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Extension > Agriculture > Dairy Extension > Milk quality and mastitis > Mastitis effects on reproduction

Mastitis effects on reproduction

Ricardo C. Chebel
DVM, MDVM, Dept of Veterinary Population Medicine

October 8, 2011

Mastitis is the inflammation of the mammary gland, which is usually correlated with intramammary infection by bacteria or fungi, the first being more common. It is accepted that the normal mammary gland has a somatic cell count (SCC) <200,000 cells/mL and no bacterial infection. Cows that have a SCC >200,000 cells/mL (LSCC or linear score >4) and/or have bacteria in the milk but do not have clinical signs of mastitis are considered to have subclinical mastitis. The costs associated with mastitis are innumerous and include antibiotic treatment, discarded milk, reduced milk quality, increased culling rates, and reduced milk yield. Another cost of mastitis that is often taken for granted, however, is reduced fertility.

Existing literature demonstrates the association between mastitis and reproductive performance. It is important to note that these studies are mostly retrospective in which diagnosis and recording of mastitis events were performed by farm personnel. Therefore, inherent variations among dairies/studies exist and may account for differences in reported findings. Regardless, compelling data demonstrate a significant negative association between mastitis and reproductive performance.

A few studies evaluated the association between mastitis and reproductive performance in a similar way. Cows were classified as having had mastitis before first postpartum AI (MG1), between first postpartum AI and pregnancy diagnosis (MG2), after pregnancy diagnosis (MG3), or no mastitis (control). These studies demonstrate that occurrence of mastitis is associated with prolonged interval to first postpartum AI, increased service per conception, and prolonged interval from calving to conception (Table 1).

Table 1. Association between occurrence of mastitis and reproductive parameters.

 

Classification of Mastitis Occurrence

Reference code*

MG1               (mastitis between calving & first AI)

MG2                       (mastitis between first AI & PG diagnosis)

MG3                 (mastitis after PG diagnosis)

Control                   (no mastitis)

MG3 + Control
(after PG diagnosis + no mastitis)

Days in milk at first AI

75.7

75.2

---

---

67.8

1

68

58.5

62.3

64

---

2

 

 

 

 

 

 

 

Services per conception

1.6

2.9

---

---

1.7

3

2

3.1

---

---

1.6

1

2.6

3.1

2.5

2.6

---

2

 

 

 

 

 

 

 

Interval to conception

113.7

136.6

---

---

92.1

3

106.2

143.5

---

---

85.4

1

165

189.4

118.4

139.7

---

2

*Reference code: ¹Schrick et al. (2001); ²Santos et al, 2004; ³Barker et al (1998)

In the study by Santos et al. (2004), it was also demonstrated that the percentage of cows pregnant to first AI and percentage of cows pregnant at 320 days postpartum was smallest for MG1 (22.1 and 72.3%) and MG2 (10.2 and 58.5%) cows, whereas MG3 (37.9 and 93.1%) and control (28.7 and 85.4%) cows did not differ. Furthermore, cows that had mastitis at any interval relative to first postpartum AI were more likely to have an abortion (MG1 = 11.8%, MG2 = 11.6%, MG3 = 9.7%, control = 5.8%).

Looking at other studies, Figure 1 demonstrates the association among clinical (Risco et al., 1999; Chebel et al., 2004) and subclinical mastitis, defined as LSCC > 4.5, and pregnancy losses (Moore et al., 2005). These studies demonstrated that a cow diagnosed pregnant at 30 to 45 days after AI that had mastitis before first pregnancy diagnosis were still at higher risk of pregnancy loss/abortions than those that did not have mastitis. These are important findings because they demonstrate that mastitis that occurs during early pregnancy (before 30 to 40 days after AI) may not only cause immediate embryonic death and reduced pregnancy per AI as demonstrated by Santos et al. (2004) but may also have deleterious effects to fetal development, consequently increasing incidence of abortions.

Mastitis prevents ovulation, extends the interval to first AI and reduces fertilization rates and embryo development. Mastitis compromises pregnancy establishment and maintenance and increases incidence of abortions. Cows that have mastitis at any interval after calving have reduced pregnancy rate, which results in significant economic losses to dairy herds. In a recent study evaluating approximately 9,000 lactations in two dairy herds in California, we demonstrated that the hazard ratio for pregnancy was 1.25 (95% CI = 1.19, 1.32) for cows without mastitis. This means that the speed at which cows without mastitis became pregnant was 25% faster than cows with mastitis. The median interval from calving to pregnancy (at which 50% of cows are pregnant) for cows without mastitis was 128 days, whereas cows that had mastitis had a median interval of 154 days (see Figure 2).

In this study, the yearly incidence of mastitis was approximately 35% and the median interval from calving to pregnancy was 26 days longer for cows with mastitis compared with cows without mastitis. If the average cost of one day open is $2 per day, the 'reproduction' cost of mastitis was $52 per case. Therefore, in a herd of 1,000 lactating cows, by reducing the yearly incidence of mastitis from 35% to 25% the expected savings due to improved reproductive performance is approximately $5,000. Ultimately, reducing mastitis incidence must be a goal for dairy producers not only because of milk quality and the direct costs of mastitis, but also because of the indirect cost of mastitis and its effects on reproduction.

Drs Bey (left) and Farnsworth

Figure 1. Incidence of pregnancy loss according to occurrence of mastitis.

Drs Bey (left) and Farnsworth

Figure 2. Association between occurrence of mastitis and speed at which cows become pregnant (Mendonca and Chebel et al., 2011).

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