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Breeding for disease resistance : Some examples. The context Cost of disease Definition of disease resistance Measurement of resistance to a pathogen Genetic variability of disease resistance Prospect for genetic improvement of disease resistance Conclusion. The context.

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breeding for disease resistance some examples

Breeding for disease resistance :Some examples

The context

Cost of disease

Definition of disease resistance

Measurement of resistance to a pathogen

Genetic variability of disease resistance

Prospect for genetic improvement of disease resistance

Conclusion

slide2

The context

The new European agricultural policy :Increase in production is no longer a priority Some current drugs used in animal production could be banned Research focus on ecological management and animal welfare

Resistance to diseases and product quality are becoming important goals in Europe

slide3

Economic consequences of disease include :

Loss of rabbit meat sales (mortality, lower growth rate) Increased culling rate Cost for veterinary treatment

Increased work in animal management (culling, treatment)

Cost of disease

slide4

Definition of disease resistance

Infection

Different mechanisms of the pathogen and of the host

Depending on the stage of infection the resistance appears as :

all or none : mortality/survival, morbidity/absence of symptoms …

Quantitative : number of bacteria in target organ, fever …

slide5

Definition of disease resistance

Resistance/sensitivity depending on pathogenic stage during

an infectious process (Lantier and Vu Tien Khang, 1988)

slide6

Definition of disease resistance

Resilience :

ability to continue a normal production life for an animal which has

been infected by a parasite

Potential source of infection for flock mates ?

Specificity of resistance

NRAMP1 gene (sheep) : control infection of Salmonella, Leishmania

and various Mycobacteria

Sheep with alleles ARQ are susceptible to natural scrapie but resistant

to some different scrapie strains (BSE)

slide7

Measurement of resistance to a pathogen

Direct measurement

Survival analyse : describes the length of healthy life

The survivor function gives the proportion

of animal still healthy at time t (from 4 to 10)

The statistical model includes explanatory

variables : year-season effect, size of the

originating litter, parity, genetic effect

Ex : Crusader growing rabbits : Signs of illness are recorded from week 5 to week 10

slide8

Measurement of resistance to a pathogen

Direct measurement

Total number of pasteurellosis

signs over the growing period

All or none trait : healthy/sick

slide9

Genetic variability of disease resistance

Polygenic approach

Infinitesimal model Dominant model in selection

Genetic parameters :

Heritability : The relative importance of heredity in determining phenotypic values

Genetic correlation : Genetic links between traits

The genetic value of an individual

=

Sum of independent and small effects of a very large number of genes

slide10

Genetic variability of disease resistance

Polygenic approach : Crusader rabbit survey

Heritability of resistance to pasteurelosis : 0.08

(number of weaned : 0.06 ; ADG 5-10 weeks :0.30)

Correlation with Weight Gain

Correlation with Litter size at weaning

(estimates of litter size effect in survival analyse)

Trait

Healthy

animals

Sick

animals

Weight at 10 weeks (kg)

1.94

1.80

Weight gain 5–10 w (kg)

1.10

0.96

slide11

Genetic variability of disease resistance

Major genes

Some genes have a major effect on susceptibility to disease

A large number of major genes have been found in the human

The number of such genes is limited in livestock :

. the Weaver disease in Cattle

. the scrapie in sheep (PrP gene)

. the malignant hyperthermia in pigs (halothane sensibility gene)

slide12

Prospect for genetic improvement of disease resistance

Interest of breeding for disease resistance

Economic interest in rabbit

Salmonella in sheep (abortus ovis) : efficient vaccines are available

Scrapie in sheep : no vaccination and no treatment

Selection is very appealing in this case

Reducing mortality of weaners from 15% to 10% gives an extra $ 16 per doe per year

Increasing growth rate by 3 g/hd/day gives an extra $ 11 per doe per year

Depend on the efficiency of the non genetic approach

(environment, vaccination, treatment, culling of infected animal)

slide13

Prospect for genetic improvement of disease resistance

The cost of breeding for disease resistance

Direct costs :

Challenge in controlled conditions

Laboratory analyses (antibodies level, faecal eggs count)

Genotyping (PrP gene)

Indirect costs :

Negative effects on production traits

(unfavourable correlations ; ex : mastitis in dairy cattle)

Loss in selection pressure on other traits

slide14

Conclusion

Susceptibility to disease is influenced by genetic factors

Estimation of heritability are medium, similar to the values

of production traits

A number of questions are still remaining

Correct definition of resistance measurement

Cost of resistance measurement

Consequences of selection on theses trait (resistance to other disease,

resistant animals remains diffusers of pathogens)