|
My
client, Ann, was dismayed when I told her that my
physical examination and blood tests failed to reveal
why Oscar, her 10 year-old talkative Siamese cat, had
recently taken to loudly vocalizing in the middle of the
night. Siamese cats have a reputation for being
garrulous, and Ann had always found this charming. But
not at three o’clock in the morning.
“This
has been going on for almost a week now”, said Ann,
through bloodshot eyes. “I’m trying to recover from a
cold, and I really need a good night’s sleep. Last
night I took Nyquil®, and slept through most of Oscar’s
howling. I’m tempted to give a little to Oscar, so maybe
he’ll sleep through the night as well”, she told me. I
was shocked. “Nyquil contains acetaminophen!” I told
Ann. “If you give Nyquil® to Oscar”, he may fall into a
sleep that he never wakes up from!”
Ann
prefers that I not use her last name in this article;
she is embarrassed, as a cat owner, that she didn’t know
that acetaminophen is very toxic to cats. She shouldn’t
be. Despite the dangers of acetaminophen being
published in numerous journals and magazines, many cat
owners and enthusiasts still are unaware of how toxic
this drug can be to cats, or that many common
over-the-counter (OTC) drugs contain acetaminophen as
one of their main ingredients.
Companion animals are at risk for developing toxicosis
to prescription drugs as well as over-the-counter (OTC)
medications, either by deliberate administration of the
medication by owners, or by accidental consumption of
improperly stored drugs. Dogs are more likely to chew
on pill vials or tubes of ointment, eating the pills or
ointment when the container finally breaks open. Cats,
on the other hand, are generally more discrete about
what they put in their mouths and are less likely to
voluntarily ingest medications. With cats, toxicity
tends to occur when well-intentioned owners unknowingly
administer a seemingly harmless OTC medication, often
with devastating results.
Veterinarians occasionally use OTC drugs to treat a
variety of their patients’ maladies. The most common of
these are probably the non-steroidal anti-inflammatory
drugs (NSAIDs). We prescribe these to treat
musculoskeletal inflammation and pain, control fevers,
and sometimes to inhibit blood clotting. Aspirin is
clearly the best known of the NSAIDs. Aspirin is a
non-steroidal anti-inflammatory drug (NSAID) that, for
many years, had been used to control musculoskeletal
pain and inflammation in dogs, and sometimes cats. In
recent years, a multitude of safer and more effective
oral pain relievers have been developed, making aspirin
an uncommon first choice for joint and skeletal disease
in companion animals. In cats, we still use aspirin
occasionally, mainly to inhibit blood clotting. Cats
with certain heart diseases like hypertrophic
cardiomyopathy are at increased risk for developing
dangerous blood clots. Aspirin is frequently prescribed
in an effort to inhibit blood clot formation, however,
the dose must be strictly adhered to. Acetaminophen is
often grouped with the NSAIDs, although technically it
is somewhat different. “Veterinarians occasionally
prescribe low doses of acetaminophen for pain control in
dogs, but never for cats” says Dr. Jill
Richardson, consulting editor of toxicology for the
Veterinary Information Network and Director of Consumer
Relations and Technical Services for Hartz. “In dogs,
the dose is 5 – 10 mg per kg. Dogs show signs of
toxicity when the dose exceeds 75 mg per kg.” Cats,
however, are very sensitive to acetaminophen toxicity.
“There have been reports of toxicity developing in cats
at doses as low as 10 mg/kg.” Acetaminophen is one of
the top two most common household medications, and it is
no surprise that acetaminophen toxicity is commonly
reported. In fact, between January 1998 and March 2000,
veterinarians at the ASPCA Animal Poison Control Center
consulted on over 1050 cases of accidental exposure to
acetaminophen in dogs and cats.
Although most readily recognized by the trade name
Tylenol, acetaminophen is the major ingredient of most
aspirin-free pain relievers and cold remedies, including
Excedrin®, Panadol®, Anacin®, Midol®, Pamprin®,
BromoSeltzer®, and Percogesic®. Many decongestant
products and “cold” or “flu” formulas also contain
acetaminophen. For example, a single adult dose of
Nyquil® syrup contains 1000 milligrams of acetaminophen,
the equivalent of more than three Tylenol tablets. This
is a frighteningly large dose, considering that
ingestion of one 325-mg tablet by a cat results in
severe toxicosis, and two tablets ingested within 24
hours is fatal. “Any dose is potentially
life-threatening to a cat” says Dr. Richardson.
The
enzymes responsible for the metabolism of most drugs are
found in the liver. One particular enzyme, called
glucuronyl transferase, is responsible for attaching a
large molecule called glucuronide to a drug, rendering
the drug inactive and water-soluble. The process of
attaching a large molecule to a drug is called
“conjugation”. Cats, as a species, have low levels of
glucuronyl transferase. Thus, many drugs that are
quickly excreted as glucuronyl conjugates in other
species are very slowly removed from the bloodstream in
cats. Toxic metabolites accumulate in the bloodstream,
causing severe organ damage.
At
toxic doses, acetaminophen often causes hepatic necrosis
– death of liver cells – especially in dogs. Cats,
however, are more likely to develop a condition called
methemoglobinemia, in which there is an excess amount of
methemoglobin in the bloodstream. Methemoglobin is an
abnormal form of hemoglobin that is incapable of
transporting oxygen. As methemoglobin levels start to
rise, clinical signs develop, such as chocolate-brown
mucous membranes, fast heart rate, labored breathing,
depression, vomiting, edema (swelling) of the face, neck
and limbs, hypothermia, ataxia (incoordination) and
coma. Cats may become jaundiced as liver failure
develops.
Treatment for acetaminophen toxicity involves providing
supplemental oxygen, administering intravenous fluids,
and giving several drugs intravenously, including
vitamin C, cimetidine (Tagamet), and N-acetylcysteine, a
drug that provides the body with excessive amounts of
the amino acid cysteine. This amino acid is necessary
for the liver to repair itself and counteract
acetaminophen’s toxic effects. Time is of the essence
in treating acetaminophen toxicity. Treatment tends to
be less efficacious when initiated more than 8 hours
after ingestion.
Other
over-the-counter NSAIDs such as ibuprofen (Advil®,
Nuprin®, Motrin®) or naproxen (Aleve®) should never be
given to cats. Ibuprofen has a narrow margin of safety
in dogs, and cats, with their inability to metabolize
these drugs, are thought to be twice as sensitive as
dogs to ibuprofen’s toxic effects.
Ultimately, any oral medication can potentially cause
toxicosis in companion animals, especially cats, with
their small body size and unique metabolic pathways.
Cat owners should be certain to keep all medications
safely stored, and have the phone number of their
veterinarian and national poison control center readily
accessible in case of a toxin-related emergency. No
product, over-the-counter or otherwise, should be given
to a cat without the advice of a veterinarian.
Sidebar 1: What to do if you suspect acetaminophen
toxicity
“Acetaminophen toxicity is an emergency situation” says
Dr. Jill Richardson, Director of Consumer Relations and
Technical Services for Hartz, and consulting editor for
toxicology for the Veterinary Information Network. “You
should contact your veterinarian immediately, even if
the cat ate only one pill, or even if the owner simply
suspects that it was eaten. Early aggressive treatment
by a veterinarian is extremely important.
Even
if you only suspect your cat may have eaten an
acetaminophen tablet, still see a vet immediately.
There is a test that can confirm exposure, but treatment
for poisoning should be initiated while you wait for
test results to see if the cat was actually exposed.”
Sidebar 2: Clinical signs of acetaminophen toxicity
Brownish-gray gums
Labored breathing
Swollen face, neck and limbs
Hypothermia (low body temperature)
Uncoordinated gait
Coma
Jaundice
My
source for much of the info in the article came from Dr.
Jill Richardson herself, and from an article she wrote,
which can be found here:
http://www.aspca.org/site/DocServer/veccs_july00.pdf?docID=132
Updated
4/13/06 |
ShareThis