Methanol
Methanol (also known as methyl alcohol or wood alcohol) is found most commonly in "antifreeze" windshield washer fluid and
varies in concentration from 20-100% (with 20-30% being the most common form). Methanol's metabolite, formaldehyde, is rapidly
oxidized by the enzyme aldehyde dehydrogenase to formic acid, which can cause metabolic acidosis if significant quantities
of methanol are ingested. Formic acid is subsequently converted to carbon dioxide and water. In humans and some non-human
primates, formic acid is inefficiently metabolized and accumulates causing injury to the retina and central nervous system.
Even very small amounts of methanol can be extremely dangerous to humans, but this is not an issue in non-primates. The minimum
lethal dose in dogs is 8.0 ml/kg of 100% methanol.
According to the ASPCA Animal Poison Control Center database, the most common methanol exposures occur in dogs and usually
involve chewing on containers or lapping up spills. In general, alcohols are rapidly absorbed from gastrointestinal tract,
so decontamination needs to be done within the first 20-30 minutes of an oral exposure to be effective. With small exposures
in dogs and cats, only mild gastric upset may be seen. Larger exposures result in vomiting, diarrhea, ataxia, disorientation
(inebriation), depression, hypothermia, tremors and dyspnea; in severe cases seizures, bradycardia or ventricular premature
contractions, metabolic acidosis, coma, and respiratory depression may occur. Death is usually due to respiratory depression,
hypothermia, or aspiration.
A recent small ingestion should be treated with dilution (milk or water) to minimize gastric upset. Large ingestions warrant
decontamination via emesis if less than 30 minutes have passed and the animal is asymptomatic. Activated charcoal is of questionable
value in binding small alcohols and is generally not used. Symptomatic animals should be monitored until recovered. Treatment
may entail IV fluids to enhance elimination of methanol, thermoregulation, correction of acid/base imbalances, management
of cardiac abnormalities, ventilatory support, and seizure control (anticonvulsants should be used with care as they can worsen
the CNS depression). Anecdotally, yohimbine has been used to aid in reversal of coma in alcohol-intoxicated dogs. Given prompt
and aggressive care, most dogs and cats will recover within 4-36 hours.
Propylene glycol
Propylene glycol is the primary ingredient in some forms of automotive antifreezes/coolants, as well as many recreational
vehicle antifreezes (be sure to check the label). Propylene glycol is also present in a variety of food, pharmaceutical, and
cosmetic agents, generally at low concentrations (generally less than 12-15%). Propylene glycol in injectable medications
or activated charcoal formulations may interfere with ethylene glycol test kits (i.e. result in false-positives, see ethylene
glycol below). Products containing less than 20-30% propylene glycol should not pose an acute toxicity hazard to pets if ingested.
Newer automotive antifreezes containing 50% or more propylene glycol (i.e. Prestone's Low Tox and Sierra) are indeed "safer"
than ethylene glycol-based antifreezes in that they will not cause the serious kidney damage that is seen with ethylene glycol
toxicosis. However, it is important to remember that ingestion of propylene glycol-containing antifreezes may result in serious
intoxication similar to that seen with other alcohols. A dog given propylene glycol at 10 ml/kg displayed no clinical signs;
however the LD50 (dose at which 50% of experimental dogs died) in dogs is 22 ml/kg. Therefore, doses over 10 ml/kg of 100% propylene glycol
should be considered potentially toxic and justify veterinary intervention.
Like methanol and ethylene glycol, propylene glycol is rapidly absorbed from the gastrointestinal tract. Decontamination must
therefore be accomplished within the first 20-30 minutes to be of much benefit. Clinical signs can occur as early as 20 minutes
following ingestion. Due to the risk of aspiration of vomitus, induction of vomiting is NOT recommended in animals that are
already symptomatic. As with other alcohols, propylene glycol can cause severe intoxication, ataxia, inebriation, and metabolic
acidosis (see methanol clinical signs above). Treatment is be similar to that described for methanol toxicosis—fluids to promote
excretion, treatment of CNS effects (seizures, coma) as needed, thermoregulation (hypothermia is a significant risk in comatose
patients), and management of metabolic acidosis. As with methanol, the overall prognosis is generally good provided the animal
receives prompt and appropriate veterinary care. Recovery usually occurs within 24-36 hours of ingestion.
