Reporting Adverse Drug Effects

We've all heard the scary stories about drug reactions:

"I knew something was wrong when I noticed melon-sized hives appearing on my arms."

"Suddenly, the room began spinning--yet there wasn't any tequila involved."

"I was borderline insane due to the intense itching."

The simple fact is that everyone has their own unique physiological makeup that reacts to therapeutic medications differently, and our horses are no different. Despite the myriad safety and efficacy studies and clinical trials completed before introducing a drug to market, there are always going to be few situations where the right (or wrong) combination of drugs given concurrently causes a problem, or the horse and the medication just don't mesh.

It's also important to note that there are circumstances where a drug might appear to have caused a reaction, but it's really an underlying disease that has just become apparent.

Since your horse can't do anything about an allergic drug reaction, it's vital that you know how to help him. At the same time, you just might be alerting authorities of a possible yet-undetected problem with a drug.

Reporting an Adverse Drug Experience

When a horse owner detects a possible "adverse drug experience" (ADE) with a drug labeled for horses or a drug labeled for another species that was prescribed for equine use (extra label use), he or she should first contact the horse's treating veterinarian so the horse's clinical signs can be assessed and treated. The veterinarian should call the drug's manufacturer as soon as possible, while the information is still fresh on his or her mind. Reports given to the drug company will be archived and forwarded to the Food and Drug Administration (FDA) Center for Veterinary Medicine (CVM), which monitors adverse drug experiences for animal drugs, medicated feeds, and veterinary devices. The FDA calls this activity "pharmacovigilance." Adverse drug experiences can be either undesired responses in an animal or the lack of a desired effect entirely.

Lynn Post, DVM, PhD, director of the Division of Surveillance in the Center for Veterinary Medicine, explains the reporting process: "Usually, the owner contacts the attending veterinarian, who contacts the company (that manufactured the drug in question). Once the company receives the report, they have 15 working days to forward the information to FDA on Form 1932."

The more information reported that ends up on Form 1932, the better. It's crucial to include data about the horse's medical history, all other drugs the animal was receiving at the time of the event, and all details available about the clinical findings on the animal--from physical examination to urinalysis (see a complete list and a link to Form 1932 in the "At a Glance" sidebar at right).

"Trained veterinarians may perform a causality assessment on the report and determine the strength of the association between the drug and the adverse event," says Post. "The information is captured in our ADE system. CVM will contact the reporter if necessary."  

The most frequently noted signs in the Center for Veterinary Medicine adverse drug experience database pertaining to horses include death, colic, depression/lethargy, incoordination, loss of appetite, diarrhea, inability to stand, accidental exposure to a drug or chemical, swelling around the mouth and lips, and collapse.

"This does not mean that these are actually the most common adverse drug events in horses, but they are the most frequent ones in our database," says Post.

The signs reported generally depend on the drug in question's potential toxicity.

"For example, you would expect to see colic listed as a sign for horse wormers," says Post.

Overall, there are about 5,000 reports of adverse drug events in the CVM database, says Post. "These reports contained approximately 24,000 horses that were treated with a drug; approximately 10,500 horses that may have reacted due to a drug, and approximately 1,400 horses that may have died as a result of a drug," he says.

Authorities at the Center deemed the causes of the other horses' clinical signs are probably not in any way linked to the drugs in question.

He feels that adverse events in horses are vastly underreported, and the numbers collected from year to year vary. In 2005 (the most recent year with complete records available), there were 239 adverse drug experience reports, containing 2,075 horses treated with a drug, 665 horses that might have reacted to a drug, and 60 horses that might have died as a result of a drug.

Why Pharmacovigilance?

According to the FDA, the adverse drug experience database exists to "provide an early warning or signaling system to the Center for adverse effects not detected during pre-market testing of FDA-approved animal drugs and for monitoring the performance of drugs not approved for use in animals." Scientists use the information to make decisions regarding product safety--they can decide whether a label needs to be changed or if another regulatory action needs to be made.

Therefore, it is important that horse owners and veterinarians report any detected adverse clinical events.

Seven scientists review the data (both small and large animal adverse drug experience reports) and recommend regulatory actions or label changes accordingly. How readily this happens depends on the situation. Several years ago, an equine dewormer had a faulty dosing syringe that led to the overdosing of foals, resulting in several deaths. Post explains that Center authorities were involved in a timely turnaround of changing the label and dosing syringe for that deworming paste, which prevented additional deaths.

A Collection of Events, Not a Scientific Tool

The adverse drug experience database lists events by the active ingredient, not the drug brand name. It's important to note that not every adverse drug experience appearing in the online database represents a proven problem with a particular drug. The entries represent clinical manifestations that could be scored at least "possibly drug-related" on a scale. The reported problem could have been caused by underlying disease in the horse, a combination of drugs given concurrently, or a non-drug-related cause. The database does not list information about these variables, and it doesn't reveal how the drugs were used (whether the owner or veterinarian followed label recommendations or if it was used in an extra label manner), nor does it list the final outcome of the reaction.

The FDA emphasizes that the accuracy of the information in the adverse drug experience database depends on the quality of the data provided by the attending veterinarian or the horse owner. The collection of information should never be used to calculate how many times a particular incident happened in association with a drug or whether or not there is a risk of using the drug: "The number of reports simply represents the numbers of ADEs received for a particular drug, by species, and route of administration," noted FDA officials online.

Take-Home Message

There's an established way to report a possible adverse drug experience to the manufacturer of the drug and to regulatory authorities. Horse owners and veterinarians should act expediently to submit thorough information about the event in case there is a problem on a broader scale, where many other horses might be affected.

The adverse drug experience database is useful for reviewing the types of reactions reported, but keep in mind that in general, adverse drug events are drug associations.

"There may or may not be a cause and effect relationship between a drug and an adverse event," Post says. "In other words, adverse drug events are observations and are not necessarily scientific fact, but it is the best that we can do with the information available."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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