Fatal brodifacoum poisoning in a pony

Abstract

Fatal brodifacoum poisoning in a pony is described; this condition has not previously been reported in ponies. Discussion of what factors in the pony’s history and treatment may have predisposed to the severity and ultimate death is provided.

Increased popularity of anticoagulant rodenticides has led to an increase in cases of accidental poisoning in nontarget species, including pets and farm animals (1). Brodifacoum is a second-generation anticoagulant or superwarfarin, which, even in small doses, is more toxic than most vitamin K antagonist rodenticides (1). Its LD₅₀ is estimated to be 50–100 mg/adult horse (2). Very few cases of anticoagulant intoxications in the equine species have been reported (2–4) and, in ponies, only experimental research studies, not clinical cases have been described (3,5). To our knowledge, this is the first reported case of accidental ingestion in the pony. Most of the experimental studies were done to establish a safe and effective dose of anticoagulants for use in horses, not to characterize all of the clinical features of toxicity.

Case description

A 130 kg, 3-year old, female cross-bred pony was referred to the University Veterinary Teaching Hospital with signs of abdominal distension and persistent recumbency. On arrival, the pony had slightly congested mucus membranes, abdominal distension, pawing, nystagmus and mydriasis, a heart rate of 45 beats/min, a respiratory rate of 12 breaths/min, low arterial blood pressure (systolic 39 mm Hg, diastolic 30 mm Hg, mean 32 mm Hg) and dehydration (5% bodyweight [BW]). The rectal temperature was 34.8°C; results from auscultation of the lungs and abdominal viscera were normal. Amazingly, the referred pony had a good appetite. Excessive bleeding was observed at collection of blood samples from the external jugular vein. Suspicion of ingestion of an anticoagulant rodenticide (brodifacoum) arose on questioning the owner. The specific amount of brodifacoum that the pony had ingested or the time frame between ingestion and the onset of clinical signs could not be determined exactly, but it was approximately 2 kg of pellets containing 0.005% brodifacoum (100 mg of brodifacoum), and the pony was referred 24 h after ingestion of the pellets. Hematological examination, blood biochemical analysis, and coagulation tests were performed (Table 1). Laboratory studies revealed slight anemia. Anisocytosis and platelet agglutination were seen on microscopic examination of blood smears. The serum total protein value was in the lower range of normal limits. Prothrombin time (PT) and serum aspartate aminotransferase (AST) and creatine kinase (CK) levels were increased. A fecal occult blood test and a urinalysis for blood were both positive. Toxicological analysis of blood by thin layer chromatography under UV-light showed the presence of brodifacoum (6), a substance belonging to the group of second-generation anticoagulant rodenticides with a hydroxycumarinic structure.

Table 1.

Results of hematological, coagulation, and serum biochemical analysis

	Units	Pony	Reference values		Units	Pony	Reference values
RBCa	1012/L	8.01	6–12	Bilirubin	μmol/L	40.35	8.55–34.2
PCVb	L/L	0.31	0.32–0.47	ASTe	U/L	508	226–366
Hemoglobin	g/L	97	100–160	APf	U/L	111	143–395
WBCc	109/L	7.5	6–12	CKg	U/L	4600	86–300
Neutrophils	109/L	6.05	2.7–6.7	Urea	mmol/L	19.02	8.56–17.85
Lymphocytes	109/L	1.45	1.5–5.5	Creatinine	μmol/L	53.04	61.8–176.8
Platelets	109/L	18d	100–350	Triglyceride	mmol/L	0.275	0.044–0.484
Total Protein	g/L	58	52–79	Ca	mmol/L	2.32	2.55–3.35
Fibrinogen	g/L	3	2–4	PTh	s	21.5	8–12.4
Albumin	g/L	29	26–37	APTTi	s	19.5	36–47
Glucose	mmol/L	6.43	4.21–7.04

^aRBC, red blood cells;

^bPCV, packed cell volume;

^cWBC, white blood cells;

^dplatelet count was normal (> 100), by manual quantification;

^eAST, aspartate amynotransferase;

^fAP, alkaline phosphatase;

^gCK, creatin kinase;

^hPT, prothrombin time;

ⁱAPTT, activated partial thromboplastin time

The pony was treated initially with flunixin meglumine (Finadyne; Schering Plough, Madrid, Spain), 0.25 mg/kg BW, IV, q8h, oral activated charcoal (0.5 kg) and mineral oil (2 L) via a nasogastric tube (alternating treatments every 4 h), glucose 5% fluids (G5; B. Braun, Barcelona, Spain), 3 L, IV, during the first 6 h; isotonic electrolyte solution (Lactate Ringer’s solution), 40 mL/kg BW/h during the first 2 h; and 20 mL/kg BW/h during the next 18 h, for dehydration and potential hypovolemia; and vitamin K₁ (Konakion; Roche, Madrid, Spain), 120 mg, SC q4h. Dobutamine (Dobutrex; Lilly, Madrid, Spain), 1 μg/kg BW/min, was administered until the blood arterial pressure was stabilized — systolic 122 mm Hg, diastolic 65 mm Hg, mean 93 mm Hg — because of the hypotension; fresh blood (15 mL/kg BW), tested by standard techniques for compatibility, was also administered. Prothrombin times did not return to within normal ranges. Despite therapy, the pony died within 20 h of hospitalization. Necropsy revealed multifocal hemorrhage in subcutaneous tissue, muscles, the pericardial sac, and the meninges, and multiple ecchymotic lesions, without a significant amount of blood, in liver, kidneys, the gastrointestinal tract, and bladder. Numerous hemorrhagic areas were seen in the serous membranes.

Discussion

Horses may develop hemorrhagic diathesis after consuming therapeutic warfarin, rodenticides, or moldy sweet clover (Melilotus spp.) (7). Rarely are they exposed to warfarin or other coumarin derivatives used as rodenticides around buildings or feed storage areas (8). The diagnosis in this case was made through a combination of compatible clinical signs and a history of exposure to the rodenticide. Evidence of ingestion of an anticoagulant rodenticide was demonstrated by a positive toxicological analysis and prolongation of PT, but no ecchymoses were seen upon inspection of mucus membranes. Slight congestion of the mucus membranes is not an expected clinical finding of vitamin K antagonism, but it may have resulted from dehydration. Abdominal distention may have been related to previous ingestion of highly fermentable feedstuffs. Signs like nystagmus and pawing could be related to neurologic disorders, secondary to hemorrhage of the meninges.

Brodifacoum is at least 100 times more potent than warfarin and is much more likely to cause death in rats and non-target species after a single feed (1). The LD₅₀ is estimated to be 50–100 mg/adult horse (0.1–0.2 mg/kg BW) (2). If ingestion was the 2 kg of pellets then the pony ingested 100 mg of brodifacoum (0.77 mg/kg BW), that is well over the LD₅₀ in horses, and exceeds the dosage which induced changes in coagulation times and some clinical signs in horses (2,4). These anticoagulants competitively inhibit vitamin K, which is necessary for the production of clotting factors II, VII, IX, and X (7). The result is decreased production and subsequent deficiency of the vitamin K-dependent clotting factors and eventual development of a hemorrhagic crisis (7).

Hematologic analysis revealed that brodifacoum had no significant effect on the leukocyte and erythrocyte counts, the packed cell volume (PCV), hemoglobin, total bilirubin or albumin concentrations, as previously described in horses at the first days of intoxication (4). However, the PCV and hemoglobin values were slightly decreased (the PCV might have been lower if it had not been for the dehydration and splenic contraction), and the bilirubin value increased. Fibrinogen concentration also remained within its normal range of activity, as described for warfarin (9). Lowered protein may have been first evidence of hemorrhage. The still normal PCV confirms that the ingestion and hemorrhage was recent and likely ongoing as the PCV lags behind the drop in total protein (TP) by 12–24 h in animals that have experienced recent hemorrhage. It is very unlikely that there was any pathology involving the liver which would explain the lowered proteins especially considering the course of events that ensued where the animal continued to hemorrhage irrespective of therapy. The low concentrations in TP might have enhanced the toxicity of the rodenticide, as hypoproteinemia may result in fewer binding sites, thereby increasing the amount of free circulating poison (10). Prothrombin time was increased. This parameter has been used for monitoring the effect of several anticoagulants in experimental studies. Dicumarol compounds, such as warfarin, produce hypoprothrombinemia, which can be monitored by measuring PT (5). Clinical pathologic analysis reflects prolonged PT first, because the plasma half-life of factor VII is shorter than that of the other clotting factors (7). This fact probably explains the lack of prolongation of the activated partial thromboplastin time (APTT). Other authors observed significant increases in clotting times from days 4 to 8 of brodifacoum intoxication (4).

Hepatic dysfunction may impede metabolism of coumarin derivatives (10), thus enhancing the toxicity of these compounds. While hepatobiliary disease may impede metabolism and excretion of brodifacoum, there is insufficient evidence for hepatobiliary dysfunction in the pony of this report. Increased serum concentrations of AST more likely reflects myopathy, probably because of persistent recumbency and muscular hemorrhage found in necropsy. Further serum biochemical investigation, including quantification of serum concentrations of sorbitol dehydrogenase (SDH), gamma glutamyl transferase (GGT), and bile acids, and histopathological documentation of hepatic damage would be required before hepatic dysfunction could be considered in this pony. The lack of owner compliance and financial constraints precluded further investigation in this case. The concurrent increase in serum concentration of CK suggested skeletal muscle damage and, perhaps, myocardial injury, given finding of cardiac tamponade.

The response to symptomatic medical therapy (flunixin meglumine, oral activated charcoal and mineral oil, administration of IV fluids to correct dehydration; dobutamin to increase blood pressure and fresh blood to replace clotting factors and improve oxygen carrying capacity of the blood) was not good.

The pony received a blood transfusion; however, exogenous red blood cells (RBCs) may not have been necessary given the RBC count and PCV on hematological examination of the pony. We administered a blood transfusion, instead of fresh plasma, because of the potential danger of internal hemorrhage. Vitamin K1 administration is the specific antidote for coumarin derivatives, but it was tried without success. The PT times did not return to within normal ranges, perhaps due to the early death of the animal, since returning to normal levels lasts some days (5). Vitamin K1 therapy proved to be ineffective, probably due to the quantity of toxin ingested and the acute progression of the symptoms. The potential effects of the flunixin meglumine administered on the amount of active brodifacoum in blood could have played a role in the fatal outcome. Flunixin meglumine is extensively protein bound in plasma and may displace brodifacoum from protein binding sites, resulting in a greater proportion of free active brodifacoum in blood.

Necropsy revealed changes typical of anticoagulant poisoning. The findings at necropsy suggested that the cause of death was hemorrhage in vital organs such as central nervous system, meninges, and pericardial sac. The cardiac tamponade would be sufficient to cause the death of the animal.

The prognosis for animals ingesting second-generation coumarin derivatives is more guarded than that of first-generation compounds because of the potential for a more prolonged course and more severe clinical signs (8). The major difference between warfarin and the second-generation anticoagulant rodenticides is that the latter have longer body retention times and a tendency to induce bleeding for a longer period of time. Therefore, it is necessary to continue treatment for weeks rather than days. This difference dramatically influences management in terms of duration of treatment and cost of treatment, and likely affects outcome (2).