Acute penitrem A and roquefortine poisoning in a dog

Abstract

Penitrem A and roquefortine poisonings were diagnosed in a Laborador retriever following garbage consumption. Clinical signs of mycotoxicosis included polypnea, tachycardia, and ataxia that quickly progressed to lateral recumbency and seizures. Removal of the mycotoxins from the stomach soon after ingestion allowed the dog to recover within 72–96 hours.

A 45-kg, 4-year-old, castrated male, Labrador retriever had vomited once and was presented for lethargy, excessive panting, trembling, and incoordination. That morning, the dog had been outside unsupervised, with access to restaurant garbage.

The dog was anxious, ataxic, and full-body tremors were evident. The heart rate was elevated but difficult to determine due to the polypnea. Body temperature was elevated (40.9°C) and both pupils were dilated. Mucous membranes were pink, and capillary refill time (CRT) was normal (1 s). Collection and immediate freezing of the vomitus was requested to help with diagnosis, treatment, and prognosis. Because of the severity of its condition, the dog was to be transferred to an emergency clinic for 24-hour care, but convulsions began before this was possible. Seizures, but no foreleg paddling, and hyperesthesia were controlled by administration of diazepam (Valium 10; Rocher, Mississauga, Ontario), 3 boluses, each 0.22 mg/kg body weight (BW) given, IV, at 3-minute intervals. Lactated Ringer's solution (Baxter, Toronto, Ontario) was administered, IV, at a rate of 90 mL/kg BW/h. All tremor activity ceased after administration of phenobarbitol sodium (Phenobarbitol; Abbott Laboratories, Toronto, Ontario), 2 boluses, each 2.7 mg/kg BW given, IV, at a 5-minute interval.

Acute toxicosis was suspected. Because of the possibility of septic shock, or anaphylactic shock, or both, the dog was treated with ampicillin sodium (Ampicillin; Novopharm, Toronto, Ontario), 22.2 mg/kg BW, IV, and tripelennamine HCl (Vetastim; rogar/STB, Montreal, Quebec), 0.5 mL/10 kg BW, IM. In addition, prednisolone sodium succinate (Soludelta Cortef; Upjohn, Orangeville, Ontario), 22.2 mg/kg BW, was given IV. Gastric lavage with warm water was performed, after anesthesia had been induced with a bolus of propofol (Rapinovet; Schering-Plough, Pointe-Claire, Quebec), 2 mg/kg BW, IV, and was maintained with isoflurane (Aerrane; Janssen, Toronto, Ontario). Activated charcoal (50 g) was administered. Hyperthermia was treated by placement of cold wet towels over the animal's body during anesthesia. Heart rate (96 beats/min) and body temperature (38.1οC) were normal before transport to the emergency clinic. Diazepam (Valium; Sabex, Boucherville, Quebec) was administered in an IV drip (0.2 mg/mL) before and during the 30-minute trip.

On arrival at the emergency clinic, the dog was panting and had begun to paddle with his forelegs. Rectal temperature (38.5°C), CRT (< 2 s), and mucous membranes were normal. An electrocardiogram revealed a regular cardiac rhythm with an increased heart rate (160 beats/min). The dog was maintained on diazepam, 0.5 mg/kg BW/h, IV, for 4 h. Administration rate was halved at 2-hour intervals during the next 6 h. Intravenous electrolytes (Plasma-lyte 148; Baxter, Toronto, Ontario) were administered, as required. Ampicillin sodium was continued at 22.2 mg/kg BW, IV, q8h, for 24 h. Lateral and dorsoventral thoracic radiographs taken on day 2 revealed signs of aspiration pneumonia. Ampicillin was discontinued and cefoxitin sodium (Cefoxitin; Novopharm, Toronto, Ontario) was prescribed at 22.2 mg/kg BW, IV, q6h, for 24 h, then q8h for 24 h. Pulse oximetry readings of blood oxygen saturation were below normal (80%). Nasal catheterization with 100% oxygen therapy (112 mL/kg BW/min for 20 h) increased blood oxygen saturation to levels greater than 95%. By day 4, the dog was free of abnormal neurological signs and the pneumonia was treated at home with amoxicillin and clavulinic acid (Clavamox; Pfizer, London, Ontario), 11.1 mg/kg BW, PO, q12h, for 14 d.

Gross and microscopic examination of the vomitus revealed blades of grass, pieces of raw tenderloin, and a cereal-based substance resembling flour. A thin layer chromatography screen was used for detection of penitrem A (PA), roquefortine (RQ), or strychnine. Significant levels of PA and RQ (> 10 ppm) were identified.

Poisoning is the primary differential for a dog with acute onset of vomiting, polypnea, tachycardia, hyperesthesia, and ataxia, followed by lateral recumbency, muscle fasciculations, and seizures. Metaldehyde, mycotoxins, strychnine, zinc phosphide, bromethalin, xanthines, and insecticides must be considered. The history of garbage consumption in this case suggested that mycotoxins or xanthines were the most likely toxins. The dog exhibited mydriasis and tachycardia, clinical signs not associated with insecticides that cause cholinergic stimulation; the lavage contents lacked an acetaldehyde odor, making metaldehyde poisoning unlikely; recovery from bromethalin poisoning rarely occurs in 3 d; and strychnine poisoning is a rare occurrence.

Mycotoxins are produced by specific fungus. Only certain species of Aspergillus, Claviceps, and Penicillium produce tremorgenic mycotoxins (1), such as PA and RQ, which can cause uncontrollable neuromuscular activity in vertebrates. Different taxonomic classifications of fungal species have made it difficult for researchers to agree on the species of origin for individual mycotoxins (2,3). On the basis of morphological and physiological properties, Penicillium crustosum and Penicillium roquefortii are now considered the most common producers of PA and RQ, respectively (2). Fungal synthesis of mycotoxins is factor-dependent. For example, an ambient temperature of 25°C and a pH of 5.7 optimize production of PA by P. crustosum. Similarly, a culture medium containing 2% skim milk, potato extract, and 2% lactose induces the highest levels of PA when P. crustosum isolates are incubated for 3 wk (4). Penicillium crustosum may be isolated from several types of food, including meat, fruit, cereal and cereal products, cheese, spices, and nuts (4). Lacking appropriate environmental conditions, fungal mycelia can proliferate but fail to produce a specific mycotoxin.

Treatment for PA or RQ toxicosis should begin immediately after obtaining a suspicious history and observation of characteristic clinical signs, which usually begin 30 min to 4 h after ingestion of garbage, depending on the amount of mycotoxin consumed (5,6).

Penitrem A intoxication causes ataxia, polypnea, and sustained tremors that may progress to generalized seizures and death (5,6,7). However, as in this case, animals may vomit before any of these clinical signs appear (7). Hyperesthesia may occur, indicated by muscle fasciculations in response to noise (5). Death of the animal can be avoided by administering phenobarbitol immediately to control seizures. Clinical signs may resolve within 12 h (7), but complete recovery may take as long as 21 d (5), depending on the amount of PA absorbed (6). Without treatment, animals receiving more than 0.5 mg/kg BW of PA are likely to die within 3.5 h (6). A minimum toxic dose of PA for the dog has not been determined.

Roquefortine intoxication is characterized by vomiting, panting, muscle tremors, paddling, hyperesthesia, and seizures (8). Intravenous diazepam does not alleviate the clinical signs of RQ poisoning as it does similar signs caused by strychnine and metaldehyde poisoning (8). The prognosis for dogs with RQ intoxication depends on how soon the toxin is removed from the stomach. Barbiturate therapy is effective in controlling seizures but may not prevent death if the toxin remains in the stomach (8). A complete recovery occurs within a 24- to 48-hour period following vomiting (8).

This dog showed signs typical of both PA and RQ poisoning. Death due to RQ poisoning was likely prevented by early vomiting, and barbiturate therapy probably protected the dog against death due to PA-induced seizures. If RQ levels in the vomitus had been compared with those in the lavage solution, it might have been possible to show that vomiting did protect the dog against fatal intoxication. The effectiveness of activated charcoal in preventing fatality in animals that have ingested RQ is unknown and needs to be investigated.

The exact mechanism of action by which PA causes symptoms is uncertain. Data from postmortem examination suggest that some serum enzymes may increase due to hepatic centrolobular changes (congestion and hepatocyte degeneration and necrosis) (6). Lungs and large and small intestine are often hemorrhagic and edematous (6), which may be the result of shock.

The primary target of PA may be at the biochemical level, as a neural lesion has not been identified microscopically (9). In vitro research has demonstrated that PA alters central presynaptic release of the excitatory neurotransmitters (NTs) glutamate and aspartate, and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) (9). In response to PA, release of these NTs by inactive cortical synaptosomes increases, and release by stimulated synaptosomes decreases (9). Synapses at the neuromuscular junction are not affected (9). Research is needed to identify the in vivo synaptic effects and primary site of action of PA.

In electrophysiologic and histologic studies in rodents, seizure-sensitive areas (the thalamus and hippocampus) exhibited no abnormalities to account for PA-induced seizures (10). Rather, dose-related lesions of the cerebellar cortex may be implicated. At a dose of < 0.5 mg/kg BW of PA, some degeneration of Purkinje cells were observed (10). After inoculation with a low dose of PA, animals appeared to be most severely affected within 30 min and recovered within 24 h (10). High doses of PA caused extensive loss of Purkinje cells and vacuolization of the molecular layer of the cerebellar cortex (10). Lesions of this magnitude caused significant neuromotor deficits that persisted for more than a week (10). Breton et al (10) theorized that the cerebellar lesions are caused by PA's interference with release mechanisms for glutamate and aspartate, the neurotransmitters for the primary excitatory inputs to Purkinje cells. These inputs are believed to be secreting or transmitting GABA (GABAergic). Upon exposure to PA, excitotoxic cell damage to the Purkinje cells occurs. The GABAergic axons of the Purkinje cells form inhibitory synapses with the cerebellar nuclei, which, in turn, activate the cells of the medial thalamic nucleus (10). This may be the mechanism by which lesions in the cerebellum, a structure responsible for motor coordination, cause seizures, a clinical sign specific to thalamocortical damage.

No studies have investigated the mechanism of action of RQ. Clinical signs are similar to those of strychnine poisoning. Strychnine competes with glycine at the postsynaptic site in the spinal cord and medulla. Research investigating the site of action of RQ is needed to aid clinicians further in effectively treating affected animals.

The possibility of intoxication with PA or RQ may be overlooked by clinicians presented with a dog exhibiting convulsions. A history of garbage consumption and sudden ataxia progressing to generalized tremors and seizures should raise suspicion of intoxication by PA, RQ, or both. Further investigation into the action of PA and RQ will allow veterinarians to make a faster diagnosis and more appropriate selection of treatment.