In vivo model to study the impact of genetic variation on clinical outcome of mastitis in uniparous dairy cows

Abstract

Background: In dairy herds, mastitis causes detrimental economic losses. Genetic selection offers a sustainable tool to select animals with reduced susceptibility towards postpartum diseases. Studying underlying mechanisms is important to assess the physiological processes that cause differences between selected haplotypes. Therefore, the objective of this study was to establish an in vivo infection model to study the impact of selecting for alternative paternal haplotypes in a particular genomic region on cattle chromosome 18 for mastitis susceptibility under defined conditions in uniparous dairy cows.

Results: At the start of pathogen challenge, no significant differences between the favorable (Q) and unfavorable (q) haplotypes were detected. Intramammary infection (IMI) with Staphylococcus aureus 1027 (S. aureus, n = 24, 96 h) or Escherichia coli 1303 (E. coli, n = 12, 24 h) was successfully induced in all uniparous cows. This finding was confirmed by clinical signs of mastitis and repeated recovery of the respective pathogen from milk samples of challenged quarters in each animal. After S. aureus challenge, Q-uniparous cows showed lower somatic cell counts 24 h and 36 h after challenge (P < 0.05), lower bacterial shedding in milk 12 h after challenge (P < 0.01) and a minor decrease in total milk yield 12 h and 24 h after challenge (P < 0.01) compared to q-uniparous cows.

Conclusion: An in vivo infection model to study the impact of genetic selection for mastitis susceptibility under defined conditions in uniparous dairy cows was successfully established and revealed significant differences between the two genetically selected haplotype groups. This result might explain their differences in susceptibility towards IMI. These clinical findings form the basis for further in-depth molecular analysis to clarify the underlying genetic mechanisms for mastitis resistance.

Keywords: BTA18; Escherichia coli; Genetic selection; Haplotype; Intramammary infection model; Mastitis; Somatic cell count; Staphylococcus aureus.

Fig. 1

Graphical illustration of the in vivo intramammary infection (IMI) model. Animals selected for paternal BTA 18 haplotypes favorable (Q, n = 18) or unfavorable (q, n = 18) for somatic cell count received intramammary challenge with Staphylococcus aureus1027 (n = 24) or Escherichia coli1303 (n = 12) for 96 or 24 h, respectively. During Staphylococcus aureus challenge, inoculation was performed in the hind left (HL) and hind right (HR) quarters, and the front left (FL) quarter served as a negative control inoculated with saline solution, while the front right (FR) quarter was not treated. During Escherichia coli challenge, inoculation was performed with HR, saline solution was administered HL and front quarters were untreated. Clinical examination, udder examination and blood and milk sampling were performed every 12 h. The graphical illustration of the cow and the udder has been designed by Wolfram Petzl

Fig. 2

Graphical illustration of colony forming units isolated from Q-/q-uniparous cows after intramammary challenge. Colony forming units logarithmized to the base 10 per milliliter (log (CFU/ml)) of bacteria isolated from sterile milk samples of infected udder quarters after intramammary challenge with (a) Staphylococcus aureus (Q: n = 12 versus q: n = 12) and (b) Escherichia coli (Q: n = 6 versus q: n = 6) is shown. The first sample was taken before intramammary challenge and defined as 0 h relative to challenge. Afterwards, quarter milk samples were taken every 12 h. Data are presented as the mean and standard error of the mean (a) and as the median and interquartile range (b). Differences between uniparous cows selected for favorable (Q) and unfavorable (q) haplotypes are indicated with * if P < 0.05 and with ** if P < 0.01. Significant differences within the haplotype groups over time relative to challenge are not shown

Fig. 3

Graphical illustration of somatic cell count from Q-/q-uniparous cows after intramammary challenge. Somatic cell count (SCC) logarithmized to the base 10 in ml (log SCC 10³/ml) determined in milk sampled under sterile conditions from infected udder quarters after intramammary challenge with (a) Staphylococcus aureus (Q: n = 12 versus q: n = 12) and (b) Escherichia coli (Q: n = 6 versus q: n = 6) is shown. The first sample was taken before intramammary challenge and defined as 0 h relative to challenge. Afterwards, quarter milk samples were taken every 12 h. Data are presented as the mean and standard error of the mean (a) as the median and interquartile range (b). Differences between uniparous cows selected for favorable (Q) and unfavorable (q) haplotypes are indicated with * if P < 0.05 and with ** if P < 0.01. Significant differences within the haplotype groups over time relative to challenge are not shown

Fig. 4

Graphical illustration of total milk yield from Q-/q-uniparous cows after intramammary challenge. Total milk yield in percent (%) relative to total milk yield at the start of the challenge (0 h) of uniparous cows after intramammary challenge with (a) Staphylococcus aureus (Q: n = 12 versus q: n = 12) and (b) Escherichia coli (Q: n = 6 versus q: n = 6) is shown. Milking was performed every 12 h after challenge, and total milk yield was determined. Data are presented as the mean and standard error of the mean (a) as the median and interquartile range (b). Differences between uniparous cows selected for favorable (Q) and unfavorable (q) haplotypes are indicated with * if P < 0.05 and with ** if P < 0.01. Significant differences within the haplotype groups over time relative to challenge are not shown