Equine Diseases

Upper Respiratory and Guttural Pouch Diseases

• Strangles
    • Definition: A highly contagious disease caused by the bacterium Streptococcus equi
    • Clinical Signs: Characterized by sudden onset of fever, mucopurulent nasal discharge, and abscessation of the submandibular and retropharyngeal lymph nodes. It can be difficult to treat successfully.
    • Management: Requires maintenance under a strict isolation protocol to prevent transmission.
    • Immunization: Modified live virus intranasal vaccine is available and can be helpful.
    • Diagnosis: Achieved through culture, PCR (Polymerase Chain Reaction), and serology.
    • Treatment: Involves antibiotic therapy and the lancing and flushing of abscesses.

• Guttural Pouch Empyema
    • Etiology: Often a sequela to strangles or regional lymph node abscesses.
    • Condition: A bacterial infection resulting in the accumulation of pus in the guttural pouch.
    • Chondroids: Chondroids are abscesses that have turned into concretions. Resolution of the disease requires the physical removal of these chondroids.
    • Treatment: Treated with antimicrobials and lavage of the pouch.

• Guttural Pouch Mycosis
    • Etiology: A fungal infection of the guttural pouch, most commonly caused by Aspergillus spp.
    • Characteristics: Results in the formation of a fungal plaque within the pouch.

Viral and Inflammatory Respiratory Conditions

• Equine Influenza
    • Causative Agent: Viruses belonging to the Orthomyxoviridae family.
    • Clinical Signs: Fever, anorexia, weight loss, mucopurulent nasal discharge, increased respiratory rates, and retropharyngeal lymphadenopathy.
    • Diagnosis: Presumptive diagnosis based on signs, confirmed via virus isolation, immunoassay, immunofluorescence, PCR, and antibody detection.
    • Treatment: Primary treatment is supportive care.

• Rhinopneumonitis (Equine Herpes)
    • Causative Agents: Equine herpesvirus $1$ and equine herpesvirus $4$
    • Clinical Range: Contagious virus causing respiratory disease, abortion, and neurological issues. Respiratory signs are almost indistinguishable from influenza.
    • Incubation Period: Ranges from $2$ to $10$ days.
    • Transmission: Occurs via aerosol transmission, respiratory secretions, and fomite transmission.
    • Reproductive Impact: Causes abortion in mares.
    • Neurological Signs (Myeloencephalopathy): Clinical signs include ataxia, fever, loss of anal tone, paralysis of the tail, urinary incontinence, and recumbency.
    • Male Reproductive Signs: Scrotal edema, loss of libido, and reduced sperm quality.
    • General Symptoms: Mucopurulent nasal discharge, lymphadenopathy, and coughing.
    • Vaccination Protocol:
      • Inactivated univalent vaccines are administered to broodmares to prevent abortion.
      • Schedule: Vaccination occurs during the $3^{rd}$, $5^{th}$, $7^{th}$, and $9^{th}$ months of pregnancy.
      • Note: Vaccines do not provide $100\%$ protection against abortion.
    • Management: Standard quarantine protocols and supportive care are essential.

• Heaves (Recurrent Airway Obstruction - RAO)
    • Synonyms: Also known as Chronic Obstructive Pulmonary Disease (COPD).
    • Pathophysiology: An allergic airway disease characterized by airway inflammation, bronchoconstriction, and excessive mucus production. Exposure to air pollutants is a causative factor, though the exact etiology is sometimes controversial.
    • Clinical Signs: Difficulty breathing, nasal discharge, coughing, and a lack of stamina.
    • Management: The condition is never "cured." It is controlled through management practices and medical therapy as necessary.

• Exercise Induced Pulmonary Hemorrhage (EIPH)
    • Cause: Currently unknown.

• Bacterial Pneumonia and Pleuropneumonia
    • Pathogenesis: Results from the aspiration of bacteria followed by colonization.
    • Treatment: Involves antibiotics, anti-inflammatories, and pleural drainage.

Cardiovascular Diseases

• Second-Degree Atrioventricular (AV) Block
    • Description: A common arrhythmia involving altered conduction through the AV node, resulting in atrial contraction without following ventricular contraction.
    • Significance: Causes "dropped beats," which are typically not clinically significant in horses.
    • Warning: The drug Xylazine exacerbates this condition. Dexmed can also make this conditon worse.

• Atrial Fibrillation
    • Prevalance: The most common relevant arrhythmia in horses.
    • Predisposing Factors: Horses are prone due to the large size of their atria and high vagal tone.
    • Diagnosis: Detected via auscultation and a complete cardiac workup, including echocardiography.
    • Treatment: Managed via Quinidine administration or electrical conversion.

Infectious and Systemic Diseases

• Equine Viral Arteritis (EVA)
    • Characteristics: A contagious disease characterized by limb swelling, conjunctivitis, abortion, and respiratory disease.
    • Stallion Persistence: Stallions infected after puberty develop a persistent infection in their accessory sex glands.
    • Abortion: Occurs in mares due to viral damage to the placenta.
    • Vaccination: An approved vaccine exists for stallions and non-pregnant mares under USDA supervision. Pregnant mares must not be vaccinated.

• Equine Infectious Anemia (EIA)
    • Nature: A persistent, life-long viral disease where infected horses become permanent carriers.
    • Transmission: Transmitted by biting flies.
    • Clinical Signs: Anemia, fever, and weight loss.
    • Regulations:
      • Testing: Horses not traveling or sold should be tested yearly.
      • Positive Test: If a horse tests positive, the entire herd is quarantined until all horses on the premises are tested (usually a $60$-day period). The state veterinarian releases the quarantine.
      • Coggins Test: A veterinarian must draw blood and provide a detailed description of the horse on specific forms for health certificates.

• Anaplasmosis (Equine Granulocytic Ehrlichiosis)
    • Etiology: Bacterial infection transmitted by biting ticks of the Ixodes genus.
    • Clinical Signs: Fever, anemia, icterus, lethargy, stiffness, and edema.
    • Diagnosis: Observation of White Blood Cells (WBCs) on a peripheral blood smear, PCR, and serology.
    • Treatment: Intravenous (IV) oxytetracycline for $5$ to $7$ days. Administration must be very careful and slow, as rapid administration can be fatal.

• Lyme Disease
    • Causative Agent: Borrelia burgdorferi, transmitted by Ixodes ticks.
    • Clinical Signs: Stiffness, lameness, and behavioral changes. Diagnosis is difficult due to nonspecific signs.
    • Diagnosis: Western blot, ELISA, or C-6 ELISA SNAP test.
    • Treatment: IV oxytetracycline or oral doxycycline for a duration of at least $1$ month.

Gastrointestinal Disorders

• Ulceration
    • Gastric and Colonic: Young horses are particularly prone.
    • Predisposing Factors: Stress, high-grain diets, musculoskeletal pain, and the administration of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs).
    • Phenylbutazone Toxicity: Colonic ulcers are a known secondary complication of phenylbutazone (NSAID) toxicity.

• Colitis
    • Pathophysiology: Develops rapidly, leading to hypovolemia, shock, endotoxemia, electrolyte loss, and acid-base imbalance due to severe diarrhea.
    • Etiologic Agents: Salmonella spp., Clostridium spp., and Neorickettsia risticii.
    • Treatment: IV fluid therapy.
    • Complications: Include laminitis, cardiovascular collapse, arrhythmias, and thrombophlebitis.

• Potomac Horse Fever
    • Causative Agent: Neorickettsia risticii.
    • Transmission: Ingestion of fresh water insects, snails infected with flukes, or feces from infested birds or bats.
    • Geography: Found in states east of the Mississippi River, as well as California.
    • Diagnosis: Serum testing or PCR analysis of whole blood.
    • Vaccination: Inactivated bacterins are available but are not considered preventative.

• Choke (Esophageal Obstruction)
    • Definition: Indicates obstruction of the esophagus, usually by grain or hay.
    • Predisposing Conditions: Chronic dental disease, retained deciduous caps, or eating too quickly (often in overcrowded environments).
    • Clinical Signs: Anxiety, gagging, excessive salivation, and the discharge of feed and saliva from the nostrils. The horse may attempt to eat despite being unable to swallow.
    • Complications: Aspiration pneumonia is a significant risk; esophageal stricture or rupture is less common.

• Gastrointestinal Neoplasia
    • Lymphosarcoma: The most common gastrointestinal neoplasia in horses. Diagnosed via biopsy, laparoscopy, laparotomy, or finding neoplastic cells in abdominal fluid.
    • Squamous Cell Carcinoma: The most common neoplasia specifically of the stomach. Symptoms include weight loss and anorexia. Diagnosed via gastroscopy.
    • Prognosis: There are no effective treatments; the prognosis for both is grave.

• Liver Disease
    • Clinical Signs: Weight loss, anorexia, jaundice, and fever.
    • Diagnosis: Serum chemistry, transabdominal ultrasound, and biopsy.
    • Treatment: Nutritional management, anti-inflammatories, and antibiotics.

• Colic
    • Definition: General abdominal pain in horses.
    • Causative Factors: Distention of the gut, pulling at the root of the mesenteric artery, ischemia or infarction, and enteritis or ulcerations.
    • Clinical Signs: include standing and lying down, rolling, kicking at the abdomen, pawing, looking at the abdomen, grunting, sweating, abdominal distention, straining to urinate/defecate, decreased bowel movements, rapid breathing, flared nostrils, and abnormal behavior.
    • Treatment: Administration of mineral oil, pain medication, hand walking, and IV fluids.

Neurological Disorders: Brain and Brainstem

• General Signs of Brain Damage: Altered mentation, head pressing, seizures, cranial nerve damage (affecting facial muscles, swallowing, balance, vision, taste, and ocular position), incoordination, and altered breathing.

• Rabies
    • Nature: Zoonotic infection and universally fatal. Generally acquired via a bite from a wild animal.
    • Diagnosis: No accurate antemortem (before death) test exists. Confirmed postmortem by fluorescent antibody stain of brain tissue.
    • Safety: Human exposure requires a postexposure rabies vaccine. Suspects and tissues must be handled with extreme caution.

• Viral Equine Encephalitis
    • Types: Eastern (EEE), Western (WEE), Venezuelan (VEE), and West Nile Virus (WNV).
    • Characteristics: Rapidly progressive, highly fatal neurologic disease transmitted by mosquitoes.
    • Treatment/Prognosis: Supportive care; prognosis is poor to guarded.
    • Vaccination: Vaccines for EEE and WEE are highly efficacious; VEE is not routinely given.

• Equine Leukoencephalomalacia
    • Cause: Moldy corn toxicity resulting from the ingestion of a fungal toxin produced by Fusarium moniliforme.
    • Pathology: The toxin produces liquefactive necrosis of the cerebral cortex.
    • Clinical Signs: Profound depression, head pressing, altered states of consciousness, incoordination, and aimless wandering. Prognosis is poor.

• Temperohyoid Osteoarthropathy (THO)
    • Location: Junction of the stylohyoid bone and temporal bones.
    • Clinical Signs: Occur after the joint has fused; includes sudden onset of head tilt, ataxia, nystagmus, facial paralysis, and difficulty swallowing.
    • Treatment: Anti-inflammatories and antibiotics. Early-stage surgery can decrease pressure and prevent bone fracture.

Neurological Disorders: Spinal Cord

• Cervical Vertebral Malformation (Wobbler Syndrome)
    • Type: Developmental orthopedic disease.
    • Pathophysiology: Compression of the cervical spinal cord.
    • Demographics: Males are affected four times more frequently than females; Thoroughbreds are predisposed.
    • Clinical Signs: Symmetric incoordination where hind limbs are more severely affected than forelimbs.
    • Diagnosis: Determined by radiographs and confirmed by myelographic examination.
    • Treatment: Surgical stabilization can improve neurological status.

• Equine Protozoal Myelitis (EPM)
    • Causative Agent: Sarcocystis neurona, a protozoan. Horses are dead-end, aberrant hosts.
    • Treatment: Ponazuril (Marquiz®).

• Equine Herpesvirus Myeloencephalopathy: (Reference Rhinopneumonitis section).

• Equine Degenerative Myeloencephalopathy (EDM) and Vertebral Fracture are also noted as major spinal cord disorders.

Systemic Neurological Diseases

• Tetanus
    • Clinical Signs: Stiff, stilted gait, hyperexcitability, seizures, and coma.
    • Entry Points: Subsolar abscesses, penetrating wounds, or infected IM injection sites.
    • Prevention: Tetanus toxoid is highly efficacious for prevention; healthy horses are vaccinated annually.
    • Immediate Protection: Unvaccinated horses at high risk receive both tetanus antitoxin and tetanus toxoid.
    • Warning: Tetanus antitoxin is associated with fatal serum hepatitis; its use should be limited to high-risk cases.

• Botulism
    • Characteristics: Rapidly progressive, fatal neurologic disease caused by a neurotoxin.
    • Clinical Signs: Profound weakness, muscle fasciculations, and dysphagia (difficulty swallowing).

Dermatological and Ophthalmic Diseases

• Ringworm (Dermatophytosis)
    • Etiology: Superficial fungal infection caused by Trichophyton and Microsporum spp.
    • Susceptibility: Younger animals are more likely to be affected.

• Dermatophilosis (Rain Scald/Rain Rot)
    • Etiology: Bacterial infection caused by Dermatophilus congolense.
    • Clinical Signs: Produces crusty lesions.

• Culicoides Hypersensitivity
    • Etiology: Syndrome secondary to the bite of Culicoides flies.
    • Signs: Severe pruritus (itching) affecting the face, mane, tail head, and ventral abdomen.

• Sarcoid
    • Definition: Benign but locally invasive skin tumor; the most common tumor in horses.
    • Forms: Raised, hairless lesions with a corrugated surface, or a flattened form known as verrucous sarcoids.
    • Cause: Unknown, though a viral agent is suspected.
    • Treatment: Surgical resection, cryotherapy (freezing), laser therapy, immunotherapy, radiotherapy (iridium $191$), and chemotherapy.

• Melanomas
    • Characteristics: Common skin tumors occurring in the perineal region; usually benign.
    • Note: Surgical removal is generally not recommended.

• Equine Recurrent Uveitis (Moon Blindness)
    • Significance: The most common cause of blindness in horses.
    • Nature: An immune-mediated condition characterized by episodes of intraocular inflammation that become more frequent and severe over time.

Diseases of the Foot

• Thrush
    • Causative Agent: Fusobacterium necrophorum.
    • Clinical Signs: Foul odor, black discharge from the frog, and lameness.
    • Treatment: Trimming the affected area and applying antiseptic.

• Laminitis and Founder
    • Laminitis: The acute phase of the disease characterized by pain, reluctance to walk, bounding digital pulses, and sensitivity to hoof testers.
    • Founder: The chronic phase where the coffin bone separates from the hoof wall.
    • Etiology: Many causes are implicated in this painful condition of the foot.