Acute onset of generalized tremors, mild ataxia, and hyperesthesia in a young dog after presumptive ingestion of a giant sea hare belonging to the genus Aplysia

Introduction

Giant sea hares are marine mollusks that can grow to 40 centimeters or more in length and are usually light to dark brown or black in color. They are aptly named as they typically have a rounded body shape with long rhinophores on their heads, giving an overall appearance of a sitting rabbit or hare. Giant sea hares have large wing-like lateral body flaps known as parapodia that are used to assist them in swimming for limited periods of time. They have been reported to aggregate in large numbers to spawn in shallow waters during summer months. Mass beach stranding incidents involving hundreds of giant sea hares have been reported during this period of increased spawning activity.1 If washed ashore, sea hares may present as a potential food source to unsuspecting mammalian scavengers. Even though many different bioactive substances and potential toxins have been isolated from sea hares,2-6 sea hare toxicosis in mammals is rarely reported. To the author's knowledge, there are only 3 medical case reports of sea hare toxicosis in people7-9 and no case reports in the veterinary literature. The author reports the clinical course and successful treatment of a young dog with acute sea hare intoxication.

Case Report

A 1-year-old neutered male Poodle cross weighing 6 kg presented approximately 30 minutes after he was seen partially ingesting a giant sea hare washed up at a local beach. The owners had taken their dog to the beach for a short walk of less than 30 minutes duration and reportedly kept him in close sight despite being let off the leash. The dog walked along the beach sand throughout the walk, did not enter the water and was not seen drinking any sea water at any point. The owners report that their dog ran ahead of them and witnessed him ingesting part of a washed up dead giant sea hare before they could stop him. The owners described the sea hare as a large dead specimen longer than at least 30 centimeters and brown in color. There were several other similar specimens on the same beach, suggestive of a mass stranding incident. The specimens were later positively identified by a local marine biologist as giant sea hares belonging to the genus Aplysia. The owner noticed the dog shaking uncontrollably within 20 minutes after the sea hare ingestion. The dog had no prior history of any neurological problems and was otherwise a healthy dog. There was no known exposure to tremorgenic mycotoxins (eg, moldy foods) or household/gardening toxins (eg, metaldehyde or methiocarb) and the house had not been treated with any pesticides (eg, pyrethrins/pyrethroids). There was no vomiting or diarrhea prior to the development of the tremors.

On physical examination, the dog was mildly ataxic with generalized mild tremors and mild hyperesthesia. The tremors did not resolve at rest and appeared to be exacerbated with handling and movement. He had a heart rate of 168/min and was panting heavily. His rectal temperature was 38.8°C. His mucous membranes were moist and pink and he had a capillary refill time of < 2 seconds. Oral examination did not reveal any foreign material. A normal gag reflex was still present. He had normal sized pupils and normal pupillary light reflexes. Abdominal palpation did not localize any pain. The skin and fur did not have any obvious characteristic smell or other physical evidence of dermal exposure to chemicals. Baseline blood results were unremarkable and included a packed cell volume of 0.42 L/L [42%] (reference interval, 0.37–0.55 L/L [37–55%]), sodium 148 mmol/L (148 mEq/L) (reference interval, 136–154 mmol/L [136–154 mEq/L]), potassium 4.0 mmol/L (4.0 mEq/L) (reference interval, 3.4–5.3 mmol/L [3.4–5.3 mEq/L]), chloride 109 mmol/L (109 mEq/L) (reference interval, 96–113 mmol/L [96–113 mEq/L]), glucose 4.8 mmol/L (87 mg/dL) (reference interval, 3.6–6.8 mmol/L [65–123 mg/dL]), total calcium 2.8 mmol/L (11.2 mg/dL) (reference interval, 2.0–3.0 mmol/L [8–12 mg/dL]), phosphorus 1.32 mmol/L (4.1 mg/dL) (reference interval, 0.8–2.2 mmol/L [2.5–6.8 mg/dL]), blood urea nitrogen [BUN] 5.9 mmol/L (16.5 mg/dL) (reference interval, 2.5–9.6 mmol/L [7.0–26.9 mg/dL]), creatinine 67 μmol/L (0.8 mg/dL) (reference interval, 44–159 μmol/L [0.5–1.8 mg/dL]), cholesterol 5.4 mmol/L (209 mg/dL) (reference interval, 2.9–8.3 mmol/L [112–320 mg/dL]), amylase 825 U/L (825 units/L) (reference interval, 500–1500 U/L [500–1500 units/L]), alanine aminotransferase [ALT] 61 U/L (61 units/L) (reference interval, 10–100 U/L [10–100 units/L]), alkaline phosphatase [ALP] 149 U/L (149 units/L) (reference interval, 23–212 U/L [23–212 units/L]), total bilirubin 12 μmol/L (0.7 mg/dL) (reference interval, 0–15 μmol/L [0–0.9 mg/dL]), total protein 73 g/L (7.3 g/dL) (reference interval, 52–82 g/L [5.2–8.2 g/dL]), albumin 36 g/L [3.6 g/dL] (reference interval, 27–39 g/L [2.7–3.9 g/dL]), globulin 37 g/L [3.7 g/dL] (reference interval, 25–45 g/L [2.5–4.5 g/dL]).

Successful emesis was achieved via application of subconjunctival apomorphine1 at 0.5 mg/kg body weight. The conjunctiva was flushed thoroughly with sterile 0.9% saline after emesis commenced. The vomitus consisted predominantly of blackish soft jelly-like portions that appeared to be sea hare remnants and a few commercial dog biscuits that the dog had eaten for breakfast. Approximately 15 minutes after emesis commenced, he was given 5 mg/kg metoclopramide2 IV and his tremors were treated with 50 mg/kg methocarbamol3 IV given slowly over 10 minutes. Upon checking again that a normal gag reflex was still present, he was then given oral activated charcoal4 1 g/kg and 40% (0.4 g/mL) sorbitol^d at a dose of 5 mL/kg (equivalent to 2 g/kg).

The symptoms had improved within 20 minutes after treatment but were not completely resolved. After 45 minutes, there was clinical improvement and only very mild fine muscle tremors were observed in the hindlegs. A second dose of 20 mg/kg methocarbamol was administered orally approximately 2 hours after the first dose was given IV. The mild muscle tremors were completely resolved within an approximate 6 hour period. The patient was monitored in hospital and was discharged to the owner approximately 6 hours after admission with normal mentation and normal gait. The following day, the owner reported that the dog was walking and acting normally. He was examined 6 months later during a routine health check and appeared to be bright, alert, and active. The owner reported no residual effect of the initial episode and no further recurrence of symptoms.

Discussion

Acute generalized tremors in a young and previously healthy dog is highly suggestive of toxicosis. A presumptive diagnosis of acute sea hare toxicosis was made in this case due to the history of the dog being seen by the owner to ingest a portion of a sea hare at the beach, the acute onset of symptoms, the finding of sea hare remnants in the vomitus, and the lack of any known exposure to other potential toxins. There are no known clinical or blood biochemistry findings pathognomonic for sea hare toxicosis. Sea hares are commonly found in tropical and temperate waters and belong to the family Aplysiidae. Sea hares only have a soft body and an atrophied inner shell. For example, in the genus Aplysia, the shell is a soft flattened plate over the visceral rear end, where it is fully or partially enclosed in the mantle skin. In Dolabella auricularia, the shell is ear-shaped. The shell is only present in the larval stage of the 2 genera Bursatella and Stylocheilus. The apparent lack of a physical defense against marine predators appears to be balanced by a host of chemical defenses instead, comprised of an array of toxic compounds that have been isolated from various species of sea hares.2-6 Depending on the sea hare species and potentially the geographic location, sea hare toxicity cases in humans appear to be divided into 2 distinct presentations, the first consisting primarily of acute neurological disturbances7 and the second consisting of liver damage without neurological symptoms.8, 9 Nausea was the only common symptom that accompanied all 6 human cases.7-9

Sorokin7 first reported a human case of toxicosis after ingestion of the sea hare Dolabella auricularia. The human patient presented with ataxia and muscular twitching, pyrexia, vomiting, and leukocytosis. Sorokin suggested that the neurological symptoms were due to a type of naturally occurring bromine in the sea hare's digestive gland, derived from the bromine-containing diet. Hino et al8 reported 4 cases of acute liver damage in people following the ingestion of a sea hare egg belonging to the Aplysiidae family. All 4 patients developed nausea and dark urine approximately 12 hours after eating a boiled sea hare egg with some beer and food. Two of the patients developed jaundice 2 days after ingestion. Laboratory tests showed increased concentrations of serum ALT and lactate dehydrogenase (LDH) in all 4 patients, and hyperbilirubinaemia, positive urine bilirubin, and urobilinogen in the 2 jaundiced patients. One of the jaundiced patients had a liver biopsy performed on the 8th day after the onset of symptoms. The biopsy findings revealed lipofuscin-laden macrophages, indicating the presence of acute liver necrosis. All the patients recovered within 1 month after conservative treatment. Sakamoto et al9 reported a case of a man presenting with nausea, vomiting, and abrupt chilling 6 hours after eating 2 pieces of the sea hare Aplysia kurodai. He rapidly developed acute liver damage with characteristic histological findings presenting apoptosis of numerous hepatocytes. It was suggested that bioactive substances such as aplysianins might have caused the liver damage. No neurological disturbances were reported in the 5 human cases that developed liver damage.

The exact toxin responsible for the neurological symptoms in this human case is unknown, although Sorokin7 suspected an organobromine as the neurotoxin. Kicklighter et al3 reported that when under attack by spiny lobsters, sea hares of the genus Aplysia will secrete a defensive ink and opaline combination cloud that can cause massive and sustained excitation of the chemosensory neurons in the attacking lobsters. These ink-opaline secretions contain millimolar quantities of amino acids that stimulate chemoreceptor neurons in the lobster's nervous system. The neuroexcitatory effect of these amino acids on mammalian cells is still unclear.

Kato and Scheuer6 extracted aplysiatoxin (C₃₂H₄₇BrO₁₀) and debromoaplysiatoxin (C₃₂H₄₈O₁₀) from the sea hare Stylocheilus longicauda and reported that accidental contact with the toxic extract caused redness to human skin as well as redness and swelling of ocular and nasal mucous membranes. Approximately 0.25 g of aplysiatoxins were derived from 1 kg of wet sea hare. The purified toxin was shown to have an LD₁₀₀ of 0.3 mg/kg in mice when injected intraperitoneally.6 Aplysiatoxins have also been shown to be potent protein kinase C (PKC) activators.10, 11 PKC is a multifunctional family of isoenzymes that phosphorylate hydroxyl groups of the serine and threonine amino acid residues on many target proteins, resulting in a wide variety of cellular processes via signal transduction.12 These vital cellular processes include neuronal excitability and neurotransmitter release. The high concentrations of PKC in the nervous system compared to many other tissues suggest that PKC plays an important role in the regulation of neuronal activity.13 Previous studies using PKC activators have indicated that PKC regulates calcium, potassium, and chloride channels, and that increased PKC activity appears to enhance neurotransmitter secretion by mammalian nerve cells.13-15 Increased PKC activation may therefore potentially result in neurological symptoms. Nagai et al5 reported that mice poisoned with aplysiatoxin and debromoaplysiatoxin showed diarrhea, lethargy, muscular contractions, and sometimes hind leg paralysis. The lethal doses of aplysiatoxin and debromoaplysiatoxin in mice were demonstrated to be approximately 0.25 mg/kg and 0.5 mg/kg, respectively.5 Aplysiatoxin and debromoaplysiatoxin compounds isolated from the sea hare Stylocheilus longicauda have also been found in the red algae Gracilaria coronopifolia and certain marine blue-green algae.5 Nagai et al5 also observed blue-green algal colonization of red algae and suggested that the marine blue-green algae may be the true origin of the toxins. Capper et al16 reported varying high concentrations of lyngbyatoxin-a (C₂₇H₃₉N₃O₂) and debromoaplysiatoxin in the body tissues and excretions of 2 species of sea hares (Stylocheilus striatus and Bursatella leachii) that grazed directly on the marine blue-green algae Lyngbya majuscula. Lyngbyatoxin-a is a PKC activator12 and a highly inflammatory and vesicatory indole alkaloid toxin.17 A recent study also showed a possible route for potent freshwater blue-green algal hepatotoxins to contaminate marine coastal ecosystems. Miller et al18 demonstrated strong evidence for significant and recurrent marine pollution by freshwater-derived microcystins within North America's largest national marine sanctuary, resulting in deaths of marine species including threatened southern sea otters. Analysis of marine bivalves in the sanctuary showed bioaccumulation of microcystins, which are potent hepatoxins produced by freshwater blue-green algae Microcystis sp. Although sea hares were not analyzed in that study, it showed a potential route for freshwater blue-green hepatotoxins to accumulate in sea hares. The most commonly reported signs in dogs following recent exposure to hepatotoxic blue-green algae include lethargy, vomiting, diarrhea, depression, weakness, pallor, and shock. The signs usually appear within 1 to 4 hours of exposure. Death can occur within 1 to 5 days.19 Signs of exposure to neurotoxic blue-green algae include an abrupt onset (within 30 to 60 min of exposure) of salivation, lacrimation, urination, defecation, muscle rigidity, tremors, seizures, paralysis, respiratory paralysis, and death within 1 hour.19 The apparent adaptation of sea hares that allows them to feed directly or indirectly on otherwise toxic blue-green algae and subsequently sequester the toxic metabolites may suggest a possible role in their natural defense mechanisms. It has therefore been postulated that sea hares may derive their toxic compounds directly from their algal grazing diet.12, 16

The rate of gastrointestinal absorption of sea hare toxins in dogs is considered to be rapid, based on the author's observation that symptoms developed within 20 minutes after sea hare ingestion. The author suggests that the treatment in the early stage of sea hare toxicosis involve preventing further toxin absorption by gastric emptying (either via emesis or lavage), and the use of activated charcoal and cathartics. Gastric emptying methods are most effective if performed within the first 60 minutes of exposure of toxin ingestion; otherwise, the efficacy of the procedure is greatly reduced.20 Emesis has been reported to be more effective at emptying the stomach than gastric lavage;21 however, there is an inherent risk of aspiration with inducing emesis in patients that are already exhibiting neurological symptoms. In this case, the patient still had a strong gag reflex on examination and the neurological symptoms were deemed to be only mild at the time of emesis. A decision was therefore made to induce emesis as it was considered the more effective and less risky method of emptying the stomach contents (compared with gastric lavage under general anesthesia). Apomorphine is a synthetic opiate that stimulates dopamine receptors in the chemoreceptor trigger zone to induce emesis.20-22 It can be administered at a dose of 0.02 to 0.04 mg/kg either via the IV, IM, SC, or subconjunctival route.20 If administered conjunctivally, the conjunctival sac should be rinsed gently with sterile saline once emesis has commenced to prevent ongoing vomiting and to minimize ocular irritation.20, 22 Known side effects of apomorphine include protracted vomiting, restlessness, excitement, and central nervous system depression.22 Gastric lavage may be indicated when emesis is contraindicated (eg, loss of gag reflex, severe neurological symptoms) or when emesis has failed. The risks of gastric lavage include aspiration pneumonia, postlavage vomiting, and mechanical injury to the throat and esophagus. Adverse effects such as electrolyte imbalances, hypoxia, and hypercapnia have also been reported in humans undergoing gastric lavage.22 In human patients, gastric lavage is no longer recommended in the routine treatment of poisoned patients due to the lack of evidence that it improves clinical outcomes and because of the potential for increased morbidity.22 Activated charcoal is a carbonaceous compound that is activated by an oxidizing gas at high temperatures to break down the carbon into smaller granules with larger surface areas. The larger surface area allows many binding sites for adsorption of toxins.20-22 Activated charcoal can be given every 4 to 8 hours and could potentially be given for several days. The recommended dose is 1 to 4 g/kg.22 Not every toxin is adsorbed effectively by activated charcoal and it is unknown how effectively activated charcoal adsorbs sea hare toxins. Activated charcoal administration may cause constipation and if the activated charcoal movement through the gut is delayed, the adsorbed toxins can move off the charcoal and can be absorbed by the gut. The benefits of using activated charcoal is threefold—it directly adsorbs any potential toxins ingested in the stomach, it may also help reduce enterohepatic recirculation of toxins and finally, it helps to adsorb any toxins that may have moved down their concentration gradient from the bloodstream back into the gut lumen.20-22 A cathartic (sorbitol) was used to speed gastrointestinal transit time and subsequent elimination. Care must be taken particularly in smaller patients as excessive use of sorbitol can lead to diarrhea and potential dehydration.20, 22 Multiple dosing of sorbitol can be potentially dangerous and intensify side effects such as vomiting, nausea, abdominal cramps, dehydration, and possible hypotension.20 Tremors resulting from sea hare toxicosis may be controlled initially with methocarbamol administration. The author also noted that keeping the patient in a quiet and dark environment and minimizing handling once the initial decontamination procedures were performed, assisted in reducing the excitement of the hyperesthetic patient and the overall severity of the tremors. Methocarbamol is a centrally acting muscle relaxant and a carbamate derivative of guaifenesin; however, the exact mechanism of action is not well established.23 The recommended dose is 44–220 mg/kg PO or IV. The dose used is dependent on the severity of the symptoms and can be repeated every 6 to 8 hours up to a maximum daily dose limit of 330 mg/kg.23, 24 If given via the IV route, the solution can be diluted with 0.9% saline to reduce the risk of thrombophlebitis. The rate of intravenous methocarbamol administration in human patients has been suggested to be < 5 mg/kg/min to prevent significant hypotension.23 Currently available formulations of injectable methocarbamol contain polyethylene glycol (PEG) 300 as a solvent vehicle and should not be administered in patients with renal disease.23 The efficacy of diazepam or midazolam to control tremors associated with sea hare toxicoses is unknown. There were several benefits of administering an antiemetic in this case. First, it was administered to reduce the emetic effects of apomorphine once the gastric contents were emptied. Second, the administration of activated charcoal can sometimes cause nausea. Third, acute vomiting is a potential side effect of administering high doses of methocarbamol.24 It is anticipated that more severe cases of sea hare toxicosis may require general anesthesia, decontamination procedures such as gastric lavage and enema, supportive therapy such as IV fluids and positive ventilation if apnea develops. At the time of the dog's presentation, the author was not aware of potential sea hare hepatotoxicity and recommends assessing ongoing liver function with future cases of sea hare toxicosis.

In conclusion, sea hare toxicosis is rarely reported and although a few potential sea hare toxins have been isolated in previous studies, further research is required to determine the exact nature and mechanism of their toxicity in people and dogs.