Lec 21

• Disease Resistance

  Host – able to moderate pathogen lifecycle<|>Stop establishment of infection<|>Cause incomplete development of an infection<|>Control or get rid of infection if developed
• Tolerance

  Host – able to withstand infection and disease effects
• Resilience


  Host – able to maintain performance in the face of disease challenge
• Future strategy in breeding for disease resistance

  1. Awareness of pathogen resistance to therapeutic agent – less use of antibiotics and drugs

  2. Possibility of between species transfer of such resistance – from livestock pathogens to human pathogens

  
• Future Strategy in Breeding for Disease Resistance

  1. Biological control – use of bacteriophages as in the control of E. coli septicemia and meningitis in chickens and calves – targeted at quarry bacterium than broad spectrum of antibiotics

  2. Reducing need for therapy – moderate therapy using lower stocking rate and less frequent strategic drug intervention (organic approach)

  3. Selection pressure for general immunological competence and targeted resistance tests for specific major pathogens – marker assisted selection on candidate genes

  
• Anthelmintic Resistance


  Ability of worms to survive a dose of anthelmintic which would normally effective<|>Or 'drug tolerance'<|>Inherited

  
• Malaysia: since 1990s: Thiabendazole resistance (Rahman, 1993)
• Alternative Approach for GIN Control

  1. To minimize the contact between host and infective larvae through grazing management

  2. To control the parasite with alternative treatment such as natural plants and minerals

  3. To boost host immunity

  
• Variation in helminth resistance in goats and sheep


  • Individual variation exists – a minority of animals harbouring a majority of the nematodes
  • Evident from pedigree information where progeny have been tested for resistance under uniform conditions

  
• Variation in helminth resistance in goats and sheep

  • h2 for resistance to H. contortus infection (FEC) varied from 0.22 to 0.43 – from challenge studies and naturally infected flocks
  • There is prospect for selective breeding for improved resistance – some sheep commercial flocks in New Zealand and Australia adopt structured breeding programmes for parasite resistance

  
• Selective Breeding of Genetically Resistant breed


  Common practice in the livestock industry<|>Sustainable, feasible and desirable approach<|>One example of selective breeding implemented in the sheep industry is breeding for nematode resistance<|>Richer countries eg: Australia and New Zealand, sheep breeding companies have been breeding for nematode resistance for a long time

  
• Nematode Resistance


  Resistance: ability of the host to carry a reduced parasite burden<|>Three consequences observed after parasite entry into host, the parasites: unable to establish infection, incapable of completing their life cycle, rejected from host<|>Exact and precise mechanism of resistance is not entirely understood for all GIN, the general principle applied is an increase in host resistance associated with a better immune resistance against parasite

  
• Breeding of genetically resistant sheep is a better option

  • Permanent solution
  • Demanding no extra resources and inexpensive method
  • Slow down the development of anthelmintic resistance
  • Meeting the demand for drug free residues in meat for customers
  • Enhances resistance to other GIN (Gruner et al. 2004)
  • Strong favourable genetic correlation between resistance (FEC) and growth rate
  • The heritability of nematode resistance is one-third suggesting that the breeding of nematode resistance is feasible
  • The ultimate 'instrument' for controlling parasites in the long term

  
• Resistance


  Negative genetic correlation between resistance and resilience (Rashidi et al. 2013)
• The main benefit of resistance compared with resilience is reduce contamination of larvae in the field
• Other non-resilient sheep have a lower risk of parasite challenge
• Evidence of genetic variation in nematode resistance among breeds

  • Africa: Red Masai, Sabi, Djallonke, Dorper and Menz
  • Asia: Sumatra, Garole, Lohi and Local Kashmiri crosses
  • America: St Croix, Blackbelly, Katahdin, Gulf Coast Native, Santa Ines and Criollo
  • European : Canarian Hair Breed (CHB), Merinoland and Texel
• Selection within Breeds


  Farmers can substitute susceptible breeds with resistant breeds in GIN endemic areas<|>Use of a resistant breed that has already been adapted is simple<|>Obstacles: unfavourable response from sheep farmers and good performance in economic traits of the nematode susceptible breeds<|>Substitution of breeds is not a viable option in all farms
• Selection within Breeds


  Selection within a breed is can be adopted<|>Set up a selection scheme: identify a selection objective<|>In big livestock producer countries such as Australia and New Zealand, nematode resistance is one of components of the selection objective in commercial farms
• What Traits Reflect Resistance to GIN?

  Resistance to GIN is complex physiological characteristic<|>Nematode resistance status: parasitological, immunological and/or pathological parameters<|>Low FEC : sign of a nematode resistant animal<|>Typically, higher abilities of immunological and pathological responses are associated with low FEC, thus immunological and pathological parameters become the indicator of resistance
• FEC :Advantages


  • FEC is a chosen parameter: simple to measure and highly repeatable, the heritability of the single FEC ranging from 0.2 to 0.4, and this value is similar to heritability of milk production in dairy cattle, selection of low FEC is a part of breeding programmes and has been proven to be successful in Australia and New Zealand, without any adverse genetic correlation with important economical traits, computer simulation model has shown that selection of resistance based on FEC is promising

  
• FEC: Disadvantages


  • Drawbacks of FEC include time-consuming and labour intensive processes, Animal needs to have encountered the parasitic challenge. This compromises the health and welfare of the animal, Less fecund GIN such Teladorsagia circumcincta, the number of eggs and worm burden are poorly correlated, Density-dependent relationship contributes to a low FEC even though the animal is heavily infected
• Genetic Markers


  Genetic markers or DNA-based tests have been proposed as a marker for nematode resistance<|>Genetic markers : 1970s, with the aim to detect quantitative trait loci (QTL) or loci that control genetic variation<|>Genetic markers can avoid problems encountered when phenotypic marker is not efficient anymore due to absent of parasite challenge in field
• Hunting Genes Underlying Nematode Resistance

  Identification of genes or QTL, which code for control nematode resistance, aid in selective breeding<|>Two methods in genetics to identify the gene associated nematode resistance: QTL mapping and candidate gene analysis

  
• Candidate gene: MHC


  Testing or analysis of candidate genes is useful approach : prior knowledge of the genes or pathways in biological, physiological and functional of disease in question<|>The aim of the candidate gene analysis is to determine the relation between a particular phenotypic trait and mutation in a given gene<|>Advantage of candidate gene analysis compared to QTL mapping is powerful due : ability to detect trait loci with even a small effect
• Candidate gene: MHC


  Two important genes have been used for candidate gene analysis for identifying QTL underlying resistance to nematodes, predominantly Teladorsagia circumcincta: interferon gamma and MHC genes
• Strategies in animal breeding for disease resistance

  • Breeding objective focus on higher level of production
  • Selection objective is not solely for nematode resistance but as indirect response to higher yield (10 trait) – economically important traits
  • Commercial attraction when high resistant animals are also the high producers
  • Red Maasai more resistant than Dorper and also equally productive but not in all resistant breeds
  • Perceived higher nematode resistance of native breeds compared with imported breeds may be due to few resistant sires
  • No consistent pattern in genetic association between FEC and production traits: genetic correlation may strengthen when parasite challenge increases