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Incidence vs. prevalence: Know where your herd sits with subclinical ketosis

Chris Church for Progressive Dairyman Published on 04 October 2017

Ketosis is a very common disease in dairy cattle. Various testing methods are available to help dairy producers understand how it affects their herds. Here are a couple of points to consider in selecting the method that is right for you.

Firstly, ketosis is the result of a normal process that has gotten carried away. When a cow freshens, she is unable to eat enough calories to match milk production, so her body starts to mobilize fat stores. Some of that mobilized fat, called non-esterified fatty acids (NEFA) when circulating in the blood, can be turned by the liver into a type of molecule called ketone bodies, or ketones for short. If we measure her blood, we can find these ketones, such as beta-hydroxybutyrate (BHB). In moderation, this is perfectly healthy. Levels of BHB under 1.2 mmol/L do no harm, and are normal in high-producing dairy cows. The problem arises as these levels increase. Above 1.2 mmol/L, the cow may not appear any different, but the ketones will (invisibly) start to cause issues. We call this subclinical ketosis (SCK). As the levels go higher still, the cow will start to show clinical signs, such as reduced milk production, appetite and manure. This also drives up the risk of other diseases and problems, like displaced abomasum (DA) and ovarian dysfunction.

The latest studies from Cornell put a dollar figure of $400 for a case of SCK, but I prefer to use this example: Researchers were investigating the impact of using various timed artificial insemination programs on first conception rates across 6,000 cows. During the study, they noticed some cows responded poorly compared with their herdmates. They were able to trace the differences in conception to diseases suffered during transition. Cattle that were healthy post calving had a first conception rate of 51 percent, whereas those diagnosed with clinical ketosis had a rate of 29 percent. This would have a significant financial impact for any dairy.

Before we go further, it is important to discuss some of the different terms that are used to describe ketosis testing. If you have never tested before and want a baseline on where your herd stands, you can use the Ketoscreen test available from CanWest DHI (or Cétolab from Valacta). These programs test any fresh cows present on test day. This is very inexpensive and requires little work, but it only gives you a “snapshot” of your herd every five to six weeks. Because the duration of an average case of SCK is only about one week, you will not be able to use Ketoscreen or Cétolab to find every SCK case to treat, but you can track the herd over time. The proportion of positive cows on a given day is called a prevalence test. If we want to know what is occurring with every cow at the beginning of her lactation, we would have to use a cow-side test more regularly.

A monitoring program performed twice a week that captures all fresh cows will tell us the proportion of cows that have been positive at least once in early lactation; this is called an incidence rate. A rough guide is that your incidence will be 2.4 times higher than what the prevalence rate is for your herd. For example, if your recent KetoScreen or Cétolab shows your herd has 25 percent SCK prevalence, your incidence in the herd would be 60 percent, which means that six out of 10 cows are affected at least once in early lactation. Studies and articles may quote SCK rates associated with prevalence or incidence, so it is important to know the difference. Moreover, because the riskiest period for SCK is in the first 15 days in milk (DIM), it is important to know the range of DIM used in your risk calculation. For example, if the prevalence is calculated on all cows at their first DHI test (with cows roughly between five and 40 DIM at first test), cows with lower SCK risk will be included in the calculation and your reported prevalence may then be artificially low.

How are we doing?

Valacta and CanWest DHI have been monitoring the ketone herd prevalence levels across Canada for the last few years. The results for CanWest DHI in 2015 are listed in Figure 1. Herds that have a prevalence in the first 21 DIM above 20 percent are considered high risk for disease. Using that standard, more than 50 percent of herds tested fall in that category. Results for herds analyzed by Valacta are similar.

Subclinical ketosis figure 1

Subclinical ketosis table

How are you doing?

I always believe that prevention is better than treatment, but before we can get to that, you need to know if your herd is struggling with SCK. Having a baseline will also allow you to see if changing other areas actually make a measurable difference. For the individual cow, testing for SCK is easy and can act as a marker of success or failure of your transition program before you see more costly diseases such as LDAs (left abomasal displacement).

Assuming that we want to pick the best program available, let’s aim for an incidence test that will find every cow affected. Commercial cow-side tests range in ease of use, price and accuracy. For accuracy, a blood test is always best. However, for freestalls that do not have headlocks or for those who don’t like the idea of collecting blood, the milk strip test (Keto-Test from Elanco) can be used in the parlour or tiestall. SCK may affect individual cows at different days in milk, so the best system requires testing fresh cows twice a week. For example, every Monday and Thursday, any cows fresh in the last two weeks would be tested. I know this seems like a big investment, but any positive cows can then be treated before they drop in milk and, hopefully, before future reproduction is impacted. If more than 50 percent of cows are positive, it then becomes more cost-effective to stop testing and just treat all fresh cows with propylene glycol until the root of the problem can be solved.

Besides treating the positive individuals, routine herd testing can help raise the alarm if the transition period goes off track. Often, we don’t realize there is an issue until there is a cluster of displaced stomachs. Suddenly finding a high rate of cows with SCK can allow you to call your team – nutritionist and veterinarian – in to troubleshoot before bigger problems arise.

Finally, remember to record the outcomes of your testing. This can be in a computer system or on paper charts (ask your veterinarian or Elanco representative for examples of paper charts). For those who prefer a mobile option, the iKetone app is available for Apple and Android phones. It was developed by the Ontario Veterinary College and is free of charge.

Ketosis is a costly disease that affects the majority of Canadian herds. Routine testing can help you treat individual cows before they lose milk and help monitor overall transition success before other diseases tell you there is problem. Talk with your veterinarian about finding a herd screening protocol that fits best for you.  end mark

References omitted but are available upon request. Click here to email an editor.

Chris Church
  • Chris Church

  • Dairy Technical Consultant
  • Elanco Animal Health Canada

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