Dogs

One 24.3-lb (11-kg) dog showed signs of vomiting within minutes, lethargy and weakness within two hours of exposure, and hypothermia, muddy mucous membranes (suspected methemoglobinemia), and recumbency within nine hours of exposure after potentially ingesting four 500-mg capsules (approximate dose = 182 mg/kg).⁶ This dog was euthanized about 10 hours after exposure because of poor response to treatment.

The most commonly reported clinical signs in the 13 dogs were vomiting (six dogs), methemoglobinemia indicated by changes in mucous membrane color (six dogs), cyanosis (five dogs), tachycardia (five dogs), ataxia (four dogs), and weakness or recumbency (two dogs).⁶ Of the seven dogs with a known outcome, five recovered with treatment and two were euthanized. The onset of clinical signs of the five dogs that recovered was within 30 minutes, and the duration of signs in one dog was two weeks.

In the three cats, dyspnea was the most common clinical sign (all three cats), followed by methemoglobinemia or changes in mucous membrane color (one cat).⁶ The minimum dose at which dyspnea was reported in one cat was 79 mg/kg. The final outcome was unknown in all three cats. The clinical signs were noticed within 30 minutes to two hours after exposure. The durations of the clinical signs were unknown.

TREATMENT

If the exposure is within two hours of presentation and no clinical signs are present, induce emesis by using 3% hydrogen peroxide (2.2 ml/kg orally; repeat once in 10 to 15 minutes if no emesis). Alternatively, emesis may be induced in dogs by administering apomorphine (0.03 mg/kg intravenously or dissolving a pill in saline solution and instilling the resulting solution in the conjunctival sac) or in cats by administering xylazine (0.4 to 0.5 mg/kg intramuscularly or intravenously). Any central nervous system or respiratory depression resulting from xylazine administration in cats may be reversed with yohimbine (0.1 mg/kg intravenously). Then administer activated charcoal (1 to 3 g/kg orally, or a labeled dose if using a commercial product) with a cathartic such as sorbitol (70% solution at 1 to 3 ml/kg orally).

The agent of choice for treating methemoglobinemia is 5% N-acetylcysteine at a dosage of 140 mg/kg intravenously or orally followed by 70 mg/kg orally every six to eight hours for five to 15 treatments. To obtain the 5% concentration, dilute in 5% dextrose or sterile water. Methylene blue is not commonly administered anymore because injectable methylene blue is not readily available to veterinarians, but the dosage is 1 to 1.5 mg/kg in a 1% solution or as provided by the manufacturer intravenously, repeated once in 30 minutes if needed in dogs only since cats treated with it can show increased oxidative damage to their red blood cells.

Administer intravenous fluids, oxygen, and whole blood or packed red blood cells to improve oxygen-carrying capacity as needed. Filgrastim (granulocyte colony-stimulating factor [Neupogen—Amgen] 1 to 5 µg/kg subcutaneously once daily) can be administered proactively or if myelosuppression becomes evident.²