Selecting Worm Resistant Animals

www.agriculture.vsu.edu
Selecting Worm Resistant Animals
Dahlia O’Brien
Small Ruminant Specialist
Virginia Cooperative
Extension, Virginia State
University
dobrien@vsu.edu
(804) 524-6963

Internal Parasite Control
 In the wake of increasing drug resistance,
alternative are needed for effective worm
control
 Alternatives include:
 Targeted Selective Treatment
 Pasture Management
 Nutritional Supplementation
 Genetic Selection
 No alternative by itself will likely be
effective
 An integrated approach to worm control is
needed

Genetic Selection
 Genetic selection for resistance or
resilience to internal parasites is a
very important strategy
 Identifying and selecting the best
animals for long-term health
builds a more resistant herd/flock
 This presentation will focus on
how best to select those animals
with traits of resistance to
internal parasites

Resistance vs. resilience
Resistance
 Ability of host/animal to limit
infection
 Suppressing establishment of
parasite inside the body
 Suppressing egg laying
 Expelling adults worms (“self-
cure”)
 Assessed by FEC
Resilience
 Ability of host/animal to withstand
challenge/ infection and still
perform
 Assessed by:
 Packed cell volume (PCV)
 FAMACHA© score
 Body condition score (BCS)
 Performance indicators
Resistance and resilience are the two major concepts relating to the animal’s ability to
limit/withstand a worm infection

 Which is better?
 Resilient animals are productive
even under parasite challenge
 Require less deworming
 Resilient animals spread a lot of
internal parasite eggs onto pasture
 Causing parasitism in more
susceptible animals (young, old,
non-resistant)

 Which is better?
 Resistant animals reduce the
need for deworming
 Resistant animals reduce the
contamination of pasture
 Resistant animals pass their
resistance genes onto the
next generation

Heritability
 Heritability is the portion of trait variation
in a population of animals that are due to
genetic variance
 Parasite resistance and resilience are
quantitative traits
 Controlled by many different genes
 Parasite resistance is a moderately heritable
trait
 Heritability estimates for FEC is 20 – 30%
 Parasite resilience is less heritable
Selecting lines of animals that have an improved ability to regulate their parasite populations
increases immunity against parasites at the herd level

Host Immunity
 Most Susceptible
 Kids/lambs
 Periparturient females
 Old animals
 Less Susceptible
 Bucks
 Open does
 Pets

Host Immunity
 Key factor in determining whether an
animal can resist or allow parasite
infection to establish is host immunity
 Controlled by animal genotype
 More effective immune response results in
low larval establishment or worm egg
shedding
 There are two kinds of immunity:
 Innate – present at birth
 Acquired/Adaptive – develops as animals
are exposed to parasites

Host Immunity
 Young start to develop immunity
to parasites slowly with exposure
 Lambs acquire immunity 4 – 9
months of age
 Takes longer in goats (≥ 1 yr. old)
 Breed dependent
 Regular exposure is necessary to
develop immunity
 Immunity may only last for weeks
in the absence of infection

Host Immunity
 Adults (especially sheep) tend to
remain relatively resistant to
infection
 Immune response is suppressed
under stress
 Low nutrition
 Disease
 High producing females (multiples,
heavy milkers etc.)

Host Immunity
 Ways to boost immunity
 Not placing very young
animals on heavily
infested pastures
 Always ensure quality
nutrition to susceptible
animals
 Separate and feed females
based on litter size
 Use low-stress handling
techniques

Breed Resistance
 There are some breeds that have
been naturally selected for
resistance to internal parasites
 Typically, these breeds were
developed in situations and
climates that favored internal
parasites
 These animals were selected by
natural selection (“survival of the
fittest”), and are generally more
resistant to worms

Breed Resistance
 Traditionally, identifying animals with
lower levels of infection has been
accomplished by fecal egg counts (FEC)
 FEC has allowed us to identify parasite
resistant breeds
 Hair sheep – St. Croix, Barbados Blackbelly,
Katahdins
 Wool sheep – Texels, Gulf Coast Native, and
Florida Native, Hog Island???
 Goats – Spanish, Kiko and Myotonic

Breed Resistance
 Tennessee State University (Dr. Richard
Browning)
 Compared reproductive, growth and health
traits among Boer, Spanish and Kiko meat
goats
 Annual rates of parasitism were greater
for Boer does (53%) than for Kiko (24%)
and Spanish does (22%)
 Postpartum FEC were less for Spanish
and Kiko dams compared to Boer dams

Crossbreeding
• Mating animals of different breeds is
one way to adding parasite resistance
to your herd/flock
• Breed complementarity - Balance
strengths and weaknesses of different
breeds
• Heterosis (hybrid vigor) - superiority
of crossbred animal as compared to
(weighted) average performance of
purebred parents

Animal Selection
 Selection based on breed
can be an effective tool in
reducing worm loads
 Especially in purebred flocks
 Within every breed there is a
range in FEC
 Individual animals should
always be monitored for
their own merit

Animal Selection
 Important to identify animals that have
lower numbers of worms
 More parasite resistant animals
decrease the need for deworming
 Culling parasite susceptible animals
will increase flock resistance, reduce
pasture contamination and decrease
deworming frequency
 80/20 rule
 20% of herd/flock shed 80 percent of the
parasite eggs
Susan Schoenian, Meat Goat Test

 The most accurate way to
identify individual animals
that are parasite resistant is
to measure FEC
 Resistant animals will have
low FEC
 Do not shed many eggs in
their manure
 Immune system

 Accuracy of FEC increases
with multiple samples
 Compare samples taken
from similar groups of
animals, during the same
season and year
 Information gained from
FEC is very useful and the
best assessment of
resistance in a flock/herd

 Producers can learn to do
their own FEC (workshops
offered)
 Offered by many
veterinarians and labs
 It can be labor-intensive and
costly

 Measure and record
FEC during the
parasite season
 Take at least 2 fecal samples
a month apart
 Use when selecting sires
 Small differences between
FEC results are not
meaningful

 All kids and lambs
 Use contemporary
groups (same age,
management etc.)
 Collect fecal samples
when >10% have
FAMACHA © scores ≥3
 Leave out animals that
were previously
dewormed

Kid ID FEC (June, 2017) FEC (July, 2017)
101 500 600
102 450 300
103 No eggs 150 More R
104 800 950
105 2,000 4,750 More S
106 200 700
107 450 1,100
Queen 3,500 6,000 More S
Dixie 900 1,100
111 150 200 More R
112 5,000 2,200 More S

 The FAMACHA© system is an
alternative tool for determining
resistant or resilient animals
 Developed in South Africa in response
to the emergence of severe
anthelmintic resistance
 Validated for sheep and goats in the U.S.
 A system to assess anemia due to
Haemonchus contortus (barber pole
worm) infection in sheep and goats
and the need for deworming
individual animals

 Simple and inexpensive way to
identify and select animals with
parasite infections (in areas where
barber pole worm is the main
parasite)
 Resilient animals will also have a
good FAMACHA© score even
though they have high FEC
 FEC still recommended to determine
resistance vs. resilience
 FAMACHA© scores and hematocrit
values are highly correlated with
FEC

 The Five Point Check©
 Addresses limitations of
FAMACHA by determining
need for deworming for all
internal parasites that affect
sheep and goats
 Useful in deciding whether or
not to deworm FAMACHA
scores of 3’s
 First developed in sheep and
involves 5 check points
 Replace coat (appearance) for nose
in goats

 What data should I be recording?
FAMACHA scores and
deworming frequency
Cull those individuals needing
more than three treatments a
year
Don’t select offspring (especially
males) from animals that require
frequent treatment

 Record weights of all
lambs and kids
 To help select which animals
do well under your
production system

 Select the BEST!
 The male represents half the genetics
your flock/herd. One male may
influence the genetics of 50 or more
offspring.
 If you are saving replacements, the
male will influence 90% of the genetics
in your flock/herd after several years
of use
 Why select more resistant females and
breed them to a susceptible or
unknown male?

 Cull the WORSE!
 A female only influences
the genetics of her own
offspring, 1-4 per year
 Higher producing females,
especially yearlings, are
more likely to have higher
FEC and require
deworming

 National Sheep Improvement Center (NSIP,
http://nsip.org) calculates estimated breeding values
(EBV) for sheep producers and breed associations
 EBV’s are science-based, industry-tested
measurements of heritable traits that can be tracked
and measured
 EBVs are proven to improve on-farm productivity and
enhance breeding decisions
 Currently, Katahdin and Polypay have significant FEC
data for EBVs for parasite resistance
 Producers can submit FEC data, regardless of breed

If you were to breed these rams you would
expect FEC of lambs sired by Ram A to be
25% lower than FEC of lambs sired by B
Adapted from Genetic Selection; Bowdridge
and Weaver

 Buck kids and ram lambs
 Select and collect fecal samples from
the best 5 – 10
 Deworm to get them clean and at a
similar starting point
 Collect FAMACHA © scores and fecal
samples to measure FEC at 4, 6 and 8
weeks after deworming
 Calculate own “estimated breeding
values” (EBV)
 (Individual FEC – group mean)
X heritability

Take away points
 Genetic selection for parasite
resistance is one of the most
promising means to control worms in
a flock/herd
 Keep production traits in mind as well
 Resistant animals, especially the use of
resistant sires leads to lower FEC and
FAMACHA© scores
 Pasture contamination is reduced
when resistant animals are present
 Subsequently reducing the use of
dewormer and death losses

Selecting Worm Resistant Animals

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