinct species infect horses: Equine Rhinitis A Virus (ERAV) and Equine Rhinitis B Virus (ERBV). These viruses cause acute upper respiratory tract disease that is clinically indistinguishable from equine influenza virus (EIV) and equine herpesvirus (EHV) infections.

Despite seroprevalence rates of 20-90% in horse populations worldwide, ERV infections are often underdiagnosed due to limited availability of diagnostic tests and the predominant focus on EIV and EHV. ERV is now recognized as an emerging cause of respiratory disease outbreaks, particularly in young performance horses. ERV should NOT be confused with rhinopneumonitis (caused by EHV-1 and EHV-4).

Etiology and Classification

Viral Taxonomy

Equine rhinitis viruses are small, non-enveloped, positive-sense single-stranded RNA viruses. They were originally classified as equine rhinoviruses but have since been reclassified based on genomic analysis.

Epidemiology

Prevalence and Distribution

ERV infections occur worldwide with seroprevalence ranging from 20-90% depending on the horse population, age, and geographic location. Studies from the United States, United Kingdom, Canada, Australia, New Zealand, Germany, and Japan have confirmed global distribution.

Age of Primary Infection

• ERAV: Primary infection typically occurs between 3-9 months of age
• ERBV: Primary infection typically occurs between 4-6 months of age
• Most clinical exposure to ERAV occurs in late winter to early spring

Risk Factors

• Age: Young horses (less than 1 year) are most susceptible; yearlings show 87.9% clinical incidence
• Use: Competition and performance horses at higher risk due to transport and commingling
• Season: Outbreaks more common during colder months
• Environment: Multi-barn training facilities with high horse turnover

Transmission

ERAV transmission: Primarily through inhalation of respiratory aerosols and direct contact with nasal secretions. Uniquely, ERAV is also shed in urine for prolonged periods (greater than 37 days post-infection), making urinary contamination an important source of environmental spread.

ERBV transmission: Believed to occur via direct or indirect contact with nasal secretions or aerosols. ERBV has also been detected in fecal samples, suggesting potential fecal-oral transmission route.

Fomite transmission: Both viruses can survive for weeks on environmental surfaces (stall doors, buckets, tack, troughs), making indirect transmission via contaminated objects possible.

Pathogenesis

ERAV Pathogenesis

• Initial infection: Virus replicates in the upper respiratory tract epithelium (nasal turbinates)
• Viremia: Cell-associated viremia develops within days and typically persists for 4-5 days
• Dissemination: Virus spreads to distal sites including the urinary system
• Persistence: ERAV establishes persistent infection in the urinary tract with prolonged viral shedding in urine
• Resolution: Neutralizing antibodies develop 1-2 weeks post-infection, reducing nasopharyngeal shedding and viremia

ERBV Pathogenesis

The exact replication site of ERBV remains unknown. Given that acid-stable picornaviruses typically replicate in the gastrointestinal system, it is hypothesized that ERBV may vary in tropism depending on the serotype. ERBV may also cause immunomodulation, potentially increasing duration or severity of coinfections with other pathogens. Persistent infection has been documented for up to 2 years.

Clinical Signs and Presentation

Clinical signs of ERV infection are non-specific and indistinguishable from other common equine respiratory pathogens. The incubation period is typically 2-8 days. Many infections are subclinical or mild.

Common Clinical Signs

Disease Course

• Uncomplicated cases typically resolve within 7 days
• Persistent pharyngitis may cause coughing to continue for 2-3 weeks
• Mean clinical disease duration is 11 days (range: 1-40 days)
• Secondary bacterial infections may complicate and prolong recovery

Complications

• Secondary bacterial pneumonia or bronchopneumonia
• Equine asthma (inflammatory airway disease)
• Reduced athletic performance
• Temporary cell-mediated immune suppression (documented with ERAV)

Diagnosis

Clinical diagnosis of ERV is challenging because signs overlap with other respiratory pathogens. Laboratory confirmation is required for definitive diagnosis.

Diagnostic Methods

Sample Collection

• Nasal/nasopharyngeal swabs: Collect during febrile stage for best results
• Blood: For serology; collect acute sample at onset and convalescent sample 2-4 weeks later
• Urine: Particularly useful for ERAV due to prolonged shedding
• Transport: Use viral transport medium; maintain cold chain (4°C)

Differential Diagnosis

ERV infection must be differentiated from other causes of acute febrile respiratory disease:

Treatment

There is no specific antiviral therapy for ERV infection. Treatment is symptomatic and supportive.

Treatment Approach

Prognosis

• Excellent: Most horses recover fully with appropriate rest and supportive care
• Complications are more likely if inadequate rest is provided during recovery
• Rarely fatal except in severely compromised, stressed, or immunosuppressed horses

Prevention and Control

Vaccination

A conditionally licensed, adjuvanted, inactivated ERAV vaccine (Boehringer Ingelheim) has been available in the United States since 2012. No vaccine is currently available for ERBV.

Biosecurity Measures

• Quarantine: Isolate new horses and those returning from shows for 2-4 weeks
• Disinfection: Use disinfectants with proven efficacy against picornaviruses (viruses can survive weeks on surfaces)
• Equipment: Do not share tack, buckets, or grooming supplies between horses
• Hand hygiene: Wash hands between handling horses
• Isolation protocol: Handle sick horses last; use separate clothing/footwear
• Facility management: Ensure good ventilation; avoid overcrowding; separate age groups

Zoonotic Potential

There is evidence that ERAV can infect humans, both naturally and experimentally. In an experimental human infection, a volunteer developed severe pharyngitis and fever with virus isolated from blood. However:

• Natural human infections do not appear to cause clinical disease
• Human-to-human transmission has not been documented
• Seroprevalence studies in veterinarians show low neutralizing antibody rates (less than 4%)
• Risk of acquiring ERV as a zoonotic infection appears low

Coinfection Patterns

ERBV is commonly detected alongside other respiratory pathogens. In biosurveillance studies, approximately 34% of ERBV-positive cases had coinfection with at least one other pathogen:

• Streptococcus equi: 58% of coinfections
• EHV-4: 32% of coinfections
• EIV: 16.7% of coinfections

Coinfection may increase disease severity or prolong clinical signs through ERBV-mediated immunomodulation.