Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

EXTERNAL PARASITICIDES
Collar technology has advanced significantly since the
original vinyl matrices charged with the volatile organophosphate
dichlorvos. More recent collars rely on alternative
polymers that are hydrophobic and resist swelling
and degradation when exposed to water. In addition to
the active constituent, collars also may contain spreading
agents such as silicone oils and fatty acids to assist
dispersion over the coat of the host. Nonetheless, the
release characteristics of the active constituent from
collars are erratic and unpredictable and for this reason
high levels of efficacy are seldom encountered. However,
where subtotal parasite control is acceptable there is a
valuable role for collars. Concerns have been expressed
that the gradual decay in release rate over time may
accelerate the likelihood of selection of parasite resistance.
While this is plausible and strongly suspected,
direct proof is unavailable.
After swimming or washing, re-establishment of pesticidal
activity within the pelage may take 24–48 h. It is
often recommended that collars be removed during
these activities.
Particularly effective use of collars includes the use of
deltamethrin collars for protection of dogs from the
bites of sandflies and transmission of Leishmania spp
and the protection of dogs from Ixodes holocyclus
infection, given the usual preference of this tick to attach
to the neck and head. However, there may be selection
of ticks that prefer to attach at alternative sites.
The packaging of most collars should not be opened
until ready to use, as the release of active agent is initiated
by exposure to unsaturated conditions.
Precautions specific to collars include the following.
● Skin irritation. Regular inspection of neck recommended.
Collar should be removed if irritation is
observed.
● Consumption. While bioavailability varies from
product to product, collars are nevertheless concentrated
depots of pesticide. Measures should be instituted
to ensure that animals do not chew or swallow
collars.
Spot-ons
Topical high-concentration, low-volume preparations
have been designed to increase both the convenience of
treatment and compliance. Topical spot-on products are
either absorbed systemically or distribute laterally and
remain within the skin. Systemically active products
include selamectin and fenthion. The products either
rely on the lipid solubility of the drug or include permeation
enhancers to ensure satisfactory transcutaneous
passage. Once absorbed, the drugs are redistributed
throughout the body and can be detected throughout
the skin. The process of distribution throughout the skin
of drugs that are not absorbed is poorly understood.
However, imidacloprid, fipronil and permethrin are
nevertheless active at skin locations remote from the site
of application.
As has been described in other species, a decreasing
drug concentration gradient would be expected between
the site of application and remote skin locations, particularly
with unabsorbed preparations. The impact that
this may have on selection of resistant parasites is not
known. Also unknown is the impact of skin lesions on
drug distribution. Potential problems associated with
spot-on products include toxicity from ingestion (either
from close contact with treated animals or from self
grooming which is particularly important in cats exposed
to high-concentration permethrin products), skin lesions
at the site of application, hair loss, change of hair color
and, for products with flammable vehicles, temporary
risk of ignition. The solvents in some products (e.g.
imidacloprid spot-on) may be deleterious to domestic
surfaces. Precautions should be taken at the time of
administration by those applying products to ensure
that human skin contact and inhalational exposure are
minimized. The effect of wetting, by bathing, shampooing,
swimming or rain, varies from product to product
and labeled advice should be observed.
Dips and sprays
Products for use as immersion dips usually require dilution
in water prior to use and are available as emusifiable
concentrates (ECs) or wettable powders. ECs are
formulated to provide a stable suspension of pesticide
once diluted in water. It is preferable that, when applied,
the pesticide preferentially binds to skin and hair,
leaving the wash that runs away depleted of chemical.
This ensures greater pesticide retention and higher concentrations
in the coat. Sprays are generally ready-touse
solutions or suspensions of pesticide, requiring no
dilution.
There is a higher likelihood of operator exposure
during use of dips and sprays and particular attention
should be paid to protective clothing and ventilation.
With sprays particularly, care with flammable products
is necessary. Storage and disposal of dips should be
guided by labeled instructions. Some dip concentrates
(especially certain diazinon products) can become highly
toxic if stored inappropriately. The solvents in some
products (e.g. fipronil spray) may be deleterious to
domestic surfaces.
Resistance
Parasite resistance can be expected to arise wherever
parasiticide application is most intense. At present,
resistance of the cat flea, Ctenocephalides felis, has been
described to the SPs, OPs and fipronil in field isolates
and to imidacloprid and lufenuron in laboratory strains
and can be predicted to eventually include all available
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