Pyrethroid toxicity in a Vietnamese pot-bellied pig

Abstract

A 7-month-old spayed female Vietnamese pot-bellied pig (VPBP) was presented for diffuse muscle fasciculations and seizure-like activity that had started 4 hours before presentation. The pig was stuporous and displayed diffuse involuntary gross motor movement and muscle fasciculations, as well as hypertonicity of all 4 limbs. Hematologic analysis revealed hemoconcentration, severe hyperlactatemia, moderate metabolic acidosis, increased creatine kinase and gamma-glutamyltransferase. The pig failed to respond to diazepam, propofol, methocarbamol, and supportive care, followed by general anesthesia, and was euthanized. Bifenthrin, a pyrethroid insecticide, was identified by gas chromatography-mass spectrometry of stomach contents. Pyrethroid insecticide toxicity in VPBP may result in clinical signs similar to those seen in companion animals.

Case description

A 7-month-old, 21.4 kg (47 lb) spayed female Vietnamese pot-bellied pig (VPBP) was referred to the large animal internal medicine service at a veterinary teaching hospital for evaluation of seizure-like activity. The pig had acute onset of severe tremors and hyper-responsiveness approximately 4 h prior to presentation. The pig lived indoors but was allowed loose in the backyard with dogs. The grass in the yard had been treated with a weed killer containing nitrates, but no herbicides or insecticides, approximately 1 wk before presentation. The pig was fed a commercial miniature pig feed and received fruit and vegetables as treats. The pig had no known access to toxic plants. At the time of presentation, the owners were unable to identify access to any other toxins including pharmaceuticals, household products, agricultural products, or recreational drugs. The pig was unvaccinated and had no relevant medical history.

On physical examination, the pig was stuporous and displayed diffuse involuntary gross motor movement and muscle fasciculations (Video available upon request from the corresponding author). Rectal temperature was 40.0°C, respiratory rate was 120 breaths/min, and heart rate was 176 beats/min. Cardiac auscultation was unremarkable. The pig displayed hypertonicity of all 4 limbs and diffuse muscle fasciculations, including palpebral fasciculations, which, along with the specific anatomical features of the VPBP palpebral fissures, precluded ocular examination. The pig appeared to respond to stimuli, with more pronounced fasciculations when handled. Cranial nerve examination was unremarkable but incomplete due to significant facial twitching.

Laboratory abnormalities included hemoconcentration [packed cell volume 55%; reference range (RR): 31% to 45%], neutrophilia (16 884 neutrophils/μL; RR: 3100 to 9600 neutrophils/μL), lymphopenia (2414 lymphocytes/μL; RR: 4600 to 10 000 lymphocytes/μL), hyperlactatemia (17.1 mmol/L; reference value < 2 mmol/L), hyperglycemia (7.2 mmol/L; RR: 2.4 to 4.3 mmol/L), azotemia (creatinine 168 μmol/L; RR: 44.2 to 97.2 μmol/L), hypernatremia (146 mmol/L; RR: 134 to 144 mmol/L), metabolic acidosis (total CO₂ 11.9 mmol/L; RR: 19 to 29 mmol/L), hypercalcemia (2.7 mmol/L; RR: 2.1 to 2.5 mmol/L), hyperalbuminemia (45.4 g/L; RR: 24 to 35 g/L), increased aspartate transaminase (150 U/L; RR: 58 to 100 U/L), increased creatine kinase (14 264 U/L; RR: 56 to 1236 U/L), and increased gamma-glutamyltransferase (223 U/L; RR: 22 to 64 U/L)].

Attempts were made to achieve venous access in the superficial epigastric vein but were unsuccessful due to the patient’s continuous gross motor movements. The pig was induced with midazolam (Midazolam injection, 5 mg/mL; West-Ward, Eatontown, New Jersey, USA), 0.5 mg/kg body weight (BW), IM, and then masked with inhaled isoflurane and 100% oxygen for lumbosacral cerebrospinal centesis. Under anesthesia, muscle fasciculations resolved. An intravenous catheter was placed in the right auricular vein under anesthesia. Cerebrospinal fluid (CSF) analysis revealed 2 WBC/μL and total protein of 0.66 g/L, with RBCs too numerous to count (presumed to be secondary to blood contamination of the CSF sample). Following recovery from anesthesia, fasciculations returned and the pig was treated initially with midazolam (0.4 mg/kg BW, IV), followed by phenobarbital (Phenobarbital sodium injection, 65 mg/mL; West-Ward), 4 to 5 mg/kg BW boluses, IV, up to 24 mg/kg BW in 24 h, and then propofol (PropoFlo28, 10 mg/mL; Zoetis, Kalamazoo, Michigan, USA), 1 mg/kg BW, IV, was added but there was little change in the pig’s fasciculations. Propofol continuous rate infusion (CRI) at 3 mg/kg BW per hour was initiated. Methocarbamol (Methocarbamol tablets 500 mg; West-Ward), 150 mg/kg BW, was administered per rectum, which resulted in no apparent decrease in muscle tremors. Carprofen (Rimadyl chewables, 25 mg; Zoetis, Lincoln, Nebraska, USA), 2 mg/kg BW, q24h, was administered intramuscularly as an analgesic, and pantoprazole (Pantoprazole sodium injection, 40 mg vial; West-Ward), 1 mg/kg BW, q24h, was administered intravenously as a gastroprotectant. The pig’s tremors continued to increase, in particular with noise stimuli. The pig’s creatine kinase increased to 125 848 U/L the day after admission, suggesting that ongoing muscle damage was occurring. As the tremors were refractory to more conservative management, the pig was masked with inhaled isoflurane and 100% oxygen and then maintained on inhaled isoflurane and 100% oxygen with dobutamine (DOBUTamine injection, 250 mg/20 mL; Hospira, Lake Forest, Illinois, USA), 1 to 2 μg/kg BW per minute CRI to maintain mean arterial pressures of 60 to 70 mmHg as measured by invasive blood pressure monitoring via the auricular artery. Over the following 4 h, the concentration of isoflurane needed to control the tremors steadily increased from 0.8% to 1.5%. Based on the worsening of clinical signs despite treatment, euthanasia was elected, and the pig was euthanized with pentobarbital (SomnaSol; Henry Schein Animal Health, Dublin, Ohio, USA). A postmortem examination was conducted.

Gross and histologic examination [hematoxylin and eosin (H&E) staining] revealed no lesions in the cerebellum, brain stem, cerebrum, or spinal cord. There was mild alveolar edema and histiocytosis in the pulmonary parenchyma. There was mild centrilobular to midzonal microvesicular lipidosis in the hepatic parenchyma. No additional gross or histologic lesions were noted. Direct fluorescent antibody test on fresh brain tissue detected no rabies virus antigen (Rabies virus FA; Pennsylvania Department of Health, Bureau of Laboratories, Lionville branch, Exton, Pennsylvania, USA). An organic chemical screen using gas chromatography-mass spectrometry (GCMS) was performed on stomach contents, liver, and serum according to a previously published method (1). Bifenthrin was detected in all samples as determined by both a NIST (National Institute of Science and Technology) spectral library and spectrum generated within the laboratory from previous bifenthrin analyses. Evaluation of peak height (PH) and approximate amount as compared to stomach contents (%) for extracted ion 181 indicated that largest recovery was from stomach contents (PH = 6.0e7, 100%), followed by liver (PH = 2.5e5, 41.6%), then serum (PH = 1.0e5, 16.7%). No unidentified peaks were noted on the chromatographic analysis, suggesting no additional organic compound exposures. Quantitative analysis was not performed.

Further evaluation for possible toxins in the home revealed that the pig had gained access to an ant-killer insecticide containing bifenthrin (exact brand, amount, and concentration unknown, as the owner disposed of the container immediately following discovery), with presumed ingestion of the product having occurred. No other additional potential toxin history was discovered.

Discussion

Analysis of stomach contents confirmed the presence of bifenthrin in this case, suggesting ingestion and subsequent toxicosis in the pig of this report. Bifenthrin, a type II pyrethroid, is a contact insecticide and acaricide used to treat crops and stored grain and to prevent termite infestation (2). In Canada, bifenthrin is used to control weevils in raspberries and wireworm on potatoes. However, bifenthrin registration was cancelled by the Pest Management Regulatory Agency in Canada in 2017. Bifenthrin acts on voltage-gated sodium channels as well as chloride channels of neuronal axons, modulating the opening and closing of these sodium channels and inhibiting chloride channels (3). The modulation of sodium channels results in decreased sodium movement across cell membranes and inactivation of action potentials, eliminating depolarization and therefore causing repetitive firing of the nerve (4). In addition, inhibition of chloride channels decreases the threshold for an action potential and increases membrane excitability (5). The mechanism for modulation of these ion channels is not fully understood, but type II pyrethroids have been shown to stimulate protein kinase C-dependent protein phosphorylation in an in vitro study (6). Reported signs of bifenthrin toxicity in mammalian species include hyperactivity, seizures, head shaking, and listlessness prior to death (7). Pyrethroid toxicity results in motor signs generated at the spinal cord, with brain regions showing secondary responses; it also causes adrenal activation (4). Pyrethroids may act on gamma-aminobutyric acid (GABA) and glutamate systems as proconvulsant agents (4).

There is little information on bifenthrin toxicity in veterinary species, but bifenthrin toxicity has been reported and successfully treated in a dog (8). There is more literature available on other pyrethroids, namely permethrin, in cats (9,10). Cats appear to be particularly sensitive to pyrethroids, and this increased sensitivity has been postulated to be due to lack of a hepatic glucuronosyl transferase. Similar to bifenthrin, permethrin interferes with sodium channels and results in neurotoxic effects. A retrospective study evaluating permethrin toxicity in cats found that 75% of cases occurred following dermal application of a product intended for use in large-breed dogs (10). Clinical signs in cats are similar to the signs seen in the pig of this report, and include muscle fasciculations, tremors, seizures, hyperesthesia, ataxia, ptyalism, and hyperthermia (9,10). Administration of cypermethrin, a related pyrethroid insecticide, in goats in 1 study induced restlessness, salivation, pruritus, and head shaking at high doses (11). Clinical signs reported in humans with type II pyrethroid toxicity include profuse salivation, coarse tremor, increased extensor tone, reflex hyperexcitabillity, choreoathetosis, and seizures (4). Status epilepticus has been reported in a human following inhalational exposure to bifenthrin, although pyrethroid toxicity in humans more commonly results from intentional or accidental ingestion (5). One retrospective case series in humans found that approximately 40% of patients had atypical presentations, most notably respiratory failure (12).

Previously described biochemical abnormalities following bifenthrin exposure in fish include increased plasma glucose, ammonia, aspartate aminotransferase (AST), and creatine kinase (CK) (2). Administration of cypermethrin, a synthetic pyrethroid insecticide, to male dwarf goats in 1 study revealed decreased erythrocyte counts, hemoglobin, hematocrit, total protein, globulin and fibrinogen, with increases in alanine aminotransferase and aspartate aminotransferase (11). Similar findings were observed in the pig of this report. Hyperglycemia in this pig was attributed to stress (13). Increased activity of CK and AST indicate muscle damage secondary to significant tremors, while the AST may also be related to the direct effects of the pyrethroid or from rhabdomyolysis. No skeletal muscle pathology was noted on postmortem examination in this case.

Treatment reported in feline pyrethroid toxicity includes stabilization, decontamination, seizure and tremor control, and supportive nursing care. Diazepam (0.5 mg/kg BW, IV or PR bolus or 0.1 to 0.5 mg/kg BW per hour, IV, CRI) and propofol (4 to 6 mg/kg BW, IV bolus or 0.1 to 0.6 mg/kg BW per hour, IV, CRI) were commonly administered to control seizures and muscle fasciculations in 1 feline retrospective study, with cats which were refractory to diazepam receiving propofol and/or methocarbamol (10). Although diazepam is often considered a good initial anticonvulsant choice, studies in cats have found that tremors and seizures due to permethrin toxicity tend to be refractory to diazepam (10). Methocarbamol is often added at an initial dose of 55 to 200 mg/kg BW orally or rectally every 6 to 8 h to reduce muscle fasciculations following initial stabilization of seizures (10). Treatment was successful in a dog with bifenthrin toxicity; treatment included intravenous fluids, metoclopramide, diazepam, activated charcoal, methocarbamol, propofol CRI, and inhalant anesthetics (8). Hypersalivation may be controlled with administration of atropine, and a study showed that in rats ivermectin, a chloride channel agonist, reduced salivation induced by deltamethrin, a pyrethroid (14). The same study also showed that pentobarbitone (15 mg/kg BW) protected against the central signs of deltamethrin toxicity, supporting the use of chloride channel agonists in treatment of pyrethroid toxicity (4,14). In addition, studies in rats have shown efficacy of lidocaine for antagonizing the toxic pyrethroid effects on sodium channels (4). Intravenous lipid therapy has also been used successfully for treatment of permethrin toxicity in cats (15).

Prognosis following pyrethroid toxicity in cats has been reported to be generally good. The poor prognosis in the pig of this report may be related to a species variation in response to toxicosis or may be related to the dose that the pig ingested. Unfortunately, questioning of the owner did not reveal an estimate of amount of insecticide that may have been ingested. A worse prognosis may be encountered when uncontrolled seizures result in cerebral edema and brain damage or when nephropathy results secondary to muscle breakdown and myoglobinuria. Neither cerebral edema nor nephropathy was noted in the pig of this report to explain its poor prognosis. Studies in rats have found a neurotoxic oral bifenthrin dose of 4.6 mg/kg BW for motor activity (16). In another study in rats, plasma concentrations of 40 μg/L resulted in 20% decrease in motor function 4 h after exposure, while plasma concentrations of 269 μg/L resulted in an 80% reduction (17). Tissue levels of bifenthrin in samples taken postmortem in the pig of this report were unfortunately qualitative, not quantitative. Quantification of the stomach bifenthrin concentration indicates exposure; however, it cannot be used to estimate an ingested dose.

The toxic effects of pyrethroids on voltage-gated sodium channels are dose-dependent; however, duration of hyperexcitability depends on the specific pyrethroid structure and not the dose. Toxicity may be achieved at sublethal doses when exposed to other toxic agents, such as organophosphates (4). The GC/MS analysis in this case did not reveal evidence of organophosphate exposure. Pyrethroid insecticides are eliminated in the first 12 to 14 h following absorption, with metabolism occurring in the liver by a conjugation pathway (10). In a report on intoxication in a dog, clinical signs lasted for approximately 30 h following elimination of bifenthrin from plasma, and the authors postulated that cerebrospinal fluid levels may be used as a monitoring tool in bifenthrin toxicity (8).

In conclusion, the clinical signs of pyrethroid toxicosis seen in the pig herein are similar to reported clinical signs in other species, including muscle fasciculations, tremors, and hyperesthesia. Pyrethroids have been reported to cause toxicosis in cats and dogs but have not been previously reported in potbellied pigs. Because VPBPs are often kept as house pets and can be indiscriminate eaters, intoxication with novel compounds should be considered in cases of neurologic abnormalities. CVJ