Viral Diseases of Horse

Horses are susceptible to a variety of viral diseases, some of which can lead to severe health problems or even death.

Causative Agent
- Virus: Influenza A virus
- Family: Orthomyxoviridae
- Genus: Alphainfluenzavirus
Transmission
- Aerosol transmission via respiratory droplets from coughing or nasal discharge.
- Contaminated equipment, feed, and water.
Replication Cycle
- The virus infects respiratory epithelial cells, replicates in the nucleus, and is released by budding from the cell surface.
Pathogenesis
- The virus primarily infects the respiratory tract, causing inflammation and epithelial damage, leading to impaired mucociliary clearance.
Clinical Signs
- Fever, cough, nasal discharge, lethargy, anorexia, muscle soreness.
- Rapid onset and resolution in a few weeks with supportive care.
Control
- Vaccination: Inactivated and modified-live vaccines.
- Biosecurity: Isolate affected animals, quarantine new arrivals, avoid crowded conditions.
- Supportive care: Hydration, anti-inflammatory drugs.

Causative Agent
- Virus: Equine Herpesvirus (EHV-1 and EHV-4)
- Family: Herpesviridae
- Genus: Varicellovirus
Transmission
- Direct contact with infected animals.
- Aerosol transmission via respiratory secretions.
- Indirect transmission through contaminated equipment.
Replication Cycle
- The virus enters respiratory epithelial cells, where it replicates and spreads locally. It can establish latency in nerve ganglia and reemerge under stress.
Pathogenesis
- EHV-1 can cause abortion, respiratory disease, and neurological disorders (equine herpesvirus myeloencephalopathy, EHM). EHV-4 causes primarily respiratory signs.
Clinical Signs
- Fever, nasal discharge, coughing, abortion (EHV-1), neurological signs (ataxia, paralysis), respiratory distress (EHV-4).
Control
- Vaccination: Available for both EHV-1 and EHV-4, but does not prevent latency.
- Biosecurity: Quarantine new arrivals, isolation of infected animals, proper hygiene.
- Management: Minimize stress, support care for neurological cases.

Causative Agent
- Virus: Equine infectious anemia virus
- Family: Retroviridae
- Genus: Lentivirus
Transmission
- Biting flies (e.g., horseflies, deerflies) are the primary vectors.
- Blood-to-blood contact (via needles or surgical instruments).
Replication Cycle
- The virus enters macrophages and lymphocytes, where it integrates into the host’s genome and remains latent in the host’s blood.
Pathogenesis
- Persistent infection leads to episodic clinical signs (fever, anemia, weight loss), with possible acute or chronic forms.
Clinical Signs
- Fever, weight loss, anemia, edema, lethargy, jaundice, and occasional hemorrhages.
- Acute cases may die within days, while chronic forms cause intermittent flare-ups.
Control
- No vaccine is available.
- Testing: Coggins test (AGID test) for diagnosis.
- Control: Quarantine infected horses, vector control (insecticide treatment), and avoid sharing needles or equipment.

Causative Agent
- Virus: West Nile virus
- Family: Flaviviridae
- Genus: Flavivirus
Transmission
- Mosquitoes are the primary vectors, which transmit the virus from infected birds to horses.
- Horses cannot transmit the virus to other horses or humans.
Replication Cycle
- After infection through mosquito bites, the virus enters the bloodstream, spreads to various tissues (especially the central nervous system), and replicates in neurons.
Pathogenesis
- The virus causes encephalitis, affecting the central nervous system, leading to neurological symptoms.
Clinical Signs
- Fever, ataxia, weakness, incoordination, muscle tremors, paralysis, and in severe cases, death.
Control
- Vaccination: Available and recommended in endemic areas.
- Vector control: Reduce mosquito populations by using insecticides and eliminating standing water.
- Management: Supportive care for affected horses.

Causative Agent
- Virus: Rabies virus
- Family: Rhabdoviridae
- Genus: Lyssavirus
Transmission
- Bite or saliva from infected mammals (e.g., bats, raccoons, skunks, foxes).
- Can also occur via open wounds or mucous membranes if exposed to infected saliva.
Replication Cycle
- The virus replicates in muscle cells at the site of entry, spreads to the central nervous system, and causes encephalitis.
Pathogenesis
- Rapid progression of neurological disease leading to death, usually within days after the onset of symptoms.
Clinical Signs
- Behavioral changes, aggression, ataxia, paralysis, difficulty swallowing, and eventual death.
- Horses may show signs of fear or aggression, drooling, or difficulty moving.
Control
- Vaccination: Rabies vaccine is available and recommended.
- Management: Avoid contact with wildlife, especially during outbreaks.
- Isolation: If rabies is suspected, quarantine the horse immediately and consult with veterinarians.

Causative Agent
- Virus: Equine parvovirus hepatitus
- Family: Parvoviridae
- Genus: Bocavirus
Transmission
- Spread via fecal-oral transmission or contaminated environments.
Replication Cycle
- The virus infects liver cells and replicates in the nucleus of hepatocytes, causing liver damage.
Pathogenesis
- The virus leads to liver inflammation (hepatitis) and possible liver failure.
Clinical Signs
- Lethargy, jaundice, anorexia, abdominal pain, and, in severe cases, acute liver failure.
Control
- No specific vaccine is available.
- Supportive care: Liver function support, hydration, and nutrition.

Vaccination: Vaccines are available for several viral diseases (Equine Influenza, Herpesvirus, West Nile Virus, Rabies).
Biosecurity: Strict isolation of new or infected horses, proper sanitation, and vector control are essential.
Surveillance and Testing: Regular testing for diseases like Equine Infectious Anemia and Equine Herpesvirus.
Supportive Care: Immediate veterinary care for symptomatic horses (hydration, anti-inflammatory treatments, and pain relief).

Effective management of viral diseases in horses relies on vaccination, good biosecurity practices, and early diagnosis.