Examples

Examples of genetic variation in disease resistance span all major livestock species and pathogen types. The strength of evidence varies from anecdotal to precise, and from population-level descriptions to causative genes. Some examples are classified by host species and pathogen type in Table 1, most of which are summarized in dedicated texts.[1,2]

Of the diseases in Table 1, precise effects have been ascribed to specific genes for only a small number of cases, such as the receptor genes coding for resistance to neonatal (F4) and postweaning (F18) E. coli diarrhoea in pigs;[3] MHC effects associated with resistance to Marek's disease[4] and dermatophilosis in cattle;[5] and scrapie, where genetic variation in susceptibility is predominantly due to variability at PrP gene codons 136, 154, and 171.[6] More usually, genetic variation in resistance to the diseases shown in Table 1 is described at the breed level, by within-breed variation, as quantified by the heritability

Table 1 Examples of diseases for which there is documented or strong anecdotal evidence of genetic variation in host resistance or tolerance

Pathogen or parasite type

Host species Prion and virus Bacteria Protozoa Helminth and ectoparasite

Table 1 Examples of diseases for which there is documented or strong anecdotal evidence of genetic variation in host resistance or tolerance

Host species Prion and virus Bacteria Protozoa Helminth and ectoparasite

Chickens

Marek's disease Infectious laryngotracheitis Avian leucosis Infectious bursal disease Avian infectious bronchitis Rous sarcoma Newcastle disease

E. coli Pullorum Fowl typhoid Salmonellosis Campylobacter

Coccidiosis

Ascaridia galli

Pigs

African swine fever Foot and mouth disease Atrophic rhinitis Pseudorabies

Neonatal diarrhoea Postweaning diarrhoea

Cattle

BSE

Paratuberculosis

Trypanosomosis

Helminthosisa

Foot and mouth disease

Mastitis

Theileria annulata

Ticks

Bovine leukemia

Bovine tuberculosis

Salmonellosis

Dermatophilosis

Cowdriosis

Brucellosis

Theileria segenti Theileria parva Babesia

Sheep

Paratuberculosis Dermatophilosis Salmonellosis Cowdriosis

Trypanosomosis

Helminthosisa Liver fluke Flystrike

aHost resistance to many species of nematode helminths has been described.

aHost resistance to many species of nematode helminths has been described.

of an indicator trait, or by quantitative trait loci (QTL) that imply the existence of genes influencing resistance in specific chromosomal regions. For cases such as foot-and-mouth disease and African swine fever, the evidence is anecdotal, arising from field observations following epidemics.

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