Equine Insect Toxicity Study Guide

Overview and Clinical Importance

Insect toxicosis represents a critical category of equine poisoning that practitioners must recognize rapidly. Two primary insect-related toxicoses dominate NAVLE content: cantharidin toxicosis (blister beetle poisoning) and Mare Reproductive Loss Syndrome (MRLS) associated with Eastern tent caterpillar ingestion. Both conditions can cause severe morbidity and mortality, making early recognition and intervention essential.

These toxicoses represent distinctly different pathophysiological mechanisms: cantharidin acts as a direct vesicant causing mucosal damage and electrolyte derangements, while tent caterpillar setae penetrate tissues and introduce bacteria systemically.

Part I: Blister Beetle Toxicosis (Cantharidin Poisoning)

Etiology and Epidemiology

Cantharidin toxicosis results from ingestion of blister beetles (family Meloidae), which contain the toxic compound cantharidin. More than 200 species exist in the continental United States, with the genus Epicauta most commonly implicated in equine poisoning cases.

Common Blister Beetle Species

Cantharidin: Toxin Characteristics

• Produced by adult male beetles and transferred to females during copulation
• Terpenoid compound that is odorless, colorless, and highly stable
• Toxicity NOT diminished by drying, storage, or processing
• Rapidly absorbed from GI tract and excreted unchanged by kidneys
• Even crushed beetle fragments or body fluids retain toxicity

Source of Exposure

The primary source of exposure is alfalfa hay contaminated with blister beetles. Beetles swarm in alfalfa fields during flowering and are crushed during hay crimping and baling operations. Key risk factors include:

• Second and third cuttings of alfalfa (peak beetle activity: June-September)
• Hay that has been crimped or conditioned during harvest
• Alfalfa products including cubes, pellets, and treats
• Geographic areas with both blister beetles and alfalfa production (Western/Southwestern US)
• Proximity to grasshopper populations (blister beetle larvae feed on grasshopper eggs)

Pathophysiology

Cantharidin is a potent vesicant (blistering agent) and irritant that causes cellular damage, vesicle formation, and necrosis on contact with skin or mucous membranes. The mechanism involves inhibition of protein phosphatases 1 and 2A, disruption of cellular metabolism, and acantholysis leading to blister formation.

Organ Systems Affected

Clinical Signs

Clinical signs typically appear within 3-18 hours of ingestion and severity depends on the amount of toxin consumed. Horses ingesting large doses may die suddenly without premonitory signs.

Synchronous Diaphragmatic Flutter (Thumps)

Synchronous diaphragmatic flutter (SDF) is a characteristic finding in cantharidin toxicosis resulting from severe hypocalcemia:

• Hypocalcemia causes hyperexcitability of the phrenic nerve
• Phrenic nerve runs over the right atrium before innervating the diaphragm
• Cardiac electrical activity stimulates the hyperexcitable phrenic nerve
• Diaphragm contracts synchronously with heartbeat, appearing like hiccups

Diagnosis

Laboratory Abnormalities

Definitive Diagnosis

• Detection of cantharidin in urine or gastric contents using HPLC or GC-MS
• Urine sample must be collected within 3-4 days (concentration becomes undetectable)
• Identification of beetle fragments in hay or gastric contents
• Post-mortem examination of GI tract and kidneys

Treatment

There is NO specific antidote for cantharidin toxicosis. Treatment is symptomatic and supportive.

Prognosis

• Prognosis is guarded to poor; depends on amount ingested and timing of treatment
• Fatality rate without treatment: up to 65%
• With prompt treatment: mortality reduced to approximately 20%
• Up to 10% of survivors develop acute laminitis
• Poor prognostic indicators: persistent tachycardia, increasing CK, deteriorating condition over 48 hours

Prevention

• Purchase first cutting alfalfa (beetles dormant until late May-June)
• Avoid hay that has been crimped or conditioned
• Source hay from regions with low blister beetle populations
• Inspect each hay flake before feeding
• Consider eliminating alfalfa from the diet entirely

Part II: Mare Reproductive Loss Syndrome (MRLS)

Etiology and Epidemiology

Mare Reproductive Loss Syndrome (MRLS) is associated with ingestion of Eastern tent caterpillars (Malacosoma americanum). First recognized during a devastating outbreak in Kentucky in 2001, MRLS caused the loss of approximately 25-30% of the foal crop with economic losses estimated at $336-500 million.

Eastern Tent Caterpillar: Key Facts

Pathophysiology

Septic Penetrating Setae Hypothesis (Primary)

• ETC are hirsute (hairy) caterpillars covered with barbed setae (hairs)
• The cuticle/exoskeleton containing setae is the abortigenic component
• Barbed setal fragments penetrate the intestinal wall and blood vessels
• Setae carry commensal bacteria (Actinobacillus, Streptococcus) from the GI tract
• Bacteria spread hematogenously to poorly immunoprotected tissues
• Fetus/placenta, pericardium, eye, and CNS are particularly susceptible

Clinical Syndromes of MRLS

Prevention of MRLS

• Remove black cherry and wild cherry trees from pastures
• Scout pastures for ETC tents in early spring (February-March)
• Remove tents manually before caterpillars disperse
• Remove pregnant mares from pastures with ETC activity
• Keep horses in dry lots during peak dispersal (late April-May)

Comparison: Blister Beetles vs. Tent Caterpillars