Todd C. Holbrook, OSUCVHS
Cantharidin, the toxic compound present in blister beetles, has had a wide range of medicinal uses in China for over 2000 years.1 In some cultures it has even been used as an abortifacient.2 Cantharidin's most prominent common name "Spanish Fly" and its reputation as an aphrodisiac is well known.3 The use of cantharidin in western medicine began in the 1950's, its vesicant chemical property has been utilized to treat human dermatologic diseases including warts, and molluscum contagiousum (pox virus).4 There is a growing body of recent research literature on the molecular mechanisms of the compound and potential applications in the field of oncology.5,6,7
The first reported clinical case of blister beetle toxicosis in a horse was reported in 1963.8 Although the horse is the most commonly affected veterinary species, the toxicosis has been reported or induced in other animals including cattle, goats, sheep, rabbits, rats, dogs, cats, emus and chickens, as well as people.9,10,11,12,13,14,15
Cantharidin is an anhydride derivative of cantharidic acid similar in structure to endothall, a commercial herbicide.18 The male beetle produces the toxin as a defense mechanism, storing it in the hemolymph and genitalia, transferring it to the female beetle during copulation.18 The concentration of toxin in the beetle varies widely. In some male beetles, it may represent up to 12% of body weight.19 Therefore, the number of beetles required to induce toxicosis varies. The lethal toxic dose of cantharidin (approximately 0.5-1 mg/kg) may require the ingestion of over 100 to 150 beetles.16,20 The toxin apparently is not degraded appreciably by storage in hay, or by processing of alfalfa products into other forms of feed.17 The toxin is absorbed directly through the skin and mucus membranes and can result in irritation and acantholysis from the activation or release of proteases.1 Specifically, cantharidin binds to and inhibits protein phosphatase 2A (PP2A) receptors in a variety of tissues.18 Protein phosphatases are an integral part of reversible protein phosphorylation processes that regulate a variety of cell functions including gene transcription, protein-protein interactions cell cycle progression and apoptosis.21 Microarray technology has revealed a large number of genes are upregulated and downregulated upon exposure to cantharidin. Alteration of genes regulating the cell cycle and apoptosis are most likely involved in cytotoxicity.22 Other studies indicate that oxidative stress and DNA damage may be important toxic mechanisms of cantharidin.7
On a tissue basis, protein phosphatase inhibition may explain a number of clinical findings notable in horses with cantharidin toxicosis. The hyposthenuria that often occurs in the face of dehydration in horses with cantharidiasis is most likely due to the result of PP2A inhibition in the renal cortical collecting ducts. Experimental models in rats have shown that PP2A is important in regulating the action of vasopressin on Na-K ATPase activity and thus water and sodium absorption in the kidney.23 In the cardiovascular system PP2A inhibition can cause increased contractility, vasoconstriction, and endothelial cell leakage.24,25,26 Clinical findings in some horses with blister beetle toxicosis that may be related to these cardiovascular effects of cantharidin include forceful cardiac contractions notable upon auscultation, poor tissue perfusion, hypoproteinemia, and laminitis.14 Clinicopathologic data on affected horses frequently reveals a neutrophilic leukocytosis, and recent studies have revealed that cantharidin upregulates cytokine genes important in the inflammatory response.22
Clinical Signs, Clinical Pathology, and Differential Diagnosis14,20
A wide range of clinical signs in horses can result from toxicosis, depending on the number of beetles ingested. The signs can vary from mild lethargy and anorexia, to acute severe colic, hypovolemic shock and death. Clinical signs of colic usually predominate; ranging from mild discomfort, to more severe signs of pain including sweating, trembling and anxious behavior. Evidence of dehydration and poor tissue perfusion including injected oral mucus membranes, prolonged capillary refill time and cool extremities usually occurs. Low grade fever is very common in horses that have not received nonsteroidal anti-inflammatory drugs (NSAIDs); however, in severe cases with profound shock hypothermia may be present. Tachycardia and tachypnea are invariably present with significant cantharidin ingestion. Often, early in the clinical course there is ptyalism and horses may play in water buckets submerging their muzzle, and drinking frequently. Some horses display bruxism. Oral ulcerations may be present but in the author's experience are rare.
The toxin is excreted in the urine resulting in urinary tract irritation. Signs of dysuria are often present, frequently noted as pollakiuria initially. Some horses develop polyuria. Frequently, the urine is clear and dilute in character prior to fluid therapy. This is common in the face of dehydration with this toxicosis. Microscopic hematuria and positive blood on dipstick analysis is common. Geldings and stallions may exhibit paraphimosis.
Differentials for the dysuria include urinary tract infection, urolithiasis, and causes of polyuria. Polyuria can accompany renal failure, pituitary adenoma, endotoxemia, psychogenic polydipsia, and diabetes insipidus. Whereas the urine specific gravity is hyposthenuric in cantharidin toxicosis, it is usually isosthenuric (specific gravity 1.008-1.014) in horses with renal failure.27
Although the differential diagnosis list for colic can be quite extensive, one should primarily focus on cause's representative of the clinical signs presented. In horses with concurrent fever one should consider colitis such as that caused by salmonellosis, or clostridiosis, proximal enteritis, and peritonitis. In horses with clinical signs of profound shock the list broadens to include causes of colic that result in endotoxemia such as gastrointestinal rupture, severe colitis, proximal enteritis, NSAID toxicosis, and prolonged strangulating obstructions. Occasionally, cantharidin intoxicated horses exhibit extreme pain that is difficult to control with analgesics; these horses certainly mimic surgical colic cases, making the decision process a challenge.
On occasion the differential diagnosis list may include specific diseases other than colic depending on the coincident signs displayed. Horses with congested mucous membranes, tachycardia, lethargy, muscle fasciculations, and sweating, with or without signs of colic may appear similar to horses with monensin toxicosis. The presence of arrhythmias would make one more suspicious of monensin ingestion. Occasionally, cumulative signs such as anorexia, lethargy, fever, tachycardia, tachypnea, mild pain, and stilted gait may resemble pleuritis. These same signs in the absence of fever may suggest the presence of myositis. Other cases presenting with lethargy, anorexia, muscle fasciculations, ataxia, and mild fever mimic signs typical of West Nile encephalitis.
One of the most consistent clinical pathology findings is hypocalcemia, signs can include muscle fasciculations, a stilted gait, weakness or ataxia, synchronous diaphragmatic flutter, or abnormal facial expression. Other conditions that can cause hypocalcemia are listed in table 1.28
Table 1. Conditions associated with hypocalcemia in horses
Acute or chronic renal failure
Dystocia, retained placenta
Other common abnormalities on laboratory analysis include hyposthenuria (specific gravity 1.003-1.006) in the face of hemoconcentration, hypomagnesemia, azotemia, and increased serum creatine kinase (CK). Neutrophilic leukocytosis is very common, although occasionally horses will be neutropenic and lymphopenic. After 24 to 36 hours, many horses develop hypoproteinemia, most likely from gastrointestinal protein loss. Although metabolic acidosis can occur in cantharidiasis, mixed acid-base responses appear more common. Most of the aforementioned lab findings may occur in a number of gastrointestinal diseases. However, the hyposthenuria and increased CK are notable to remember. Hyposthenuria in the face of dehydration rarely occurs in other gastrointestinal diseases. The hyposthenuria in blister beetle toxicosis most likely results from cantharidin's affect on ADH action on the distal collecting duct as described above. Although some horses with colic, regardless of cause may have laboratory evidence of traumatized muscles from violent thrashing, increased serum CK is fairly consistent in horses with cantharidiasis. Some cases may present with clinical and laboratory findings that suggest primary myositis, thus rhabdomyolysis is also a differential. Because evidence of myocardial necrosis has been noted in some horses with cantharidin toxicosis at necropsy, cardiac sources of CK must also be considered.29 Abdominocentesis results may reveal moderate increases in total protein, and normal to mild elevations in white blood cells.20
Definitive Diagnosis and Treatment
Diagnosis is often straight forward when incriminating beetles are found in the alfalfa hay source. The absence of this information certainly should not rule out the diagnosis. However, feeding areas as well as all recently opened bales should be closely examined. Hay should be discarded to eliminate risk to other livestock. All members of the herd fed from the same hay source should be monitored closely.
The toxin can be quantitated by gas chromatography-mass spectrometry in urine or gastrointestinal contents for confirmation of the diagnosis; however this sometimes takes 24 hours from the time of sample submission.30 It is recommended that at least 500 mls of urine should be collected, although I have received positive results with smaller volumes when more was not available.
Because of the acantholytic effect of the toxin on the squamous gastric mucosa, gastroscopy is very helpful in the early diagnosis of cantharidin toxicosis, and is a routine procedure in our clinic in suspected cases. Supportive evidence can also be obtained by endoscopic examination of the urethra and bladder. Petechial to ecchymotic hemorrhages of the urethral and bladder mucosa are typically present.
Overall treatment goals include eliminating continued toxin exposure and reducing toxin absorption, pain management, correction of fluid and electrolyte deficits, and gastrointestinal mucosal protection. The administration of activated charcoal (1-3g/kg) or mineral oil (1gallon) may enhance toxin elimination if given early. Cantharidin is lipid soluble and mineral oil is poorly absorbed, therefore it may potentiate toxin removal.14 If the clinician decides to use both compounds in clinical management, they should be administered several hours apart to avoid interaction.20
Pain control may be achieved with flunixin meglumine in some cases, but often additional analgesics are necessary. Flunixin meglumine dose and frequency should be minimized in most cases due to concurrent gastrointestinal irritation and dehydration. Xylazine and/or detomidine are also often used for analgesia. Occasionally in severely painful horses butorphanol is more useful, especially as a continuous rate infusion. Recent studies in rats suggest that cantharidin interferes with the analgesic properties of alpha 2 antagonists, but not narcotic kappa agonists (butorphanol).31,32 Acepromazine should be avoided because it may potentiate hypotension.
Fluid therapy is indicated to correct dehydration, electrolyte (hypocalcemia and hypomagnesemia most commonly) and acid base abnormalities, and promote diuresis. Fluid administration rates are dictated by the severity of hemoconcentration and shock, if present. In severely affected horses large bore (10-12 gauge) catheters allow more rapid delivery of balanced electrolytes. Because ionized calcium is the physiologically active form and it can be influenced by acid-base status; serum ionized calcium and blood pH should be assessed when possible. Frequently the severity of hypocalcemia requires large (500ml 23% calcium gluconate) repeated doses for correction of serum calcium concentrations. This may need to be continued for several days, based on repeated chemistry analysis. It is safer to administer calcium diluted in non-bicarbonate containing isotonic fluids through a separate catheter at a slower rate than fluids administered for shock support to avoid cardiac toxicity.
Gastrointestinal lesions commonly include gastric squamous epithelial ulceration and glandular mucosal irritation, as well as small intestinal and colonic hemorrhage, edema and inflammation.29 I typically administer sucralfate orally (10g q 6 hr) as a gastrointestinal protectant. In addition, omeprazole (4mg/kg PO q 24h) may be given to reduce gastric acidity, although long term treatment may not be indicated.
A number of case studies of horses affected with cantharidin toxicosis have been published.14,33,34 Later studies, and clinical experience suggests that with early diagnosis, and aggressive treatment, the majority of horses with cantharidin toxicosis recover.
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