Deciphering digital dermatitis

ANIMAL HEALTH
WHEN NOT TO USE
IVERMECTIN
Cost-cutting in dairies could motivate illegal use, risk of residues.
BY JOHN MADAY
Since its introduction in the early
1980s, ivermectin has become a
go-to product for broad-spectrum
control of internal and external parasites
in cattle and other species. Today, the
legacy branded product, Ivomec, continues
to play a significant role in control of
cattle parasites, while generic versions of
ivermectin tempt producers with their
lower prices.
Veterinarians advise caution, however,
in the use of those generic products, and
avoidance of any use of ivermectin, even
the branded version, for parasite control
in dairy herds.
Dairy producers typically treat cows
for internal parasites at dryoff and, in
some regions, treat the herd using a pouron
for lice during the late fall and winter,
says Kim Bridges, DVM, a beef and dairy
practitioner in Virginia who also serves
as a drug residue-avoidance consultant.
Low profit margins in dairy production,
coupled with the low prices for generic
animal-health products, Bridges says,
could tempt producers to use ivermectin
at that stage, believing the dry period is
sufficient to clear any drug residues that
might affect milk once those cows return
to lactation.
That, says Merial manager of veterinary
services Tom Van Dyke, DVM,
would be a serious mistake. Simply put,
ivermectin is not approved for use in
lactating dairy heifers or cows under any
conditions. Any amount of ivermectin
detected in milk is considered an FDA
violation, and milk from cows treated
with ivermectin at dryoff may later test
positive when they return to lactation.
Van Dyke says scientists use pharmacokinetics
studies to determine meat and
milk withdrawal times for FDA drug
approvals and labeling. To establish a
withdrawal time of any drug, the FDA
must first accept a tolerance level of drug
residue that is deemed ultra-safe for consumers,
and then determine how much
time must go by before the drug concentration
is below the tolerance, he explains.
Avermectins, in general, and
ivermectin, in particular, are “lipophilic”
chemicals, meaning they readily attach to
fat molecules and tend to accumulate in
fatty tissues. Based on results of pharmacokinetic
research, all ivermectin products
carry long pre-slaughter withdrawal
times before using treated animals for
meat. For injectable ivermectin (subcutaneous
injection), the meat withdrawal
time is 35 days. For pour-on, topical
ivermectin, the withdrawal period is even
longer, at 48 days.
Because ivermectin is not approved for
use in lactating dairy heifers or cows, no
milk withdrawal time has been established.
The Food Animal Residue Avoidance
Databank (FARAD), a consortium of
faculty from several veterinary universities,
serves as the primary source for
science-based recommendations regarding
safe withdrawal intervals of drugs and
chemicals in food-producing animals.
Van Dyke says that for situations where
ivermectin gets used in lactating dairy
Virginia practitioner Dr. Kim Bridges says low dairy margins could
motivate some producers to use generic ivermectin in their drycow
programs.
Long withdrawal times could result in
violative ivermectin residues in milk
from cows as they return to lactation.
PHOTO: WYATT BECHTEL
PHOTO: WYATT BECHTEL
14 OCTOBER 2016
bovinevetonline.com