Vet Partners have recently acquired the ‘Grassroots Herdcheck’ software for Cell Check and Feed Check reports, which have been very popular at LLM over the past twelve years. The platform is being hosted at our Whitchurch branch has been renamed ‘Dairy Monitor’ Cell Check, so look out for this on your reports. With antibiotic usage under increasing scrutiny both during lactation and at drying off, Bill looks at one of the infected cow categories referred to in the reports and discusses both the monitoring and the management of these cows.

What are chronic infections?

Cows that have been above the 200,000 cells/ml threshold for two consecutive recording months or more are referred to as ‘Chronics’. Whilst some of these will have only just joined the list of Chronics having appeared on the New, First or Repeat lists in the previous month, others will be making a regular appearance month after month and some may even have been infected for most of their lactation to date.

Alongside the Chronics described above, there are two other sub-categories of infected cows that could also be Chronic infections. These are the ‘Legacy First’ infections and the ‘non-True Repeat’ infections:

The Legacy First infections are cows that recorded a high cell count at the last recording of their previous lactation as well as at the first recording of the following lactation. Many of these cows will have failed to cure an existing infection over the dry period (hence ‘Legacy’ First), although there is also the possibility that they may have been cured but then become re-infected before the first recording of the following lactation.

The non-True Repeats are cows that have had one high cell count in a lactation followed by only one or two low cell counts before another high cell count recording. Whilst these could be cows with genuine repeat infections having been uninfected in-between, there is also the possibility that they have not cured at all in-between high readings but just had one or two cell counts that have crept under the 200,000 cells/ml threshold.

Are the Chronics important?

Focus for the longer term cell count management of a herd should always be on the prevention of Fresh infections (‘News’, ‘True Repeats’ and ‘Fresh Firsts’) rather than the treatment of these cows if and when they become Chronics. For any particular herd, using identifiable trends together with monthly benchmarking data in the cell count report can help to identify which if any of the Fresh infections are the most problematic.

In most herds, the Chronics tend to make up a significant part of the ‘pool’ of infected cows that exist at any one time and can show a steadily increasing cell count the longer they are infected (jumping on the so called ‘infection escalator’). However, this is not always the case and whilst up to seven out of ten cows start their chronic infection with a cell count of under 500,000 cells/ml, three out of every five remain below this level even after they have been infected for the whole of a lactation. Whereas two out of every five move up the infection escalator with one in every six reaching a cell count of one million cells/ml or more. Either way, the more Chronics there are in a herd, the more unstable the average herd recorded cell count (or bulk cell count) tends to be from day to day.

Header tank analogy

The ‘water’ in the tank represents the ‘pool’ of infected cows in the herd at any one time, which comprise a mixture of Fresh infections (News, True Repeats, Fresh Firsts) and Chronics (Legacy Firsts, non-true Repeats and Chronics). The tank is constantly replenished with Fresh infections (the inlet pipe) and continually empties (the outlet pipe) as a result of either culling cows or by cows curing (self cures; lactational treatments; dry treatments). The water level can be lowered by either slowing down the rate of the inlet pipe (prevention of Fresh infections) or by speeding up the rate of the outlet pipe (culling or treating infected cows). Ultimately the balance between the inlet and the outlet pipes will determine the water level, which in this analogy is the percentage of the herd with a cell count of >200,000 cells/ml. This in turn will largely determine the average herd recorded cell count detailed on the Cell Check report (and likewise the milk buyer’s bulk milk cell count).

Chronic Infections - Header Tank Analogy

Benchmarking Chronic infections

There are several areas on the benchmarking page of the Cell Count report that relate to Chronics. The first is the current recording month’s number of Chronics, expressed as a percentage of the total number of milking cows recorded that month. The target is <15%, although a good herd will often have <10%. Next is the six-month average, which again should be <15%. Thirdly, the proportion of First infections that are ‘legacy Firsts’ over the last twelve months is given, which should be <30% (remember these can be a measure of the level of chronic infections in the herd alongside the Chronics themselves). Finally, there is the proportion of the total number of cows that are dried off with a high cell count which have been >200,000 cells/ml for >3 months and >6 months.

In a herd carrying a lot of Chronics and relying on dry cow therapy to cure a high proportion of them, these figures will be high – they should be regarded together with the dry period cure rate and the (more important) ‘rate of failure of dry period cure’, which takes into account both the dry period cure rate AND the proportion of the herd that is dried off with a high cell count.

Ways to ‘empty the tank’

Work has shown that cows that have been infected for longer tend to be more difficult to treat successfully, as are older cows, cows infected in more than one quarter and cows infected in the rear quarters. All of these factors should be taken into account before treatment of a chronic infection.
Knowledge of the pathogen involved also helps to decide which treatment option might be most suitable, although this is usually NOT available at an individual cow level. Recent ‘herd level’ mastitis pathogen surveillance is a ‘second best’ option or lacking this any indicator of likely pathogen(s) is better than none. Remember that the goal is to return the cow to a cell count of <200,000 cells/ml permanently.

Short guide to the options for treatment of Chronics

Do nothing! – This is an option but note that self-cure rates are much lower for Chronic infections than they are for Fresh (New, Repeat or First) infections.

Treatment during lactation – This is often disappointing especially when ‘sticky’ infections such as Staphylococcus aureus or Streptococcus uberis are involved. Standard courses of lactating cow tubes may sometimes achieve cure rates of only 20% during lactation.

Dry off – dry cow antibiotic therapy is arguably the most effective way of curing Chronics, with a cure rate typically >70%.

Culling – This is the ultimate treatment for chronic infections but also the most costly. Some Chronics are simply incurable and whilst in the herd they may represent an infection ‘risk’ to other cows.


  • Manage chronic infections so that they remain <15% of cows at any one recording and preferably <10% Use the ‘Herd Monitor Cell Check’ cell count report’s Action List and develop a farm-specific decision tree for selecting appropriate Chronics to treat. Treat earlier versus late. Remember that in most circumstances standard antibiotic dry cow therapy by itself is usually reasonably effective at treating chronic infections (>70% successful)
  • Be selective and adopt a targeted approach to using aggressive treatment options
  • Prevention is always preferable to cure!