Clinical and subclinical symptoms
What is the difference?
In case of illness or disease we often talk about certain symptoms or behaviour we observe in animals. These external signs provide insight to obtain a diagnosis. However, a distinction can be made between clinical and subclinical symptoms. Clinical symptoms are signs that are directly visible. Subclinical signs cannot (yet) be seen from the outside of the animal.
Subclinical diseases do have impact!!
Subclinical milk fever, e.g., can only be diagnosed from a low calcium level in the blood; there are no clinical symptoms! A calcium concentration lower than 2.10 mmol calcium per liter blood is considered to reflect subclinical milk fever. The costs of subclinical milk fever and subclinical ketosis on a dairy farm should not be underestimated. These subclinical health issues may open the gateway for other diseases. Figure 1 shows a schedule containing the most relevant factors and processes related to health problems in lactating dairy cows. Subclinical illnesses that remain undiagnosed may lead to a higher risk of other diseases.
The above image shows the effects on a dairy cow where feed intake, feed quality, hygiene and housing are “not optimal”.
▲ = increase/positive effect | ▼ = decrease/negative effect
Risk of health problems
One problem may lead to another. Cows suffering from, e.g., subclinical milk fever have an increased risk of developing other diseases, because a sufficient amount of calcium is required for a proper muscle function. A low calcium level in the blood may lead to:
- An increased risk of mastitis
- An increased somatic cell count
- Retained placenta and/or metritis
- Displaced abomasum
- Increased risk of ketosis
Percentage of subclinical milk fever increases with the number of lactations
Research shows that the percentage of subclinical milk fever increases with each lactation1. In the first lactation, around 25% of dairy cows suffers from subclinical milk fever, in the third lactation the incidence is around 49%!
Next to milk fever, also ketosis is a frequently seen problem in dairy cattle. Research has shown that more than one out of ten cows suffers from clinical ketosis2. The number of cows suffering from subclinical ketosis is even higher. Costs of ketosis are estimated between € 250 and € 600 per cow per lactation. Reason enough to do whatever it takes to tackle ketosis on the farm.