Exam 2 - Lecture 10

Question 1

Parasitic Infections in Horses Ecology, Epidemiology & Disease Risk

Answer 1

-Herd animals w/ long
evolutionary history of
success: feral/nomadic horses w/ little contact w/ infective pastures

-Parasite challenges at low
levels throughout life: facilitate general resistance to disease risk and low FECs throughout life

-Natural aversion to grazing
fecally contaminated loci w/in pastures: roughs & lawns

Question 2

Principles of Parasite Control

Answer 2

-Pasture contamination: pastures with low stocking density (1‐2 horse/acre)

-Pasture infectivity: horses with low FEC have minimal effect on pasture infectivity; parasite challenge, immunity w/ exposure

-Differential risk of infection/disease: young animals associated w/ high FEC; Parascaris equorum and undeveloped immunity & increased susceptibility

-Pasture hygiene & management: remove feces (optimal), cross‐species grazing schema, and graze on hay aftermath

-No pharmacologic solutions for poor management: drugs used effectively “Smart Deworming”

Question 3

Resistance, Refugia, and
Drug Selection Pressure

Answer 3

-Resistance:
-Drug treatment (selection) increases the frequency of naturally occurring “resistance genes” in the parasite population
-Parasites that are treatment
resistant reproduce differentially
w/out competition from other worms

-Management factors promote
resistance: exclusive use of single drug; resistant worms reproduce
continually through the ERP
-Frequent use of a drug(s); denies susceptible worms an opportunity to reproduce

Question 4

Refugia & Drug Selection Pressure

Answer 4

-“Refugia”:
-Proportion of the worm population
not subject to drug selection pressure @ time of treatment; encysted in host & free‐living on pasture and allowed to reproduce and contribute to the population genome

-Differential selection by drug used: Pyrantel, Adults only, no effect on
encysted stages
-Ivermectin, Late L3 & Adults
-Moxidectin, Early & Late L3 & Adults

-Selective treatment: maximize “Refugia”, mitigate negative selection pressure
of treatments, and facilitates heterogeneous mix of `susceptible & resistant parasite types

Question 5

Parasite Infections in Horses
Differential Disease Risk by Age

Answer 5

-Disease Risk & Parasite Targets: Foals & Juveniles (Age based susceptibility, contribution to pasture contamination)
-Adults, > 3 years old: acquired immunity and premuntion (infection‐immunity); stable reproducing population of
worms and host is refractory to hyper‐infection

-Treatment Goals:
-Reduce pasture contamination
-Inhibit intensity of infections
-Prevent disease

Question 6

Parasite Control & Disease Risk: Parasite control for 1st year of life (foals & weanlings)

Answer 6

-Deworm @ 8 weeks
-FBZ target Parascaris
-Pyrantel also effective if worm burden is low

-Deworm @ 16 weeks
-Pyrantel to target Parascaris IVM @ Cyathostomes

-Deworm @ 24 weeks
-Macrocyclic lactone to target Small Strongyles, perhaps Parascaris

-Deworm @ 32 or 34 weeks
-Macrocyclic lactone to target Small Strongyles, perhaps Parascaris

-Deworm @ 52 weeks

Question 7

Parasite Control & Disease Risk: Parasite control for 2nd year of life

Answer 7

-Consider animals as high egg
shedders: Treat @ 10‐12 week intervals; Fall, Winter, Spring, seasons w/ highest pasture
transmission, largest possible REFUGIA (worms not subject to treatment)

-Parasite control animals > 3 years: treat at least once annually for Large Strongyles & Tapeworms (Macrocyclic lactone & Praziquantel)
-Identify and treat high FEC shedders by selective deworming
-No more than x3 @ 12 week
intervals (Fall, Winter, Spring)