Cross-sectional study on the prevalence of contagious pathogens in bulk tank milk and their effects on somatic cell counts and milk yield

Figures & data

Figure 1. Relationship between estimated bacteria counts and frequency of positive BTM (1,2,3). The frequency of positive results among each pathogens have a statistically significant trend at Cochrane-Armitage test (p < .0001). BTM: bulk tank milk.

Table 1. Prevalence of positive bulk tank milk samples for the four bacteria species considered.

District	Bacteria species
	S. aureus	Str. agalactiae	Prototheca spp	Mycoplasma bovis
1	39.07%^b,c,1	8.37%^a	8.37%^a,b	1.39%^a
2	42.94%^c	9.22%^a	12.96%^b	1.75%^a
3	30.29%^b	12.98%^a	8.65%^a,^b	0.00%^a
4	58.62%^a	10.34%^a	2.59%^b	2.56%^a
Total	42.00%	9.64%	11.19%	1.55%

¹ Column means with different letters are statistically different (α < 0.05).

Table 2. Analysis of variance of the GLM model applied to assess the role of the factors considered on SCC and milk yield.

Factors	SCC, log10 cell/ml		Average milk yield, kg
	F	P	F	P
District	2.1400	0.0935	4.3200	0.0048
Herd size (HS)	5.0900	0.0004	29.5900	<0.0001
Bulk tank milk results (BTM)	5.6300	<0.0001	6.3800	<0.0001
District * BTM	2.0900	0.0101	2.1700	0.0070
HS * BTM	2.2900	0.0007	2.2000	0.0014
Model R^2 (P=)	0.15 (<0.0001)		0.40 (<0.0001)

GLM: general linear model; SCC: somatic cell count.

Table 3. Results of contrast analysis related to the three main factors considered for SCC and milk yield.

Factor		SCC, log10 cell/mL		Average milk yield, kg
District	Frequency	Mean	Std. Dev.	Mean	Std. Dev.
District
1	72	5.52^a,1	0.18	26.13^a	6.28
2	401	5.50^a	0.19	30.18^b	5.00
3	69	5.58^b	0.19	27.22^a,c	6.75
4	39	5.53^a	0.22	29.33^b	5.45
Herd size (n.cows)
1–45	115	5.55^a	0.23	23.79^a	6.10
46–80	119	5.55^a	0.20	28.68^b	4.49
81–120	113	5.52^b	0.19	28.95^b,c	4.28
121–180	109	5.46^c	0.16	31.72^d	4.13
>180	125	5.48^c	0.16	33.00^e	3.96
BTM results
S.aureus	244	5.56^b	0.19	27.97^b	5.60
Str.agalactiae	56	5.63^c	0.19	26.75^b	5.78
Prototheca spp	65	5.50^a,b	0.14	30.68^a	4.54
Mycoplasma bovis	9	5.55^a,b,c	0.21	30.71^a	4.40
Negative	207	5.48^a	0.21	29.96^a	5.57

¹ Column means with different letters for each factor are statistically different (α < 0.05).

BTM: bulk tank milk; SCC: somatic cell count.

Table 4. Results of contrast analysis related to the interaction of BTM results and herd size and for SCC and milk yield.

Factor 1	Factor 2	SCC, log10 cell/mL		Average milk yield, kg
Herd size (n.cows)	Pathogen	Mean	Std. Dev.	Mean	Std. Dev.
1–45	S. aureus	5.59^b,1	0.20	22.48^b	5.38
	Str. agalactiae	5.76^a,c	0.12	19.57^b	4.81
	Prototheca spp	n.e.^2		n.e.
	Mycoplasma bovis	n.e.		n.e.
	Negative	5.62^a	0.23	24.81^a	6.45
46–80	S. aureus	5.62^b	0.21	28.46^a	4.30
	Str. agalactiae	5.51^a	0.18	27.12^a	3.92
	Prototheca spp	5.58^a	0.13	29.56^a	3.67
	Mycoplasma bovis	n.e.		n.e.
	Negative	5.51^a	0.19	29.02^a	4.45
81–120	S. aureus	5.54^b	0.17	28.98^b	4.07
	Str. agalactiae	5.69^b	0.24	25.98^c	3.69
	Prototheca spp	5.53^b	0.11	28.64^b	4.16
	Mycoplasma bovis	n.e.		n.e.
	Negative	5.48^a	0.17	29.69^a	4.10
121–180	S. aureus	5.47^a	0.17	31.44^a	4.79
	Str. agalactiae	5.55^a	0.07	30.08^a	2.57
	Prototheca spp	5.46^a	0.13	30.82^a	2.67
	Mycoplasma bovis	5.67^a	0.07	29.20^b	3.82
	Negative	5.45^a	0.17	31.98^a	4.14
>180	S. aureus	5.51^b	0.15	31.41^b	3.43
	Str. agalactiae	5.57^b	0.14	31.40^b	3.86
	Prototheca spp	5.50^b	0.16	34.42^a	3.30
	Mycoplasma bovis	5.47^a	0.17	33.32^a	2.32
	Negative	5.45^a	0.16	33.75^a	3.89

¹ Column means with different letters for each herd size are statistically different (α < 0.05).

² Not evaluable.

SCC: somatic cell count.

Table 5. Odds ratio (confidence limits 95%) for risk factors associated to the presence of the four pathogens considered in BTM.

Factor	Pathogen
	S. aureus	Str. agalactiae	Prototheca spp	Mycoplasma bovis
District (district 1 as reference)
2	1.005 (0.743–1.359)	10.466 (7.083–15.463)*	8.016 (5.559–11.558)*	49.351 (29.216–83.365)*
3	0.624 (0.460–0.845)*	4.578 (3.262–6.424)*	3.622 (2.609–5.030)*	11.500 (7.578–17.453)*
4	3.631 (2.552–5.166)*	39.912 (25.773–61.807)*	21.592 (14.452–32.260)*	282.869 (155.415–514.847)*
Herd size (1–45 cows as reference)
46–80	0.936 (0.673–1.301)	4.726 (2.934–7.613)*	3.579 (2.304–5.558)*	8.326 (4.642–14.932)*
81–120	1.422 (1.013–1.995)*	12.607 (7.535–21.094)*	4.873 (3.058–7.766)*	19.706 (10.321–37.627)*
121–180	1.314 (0.912–1.894)	17.63 (9.981–31.155)*	12.820 (7.731–21.259)*	89.137 (41.944–189.433)*
>180	1.265 (0.871–1.836)	19.489 (10.920–34.783)*	10.275 (6.142–17.188)*	69.512 (32.584–148.292)*
Average milk yield (>34 kg/d as reference)
<21	3.174 (1.955–5.154)*	17.305 (9.307–32.178)*	6.351 (3.632–11.104)*	25.358 (12.353–52.052)*
21–28	2.352 (1.682–3.289)*	1.595 (1.035–2.457)*	0.989 (0.669–1.462)	0.711 (0.425–1.189)
29–31	1.512 (1.064–2.150)*	1.326 (0.843–2.086)	0.889 (0.588–1.344)	1.356 (0.804–2.285)
32–34	1.450 (1.047–2.008)*	1.377 (0.901–2.104)	1.329 (0.919–1.922)	1.506 (0.935–2.423)
SCC (<200 cells/μL as reference)
201–300 cells/μL	0.979 (0.668–1.435)	0.718 (0.438–1.177)	0.948 (0.608–1.479)	0.311 (0.176–0.551)*
301–350 cells/μL	1.430 (1.006–2.032)*	0.618 (0.386–.991)*	0.664 (0.431–1.024)	0.296 (0.172–0.509)*
351–400 cell/μL	1.005 (0.695–1.454)	1.018 (0.643–1.612)	1.295 (0.850–1.974)	0.427 (0.251–0.724)*
401–500 cell/μL	1.367 (0.931–2.005)	0.757 (0.459–1.249)	1.419 (0.910–2.213)	0.556 (0.320–0.967)*
>500 cell/μL	1.767 (1.186–2.632)*	1.620 (0.992–2.647)	1.057 (0.658–1.698)	0.566 (0.319–1.004)

* Odds ratio statistically different (α < 0.05).

BTM: bulk tank milk.