An interesting paper was presented at the National Mastitis Council Regional meeting by Dr. Ricardo Chebel from the University of California – Davis Veterinary School.

He has studied and also done a good review of other studies regarding whether mastitis is associated with reproductive performance of dairy cattle. This article contains abstracts from his paper.

Chebel points out that most diagnosis of clinical mastitis and recording of mastitis events is done by farm personnel on commercial farms, not necessarily by some standardized protocols or regular observation on research projects. This is valid and like the “real world,” but as he says it means that not every study’s definition of mastitis is exactly the same. However, most research has suggested that mastitis is indeed associated with reduced fertility of cows.

Abnormal estrus and mastitis
A study by Moore et al. in 1991 looked at intervals between estrus in cows with different types of mastitis. Cows in a herd that contracted cases of Staph aureus mastitis did not differ from other cows without mastitis, but in another herd with cases of Gram-negative bacterial mastitis (coliform mastitis is mentioned as an example of this but no detailed culture results are shown), mastitic cows had more intervals less than 18 days or greater than 24 days between heats.

Successful pregnancy and mastitis
Barker et al. studied reproduction in Jerseys in 1988. The days until first insemination after calving averaged 94 days for cows that “experienced mastitis” before the first postcalving A.I. and 71 days for cows with “no mastitis” before the first postcalving A.I. Services per conception (SPC) were similar in cows that had mastitis before the first postcalving A.I. (1.6), after pregnancy confirmation (1.7) or had no mastitis (1.7). However, cows that “experienced mastitis” between first postcalving A.I. and pregnancy confirmation averaged 2.9 SPC, significantly higher than non-mastitic cows. Days open averaged 114 days for cows with mastitis before first A.I., 137 days for cows with mastitis between first postcalving A.I. and pregnancy confirmation and 92 days for cows with no mastitis.

Readers will note that all of the above numbers are not extremely poor, but this study only examined first insemination after calving, and as always, days open (the following is also true for calving interval) is a statistic that usually only includes cows that become pregnant.

A follow-up study by the same group reached similar conclusions. Cows that had either culture-positive milk with no clinical mastitis or had clinical mastitis between first postcalving A.I. and pregnancy diagnosis averaged 3.1 SPC and 144 days open compared with 1.6 SPC and 85 days for cows with no mastitis or with mastitis only after pregnancy confirmation. In both of the above studies, the association between mastitis and reproductive efficiency was the same whether Gram-negative or Gram-positive bacteria were cultured from milk.

Two Holstein herds were studied in California. Not only were conception rates lower for cows with clinical mastitis, but percentage of cows pregnant by 320 DIM was lower also. Abortions from 42 to 180 days “after A.I.” (probably this was estimated as days of gestation, i.e. days after the insemination considered to result in conception) were also higher in mastitic cows. Again, the type of bacteria isolated from milk was not associated with differences in reproductive effects of mastitis.

Mastitis and abortions
Two other studies looked at loss of pregnancy after conception using ultrasound. Cows with clinical mastitis during the 31 days after A.I. were not less likely to conceive (24 percent versus 25 percent for non-mastitic cows) but were 2.8 times more likely to lose pregnancy from 31 to 45 days after A.I. Cows with subclinical mastitis (defined as Linear Score of SCC greater than 4.5 at last test day before A.I.) were 2.4 times more likely to lose pregnancy from 28 to 35 days after A.I.

Possible reasons why mastitis may increase loss of pregnancy
The article also includes some interesting speculation regarding the mechanisms by which mastitis, especially mastitis contracted between conception and approximately 40 days later, might cause loss of embryos. These possibly include fever, endotoxins from mastitis bacteria, production of cytokines as part of the cow’s immune response and prostaglandins in the blood of the infected cows. However, there has not been conclusive evidence of exactly how mastitis affects reproduction. The main evidence is associations like those noted above between clinical or subclinical mastitis and reduced pregnancy, including increased loss of established pregnancy.

What can we do about mastitis and its effects on reproduction?
Based on this excellent review article and the literature, I do not think we have a definitive answer to this question. Chebel mentions that the financial importance of mastitis is even greater than previously thought, considering its apparent reproductive effects, which is an excellent point. However, most dairy producers and their herd advisors already acknowledge mastitis as an important disease, and the vast majority of herds have some form of mastitis control program.

It seems to me that one thing suggested by these findings is that reducing inflammation from mastitis could be beneficial to pregnant cows. Fever, cytokines and prostaglandins are among the speculative causes of reproductive loss from mastitis. Reducing fever and clinical severity with anti-inflammatory and supportive therapy may help reduce embryo loss in cows with mastitis during early pregnancy. Further studies in this area are needed. PD

References omitted but are available upon request at editor@progressivedairy.com

—Excerpts from Utah State University Dairy Newsletter, Vol. 30, No. 5

David Wilson
Extension Veterinarian
Utah State University