Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

10
Antiparasitic drugs
Stephen W Page
INTRODUCTION
Helminth, arthropod and protozoal infections of companion
animals continue to cause significant morbidity
and mortality in dogs and cats and frequently present a
zoonotic hazard with public health implications. Infection
occurs frequently despite the availability and use of
a number of very potent and selective antiparasitic
drugs (Table 10.1). This highlights the fact that, in addition
to the type of drug selected, the way in which drugs
are used and integrated with other approaches to parasite
control is critical to successful and sustained management
of parasite infection. This chapter provides
information on the clinical pharmacology of the major
antiparasitic drugs together with a summary of the most
important epidemiological and public health considerations
associated with the parasites of dogs and cats.
PUBLIC HEALTH CONSIDERATIONS
Close contact of humans with dogs and cats as true
companions in an aging and increasingly immunocompromised
human population has increased the
importance and likelihood of transmission of parasitic
infections with zoonotic potential. As well as direct
transmission of parasitic diseases from dogs and cats to
humans, the diagnosis of a specific parasitosis in dogs
and cats is frequently a warning or signal that humans
too may be exposed to a common source of infection.
The list of parasitoses is growing in size and many
known associations with human disease are identified
in the Appendix to this chapter (p. 245). Veterinarians
should include discussion of public health implications
of diagnosed parasite infections with clients wherever
appropriate, providing advice concerning measures to
minimize or exclude the possibility of transmission.
Antiparasitic drug use in dogs and cats has the potential
to select resistant parasites. If resistant parasites can be
transmitted to humans then treatment of infected
humans may be jeopardized. Dogs are the major reservoir
of Leishmania infantum and the public health
implications of use of pentavalent antimonial drugs and
other agents that are also used in humans have been
raised as an issue for consideration when selecting the
most appropriate therapy of leishmaniosis in dogs.
APPARENT INEFFICACY
When interventions to treat parasite infections do not
meet the therapeutic objective it is important to investigate
and find the cause, rather than to simply assume
drug failure and change to an alternative drug. Common
causes of apparent inefficacy include:
● incorrect diagnosis
● inappropriate or unrealistic therapeutic objective
(e.g. the animal may have had a lethal infection and
could not have been expected to respond to treatment;
control of clinical signs may be possible when
eradication of infection is unlikely)
● inappropriate drug prescribed (target disease agent
resistant or not inherently susceptible)
● incorrect dose regimen recommended or implemented
(inadequate dose rate, frequency or duration of
treatment)
● compliance failure (a large category that includes
insufficient mixing of suspensions, inadequate application
of externally applied products, incorrect
dosing frequency and many more)
● reinfection by continuous or intermittent exposure
(very common with many parasites such as fleas,
Trichuris and Giardia)
● insufficient supplementary specific or supportive
treatment (e.g. with Babesia infection, Toxoplasma
uveitis, tick paralysis, flea allergy dermatitis, adult
heartworm infection)
● multiple infection (e.g. concurrent infection with
ticks, Babesia and Ehrlichia spp)
● physiological or pathological conditions (e.g. gut
stasis and piperazine may allow ascarids to recover
from drug exposure, increased gut motility may not
allow sufficient time for drug absorption)
● underlying disease (e.g. immunodeficiency and infections
with Pneumocystis or Demodex spp)
● drug interactions (e.g. corticosteroids and praziquantel,
piperazine and pyrantel)
● drug resistance
● out-of-date or incorrectly stored product.
Knowledge of the actual cause of apparent inefficacy
permits appropriate revisions to the therapeutic plan to
be made, elevating the chances of future success.
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