Recently I was called by a dairy producer asking about a "chronic PI (preliminary incubation) count problem.”
Reviewing the bacteria counts from this dairy indicated that the PI counts were averaging around 30,000 cells/ml, with samples that would periodically spike to 150,000. Rarely, there would be a PI count that was in the high hundreds of thousands.
The high counts began occurring around the first of the year. The producer said he had looked at equipment sanitation multiple times and could find nothing wrong with the way the milking system and bulk tank were washing. He did not know where to look next.
Each bacteria count issue tends to have its own unique set of circumstances. But there were some simple places to check that helped focus our efforts more effectively.
1. Always compare the raw bacteria count to the PI count for the same load. In this case, we found that while the raw counts were not as high as the PI counts, they tended to run in the 5,000 to 15,000 cells/ml range (averaging 10,000) when the PI counts were around 30,000. The raw counts would increase to the 25,000 to 35,000 cells/ml range when the PI count was higher. Also, the bulk tank somatic cell count (SCC) was running around 300,000 cells/ml, higher than this dairy's historic average.
2. Bulk tank cultures indicated that most bacteria were in relatively low numbers except for the environmental Streptococci. The environmental Strep. was present consistently in the high hundreds to low thousands of cells per milliliter in the bulk tank samples.
3. Milking time and freestall bedding procedures were probably adequate.
4. Monthly DHIA information indicated that about 30% of the lactating cows had current-month SCC counts above 200,000 cells/ml.
To further analyze this problem, we compared the change in individual cow SCC over the course of one month. We first looked at the cows that had SCC changes from below 200,000 to above 200,000 from the last month to the current month. These cows roughly represented the number of new infections created during the month.
We then looked at the cows with first-test SCCs greater than 200,000 cells/ml. These roughly represented the number of cows freshening with a subclinical infection. Next we looked at the cows whose counts were greater than 200,000 cells/ml in both the last month and the current month. These represented the number of cows with chronic infections.
We found that most of the cows with counts above 200,000 cells/ml fell into the chronic category. For the months of January and February, though, the number of new infections was quite high (almost 20% of the lactating herd for both months). In the months after February, some of these cows' SCCs came back down under 200,000 cells/ml. However, many did not, resulting in the current high levels of chronic, subclinically infected cows.
It is likely that the cold weather contributed to poor teat skin condition and some mild freezing. This, in turn, led to the subsequent high levels of new subclinical infections present in the herd.
Working our way out of this problem is likely going to take a while. By regularly monitoring bulk tank cultures and analyzing the changes in individual cow SCC, we have an effective system in place to gauge
More on environmental Strep. from NMC
More on environmental Strep. from the University of Minnesota