OPINION: Mastitis in older cows more likely to cause pregnancy loss

Getting and keeping cows pregnant is a big part of dairy farming. A lot of research has looked at the most efficient ways to get a cow successfully bred back. Increasingly, it has become important to look at how cows lose those pregnancies too. Numerous studies have shown, for example, that cows which get mastitis either before breeding or early in pregnancy are more likely to lose that pregnancy.

The risk of mastitis causing pregnancy loss is influenced by the severity of the mastitis and by what bacteria are causing the mastitis. In general, moderate to severe mastitis and mastitis caused by gram-negative bacteria have a greater impact on fertility. Even so, research shows that cows which get subclinical but not clinical mastitis also have an increased risk that they will lose a pregnancy. Recently, Florida researchers were interested in looking at whether older, mature cows were more likely to suffer pregnancy loss with mastitis than first, second or third-lactation animals.

They followed Holstein cattle in one 500-cow dairy in Florida to compare the rates of pregnancy loss following mastitis. Cows were bred following timed AI (artificial insemination) and were confirmed pregnant by ultrasound at 33 days post-breeding, then rechecked at day 47 and day 75 post-breeding. All cows were screened for clinical mastitis through visible changes in the milk and quarter at twice-daily milking and for subclinical mastitis using DHI SCC (a linear score of more than 4.5 was considered as positive for mastitis.) They did not try to determine what was causing the mastitis.

Older cows were more likely to develop both clinical and subclinical mastitis. Regardless of parity, being lame before breeding was also associated with an increased chance of losing the pregnancy between 33 and 75 days after breeding. Also, cattle that became pregnant using ET (embryo transfer) were more likely to lose the pregnancy than if they got pregnant with AI.

Even considering the increased risks of losing a pregnancy because of pre-breeding lameness or ET breeding, cows in their fourth or more lactation were more likely to lose their pregnancy if they got mastitis than were younger cows.

The researchers tried to estimate the costs of the pregnancy loss if the mastitis occurred either before breeding or after breeding. Economic losses were highest for mature cows (those in their 4th or more lactation) that got clinical mastitis before they were bred. Losses were about half as much if a mature cow developed subclinical mastitis (linear score >4.5) or clinical mastitis and lost their pregnancy after they became pregnant.

This study reinforces previous studies that looked at the association between mastitis and fertility and it added a new dimension by highlighting the fact that older cows seem to be at an even greater risk of losing a pregnancy. Nobody knows exactly how a case of mastitis is able to affect pregnancy, even when the mastitis is mild and is subclinical. There are several theories to explain how mastitis could affect both the ovaries and the uterus.

What this does highlight, though, is that investing in mastitis prevention is likely to pay back not just in better milk quality and udder health but can also lead to improved herd fertility.