Equine Bacterial Diseases: Strangles

Bacterial Diseases in Horses

Strangles (Distemper)

• Caused by Streptococcus equi, a Gram-positive, capsulated, beta-hemolytic, Lancefield group C coccus
• Highly contagious and host-adapted, affecting only horses, donkeys, and mules
• Pathogenesis: transmission via fomites and direct contact with infectious exudates, carrier animals are an important source of infection
• Organism is susceptible to desiccation, extreme heat, and exposure to sunlight
• Clinical findings:
  • Incubation period: 3-14 days
  • Fever: 39.4-41.1°C
  • Mucoid to mucopurulent nasal discharge
  • Depression
  • Submandibular lymphadenopathy
  • Difficulty in swallowing, inspiratory respiratory noise, and extended head and neck in horses with retropharyngeal lymph node involvement
• Metastatic Strangles (Bastard Strangles): characterized by abscessation in other lymph nodes, particularly in the abdomen and less frequently in the thorax

Diagnosis and Treatment

• Diagnosis: bacterial culture of exudates from abscesses and nasal swab samples, CBC, endoscopic examination of the upper respiratory tract, and ultrasonography of the retropharyngeal area
• Treatment: systemic administration of antimicrobials, fluid therapy, corticosteroid and cyclosporine for temporary relief, and vitrectomy with gentamycin lavage

Prevention

• Isolation of acutely infected horses for 14-16 weeks
• Detection of urine by FAT
• Limiting exposure to stagnant water and potential maintenance hosts
• Vaccination

Salmonella and Nosocomial Infection

• Caused by Salmonella enterica subspp. Enterica serotype Typhimurium, Newport, Anatum, and Agona
• Clinical signs:
  • Enterocolitis
  • Diarrhea
  • Fever
  • Leukopenia
• Treatment: antimicrobial treatment
• Prevention:
  • Cleaning and disinfecting of horse facilities
  • Temporarily closing and emptying wards, and instituting thorough and intensive cleaning and disinfecting procedures
  • Strict traffic of humans and animals
  • Adequate ventilation and distance between cases

Tetanus

• Caused by Clostridium tetani
• Clinical signs:
  • Fever
  • Muscle stiffness
  • Hyperesthesia
  • Opisthotonus
  • Respiratory failure
• Diagnosis:
  • Clinical signs and history of recent trauma
  • Demonstration of toxin in serum of affected animal
  • Anaerobic culture and demonstration of bacteria from wound
• Treatment and control:
  • Curariform agents, tranquilizers, or barbiturate sedatives in conjunction with tetanus antitoxin
  • Administration of tetanus antitoxin into the subarachnoid space
  • Draining and cleaning of wounds, and administering penicillin or broad-spectrum antibiotics
  • Placing in a quiet, darkened stall box with feeding and watering devices
  • Sling for horses with difficulty in standing or rising
• Prevention:
  • Active immunization with tetanus toxoid
  • Wound management and immunization
  • Yearly toxoid booster
  • Vaccination of mares during the last week of pregnancy
  • Vaccination of foals at 5-8 weeks of age

Botulism

• Caused by toxins produced by Clostridium botulinum
• Botulinum toxin acts on the peripheral nervous system by preventing transmission of nervous impulses
• Found in soil and decaying plant or animal matter
• Adult horses and foals less than 8 months old affected
• Clinical signs:
  • Foals: impaired suckling, inability to swallow, decreased eyelid and tail tone, dilated pupils, and respiratory paralysis
  • Adults: parenteral nutrition
• Treatment: supportive care and antitoxin administration

Suppurative Bronchopneumonia

• Caused by Rhodococcus equi in foals
• Also causes ulcerative enterocolitis, colonic-mesenteric lymphadenopathy, immune-mediated synovitis and uveitis, osteomyelitis, pyogranulomatous dermatitis, brain abscess, immune-mediated anemia, and septic arthritis
• Inhalation of contaminated dust is the most important route for pneumonic infection in foals
• Facultative intracellular parasites of monocytes and macrophages
• Clinical signs:
  • Suppurative bronchopneumonia with extensive abscess formation and suppurative lymphadenitis
  • Early signs: slight increase in respiratory rate and mild fever
  • More commonly: acute respiratory distress and high fever
  • Intestinal manifestations: granulomatous or suppurative inflammation of Peyer's patches and mesenteric or colonic lymph nodes
• Treatment: administration of erythromycin and rifampin, or azithromycin and clarithromycin in combination with rifampin
• Prevention:
  • Husbandry: stocking density, proper ventilation, and dust control
  • Passive immunity: transfusion of hyperimmuned plasma
  • Chemoprophylaxis: development of superinfections, bacterial resistance, and antimicrobial-induced colitis

Lyme Disease

• Caused by Borrelia burgdorferi
• 2-year enzootic dual infection with Anaplasma phagocytophilum cycle involving Ixodes ticks and mammals
• Ticks must be attached to a mammal for at least 24 hours
• Clinical signs:
  • Low-grade fever
  • Stiffness and lameness in more than one limb
  • Muscle tenderness, hyperesthesia, and swollen joints
  • Lethargy and behavioral changes
• Treatment:
  • Tetracycline
  • Doxycycline
  • Ceftiofur
• Prevention:
  • Preventing tick exposure or prolonged attachment
  • Early antimicrobial treatment after exposure
  • Vaccination

Leptospirosis

• Caused by highly invasive Leptospira
• Clinical syndrome:
  • Reproductive tract: Leptospira interrogans serovar Pomona, responsible for most Leptospira abortions
  • Acute renal failure: Leptospira Pomona, causing severe and acute renal failure
  • Recurrent uveitis: Leptospira interrogans serovar Pomona, causing immune-mediated keratitis and recurrent uveitis
• Treatment:
  • Tetracycline
  • Doxycycline
• Prevention:
  • Preventing exposure to contaminated urine and water
  • Vaccination