Bufadienolides

Bufadienolides, or bufagins, are structurally similar to cardiac glycosides and comprise at least 8% of the content of the dried secretions in R. marina, with marinobufagenin (also called marinobufagin) being the most common.^16,17 The chemically-related bufotoxins are also found in the secretions but are less potent than bufadienolides.^1,12 Despite being relatively lipophilic, bufadienolides are reported to be poorly absorbed from the gastrointestinal (GI) tract of dogs and cats.^1,18 What is absorbed across the GI mucosa peaks in the circulation within 30 min, and is then fairly quickly eliminated, with half-lives of 2–4 hr being reported in a rodent model, the only species in which the toxicokinetics of bufadienolides have been reported.¹⁹ Bufadienolides cross the blood-brain barrier and can inhibit their own P-glycoprotein-mediated efflux, which the authors speculate serves to concentrate these compounds in tissues including the central nervous system (CNS).^20,21

One question that has not been well addressed by prior studies is the extent to which absorption of bufadienolides occurs across the oral mucous membranes, which is arguably the region of the GI tract where exposure to toxicants in toad secretions is the greatest. Sublingual or buccal absorption bypasses metabolism in the rest of the GI tract and first-pass hepatic metabolism, resulting in greater systemic exposure to unmetabolized bufadienolides.²² While absorption from the oral cavity of the cardiac glycoside, ouabain, has been shown to occur in a canine ex vivo model and in humans, to the authors' knowledge there are no studies that provide evidence of oral absorption of other cardiac glycosides or bufadienolides.^22,23 It has been theorized that vomiting of the swallowed secretions prevents significant GI absorption of the toxic constituents of the secretions.²⁴ However, if absorption through the oral mucous membranes is the primary route of exposure, profuse salivation and vomiting seen with R. marina exposure may be important in reducing the absorption of toad secretions. Even if bufadienolides are poorly absorbed, the evidence suggests that bufadienolides have a significant role in R. marina toxicosis because of their high degree of potency, with lethal doses measured in the decigrams/kg range.¹

Bufadienolides exert similar effects to cardiac glycosides by inhibiting the sodium, potassium-adenosine triphosphatase (Na⁺, K⁺-ATPase) pump.²⁵ Pump inhibition results in intracellular sodium and calcium accumulation and potassium depletion, manifesting as increased excitability of cardiac Purkinje fibers and neurons.^13,26,27 Bufadienolides also affect voltage-gated sodium, potassium, and calcium channels, further destabilizing excitable membranes.²⁷ These changes ultimately lead to arrhythmias and CNS stimulation.^1,28 Additionally, transcellular shifting of potassium out of the cell from inhibition of the Na⁺, K⁺-ATPase pump, possibly in combination with acidemia, may result in hyperkalemia.^29,30 It should be noted that the clinical signs seen with R. marina toxicosis in dogs differ from what is seen with cardiac glycoside toxicosis in dogs and other species, with cardiac effects being more prominent when cardiac glycosides are the sole source of toxicosis.^28,31,32

Catecholamines

Catecholamines found in the secretions of R. marina include epinephrine, comprising at least 5% of the content of dried secretions, and much smaller amounts of norepinephrine and dopamine.¹ GI absorption of catecholamines is minimal due to the presence of inactivating enzymes in the GI mucosa. However, epinephrine is well absorbed sublingually, with an initial peak in the plasma occurring as early as 10 min post-administration.³³

While catecholamines alone can be arrhythmogenic, their ability to cause arrhythmias increases when combined with other arrhythmogenic compounds.²⁸ Otani et al. reported that the cardiac glycoside, ouabain, and epinephrine together better reproduced the cardio-respiratory effects of IV administration of R. marina toxins than either substance alone.⁸ Additionally, the bufadienolide, bufalin, has been shown to increase the release and decrease the reuptake of norepinephrine, elevating synaptic norepinephrine concentrations, in a canine saphenous vein model.³⁴ These findings suggest that catecholamines and bufadienolides may act synergistically to produce the clinical effects of R. marina toad toxicosis.

Indolealkylamines

Indolealkylamines are compounds derived from serotonin. Dehydrobufotenine is the most common indolealkylamine found in R. marina secretions.^14,35 Daly et al. reported large amounts of dehydrobufotenine in the parotoid glands but did not quantify the amount.³⁶ Serotonin has also been isolated from R. marina secretions, but only in small amounts that comprise 0.1% of the content of dried secretions.⁹ Because many indolealkylamines are polar compounds, they are limited in their ability to cross biologic membranes within the physiologic pH range.³⁷ This suggests that dehydrobufotenine is even more limited in the ability to traverse membranes because it is an ionized quaternary ammonium compound.

Although it is questionable whether indolealkylamines are absorbed to an extent sufficient to contribute to the toxicity of R. marina secretions, many of the clinical effects of R. marina toxicosis resemble those of serotonin excess. Serotonin intoxication or serotonin syndrome typically manifests in vomiting, mydriasis, tachycardia, tremors, and seizures in dogs.^38–41 Because the extents of neurologic and cardiovascular signs of R. marina toxicosis in dogs do not strictly resemble what is seen with cardiac glycoside toxicosis, there may be other mechanisms by which serotonergic compounds play a role in R. marina toxicosis. For example, cardiac glycosides have been shown to increase the release of endogenous serotonin in the CNS.⁴²

Decontamination

Oral lavage is the most important step in decontamination. Because it is vital to remove the toad's secretions from the oral cavity as soon as possible to limit continued absorption of the toxins, this should be performed by the owners at home if the patient is conscious and the exposure has just been witnessed.² The owner should be directed to lavage the mouth with water from a garden hose. The spray should be directed from the commissure of the mouth in a rostral direction and the patient's head should be facing down in order to avoid iatrogenic aspiration. If the patient is seizuring, oral lavage should be performed by the veterinarian using water-soaked gauze and airway protection as needed.

Although the efficacy of administering activated charcoal for R. marina toxicosis has not been evaluated, it has been shown to reduce absorption of bufadienolides in an in vivo rodent model.⁴⁷ However, administration of activated charcoal is not recommended in the typical exposure scenario because it is questionable whether a significant amount of secretions traverse distal to the oral cavity, and because administering activated charcoal to patients that are vomiting or displaying CNS signs is contraindicated. Activated charcoal may be more important if other forms of toad secretions, such as traditional Chinese medications, which contain dried secretions, or if whole toads are ingested. In the latter case, endoscopic or surgical removal, or multiple doses of activated charcoal, has been recommended.^13,48 Cholestyramine can be used to treat cardiac glycoside toxicosis but has been shown to be inferior to activated charcoal in preventing absorption of digoxin, suggesting that it may be less efficacious than activated charcoal in treating R. marina toxicosis.⁴⁹

Anti-emetic Therapy

Ptyalism and hyperemia of the oral membranes seen with oral exposure to R. marina are attributed to local irritant effects of the toxins. Atropine should not be utilized to reduce oral secretions, since hyposalivation can potentially decrease the dilution of toxins remaining in the oral cavity, and atropine may potentiate cardiac dysrhythmias.³ Possible causes of vomiting from R. marina exposure include direct stimulation of the GI tract, which afferently projects to the medullary emesis center, and stimulation of the chemoreceptor trigger zone via blood-borne toxins.^1,3 Dolasetron and ondansetron are centrally-acting anti-emetics that are antagonists of 5-HT₃ serotonin receptor subtypes. These medications may provide additional efficacy if some of the emetic effects are mediated via the indolealkylamine constituents of R. marina secretions. Maropitant may be beneficial due to its action as a NK-1 receptor antagonist both centrally and locally in the GI tract.⁵⁰ The effectiveness of specific anti-emetics has not been evaluated for treating R. marina toxicosis.

Anticonvulsant Therapy

The benzodiazepines diazepam or midazolam can be used to control seizure activity.^2,3,43 In cases that are refractory to initial therapy with a benzodiazepine, a continuous propofol drip in combination with a benzodiazepine allows for intubation.⁵¹ In cases the authors have managed, a combination constant-rate infusion of propofol and midazolam reduced seizures and anxiety associated with R. marina intoxication until the clinical signs abated, which typically occurred in 4–12 hr. Instituting long-term, anti-epileptic medications (e.g., phenobarbital, potassium bromide, levetiracetam) is not necessary as the seizures are due to an acute intoxication and completely resolve in most patients. If hyperthermia develops from tremors or seizure activity, standard cooling measures should be instituted.⁵¹

Respiratory Therapy

Significant respiratory or ventilatory compromise may be present requiring oxygen supplementation or positive pressure ventilation, respectively.⁴³ Flow-by or nasal oxygen, or an oxygen chamber, should be provided as a conservative measure in the more stable patients.^52,53 Patients should be monitored for the development of non-cardiogenic pulmonary edema and, when pre-existing occult heart disease is present, judicious use of fluid therapy and monitoring for cardiogenic pulmonary edema is important.⁴³

Treating Electrolyte and Acid-Base Imbalances

Monitoring acid-base and electrolyte abnormalities will allow for early recognition and correction of any potentially life-threatening abnormalities, as well as assist in the diagnosis of occult shock. Abnormalities, such as hyperkalemia and metabolic or respiratory acidosis, are best treated using standard therapeutic modalities for these conditions.⁴³

Cardiovascular Therapy

No specific drugs are inherently more efficacious for anti-arrhythmic therapy due to R. marina toxicosis so the choice of an anti-arrhythmic should be based upon the type of arrhythmia.⁴³ Magnesium levels should be evaluated and corrected as needed. Parenteral magnesium may be beneficial, particularly in the management of ventricular arrhythmias, because hypomagnesemia potentiates the effects of cardiac glycosides.⁵⁴ Magnesium is required for normal functioning of the Na⁺, K⁺-ATPase pump, so it is vital for maintaining the normal resting membrane potential of excitable membranes.⁵⁵ Magnesium also modulates the binding of cardiac glycosides to the Na⁺, K⁺-ATPase pump, so its presence serves to antagonize the effects of cardiac glycosides on pump functioning.^54,56 Magnesium sulfate (0.15 to 0.3 mEq/kg IV over 10 min) can be used to correct hypomagnesemia.⁴³ Ionized magnesium or, at minimum, total magnesium should be monitored to guide therapy. If monitoring is not available, magnesium sulfate should be used cautiously, especially in patients with known decreased renal function. Ionized calcium should also be monitored if magnesium sulfate is used, as chelation of calcium by sulfate anion can occur.⁵⁷

Adjunctive Therapies

Treatment with digoxin-specific antigen-binding fragments (digoxin Fab) has been used in humans experiencing cardiac effects or severe hyperkalemia due to ingestion of toad secretions. Outcomes were excellent when treatment was initiated early in the intoxication.^29,48 Although this treatment modality has not been reported for the treatment of R. marina exposure in small animal clinical cases, the use of digoxin Fab to treat severe arrhythmias or hyperkalemia due to bufadienolide exposure has been suggested for companion animals.^2,13 If digoxin Fab is instituted, serum potassium should be monitored closely over the first several hr of therapy since hypokalemia can develop due to fluid therapy enhancing renal potassium excretion during the hyperkalemic state in combination with potassium being transported back into cells as functioning of the Na⁺, K⁺-ATPase pumps is restored.⁵⁸ Also, magnesium sulfate has been shown to potentiate the effect of digoxin Fab on improving Na⁺, K⁺-ATPase pump functioning that was impaired by marinobufagenin, so extra caution is warranted if both digoxin Fab and magnesium sulfate are administered.⁵⁶

The dose of digoxin Fab to administer to dogs is uncertain. Digoxin Fab does not bind other cardiac glycosides as well as it does digoxin, so it is anticipated that higher doses of digoxin Fab are needed to treat R. marina toxicosis versus digoxin toxicosis.⁵⁹ In humans, 10 vials (38 mg/vial) are empirically recommended for toxicosis from ingestion of toad secretions.⁴⁸ Barbier et al. report an average weight of 590 mg of secretions isolated from R. marina, with bufadienolides comprising greater than 8% of the secretions, which suggests that there are over 46 mg of bufadienolides in the secretions of an average R. marina.¹⁶ If one Fab vial is required to treat an overdose of 1 mg digoxin in dogs, then it is reasonable to anticipate that multiple vials of digoxin Fab would be needed for treatment of R. marina toxicosis in dogs, although the amount required would depend on the amount of secretions absorbed.⁶⁰ One drawback is that use of digoxin Fab may be cost-prohibitive in many cases.

Intravenous lipid emulsion 20% (intralipids) has been used to treat a variety of intoxications in both human and veterinary medicine.^61,62 Intralipids may be of benefit in reducing the toxicity of the more lipophilic constituents of R. marina secretions, i.e., the bufadienolides, but there are currently no reports of its use with R. marina intoxication.

Although it is not certain whether indolealkylamines from R. marina secretions play a role in toxicosis, because some of the clinical signs suggest that there may be a serotonergic component to the clinical syndrome, therapy directed at a reduction of serotonin receptor binding may prove to be beneficial. The serotonin antagonists cyproheptadine (1.1 mg/kg per os as needed q 4-6 hr) and chlorpromazine (0.5 mg/kg IV, intramuscular, or subcutaneous q 6 hr) have been very beneficial in reducing the effects of serotonin toxicosis in dogs and should be considered as a potential adjunctive therapy in severe cases of R. marina toxicosis.^39,41,51 If chlorpromazine is employed, its potential hypotensive effect should be weighed against possible benefits.

Prognosis

With early, appropriate treatment, the outcome of R. marina toxicosis is considered excellent, with a marked resolution of signs typically occurring within the first 12–24 hr, even for severely affected patients.^2,3,10,11,13 Although early reports suggest a high mortality rate, more recent reports suggest at least 96% survival with early supportive and symptomatic therapy.^2,3 In one case series, the dogs that died developed seizures in addition to other neurologic abnormalities, but seizures do not necessarily indicate a poor prognosis since only 18% of dogs that developed seizures died.² The authors conclude that the prognosis associated with R. marina toxicosis is influenced by the amount of toxin ingested, the size of the dog, and the administration of proper, prompt and appropriate medical treatment.^2,3,10,11,13