Common Horse Diseases Quick Guide: Colic, Laminitis, Strangles, EPM, Potomac Horse Fever

1. What You Need to Know

You’re being tested on rapid recognition + first actions + key distinguishing features of five high-yield equine diseases. The exam angle is usually: Which disease fits these signs? What’s the immediate management? How do you prevent spread/recurrence?

The “big idea” for each disease

• Colic = abdominal pain (a sign, not a single diagnosis). Your job: triage severity, remove feed, call the vet early, recognize surgical red flags.
• Laminitis = failure/inflammation of the hoof laminae → severe pain, risk of rotation/sinking. Your job: freeze feet early, pain control, diet/endocrine management, mechanical support.
• Strangles = contagious upper-respiratory infection (Streptococcus equi subsp. equi) → fever then thick nasal discharge + abscessed lymph nodes. Your job: isolate immediately, confirm, manage outbreaks.
• EPM (Equine Protozoal Myeloencephalitis) = neurologic disease most often from Sarcocystis neurona (opossum-associated) → asymmetric ataxia/weakness, muscle atrophy. Your job: recognize neuro pattern, avoid over-diagnosing based on serum alone.
• Potomac Horse Fever (PHF) = infectious colitis from Neorickettsia risticii → fever, diarrhea/colitis, laminitis risk. Your job: treat early (tetracyclines), aggressive fluids, and laminitis prevention.

Critical reminder: Fever + nasal discharge = think Strangles. Fever + diarrhea = think PHF (or salmonellosis). Neuro deficits = think EPM (among differentials). Foot pain + bounding digital pulse = treat as laminitis now.

2. Step-by-Step Breakdown

These are the high-yield action algorithms you’ll be expected to know.

A) Colic: what you do first (field triage)

1. Assess severity + safety
   • Is the horse violently rolling/throwing itself? Protect people first.
2. Remove feed (hay/grain). Water is usually okay unless choking suspected.
3. Get quick vitals + pain assessment
   • Heart rate, respiratory rate, temperature, mucous membranes, capillary refill time, gut sounds.
4. Call the vet early (don’t “wait and see” if pain is moderate/severe).
5. Walk only if it keeps the horse from rolling
   • Don’t exhaust the horse with hours of walking.
6. Do NOT give additional medications unless directed
   • NSAIDs can mask severity and complicate decisions.
7. Know surgical red flags (below) and be ready for referral/transport.

B) Laminitis: immediate response (“freeze + support + remove cause”)

1. Treat as an emergency if acute lameness/stance suggests laminitis.
2. Start cryotherapy ASAP (continuous icing of distal limbs)
   • Best evidence for reducing laminar damage in early/at-risk cases.
3. Confine (deep bedding, minimize turning, no forced exercise).
4. Remove dietary triggers
   • No grain; restrict lush pasture; switch to low non-structural carbohydrate (NSC) forage.
5. Call vet/farrier team
   • Pain control + mechanical support (frog/sole support) + evaluate endocrine disease.

C) Strangles: outbreak control steps

1. Isolate immediately any horse with fever/nasal discharge/lymph node swelling.
2. Stop horse movement on/off farm until status is clarified.
3. Temperature monitoring for exposed horses
   • Twice daily temps to catch early fever (before heavy shedding).
4. Confirm diagnosis (vet-directed)
   • PCR/culture from nasopharyngeal swab or guttural pouch sampling.
5. Create groups
   • Red (sick), Amber (exposed), Green (no contact) with dedicated equipment.
6. Disinfection + hygiene
   • Hands, boots, buckets, water trough management.
7. Clear carriers
   • Some horses become guttural pouch carriers (persistent shedding).

D) EPM: how to avoid the classic testing trap

1. Recognize compatible neuro signs
   • Often asymmetric ataxia/weakness ± cranial nerve deficits, muscle atrophy.
2. Rule out common differentials
   • Cervical vertebral stenotic myelopathy (wobbler), trauma, equine herpes myeloencephalopathy, EDM, rabies, etc.
3. Test appropriately
   • Serum antibody positivity alone ≠ disease (exposure is common).
   • Best support comes from CSF testing and serum:CSF comparison (vet-run protocols).
4. Treat promptly when suspicion is strong
   • Antiprotozoals; monitor neuro improvement.

E) PHF: diarrhea + fever protocol

1. Suspect PHF with acute fever, depression, anorexia, colic, diarrhea—especially near waterways/insect exposure.
2. Call vet early: rapid deterioration and laminitis risk.
3. Start therapy early (vet-directed)
   • Tetracyclines are classically effective.
4. Aggressive supportive care
   • Fluids/electrolytes, anti-inflammatories, gut protectants as indicated.
5. Laminitis prevention
   • Cryotherapy/foot support may be used proactively in high-risk cases.

3. Key Formulas, Rules & Facts

Normal adult horse vitals (know these to spot “bad colic” and systemic illness)

Fast compare: hallmark features + first moves

Colic: “medical vs surgical” red flags (classic exam content)

More concerning for surgical/strangulating lesions when you see:

• Persistent/uncontrollable pain despite analgesia
• Heart rate persistently $>60\,\text{bpm}$ (especially rising)
• Injected/toxic mucous membranes, prolonged CRT
• Significant gastric reflux via nasogastric tube (vet finding)
• Markedly reduced gut sounds, severe distension
• Rapid clinical deterioration

Laminitis: main risk buckets

• Endocrinopathic (very common):
  • Equine Metabolic Syndrome (EMS), PPID (Cushing’s)
  • Often triggered by pasture/sugar/starch loads
• Sepsis/inflammation-associated:
  • Colitis, retained placenta, pneumonia, severe infections
• Supporting-limb laminitis:
  • Excess weight-bearing due to injury in opposite limb

Nutrition rule of thumb (commonly tested): choose forage with NSC $<10\%$ for laminitis-prone horses (work with testing/soaking protocols).

Strangles: transmission + control facts

• Spread by direct contact, shared water/feed, handlers/equipment.
• Incubation often about $3-14\,\text{days}$.
• Fever precedes nasal discharge → temperature monitoring catches early cases.
• Persistent shedders often have guttural pouch empyema/chondroids.

EPM: the diagnostic “gotcha”

• Serum antibody positive = exposure common in many regions.
• Stronger evidence uses CSF testing and interpretation in context of signs.

PHF: ecology + treatment facts

• Associated with freshwater habitats and aquatic insect vectors (mayflies/caddisflies implicated in many outbreaks).
• Often seasonal (late spring through fall in endemic areas).
• Early tetracycline-class treatment is classically effective; supportive care is crucial.

4. Examples & Applications

Example 1 (Colic triage)

Scenario: Horse is pawing, looking at flank, minimal manure, HR $68\,\text{bpm}$, sweating, repeated rolling.

• Key insight: HR $>60\,\text{bpm}$ + violent/persistent pain = possible surgical colic.
• Best actions: Remove feed, call vet urgently, prepare for referral/transport; don’t mask signs with repeated meds.

Example 2 (Laminitis recognition)

Scenario: Pony on lush spring pasture is suddenly reluctant to walk, rocks back on heels, feet warm, bounding digital pulses.

• Key insight: Classic endocrinopathic/pasture-associated laminitis.
• Best actions: Immediate icing, deep bedding, strict diet change (no pasture/grain), vet/farrier plan.

Example 3 (Strangles outbreak pattern)

Scenario: New horse arrived $7\,\text{days}$ ago. Now multiple horses have fever; one has thick nasal discharge and swollen submandibular lymph nodes.

• Key insight: Fever spreading after new arrival + abscessed nodes = Strangles until proven otherwise.
• Best actions: Isolate, stop movement, PCR/culture, temperature monitoring for exposed groups.

Example 4 (EPM testing trap)

Scenario: Horse has asymmetric hindlimb ataxia and gluteal muscle atrophy. Serum antibody for S. neurona is positive.

• Key insight: Serum positive supports exposure, not definitive disease.
• Best next step: Vet-led neuro workup; consider CSF-based testing/ratios and differentials.

Example 5 (PHF vs “just diarrhea”)

Scenario: Adult horse near a river has acute fever $39.7\,^{\circ}\text{C}$, depression, colic signs, then watery diarrhea.

• Key insight: Fever + colitis + geography/season = PHF high on list, and laminitis risk is real.
• Best actions: Early veterinary therapy (tetracyclines), aggressive fluids, laminitis prevention/monitoring.

5. Common Mistakes & Traps

1. Colic = a diagnosis (not a symptom)

   • Wrong: Treating “colic” as one disease.
   • Why it’s wrong: Colic ranges from mild gas to strangulating obstruction.
   • Fix: Think severity + cause categories and triage fast.

2. Waiting too long to call the vet for colic

   • Wrong: “Let’s see if it passes.”
   • Why it’s wrong: Strangulating lesions worsen quickly; delay worsens survival.
   • Fix: Call early, especially if HR is high or pain persists.

3. Over-walking a colicky horse

   • Wrong: Hours of forced walking.
   • Why it’s wrong: Causes exhaustion/dehydration and doesn’t fix surgical lesions.
   • Fix: Walk only to prevent unsafe rolling; otherwise keep calm and monitored.

4. Missing laminitis early because the horse ‘isn’t that lame’

   • Wrong: Waiting for severe, obvious founder stance.
   • Why it’s wrong: Early intervention (icing/support) is when you can prevent damage.
   • Fix: Treat digital pulses + heat + reluctance to turn seriously.

5. Feeding grain or turning out a laminitis-prone horse because it looks better

   • Wrong: Reintroducing pasture/grain too soon.
   • Why it’s wrong: Triggers recurrence; laminitis can rebound.
   • Fix: Follow strict diet/endocrine plan; gradual, measured changes only.

6. Not isolating immediately for suspected Strangles

   • Wrong: Waiting for lab confirmation before separating horses.
   • Why it’s wrong: Strangles spreads fast via water buckets/handlers.
   • Fix: Isolate on suspicion, then test.

7. Using a positive EPM serum test as proof of EPM

   • Wrong: “Positive antibody = disease.”
   • Why it’s wrong: Many horses are exposed without clinical disease.
   • Fix: Match neuro exam pattern + appropriate confirmatory testing strategy.

8. Underestimating laminitis risk in PHF

   • Wrong: Treating PHF as “just diarrhea.”
   • Why it’s wrong: Endotoxemia/inflammation can trigger laminitis.
   • Fix: Proactively monitor feet/digital pulses; consider preventive strategies early.

6. Memory Aids & Quick Tricks

Quick association: Strangles = abscessed lymph nodes. PHF = colitis + laminitis. EPM = asymmetric neuro. Laminitis = bounding digital pulse.

7. Quick Review Checklist

• Colic
  • You remove feed, take vitals, call the vet early.
  • You recognize surgical red flags: persistent pain, HR $>60\,\text{bpm}$, toxic signs.
• Laminitis
  • You act immediately: ice + confine + support.
  • You know main causes: EMS/PPID, sepsis/inflammation, supporting limb.
  • You remember diet focus: low NSC forage (often $<10\%$).
• Strangles
  • You isolate on suspicion.
  • You remember fever precedes nasal discharge.
  • You know complications: guttural pouch carriers, purpura hemorrhagica, bastard strangles.
• EPM
  • You look for asymmetric ataxia/weakness and muscle atrophy.
  • You don’t diagnose from serum antibody alone.
• PHF
  • You think “fever + diarrhea/colitis,” often near waterways/insect exposure.
  • You treat early and prevent/monitor for laminitis.

You’ve got this—focus on the pattern recognition + first actions, and you’ll pick up most exam points fast.