1. SELECTION OF GENETICALY RESISTANT SHEEP AGAINST GASTROINTESTINAL NEMATODES. A CASE STUDY Armando Nari FAO. Italy

2. SHEEP RESISTANCE/RESILIENCE : THE ICEBERG CONCEPT
CONVENTIONAL APPROACH
BY FARMER
POTENTIAL SELECTION FOR RESISTANCE/ RESILIENCE
Clinically
affected
Animals
at risk
Resistants
Resilients
Entire flock
treated
Treat
All
Resistant
flock 10-15
years

3. “THE GENETIC RESISTANCE APPROACH TO
MANAGING PARASITES REQUIRES SIGNIFICANT
COMMINTMENT…”
“... IT WOULD TAKE PRODUCERS YEARS TO BUILD UP SIGNIFICANT PARASITE RESISTANCE WITHIN FLOCKS”
R. Woolaston, 1997

4. can increase significantly after 8-10 years Susceptible
e.p.g
Decreased
resistance
Control
years
Increased
resistance
Resistance to worms
can increase significantly
after 8-10 years
Susceptible
Control
Resistant
CSIRO, Armidale, Australia

5. COUNTRIES WITH ANTHELMINTIC RESISTANCE IN THE AMERICAS
FAO- OIE SURVEY
2004
Nicaragua
SHEEP STOCK
1994
Uru. 21:
Braz. 18:
Arg. 17:
Per. 12:
Bol. 8:
FAOSTAT (2005)
2004
Braz. 14:
Per. 14:
Arg. 12:
Uru. 10:
Bol. 9:
ANTHELMINTIC
RESISTANCE

6. HOST RESISTANCE AND RESILIENCE
The host’s ability to limit the establishment
growth rate, fecundity and/or persistence of
a parasite population.
RESILIENCE:
The ability of the animal to grow and produce
in spite of the presence of a “normal” worm burden

7. GENETIC RESISTANCE OF SHEEP
THE RESISTANCE OF SHEEP TO INTERNAL
PARASITES IS A POLYGENIC TRAIT WITH
MEDIUM- LOW HERITABILITY
BETWEEN BREEDS. Some breeds have naturally higher
genetic resistance. These breeds have evolved different
mechanisms of resistance. By crossing resistant breeds a
synthetic population of highly resistant sheep can be
obtained (ILRI, IAEA)
BETWEEN INDIVIDUALS. There is a widely genetic
variability within flocks which allows selection for
resistance to gastrointestinal nematodes (CSIRO,
FAO/URU)

8. Leyden Baker & Alan Carles, 2001
RED MAASAI SHEEP
Leyden Baker & Alan Carles, 2001

9. Natural pasture challenge over 2 years.
HOST RESISTANCE AND BREEDS
BREED N°
Peak FEC
Deaths
Hampshire 10
20,000 10
Corriedale 10
10,000 6
Dorper 10
8,000 3
Merino 10
4,000 4
Black Head Persian 10
3,000 3
Red Maasai 10
300
Natural pasture challenge over 2 years.
Preston and Allonby (1979)

10. GENETIC ANIMAL RESISTANCE
COUNTRY
RESISTANT SHEEP BREEDS
Kenya
Red Masaai
USA
Florida Native
Lousiana Native
Navajo
St. Croix
Barbados Blackbelly
France
Lacaune
Britain
Scottish Blackface
Australia
Merino (selected lineage)
New Zealand
Romney
Perendale (Cheviot x Romney)

11. D RM D RM SEMI-ARID HUMID MORTALITY EWES LAMBS (Birth – 12 months)27
Mortality (%)
EWES 22 17
Less
mortality 12
Less
mortality! 7 2 62 52
LAMBS (Birth – 12 months) 42
Mortality (%) 32 22
Less
mortality
Less
mortality! 12 2 D RM D RM

12. RESISTANCE-6 MONTH LAMBS28
HUMID
SEMI-ARID 27 26 25
Higher
PCV
PCV (%) 24
Higher
PCV 23 22 21 20
13500
11500
GFEC (epg)
9500
7500
Smaller
GI burden
5500
Smaller
GI burden
3500
1500 D RM D RM

13. BACKGROUND
Prevalence.
Anthelmintic resistance.
Gastrointestinal nematodes impact.
Haemonchus contortus (43 %);
Trychostrongilus colubriformis (26 %);T.axei (12%);
Nematodirus spp. Nari et al (1977);
Survey at national level: 92.5 %
farms/flocks affected (BZ 86%, IMZ 71%, ML 1.2 %).
Nari et al.(1994); V
Yearling lambs (without treatment/efficacy): Mortality
50%, Live weight 23 %: wool production 29 %.
Castell et al (1995)

14. Farmer and Government interests
Technology Availability for genetic selection
Breeding programme
- Uruguayan Wool Secretariat
- DILAVE “Miguel C Rubino”
TCP/URU/8921 (A)
- DNA + PCR + gene characterization
(Lessa et al, 2002);
- Faecal Egg Count (FEC) Heredability , with
neutral or slightly negative correlation with weight
and wool traits
- Previous work. Castell & Cardellino (1994)
- TCP/URU/8921 (A). “Genetic resistance in sheep
to gastrointestinal nematodes in Uruguay” (1998)

15. DISTRIBUTION OF CORRIEDALE LAMBS IN ACCORDANCE WITH FECAL
EGG COUNTS (FEC)
SHEEP N°
26 %
0-400
epg
Arithmetic mean : 1737 (e.p.g)
n° lambs = 310
4 % > 5000 epg

16. Management requirements for measuring RESISTANCE
Ewes and lambs always graze together
All ewes and lambs get the same feed (forage and supplements)
All ewes and lambs receive the same health treatments (drugs, vaccines, dewormers).

17. Measurement of resistance in Sheep Breeds
Weaning T
T = Effective anthelmintic
FEC and PCV/ all T T
FEC and PCV all
Lambing
‘Monitoring
‘Monitoring 1 2 3 4 5 6 7 8 9 10 11 12
Months
‘Monitorig =small number of samples at two week intervals

18. Corriedale genetic lines and TCP/URU/8921
Corriedale nucleus

19. RESISTANT VERSUS SUSCEPTIBLE LAMBS
(2003 PROGENY)
29%
66%
ANOVA
FEC 1. p= 0.001
FEC 2. p= 0.006
FEC 3. p= 0.002
FEC
months
Haem. 93% Trich. 89% Haem. 54%/Trich 42%

20. ESTIMATED PROGENY DIFFERENCE (EPD) VALUES FOR FAECAL EGG COUNTS
+0.49 E P D
-0.39
H2= /- 0.03
Best Linear Unbiased Prediction (BLUP)

21. CORRIEDALE RESISTANT NUCLEUS
IN URUGUAY
Selection of the most
resistant sheep by using
Faecal Egg Count (FEC)
122 RAMS
5823 progenies
PROGENY CENTRAL TESTING FARMS
38 FLOCKS
KEY (COMMERCIAL) FARMS
OPEN NUCLEUS. URUGUAYAN WOOL
SECRETARIAT (6 rams and 140 ewes)

22. PROSPECTS FOR A BREEDING PROGRAMME OF RESISTANT SHEEP IN URUGUAY
KEY (COMMERCIAL) FARMS
Selected flock according
to production traits
Culling of more susceptible
animals (FEC)
SUL (NUCLEUS) WILL PROVIDE EXPERTISE
& PELLETS FOR ARTIFICIAL INSEMINATION
Artificial insemination with
semen of resistant rams
Test of rams at the Progeny
Central Testing farms
PROGENY CENTRAL TESTING FARMS

23. CONCLUSIONS Until novel and more practical methods for genetic
selection are developed it is possible to use FEC for
the detection of resistance to gastrointestinal
nematodes in sheep
Breeding programmes for nematode resistance
are compatible with most IPM strategies whenever
there is adequate planning and follow up
Countries and farmer associations should support
breeding strategies as a sustainable alternative to
minimize the productive impact of gastrointestinal
nematodes in sheep

24. The philosophy of
one century is the
common sense
of the next