Evidence for Disease Susceptibility Differences in Goats
Herd-level prevalence in goats varies by geographical region and breed. Few studies have investigated breed differences in susceptibility to MAP infection among goats although a study in India identified that breeds native to semi-arid climates, such as Barbari and Jamunapari, were more likely to suffer from MAP infection compared with crossbred Rajasthani goats (Singh et al., 2009).
4.4.1 Heritability estimates
In goats, there have been two studies that reported an estimated heritability for susceptibility to MAP infection. One study estimated heritability to be between 0.01 and 0.15 for susceptibility to MAP infection in India for the Barbari and Jamnapari breeds (Singh et al., 1990). A more recent study on Dutch White dairy goats reported heritability estimates for susceptibility to MAP infection that ranged between 0.07 and 0.12 (van Hulzen et al., 2012b).
4.4.2 Genome-wide association studies
Few GWAS have been conducted in goats to identify loci associated with susceptibility to MAP infection. However, a recent study identified seven loci associated (P < 5 ? 10-4) with MAP susceptibility in Italian Garfagnina sheep identified when MAP infection status was determined using a serum ELISA (Cecchi et al., 2017; Table 4.1). These seven loci were located across seven chromosomes and harboured nine positional candidate genes, three of which (LOC102187381, MANEA and PCSK5) had functions related to the Golgi apparatus (Cecchi et al., 2017). The Golgi apparatus has an important role in MAP infection as it is where TLR4 is located (Hornef et al., 2002). As previously mentioned, TLR4 is involved in MAP recognition by the host immune cells (Ferwerda et al., 2007).
4.4.3 Candidate gene studies
The number of candidate gene studies performed in goats has increased over the last several years. Now, like cattle and sheep, the association of SLC11A1 in MAP susceptibility to infection in goats has been investigated (Table 4.2).
In addition to SLC11A1, researchers have also investigated the HLA class II histocompatibility antigen, DRB1-4 β chain (DRB3) gene. The HLA system is a gene complex that plays a key role in immunity. The DRB3 gene is associated (odds ratio >0) with susceptibility to MAP infection in Holstein-Slovak crosses of cattle (Rastislav and Mangesh, 2012; Table 4.2). This gene is part of a heterodimer that has an established role in presenting mycobacterial antigens to host immune cells (Mustafa, 2000). A study in Jamunapari goats used PCR restriction fragment-length polymorphisms with the restriction endonucleases PstI and TaqI to determine variants within DRB (Singh et al., 2012). This study used multiple diagnostic methods to determine infection status (serum ELISA, faecal culture and microscopic faecal exams) and reported that two variants (P and T) differed (P < 0.001) between healthy and MAP infected goats (Singh et al., 2012). The authors reported that the recessive genotypes ‘pp’ and ‘tt’ and the recessive haplotype ‘pptt’ occurred at higher frequencies within MAPresistant goats compared with susceptible goats (P < 0.001).Another recent study investigated associations with MAP infection with 12 microsatellites in a population of 48 Garfagnina goats using a serum ELISA test to detect MAP infection (Cecchi et al., 2018). Two of the 12 microsatellites were weakly associated with MAP: ETH10 on chromosome 5 (P < 0.1) and SRCRSP05 (P < 0.05) on chromosome 21 (Cinar et al., 2018).
4.4.4 Pathway analysis
Currently, no pathway analyses in goats have been published.
4.5
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