4 NOTES Equine Viral Disease

Equine Influenza

• Equine influenza is a contagious, economically significant respiratory illness in horses
• It is an orthomyxovirus (RNA virus) characterized by two envelope antigens: neuraminidase (NA) and hemagglutinin (HA)
• These antigens determine virulence, protective immunity, and viral classification
• The subtypes A/equine/2(H3N8) or A/equine/1(H7N7) are based on NA and HA antigenicity
• North American influenza strains are prevalent in both North America and Europe
• Eurasian strains are considered extinct in North America
• Antigenic shift is less common in horses and pigs compared to humans and avian influenza A viruses

Antigenic Shift vs Antigenic Drift

• Antigenic shift involves the sudden emergence of a new strain due to significant changes in HA
• These changes are unrelated to previously identified influenza strains and can result from recombination
• Antigenic shift cannot occur through mutation alone
• Antigenic drift describes a gradual alteration in HA and NA antigens because of random mutation
• Antigenic drift occurs in equine influenza and impacts vaccine effectiveness

Epidemiology of Equine Influenza

• Equine Influenza spreads very rapidly and has a short incubation period of 1-3 days
• Morbidity is high, but mortality is low
• Common predisposing factors include being a young horse between 1-5 years old
• Crowded conditions such as sale barns, shows, vet hospitals and racetracks are common
• Transport can be a contributing factor
• Transmission occurs directly from horse to horse via aerosol
• The virus can remain stable in aerosols for up to 35 yards
• Convalescent horses shed the virus for up to 10 days

Pathogenesis of Equine Influenza

• The virus uses HA spikes to attach to epithelial cells, causing necrosis and desquamation
• This attachment interrupts the protective mucociliary blanket and impairs clearance
• Respiratory epithelium regeneration takes around 3 weeks
• Impaired mucociliary blanket function raises the risk of secondary infections (Streptococcus zooepidemicus, Actinobacillus spp.)

Clinical Signs of Equine Influenza

• Common clinical signs include pyrexia of 104F, depression, anorexia, and lymphadenopathy
• Harsh, dry, explosive and non-productive cough
• Coughing can persist for months if return to work is premature - tracheal/laryngeal pressure usually incites a cough
• Initially serous nasal discharge that becomes mucoid to mucopurulent with secondary infections
• Potential sequelae include bacterial infection, weight loss, myositis, myocarditis, purpura, reactive airway disease, and inflammatory airway disease.

Treatment of Equine Influenza

• NSAIDs are used if fever is >104 F, if the horse is anorectic, stiff, or depressed, acting as an antiinflammatory, antipyretic and analgesic
• Nonjudicial use of NSAIDs will mask signs of bacterial infection
• Horses need a minimum of 3 weeks rest to allow for regeneration of the mucociliary apparatus
• Ventilation should be improved and dust free feed provided
• Antibiotics are recommended if bacterial infection is present or likely such as with mucopurulent nasal discharge, increased fibrinogen, neutrophilia, or immunosuppression

Diagnosis of Equine Influenza

• RT-PCR is the most sensitive and is considered 100% accurate with a nasopharyngeal swab
• Virus isolation using a chick embryo is impractical, requiring a nasal swab early in the disease, 24-48 hours
• The sample must reach the lab within 24 hours on ice in virus transport medium
• Results take 10-12 days and bacterial contamination may destroy the culture with only 10% sensitivity
• Serology (hemagglutination-inhibition) is impractical and requires a four-fold rise in paired samples obtained 10-21 days apart with results in over 3 weeks
• Horse-side tests using a human influenza A test are more sensitive than culture at 68% sensitivity
• Can produce rapid results with no concern of bacterial contamination

Equine Influenza Prevention

• Three vaccine types available include inactivated vaccines, modified-live (MLV) cold-adapted vaccines, and canarypox vector vaccines
• Inactivated vaccines protect against clinical disease and viral shedding
• These contain multiple strains of equine influenza virus A2, the major circulating strains, some still contain the now extinct A1 strain
• Suited for pre-foaling boosters to increase colostral antibody levels against influenza
• Modified live vaccines replicate only in the nasal passages because they are temperature sensitive
• They provide local protection at the nasal mucosa by closely approximating natural challenge
• Has been documented to achieve onset of protection in naive horses 7 days post vaccination making it ideal for use in the face of an outbreak
• Intranasal administration to naïve horses is protective for 12 months
• Licensed for single-dose vaccination in non-pregnant animals over 11 months, followed by boosters at 6-month intervals
• Horses shed vaccine-strain virus for less than 1 week after vaccination, and it is protective for foals over six months of age
• Not recommended for late-pregnant mares to boost colostral antibodies, as circulating antibody responses are low
• Canary pox vector vaccines use intramuscular injection to provide protection for at least six months
• A two-dose priming regimen, with boosters every six months is recommended or accelerated schedule with two weeks between doses
• Safe for foals as young as four months and shows some evidence of efficacy with maternal immunity.
• Suitable for pre-foaling booster
• Used in the Australian influenza outbreak because of DIVA capability and the accelerated booster schedule

Equine Herpesvirus (Rhinopneumonitis)

• Herpesviruses establish latent infections that recrudesce with stress
• Resistance to reinfection is short-term
• Clinical respiratory disease is associated with EHV-1, EHV-2, EHV-4 and EHV-5
• Alphaherpes viruses:
  • EHV-1: Causes respiratory issues, abortion, neonatal problems, and neurologic issues
  • EHV-4: Respiratory
• Gammaherpes viruses:
  • EHV-2: Performance-limiting respiratory inflammation in young athletic horses
  • EHV-5: Interstitial pneumonia and pulmonary fibrosis

Respiratory Disease from Equine Herpesvirus

• Respiratory disease is caused by EHV-4 in 70-90% of cases while EHV-1 is less common
• Transmitted through respiratory secretions on fomites and aerosol, with an incubation period of 2-10 days
• Virus shedding occurs for 2-3 weeks after recovery

Pathogenesis of Equine Herpesvirus

• The virus replicates in lymphoreticular tissue associated with the respiratory tree
• Secondary bacterial infection of the respiratory tract is more common with EHV-1 than influenza
• Viremia via cell-associated virus does occur

Clinical Signs of Equine Herpesvirus

• Cyclic fever spikes of 104 F, depression, anorexia, and lymphadenopathy are common signs
• A nasal discharge starts serous then becomes mucoid/mucopurulent
• Cough is rare and most common in weanling foals

Treatment of Equine Herpesvirus

• NSAIDs (antipyretic, antiinflammatory, analgesic) are used when fever is >104 F, and if the horse is anorectic, stiff, or depressed
• Nonjudicial use of NSAIDs will mask signs of bacterial infection
• A minimum of 3 weeks rest is required
• Improvements to ventilation and dust-free feed should be provided
• Antibiotics are indicated if evidence of secondary bacterial infection is present

Diagnosis of Equine Herpesvirus

• PCR of tissues, exudates, or blood is most reliable and efficient
• Virus isolation can be performed using a nasal swab or buffy coat of citrated blood sample
• A four-fold rise in antibody titer indicates recent infection
• Immunohistochemistry on tissues is also used for diagnosis

Prevention of Equine Herpesvirus

• Horses that are racing, training, or showing should be vaccinated every 6 months
• Inactive, isolated, sedentary horses should be vaccinated every 12 months
• Foals should be vaccinated starting at 4-6 months of age
• The aborted fetus and membranes are a primary source of the virus

Abortion (EHV-1)

• Late gestational abortion (7-11 months) requires viremia and occurs weeks to months after EHV-1 infection
• Abortion is due to fetal or placental infection with herpesvirus
• Gross fetal lesions include pulmonary edema, congestion, ascitic and pleural fluid accumulation, and small gray necrotic foci on the liver and icterus
• Histologic fetal lesions include focal hemorrhage and degeneration of liver and lung with acidophilic intranuclear inclusion bodies
• Abortions can occur sporadically or as abortion storms within a herd
• Subsequent fertility is not affected

Equine Herpesvirus Abortion Diagnosis and Prevention

• Diagnosis is PCR or virus isolation from the fetus and placenta (lung, liver, spleen, and thymus).
• It can be identified with histopathology for intranuclear inclusions and immunohistochemistry examination
• Prevention involves vaccination at 5, 7, and 9 months of gestation with a killed vaccine abortigenic strain of EHV-1
• Brand names include Pneumabort K® and Prodigy®
• Pregnant mares should not be commingled with young horses

Neonatal Equine Herpesvirus

• Neonatal disease (EHV-1) is highly fatal and results from viral infection shortly before or immediately at birth.
• It causes destruction of lymphoid tissue, lungs, bone marrow, and adrenal glands with secondary bacterial infections
• Septicemia, bacteremia, or pneumonia can be secondary infections

Neurologic Disease related to Equine Herpesvirus

• Current vaccines ARE NOT protective for the neurotropic strain of EHV-1
• Respiratory disease may or may not be present
• Sporadic cases or herd outbreaks may occur
• Viremia is 10-fold higher than with other strains of EHV-1
• Causes ataxia and paresis, urinary incontinence, and decreased tail tone
• Vasculitis of the spinal cord and meninges
• Diagnosis includes qPCR nasal swab, buffy coat, or spinal tap, which will be xanthochromic with high protein and normal cell count
• Vaccination may reduce shedding

Equine Herpesvirus Control

• Valacyclovir may be valuable if administered prior to the first fever
• Quarantine should last a minimum of 10 days, but 28 days is recommended
• Diagnosis is difficult because of high antibody presence to EHV-1 and EHV-4,
• Abortion and neurologic disease occur after viremic phase.
• Four-fold rise in antibody titer occurred prior to clinical signs
• Control is difficult because latent infections are common in adult horses
• Infections are often inapparent with viral shedding in immunologically experienced horses

Inflammatory Airway Disease & Pulmonary Fibrosis/Interstitial Pneumonia

• Inflammatory Airway Disease (EHV-2): Mild to moderate airway inflammation limits race performance in young horses and may cause pharyngitis.
• Pulmonary Fibrosis/Interstitial Pneumonia (EHV-5): In some instances, immunosuppression and bone marrow aplasia. EHV-5 is found in some equine lymphoma samples. Often co-infected with EHV-2
• Diagnosis is via PCR on BAL sample.

Rhinovirus & Streptococcus

• Equine Rhinovirus A and B are unlikely to be important pathogens in horses and have a conditional license vaccine
• Streptococcus zooepidemicus is the most common bacterial pathogen in the equine lung
• It is a Gram-positive, beta hemolytic, cocci, susceptible to penicillin and resistant to gentamicin
• It is not a primary pathogen, and occurs secondary to impaired pulmonary defense
• Causes: viral respiratory disease, transport stress, ammonia fumes (paralysis mucociliary clearance), allergic airway disease, and aspiration.

Equine Viral Arteritis

• EVA is spread by Equine Arteritis Virus (EAV), which is an RNA togavirus.
• Major consequences are abortion in mares and establishment of a carrier state in semen of actively breeding stallions.
• An outbreak spread by AI and long-distance transport of infected mares came from New Mexico in June, 2006

Transmission of Equine Viral Arteritis

• It is transmitted through aerosolization of respiratory secretions and fomites with an incubation period of 7-19 days
• Venereal transmission occurs via stallions that develop chronic infection in the ampulla of vas deferens
• Carrier stallions are always seropositive
• Immature intact males do not develop carrier state
• Semen (fresh and frozen) is infectious, but respiratory secretions are not
• Semen quality is unaffected by EAV infection with a massive viral load
• Mares bred to affected stallions develop respiratory disease (85-100%) but do not develop long-term EAV infections
• Standardbreds have a high incidence of seropositivity and carrier infections

Pathogenesis of Equine Viral Arteritis

• Replicates in bronchial macrophages, then enters bronchial lymph nodes; viremia develops 72 hours after aerosol challenge
• Directly damages the tunica media of small arteries and venules, leading to thrombus formation and ischemic necrosis

Clinical Signs of Equine Viral Arteritis

• Most cases are mild or subclinical and mortality is rare; signs are most severe in very young and old horses
• The animal may experience fever of 104 - 106 F, anorexia, and depression
• Abortion can occur at any point in gestation late in clinical disease or the early convalescent period
• Fetal death is due to viral infection, especially pulmonary damage, and autolysis
• Abortion rate varies from < 10% to as high as 50 to 80%
• Other signs include conjunctivitis, lacrimation, palpebral and periorbital edema with a serous nasal discharge and cough, in addition to preputial, scrotal, and ventral edema, as well as limb edema

Equine Viral Arteritis Diagnosis and Treatment

• Clinical signs persist for 2 - 9 days
• Diagnosis includes laboratory findings of profound lymphopenia, and serology (Ames: Regulatory Purposes)
• Other tests you can use: virus isolation and PCR from respiratory secretions and semen; seroconversion of 2 mares 28 days after breeding to suspect stallion.
• Treatment includes supportive care, support bandages and NSAIDs for fever and inflammation

Equine Viral Arteritis Vaccination

• Highly-attenuated Bucyrus-strain, developed in 1985: is safe and effective and prevents a carrier state in stallions
• Can be serologically distinguished from natural disease
• Prior to breeding season, seronegative stallions need written certification of seronegativity, 30-60d prior to breeding season
• Isolate from nonvaccinates for 21d

Equine Viral Arteritis Vaccination Stallions & Mares

• Seropositive Stallions: must conduct a VI of semen, Serology from 2 seronegative mares (28 d)
• Stallions with a virus negative must be vaccinated annually
• Teasers should be non-shedding and vaccinated
• Colts less than 270 days old need a serology, written certification, vaccination, and isolation
• Vaccination for mares is not labeled for pregnant mares - if bred to a shedding stallion, semen virus isolation is needed
• It is recommended to vaccinate and isolate 21 days prior to breeding (1st yr), isolate for 21 days after breeding (1st yr) then isolate for 24 hrs while breeding (2nd + yrs)
• Do not vaccinate mare until foal is 2+ weeks old
• No need to test or vaccinate mares bred to vaccinated stallions

Importation of Equine Viral Arteritis

• Outbreaks have been linked to the importation of carrier stallions and infective semen
• Greater control is achievable by identifying all imported carrier stallions and determining the EAV status of imported semen

African Horse Sickness

• AHS is a foreign disease that has never been reported in the U.S.; severity depends on the horse's immune status
• It is caused by a virus in the family Reoviridae, (genus Orbivirus)
• The vector for transmission is Culicoides spp and is not contagious by direct contact
• Can be found in portions of Africa
• All equidae are susceptible, but horses are the most susceptible, followed by mules, donkeys, and zebras
• Zebras are a reservoir, with low level viremia in horses
• Epidemics occur in the Middle East, Spain, Portugal, and Morocco.
• Mortality is high among naïve populations

Forms and Diagnosis of African Horse Sickness

• Peracute mortality is 95% and includes fever, congestion of mucous membranes of eyes, nose and mouth; blood-tinged pulmonary edema, hydrothorax, and subpleural edema.
• Acute mortality is 50 - 95% and involves both pulmonary signs like cough, pulmonary edema, respiratory distress, and cardiac signs like edema of head and neck
• Subacute mortality is 50% and includes a fever, edema of head, neck, eyelids, and supraorbital fossa
• Mild mortality is seen in immune horses and resistant species (donkey and zebra)
• Affected animals will have a low-grade fever with congestion of conjunctiva and mild depression
• Diagnosis requires the state or federal veterinary officials to be contacted and virus isolation and PCR tests to be performed for diagnosis

Prevention and Control of African Horse Sickness

• Prevention is to occur in Endemic areas to prevent losses; vaccination (MLV) must occur
• Control requires eradication; quarantine, vector control, and destroying diseased animals.

Hendra Virus

• Hendra virus (HeV) infection is a rare emerging zoonosis that causes severe disease in both infected horses and humans
• The natural host is fruit bats of the Pteropodidae Family, Pteropus genus.
• Clinical signs include fever, anorexia, depression, increased respiratory and heart rates, respiratory distress, and death
• Facial edema, frothy nasal discharge, cyanotic/jaundiced mucous membranes and mild neurologic signs may also occur
• Human infection occurs after close contact with horses in end-stage disease or performing a necropsy
• The virus is fragile and easily killed by sunlight.
• Highly contagious; transmission between infected and non-infected horses occurs infrequently
• Human symptoms range from mild influenza to fatal respiratory or neurological disease
• Horses identified as the intermediate hosts, transmitting infection to humans through close contact during care of ill or dead horses
• Primary treatment for human cases is intensive supportive care and hyperimmune plasma
• The vaccine is a three-dose series that is required by many show grounds and boarding facilities and the cost is approximately $350 ($100/vaccine, three dose series) including mandatory micro-chipping ($40).