Confirmed 2,4-Dichlorophenoxyacetic Acid Toxicosis in a Dog

Introduction

The chlorophenoxy herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), is commonly used in the agricultural industry and around the home. When ingested in people, early signs include vomiting, abdominal pain, diarrhea, and gastrointestinal hemorrhage. Neurological signs such as ataxia, nystagmus, convulsions, paralysis, and coma are seen only with higher doses. Peripheral neuromuscular signs, such as loss of tendon reflexes, increased creatine kinase activity, myotonia, and peripheral neuropathy, have also been reported.^1–3 Reports of 2,4-D toxicosis in dogs have been primarily from experimental settings. Research dogs have been given various doses of 2,4-D to evaluate acute, subchronic, and chronic signs of toxicity. These experimental studies indicate that 2,4-D toxicity in dogs can lead to gastrointestinal signs and subclinical and clinical manifestations of myotonia.^4–11 The purpose of this report is to describe the presentation, diagnostic plan, and treatment for a confirmed clinical case of 2,4-D toxicosis in a dog with myotonia as the only clinical manifestation of disease.

Case Report

A 2-year-old, 35.6-kg, intact male Weimaraner was referred for episodic extensor rigidity and abnormal gait in all limbs for the previous 24 hours. Prior to this event, the dog was normal to the owners and had no previous medical history of illness. The dog was housed in an outdoor kennel and had free roam within a fenced yard, which was in close proximity to a plant nursery. The dog had no travel history outside the state ofWashington.

Upon presentation, the dog episodically exhibited an extended posture in all limbs. This posture was most pronounced immediately upon standing from a recumbent position. In some instances, the dog would fall, usually in a lateral direction, as he attempted to stand. Occasionally, the dog could stand but would immediately knuckle over in all paws and subsequently fall upon walking. While in lateral recumbency, periods of extensor rigidity lasted for 10 to 15 seconds, especially when the dog was stimulated either with physical touch or sound. When supported by the examiner, the dog could walk without knuckling but exhibited hyperextension of the limbs. This posture became less apparent as the exercise period increased in length [see Video]. Physical examination was otherwise normal, except a tick was found and removed from the cervical region. On neurological examination, cranial nerves, conscious proprioception, and spinal reflexes (e.g., withdrawl, patellar, perineal, cutaneous trunci) were normal. No hyperesthesia was noted on spinal palpation. The proximal appendicular muscles were firm and increased in size. Percussion of these muscles with a reflex hammer resulted in formation of dimples; this reaction was clinically suggestive of myotonia [Figure 1]. Although a tick was found on examination, tick paralysis was not considered a differential, because this dog did not have diffuse lower-motor neuron signs.

Diagnostics were pursued, which included a complete blood count (CBC), serum biochemical analysis, urinalysis, and electromyogram (EMG). Results of the CBC and urinalysis were normal. The serum biochemical analysis identified an elevated creatine kinase (CK) (12,000 IU/L; reference range 61 to 360 IU/L) and an elevated alanine aminotransferase (ALT) (116 IU/L; reference range 21 to 67 IU/L). The serum bicarbonate value was normal. Electromyographic evaluation in the awake dog indicated electrical activities consistent with myotonia, as evidenced by repetitive discharges that waxed and waned in frequency and produced an audible “dive-bomber” sound. Based on the signalment, history, clinical presentation, and diagnostic findings, acquired myotonia was suspected.

Due to potential herbicide exposure from the nearby nursery, serum and urine samples were submitted to a diagnostic laboratory for herbicide analysis. Serum and urine concentrations of 2,4-D, bromoxynil, 4-chloro-2- methylphenoxyacetic acid (MCPA), and 3,6-dichloro-2- methoxybenzoic acid (dicamba) were measured using gas chromatography^a and a dual electron capture and mass selective detector system.^b The estimated method detection limit was 0.05 μg/mL. Results of this analysis showed detectable 2,4-D residues in both the serum and urine [see Table]. Bromoxynil and MCPA were also detected in both serum and urine, but levels were extremely low (1.0 μg/mL).

Supportive treatment was provided with intravenous (IV) administration of lactated Ringer’s solution at 6 mL/kg per hour for the first 24 hours after presentation, and then 3 mL/kg per hour for another 12 hours. After 36 hours of fluid therapy, the dog’s gait was dramatically improved with only slight stiffness noted when initially standing up. The proximal appendicular muscles had returned to normal size and tone. Percussion of these muscles with a reflex hammer no longer resulted in the formation of dimples. No gastrointestinal signs such as nausea, vomiting, diarrhea, or abdominal pain were seen, and appetite remained normal throughout the hospitalization. At the request of the owners, the dog was discharged after 36 hours of hospitalization. Instructions were given to keep the dog indoors temporarily to avoid contact with herbicides that could have entered the owners’ yard from the nearby nursery.

The dog was reevaluated at 2 weeks and 8 weeks after initial presentation. The dog appeared clinically normal to the owners since initial discharge. Upon presentation, the dog was bright, alert, and responsive, and his gait was normal. Physical and neurological examinations were normal. Electromyography was performed with the dog awake at the 2-week interval, and no myotonic potentials were detected. Serum and urine samples were submitted to the same diagnostic laboratory for herbicide analysis, using the same method as previously described. The concentration of 2,4-D was dramatically decreased at the 2-week interval, and it was nondetectable at the 8-week interval in both serum and urine [see Table]. The calculated elimination half-life for 2,4-D in this dog with 36 hours of fluid therapy was 50.16 hours.

Discussion

Myotonia refers to a state of sustained muscle contractions after a physiological stimulus, because of failure of the muscles to relax. This condition is characterized by muscle stiffness, extensor rigidity, muscle hypertrophy, temporary inability to initiate movement, and stilted gait that improves with exercise. Myotonia can be inherited or acquired. In dogs, acquired myotonia has been associated with hyper-adrenocorticism, hypothyroidism, fibrotic myopathy, and chlorophenoxy herbicide toxicosis.^4,12–17 Because no clinical evidence of endocrine diseases or fibrotic myopathy was seen in this dog, chlorophenoxy herbicide toxicosis was the primary differential for the acquired myotonia.

Chlorophenoxy herbicides are chemical analogues of auxins, a type of plant growth hormone that causes uncontrolled lethal growth in targeted plants. These herbicides are used commonly for the control of broad-leaved weeds. The most commonly encountered herbicide of this class is 2,4- D. Other examples of herbicides in this class include MCPA and dicamba. Chlorophenoxy herbicides are often coformulated with ioxynil and/or bromoxynil, which often are more toxic and have similar mechanisms of toxicity.¹

Chlorophenoxy herbicides are absorbed rapidly following ingestion in humans. Once absorbed into the vascular system, these compounds bind extensively to serum albumin and do not display any chemical propensity for accumulation in tissues.^1,2,18 In humans, excretion of 2,4-D and its metabolites is almost exclusively renal, with an elimination half-life between 11.6 and 33 hours.^1,18 In dogs, when 2,4-D was administered orally at various dosages, it was detected in the urine within 1 hour of administration.^6,19 When 2,4-D was administered to dogs at 5 mg/kg orally, the elimination half-life was between 92 and 106 hours.²⁰ This longer elimination half-life in dogs is thought to be from a reduced ability to transport 2,4-D by the renal organic anion transport system, thereby reducing the rate of urinary excretion of 2,4-D. This results in a substantially higher body burden of these organic acids, which may be a plausible explanation for the increased sensitivity to chlorophenoxy herbicides in dogs.^5,8,18,20,21 Overall, chlorophenoxy herbicides undergo limited biotransformation and are eliminated primarily unchanged in the urine of dogs. This makes the detection of these herbicides possible in both serum and urine.¹⁹ However, the rate at which these herbicides are eliminated through the kidneys varies tremendously across species.^1,8,20,21

Although not fully elucidated, several mechanisms of chlorophenoxy toxicity have been described from experimental studies. Dose-dependent plasma membrane damage has been speculated as one such mechanism.^1,2 In rats, the breakdown of the blood-brain barrier occurs at high doses of herbicide exposure.^2,22 This allows herbicide accumulation in the central nervous system (CNS) and may explain the dose-dependent CNS toxicity. Experimentally, the appearance of blood-brain barrier damage coincided with the onset of neurological signs in rats.^2,22 Additionally, chlorophenoxy herbicides have been shown to disrupt cell membrane transport mechanisms. The organic anion transport system in the choroid plexus, which facilitates the removal of potentially toxic anions from the brain, can be affected.²

Another mechanism of chlorophenoxy toxicity is the disruption of cellular metabolic pathways involving acetylcoenzyme A (acetyl-CoA).^1,2,23 Experimentally, this leads to the production of acetyl-CoA analogues such as 2,4-DCoA. ²³ These analogues can enter the acetylcholine (ACh) synthetic pathway, forming 2,4-D-ACh, which may act as a false messenger at the muscarinic and nicotinic synapses. Lastly, chlorophenoxy herbicides may alter energy metabolism in the mitochondria by uncoupling oxidative phosphorylation. ^1,2,24 This can lead to adenosine triphosphate (ATP) depletion and compromise a variety of cellular activities.^1,2 Overall, clinical signs of 2,4-D toxicosis are most likely multifactorial. The neuromuscular signs seen may be from direct effects on plasma membranes causing ion channels to fail, interference of ACh production, and uncoupling of oxidative phosphorylation where ATP depletion causes disturbed calcium regulation in muscles and, therefore, sustained muscle contraction.^1,2

Chlorophenoxy herbicide toxicosis can lead to multiple side effects in humans. When ingested, early signs include vomiting, abdominal pain, diarrhea, and occasionally gastrointestinal hemorrhage.^1–3 Hypotension, which is a prominent early feature in one-third of the reported cases, can occur as a result of intravascular volume loss from the gastrointestinal tract, vasodilatation, or direct myocardial toxicity.² Central nervous system signs are sometimes seen, but only at higher dosages. These signs include hypertonia and hyperreflexia related to upper-motor neuron dysfunction, ataxia, nystagmus, miosis, hallucinations, convulsions, paralysis, and coma.^1–3 Hyperventilation or hypoventilation is often associated with coma.² Coma is almost an invariable feature in fatal cases and occurs in over two-thirds of reported nonfatal ingestions of chlorophenoxy herbicide.² Peripheral neuromuscular involvement includes limb muscle weakness and fasciculations, respiratory muscle weakness that can lead to respiratory distress, loss of tendon reflexes, increased CK activity, myotonia, and peripheral neuropathy.^1–3

Metabolic acidosis is also reported in several cases and is suspected to be caused by circulatory failure.² Elevated body temperature during absence of infection is noted occasionally in association with hyperventilation.² Myotonia leading to rhabdomyolysis, increased ALT activities, and renal failure are typically associated with severe poisoning. ^1–3 Less commonly, thrombocytopenia, hemolytic anemia, and hypocalcemia are seen.^1–3 Although chlorophenoxy herbicide poisoning is uncommon, toxicity can result in serious and sometimes fatal consequences in humans.^1–3 Between 1962 and 2003, 23 of the exposed 69 humans reported in the literature died following intentional ingestion of chlorophenoxy herbicide.¹ Toxicity in humans has also been reported secondary to both dermal and inhalation exposure to chlorophenoxy herbicides; however, fatalities have not been reported with these routes.¹

Management of humans exposed to chlorophenoxy herbicide includes gut decontamination and supportive therapy. Oral activated charcoal is administered in patients within the first hour of ingesting a potentially toxic amount of a chlorophenoxy herbicide. Mild toxicity can typically be managed successfully with symptomatic and supportive care alone.^1,2 Adequate urinary output (>1 mL/kg) should be ensured in all patients.³ Urinary alkalinization using sodium bicarbonate and hemodialysis have also been suggested for severely poisoned patients. Urinary alkalinization (pH >7.5) along with aggressive fluid diuresis can limit reabsorption and promote renal excretion.^1,3 Hemodialysis can increase herbicide clearance without the need for urine pH manipulation and the administration of substantial amounts of IV fluid in an already compromised patient.¹ Both of these treatments have shown some success in selected cases, but no randomized controlled trials have been carried out to critically evaluate the effectiveness of these treatments.^1–3 Further investigation is required to elucidate the potential clinical benefit of urinary alkalization and hemodialysis.

The majority of the reports of 2,4-D toxicosis in the dog are from experimental settings.^{4,6–11,19–21,25} Both subchronic (16 weeks) and chronic (1 year) dietary toxicity studies on 2,4-D have been performed in dogs at variable dosages of exposure. These experimental studies indicated that 2,4- D was well tolerated by dogs at low dosages (1.0 to 7.5 mg/kg per day).^9,10 Dogs have also been experimentally given higher dosages of 2,4-D to evaluate acute signs of toxicity. In one study, dogs given 175 or 220 mg/kg of 2,4-D orally developed gastrointestinal signs and clinical and EMG manifestations consistent with myotonia.⁵ Dogs given 8.8 to 86.7 mg/kg of 2,4-D developed subclinical manifestations of myotonia detectable only with an EMG.⁵ Steiss et al reported myotonic discharges in dogs 24 hours postad-ministration of 50 to 125 mg/kg of 2,4-D.⁴ Clinical effects and plasma concentrations of 2,4-D have also been determined in dogs after a 200 mg/kg oral dose. Clinical signs observed were vomiting in 33% (two of six) and diarrhea in 100% (all six) of the dogs. No gait abnormalities were noted, but EMG findings consistent with myotonia were seen in all six dogs. Mean total plasma concentration of 2,4- D was 511 mg/L.¹¹ The lack of observable gait abnormalities at 200 mg/kg per day in this study is in conflict with findings by other authors.^5–7,11 Overall, these experimental studies indicate that 2,4-D toxicity in dogs can lead to gastrointestinal signs and EMG evidence of myotonia. The clinical manifestation of myotonia, however, is variable and remains unclear in dogs whether it is dose dependent.

In humans, 2,4-D is not likely to have any neuromuscular toxic potential at doses below those required to induce significant systemic toxicity.¹⁸ Harrington et al reported a suspected 2,4-D toxicosis in a dog, which supported such a conclusion.¹⁷ The dog in that report had clinical signs of myotonia and systemic side effects such as vomiting, melena, anemia, and thrombocytopenia. However, the authors of that report were unable to obtain serum and urine 2,4-D levels at the time of exposure, so no correlation could be made between 2,4-D levels and the degree of clinical signs. That is the only published clinical report of 2,4-D toxicosis in the dog, and although it was not a confirmed case, it supports that systemic effects of 2,4-D toxicosis can occur in dogs similarly to the occurrence in humans.¹⁷

Conclusion

Although multiple experimental studies have shown myotonia as a sign of 2,4-D toxicity, a confirmed clinical case of 2,4-D toxicosis in the dog has not previously been reported in the literature. In addition, the dog in our report had only clinical signs of myotonia and no signs of systemic toxicity, which is different than the case reported by Harrington et al and cases reported in the human literature.^1–3,10,17 Whether the duration of exposure, the amount of toxin ingested, or the serum and urine concentration levels affect the degree of clinical signs has not been clearly evaluated in the dog and will require additional studies and clinical reports of 2,4-D toxicosis.

Chlorophenoxy herbicides are among the most common herbicides used by the agricultural industry and around the home; therefore, the risk of exposing pets can be quite high. Toxicosis involving 2,4-D should be considered a differential for dogs presenting for acquired myotonia with or without systemic signs. Exposed dogs with clinical signs of myotonia can have good clinical outcomes when treated supportively with IV fluid therapy.