This presentation was given at the Delmarva Small Ruminant Conference All Worms All Day on December 8, 2018, in Keedysville, Maryland. The presenter was Susan Schoenian.

1. It’s in the genes!
Genetic control of internal parasites in sheep and goats
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu
www.wormx.info

2. Internal parasite control in sheep and goats
▪ We need better alternatives to control
internal parasites in small ruminants.
▪ To be successful, its usually necessary
to implement various tools or
practices, including pasture
management, nutritional
supplementation, and targeted
selective deworming.
▪ Genetic selection may offer the best
long-term alternative for controlling
internal parasites in livestock.
GENETIC CHANGE = PERMANENT CHANGE

3. There are two independent traits.
RESILIENCE
▪ Ability of animal to withstand
or tolerate infection/challenge
and still perform (grow,
produce milk).
RESISTANCE
▪ Ability of animal to limit infection by
1. Suppressing worm growth
2. Suppressing larval establishment
3. Expelling adults worms (“self-cure”)
Not the same as the resistance the
worms develop to the dewormers.

4. There are two independent traits
RESILIENCE
▪ Measured by
– Packed cell volume (PCV)
FAMACHA© score
– Body condition score (BCS)
– Breech soiling
– Performance
Growth, milk production
▪ Resilient animals require
less deworming, but may
shed a lot of eggs onto
pasture, leading to
parasitism in susceptible
animals.
RESISTANCE
▪ Measured by fecal egg count (FEC).
▪ Immunity requires nutrients to be diverted
from growth, milk, and fiber production.
▪ Resistant animals reduce the need for
deworming by reducing the contamination of
pasture, as well as passing their resistance
genes onto the next generation.
▪ Resistance (FEC) is more accurate and
objectively evaluated than resilience.

5. Heritability of resistance and resilience
▪ Heritability (H2) is the portion of observed (or
measured) differences in a population of animals
that are due to genetics.
PHENOTYPE = GENOTYPE + ENVIRONMENT
▪ Parasite resistance (FEC) and resilience are
quantitative traits, meaning they are controlled by
many different genes, each with small effects.
▪ The goal of research is to find a major gene with a
large effect on parasite resistance; this would
make selection easier (hasn’t happened yet!).

6. Heritability of resistance and resilience
▪ Heritability (H2) estimates vary,
but are generally between 20
and 30% for FEC.
– Higher for resistant breeds
– Probably lower for goats.
▪ Parasite resilience (e.g.
FAMACHA©; packed cell
volume) is less heritable.
▪ There is a low to moderate
correlation between resilience
and resistance.

7. Host immunity
▪ Immunity is the means by which sheep
and goats develop resistance to
parasites.
▪ There are two kinds of immunity:
1. Innate – present at birth. Little effect.
2. Acquired – develops as animals are
exposure to parasites.
▪ Immunity develops in two stages:
1. Suppression of worm growth
egg shedding
2. Suppression of worm establishment
larvae development

8. Development of immunity
▪ Lambs/kids start to develop immunity when
they are a few months old.
▪ Significant protective immunity is usually
achieved by 12 months of age; it may take
longer in goats.
▪ Regular exposure is necessary for lambs/kids to
develop immunity; immunity may only last for
weeks in the absence of infection.
▪ Adults (especially sheep) tend to remain
relatively resistant to infection, but regular
exposure may be necessary to maintain
immunity.

9. Periparturient egg rise (PPER)
▪ Females suffer a temporary loss of
immunity around the time of
parturition, from a few weeks before
until up to 6-8 weeks after.
▪ PPER often coincides with hypobiotic
larvae resuming their life cycle.
▪ PPER is primary source of infection for
young, growing lambs/kids.
▪ PPER is a heritable trait and has a
genetic relationship with FEC in lambs
(don’t know about kids).

10. Genetics of host immunity
▪ Immunity is controlled by genotype
(genetics).
▪ It is critical to identify individuals
that are more resistant.
▪ Fecal egg counts are used to
measure resistance.
▪ Nutritional stress, ill health, and
pregnancy can all compromise
immunity.

11. Two ways to use genetics to control
internal parasites in sheep and goats
CROSSBREEDING SELECTION
Mating animals of different breeds (or types)
Choosing which
animals get to be
parents.

12. Two benefits of crossbreeding
BREED COMPLENTARITY HETOROSIS
Katahdin x Texel Boer x Kiko

13. Breed complementarity
▪ Balance strengths and
weaknesses of different breeds.
▪ Progeny are intermediate in
performance between parent
breeds.
▪ Important to use breed(s) in
appropriate role(s): sire vs. dam
▪ Examples
St. Croix ♀ x Suffolk ♂
Kiko ♀ x Boer ♂
½ Katahdin x ¼ Hampshire x ¼ Suffolk

14. Heterosis (or hybrid vigor)
▪ Superiority of crossbred animal as
compared to (weighted) average
performance of purebred parents.
▪ Heterosis is expressed in the offspring,
dam, and sire, and effects are additive.
▪ Expressed as a percentage advantage.
▪ There are limited estimates for heterosis
for parasite traits.
Suffolk x Gulf Coast Native (LSU, 2001)
+19.4% for PCV in F1’s
- 52.3% for FEC in F1’s
5/8 Kiko x 3/8 Boer

15. Using selection to improve parasite resistance
BETWEEN BREED
▪ Some breeds are more
resistant to internal parasites.
WITHIN BREED
▪ Some individuals within a
breed are more resistant.
Image: St. Croix Sheep Association Myotonic goat

16. Which breeds are more resistant to worms?
(proven, documented)
SHEEP
▪ Hair sheep with tropical origins
St. Croix (A)
Barbados Blackbelly
▪ Sheep native to the Southeast
Gulf Coast Native (B)
Louisiana Native
Florida Native
Florida Cracker
▪ Composite hair breeds
Katahdin
Dorper
▪ Terminal sire breed
Texel
GOATS
▪ Myotonic (C)
▪ Kiko
▪ Spanish
A
B
C

17. Why selection works? 70/30 rule
▪ Fecal egg counts are
not evenly distributed in
a flock or herd.
▪ Approximately 30% of
animals shed 70% of
worm eggs and vice
versa.
▪ Removing heavy egg
shedders will result in
permanent change in
flock/herd genetics and
reduce the need for
deworming (over-time).

18. Three options to select for genetic resistance
1) Quantitative genetic evaluation
Using estimated breeding values
(EBVs)
2) Central performance tests
3) On-farm performance evaluation

19. Estimated breeding values (EBVs)
▪ An EBV is a numerical estimate of an animal’s
genetic merit for a specific trait.
▪ FEC EBV is expressed as a percentage change; a
FEC EBV of 50% indicates a FEC reduction of 50%
compared to an animal with a FEC EBV of 0
(average for breed/population).
▪ Progeny from parent with an FEC EBV of -50%
would be expected to have FEC 25% lower than
progeny with FEC EBV of 0.
▪ FEC is usually accessed at weaning and post-
weaning.
▪ By selecting negative FEC EBVs, FEC is reduced in
subsequent generations, thereby increasing
resistance in flock.

20. Estimated breeding values
▪ FEC EBV’s are available for Katahdins and Polypay sheep via the National Sheep
Improvement Program (NSIP).
▪ Any sheep or goat breed can submit data to NSIP to have FEC EBV’s calculated.

21. Central performance tests
SHEEP
• TheVirginiaTech’s Southwest
REC conducts a ram test in which
rams are pastured and evaluated
for parasite resistance.
• They have an annual sale and field
day in September.
• Rams are mostly Katahdin, but
the test is open to all breeds.
MEAT GOATS
• WestVirginia University conducts
a buck performance test in which
bucks are fed in pens, but
challenged with worm larvae.
• Bucks can be purchased via
private treaty.
• Most of the bucks are Kiko, but
the test is open to all breeds
A central performance test is where animals from different flocks/herds are brought to one central location where
performance is recorded.The rationale is that observed differences are more likely due to genetic differences, which will be
passed onto offspring, rather than environmental differences, which will not be passed onto offspring.The goal of a
central performance test is to identify genetic differences among animals.

22. Principles of on-farm performance evaluation
▪ Compare animals of same contemporary group
▪ Need adequate number of animals
▪ Need sufficient parasite challenge
▪ Requires long-term commitment

23. Contemporary groups
Compare “apples to apples”
▪ Can’t compare mature animals to
young stock
▪ Can’t compare dry females to dry
females (or males)
▪ Can’t compare lambs/kids of
significantly different ages
▪ Can’t compare animals reared in
different environments.
▪ Unfortunately there aren’t any
correction factors for birth/rear type.

24. Enough animals
▪ There is a wide variation in
FEC.
▪ 10 to 15 animals
recommended; more is better
▪ To compare sires, at least 10 to
15 progeny is recommended.
▪ Don’t include recently
dewormed animals (< 30 d).

25. Sufficient parasite challenge
▪ In order to see differences in a
group, there must be a sufficient
parasite challenge.
▪ There must be enough worm larvae
on pasture to activate the immune
response.
▪ Otherwise, you don’t know if low
FEC is due to low exposure to
parasite larvae, good nutrition, etc.
– or genetics.
▪ Average FEC of group should be at
least 500 epg, preferably >1000 epg.

26. Selection for parasite resistance
STUD MALES
▪ 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?
BROOD FEMALES
▪ Cull theWORSE!
▪ 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.
▪ Unfortunately, there are no
adjustment factors for number of
offspring reared.

27. Selection of males for breeding
INDIVIDUAL
KID OR LAMB
▪ Collect weaning and
post-weaning samples.
▪ Convert data to ratios
for comparison.
▪ Select animal with
above average ratios.
▪ Consider birth type
when analyzing data.
PROGENY
MATURE MALE
▪ Collect weaning and post-weaning samples from
10 to 15 progeny from each sire.
▪ Convert data to ratios for comparison.
▪ Choose sire with lowest average progeny FEC or
one of his sons with below average individual
FEC.
▪ More fecal samples, multiple years of data, and
multiple generations will increase accuracy of
selection.

28. Selection of females for breeding
EWES
▪ The periparturient egg rise is
heritable.
▪ Collect fecal samples at
lambing and post lambing
(periparturient egg rise).
▪ Convert data to ratios for
comparison.
▪ Consider age and number of
offspring when making
comparisons.
LAMBS
▪ Collect fresh fecal samples and
determine FEC within population.
▪ Deworm all lambs at weaning and
collect fecal samples 4-6 weeks later.
Repeat in 4-6 weeks
▪ Alternatively, skip deworming and
exclude animals that have been
selectively dewormed.
▪ Consider type of birth and rearing
when making comparisons.

29. What to expect from selection
▪ Not every single offspring produced by a parent
with low FEC data will have low FEC; however,
there will be more offspring with low FEC as
compared to offspring sired by parents with
high FEC.
▪ The purpose of selection is to increase the
frequency of the genes that confer resistance.
▪ Selection takes time. Over time, you will have a
more resistant flock/herd and less pasture
contamination.
▪ There may be year-to-year fluctuations, as
influenced by the environment.
“It’s a marathon, not a sprint!”

30. Other methods of selection
▪ Deworming history
▪ FAMACHA© score
▪ Body condition score (BCS)
▪ Performance (ADG)
▪ More subjective
▪ Less accurate
▪ Does not necessarily reduce
pasture contamination, thus
continuing to expose
susceptible animals to larvae.
▪ Selects for resilience rather
than resistance.
▪ BCS score could select against
most productive females.

31. Thank your for attention. Question/comments?
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu
www.wormx.info
www.sheepandgoat.com