Transcript Document

Introduction
Bovine Nematode Resistance
An Overview
Bill Burdett, D.V.M., Intervet Inc.
Today’s
Introduction
Discussion
•Resistance (R) – what is it?
•Major nematodes of cattle
•Classes of dewormers
•Methods of R detection
•Characteristics of nematode infection
•Fecal sampling
•Case histories of R
•Using combinations of dewormers
Introduction
Defining Resistance
•What R is not: “It is not the development of
superworms.” (Anthelmintic Resistance
Roundtable, 2005)
•Some disagreement as to whether or not it
is truly mutation per se
•How does it occur? “We are simply
creating an environment where that
particular genetic makeup survives.” (Dr.
Cliff Monahan, OhSU, Anthelmintic
Resistance Roundtable, 2005)
Introduction
Defining Resistance
•“You are not changing something in the
worm. By giving dewormers, we are not
inducing resistance – we are simply
providing an environment for the resistant
parasites to flourish and multiply.” (Dr.
Dwight Bowman, Cornell U., Anthelmintic
Resistance Roundtable, 2005)
•<90% worm egg reduction following use of
a specific dewormer = resistance
Scope of the Problem
Introduction
•Huge in sheep and goats
•Becoming a major issue in cattle,
particularly to the endectocides
(macrocyclic lactones - avermectins
and milbemycins – ivermectin,
doramectin, eprinomectin, and
moxidectin)
Risk Factors for Resistance
Introduction
•Frequent use of same drug class
•Subtherapeutic drug levels from
under dosing and/or persistance in the
environment at low levels
Parasite Resistance
Introduction
Drug Level
Fenbendazole
Endectocides
Lethal Dose
24 hours
CONCEPT ILLUSTRATION
Time
70 – 80 days
Major Cattle Nematodes
Introduction
•Predominant genera = Ostertagia
and Cooperia
•Less predominant genera =
Haemonchus, Trichostrongylus,
Oesophagostomum, Strongyloides,
Trichuris, Nematodirus, and Monezia
Broad
Spectrum Nematode Dewormers
Introduction
•Group 1 = benzimadazoles (BZ) (attack
parasite’s metabolic system)
•Group 2 = imidazoles (levamisole, LEV)
and hydropyrimidines (pyrantel/morantel)
•Group 3 = macrocyclic lactones
(avermectins and milbemycins, ML) (attack
parasite’s nervous system)
Methods
of Resistance (R) Detection
Introduction
•Fecal Egg Reduction Count Test (FERCT)
for R to any dewormer (only test available
for macrocyclic lactones)
•Egg hatch test for BZ R
•Microagar larval development test for BZ
and LEV R
Methods of Resistance (R) Detection
Introduction
•PCR based test for BZ R in strongyles of
sheep and goats, cattle, and horses
•Real time PCR for quantitative
determination of gene frequency
•Controlled efficacy test – gold standard
Introduction
FERCT
•Nematode eggs counted on initial fecal
exam
•Eggs counted again at defined times
following treatment, e.g. 14 days
•% reduction determined
•Only reliable if > 25% of worms resistant
Introduction
FERCT
•Original product pour on? Confirmation of
R using an injectable or oral to eliminate
possible problems with malabsorption
•Cattle
•Ideal to use animals with minimum count of
100 epg (higher in sheep)
•≥17, e.g. 20 cattle per each anthelmintic tested
•ML’s
•persistant activity (label) second sample at X days
post-treatment depending on species
•recommended combined with larval cultures (for
appropriate species)
Introduction
Pertinent Things to
Remember
And Some Examples of
Combination Use and
Resistance
Facts
Introduction
• Calves 3-4 months of age and grazing for at
least 2 weeks most susceptible and likely to
have most eggs
• Moisture and warm temperatures (>40°F)
• Nematodirus – few eggs but very detrimental
to calves
Facts
Introduction
• Small number of animals typically shed most
of the eggs, e.g. the 80/20 rule, particularly as
animals age
• Recommended minimum number of samples
= 20
• Sampling too few animals in a group will likely
under represent the small percentage of high
egg shedding animals
• Standard deviation exceeds the mean
Facts
Introduction
• Heritability of egg shedding = .3 - .4
• Nematodes drive a Th2 immune response
which tends to counter a Th 1 response
• Th1 response necessary for good immunity to
intracellular pathogens such as viruses
• Single most significant factor of economic loss
from internal parasite infection = appetite
suppression
Facts
Introduction
• Physiological responses to GI nematodes
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Anorexia
Increased flow rate of feedstuffs through gut
Increases muscle mass in intestinal tract
Decreased fluid absorption
Combining FBZ and Avermectin
Introduction
• First feedyard trial (moderate egg
counts,e.g. in the 30’s per gram)
demonstrated extra 18 lbs. gain for FBZ +
Ivomec pour on versus Ivomec pour on
alone (+$20.08)
• Second feedyard trial (low egg counts, 7-9
per gram) showed extra 20 lbs. FBZ +
Ivomec pour on versus Dectomax
injectable alone ($9.31)
Combining FBZ and Avermectin
Introduction
• Third feedyard trial (moderate egg counts, 150’s
per 3 grams) demonstrated reduced egg counts
favoring FBZ or FBZ + Ivermectin injectable
versus Ivermectin injectable but no difference in
economic return
• Dr. Tom Elam economic evaluation of the three
trials showed an advantage of using both FBZ
and an avermectin of $22.19, $9.01, and $4.20,
respectively