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Ask the Vet, March 2023: Tell us more about Staph. Aureus

By Dayna Locitzer, DVM

In my last column I wrote briefly about the contagious mastitis causing pathogen Staphylococcus aureus or simply S. aureus. I said that if you wanted to hear more to ask and I would write a whole column… and you did, so here it is! It certainly deserves its own column. It is an insidious bug that can cause milk quality issues and take a long time to recover from if you are not on top of it. S. aureus can cause both clinical and subclinical mastitis.

Typically, cows that have it have a chronically elevated somatic cell count with outbreaks of abnormal milk and produce 15% less milk on average. It can also cause gangrene mastitis, which is a catastrophic udder infection where the skin tissue of the whole quarter starts to die and slough off. Though there is nothing that will cure a S. aureus infection, there are a lot of ways to prevent it, and all of those ways promote improved milk quality. So let’s get into what exactly it is, how it’s spread, and how we can prevent it.

S. aureus is a gram positive facultative anaerobe that is spherical in shape and lives in grape-like clusters. This means that it can live in environments with or without oxygen. It likes to live on a host, like skin or inside a mammary gland. It doesn’t survive long just hanging out on a non-living object like our environmental mastitis causing pathogens do. It has a lot of defense mechanisms to keep itself alive and evade its host’s immune system. One of these evasion strategies is living in biofilms. This is a strategy bacteria use where they band together and attach to a surface using external compounds, making it hard for them to be washed away or flushed out. S. aureus can also survive inside immune cells, avoiding detection by sentinel cells. And, most notably, it can cause microabscesses. These walled-off capsules of puss cause a perpetual inflammatory environment. This leads to chronic high somatic cell count, scar tissue and consequently decreased milk production. This walling off also makes them difficult to penetrate by antibiotics and the cow’s own immune system. The infection also goes undetectable in this stage, making it a moving target to diagnose. These microabscesses burst on occasion, leading to clinical mastitis and the characteristic intermittent shedding of S. aureus bacteria. This is why, when a cow has a S. aureus infection, she has one for life.

To understand how to prevent infection, it is important we learn how a cow becomes infected. One big factor is us, the people milking the cows. We have skin, and S. aureus likes to live on skin. It especially likes the cracked, weathered hands of farmers. We can carry it between cows. The best way to prevent this mode of transmission is by wearing gloves.

Cows also transmit it to each other or even to themselves. Cows with chapped teats or teat ends with fronds are especially susceptible to a S. aureus because the bacteria like to colonize damaged skin. S. aureus will overgrow on these damaged teats, and then through its biofilm will march its way to the teat orifice, up the canal, into the mammary gland. Therefore promoting teat end health is key to prevention. This can be accomplished by using a teat dip that doesn’t dry out skin but more importantly, making sure vacuum is appropriate.

I am going to take some time to talk about appropriate vacuum levels because this is one of the most vital factors in S. aureus prevention. Improper vacuum will cause short and long term teat end damage. It is important that you discuss the correct vacuum level with your milking system technician. Once that is correct, you must ensure each cow experiences that level of vacuum the entire time the machine is attached. To do this, milk flow must be consistent. Times of low milk flow will make the vacuum disproportionate to the amount of milk being harvested. This can happen at the beginning and end of machine attachment. If you attach the machine before the cow’s brain has signaled to the mammary gland for milk let down, the machine will be attached without much milk coming out of the udder. That is why you must allow for about 90 seconds of prep time, meaning it should take no less than 90 seconds from when you first touch her udder to when you attach the machine. It is also important to take the machine off before milk flow peters off, avoiding disproportionate vacuum. And remember, let the vacuum completely stop before removing the machine from the teats. If the vacuum is still on during machine removal, it pulls at teat ends and causes damage.

Improper vacuum also causes backflow. That means whatever is in the claw flows back into the teat canal. That could include fecal material, skin cells, and bacteria. This is how a cow can transmit S. aureus to herself but also to her neighbor cow. Let’s say you milk a cow that is shedding S. aureus, and then you milk her neighbor. If you attach the machine before she has let down her milk, she might experience backflow, and the S. aureus from her neighbor will flow up into her mammary gland. And when you hear that incorrigible noise of the inflation sliding off a teat, just know backflow is happening as well as teat damage.

How you manage your calves can also be a risk factor. When a heifer freshens with S. aureus it is potentially due to calves cross suckling either before weaning or after weaning when housed in groups. This is hard to prevent, but there are good strategies that would take another column to fully explain. Another management risk factor is when calves are kept with the milking herd so they can nurse on their dams. When calves are young, they typically stick to nursing only on their mother, but when they get older, they become indiscriminate nursers. This means that if you have S. aureus in your herd, a calf could suckle on a cow shedding S. Aureus, then with her mouth full of bacteria, go ahead and nurse on a cow that is not infected, thereby transmitting the disease. If keeping calves on mothers or nurse cows is part of your heifer raising program, consider keeping a separate nurse herd to keep the calves away from the milkers.

So you have S. aureus in your herd, now what? There are many good strategies to help keep it under control. If you have the ability, milk the S. aureus cows last and don’t touch non-infected udders after you have prepped the infected ones. If you milk in a parlor and do not control the order of cows, you can sanitize machines with a sanitizing solution after milking a S. aureus cow. Consult with your chemical supplier for an appropriate solution. Make sure to change inflation liners according to the label directions or at least twice a year. If you are using a sanitizing solution in between cows you should consider changing liners more frequently because they develop cracks, which can harbor the bacteria. Just like bare hands and cracked inflations harboring bacteria, towels can too. It is critical to use a separate towel for each cow. Additionally, I think it is valuable to screen fresh cows. This means sending in a culture of the post-colostrum milk. Sometimes you will get a false positive from a cow shedding what’s been sitting in her teat cistern for the dry period. When you get a S. aureus positive culture from screening, culture at least two more times two weeks apart, and if those are both negative you can consider her not infected. And unfortunately, since this infection will not cure, I would recommend putting that cow on the cull list.

S. aureus can be an overwhelming problem, but ignoring it will not help and will only make it worse. Your first step is to find out if it's in your herd. A simple bulk tank culture will detect it. Then look into the listed risk factors above to see where you can improve your systems. Keep on top of it, and be vigilant. You will benefit from better milk quality and more milk if you do.

Do you have a question for Dr. Locitzer, or an area you’d like her to focus on in future issue? Please send them to the NODPA News editor, noraowens@comcast.net who will share them with her.