TOTAL BACTERIAL COUNT (TBC) OF MILK

Whereas cell count measures the number of cells present in milk and is an indicator of mastitis infection, the total bacterial count (TBC) is a measure of the number of bacteria present.

• mastitis

• dirty teats

• the milking plant

Milk from a single case of mastitis, especially if Strep. uberis or Strep. agalactiae is involved, may be sufficient to increase the bulk milk TBC from 10,000 to 70,000 bacteria per millilitre. Mastitis is a com­mon cause of wildly fluctuating TBCs, especially in herds where mastitis detection is poor and the milk from affected cows enters the bulk tank. Staph. aureus produces relatively low numbers of bacteria and is unlikely to be involved.

Dirty teats, especially if splashed with faeces, will lead to an increase in the coliform count of milk and may be sufficient to incur TBC penalties. Teats which are washed but not wiped commonly lead to high TBCs. Teats with chaps, cracks and generally poor teat skin condition support increased bacterial populations and may also be involved. A change of teat dip to a higher emollient product may be required.

An inadequately cleaned milking plant is probably the most common cause of raised TBCs and this leads to an increase in the thermoduric or laboratory pasteurised count of bulk milk. Details of plant cleaning techniques and the investigation of problem herds can be found in Mastitis Control in Dairy Herds. The most common problems are inadequate use of chemicals, inadequate volumes of hot water and inadequate water temperature. Ideally 18 litres of hot water per milking unit are needed for circu­lation cleaning. With the move towards larger bore milking equipment (used to improve vacuum sta­bility), water requirements have increased.

Finally, check that refrigeration is adequate. Faulty cooling can lead to multiplication of all types of bacteria, giving an increase in TBC, thermoduric and coliform counts. When faced with TBC problems, submit a bulk milk sample to a laboratory for a differential bacterial count. If thermodurics are high, the problem is poor plant cleaning. Raised coliforms indicate poor teat preparation, and the presence of Strep. agalactiae suggests mastitis is involved. A high Staph. aureus count could indicate the cause of an elevated cell count, but is unlikely to contribute significantly to TBCs. Increased C. bovis (page 220) could indicate poor post-dipping and Strep. dysgalactiae poor teat skin condition.

Bactosan

In many countries bacteriological counts are now carried out electronically using a system such as Bactoscan. A dye which stains all living bacteria is added to the milk. The milk is then passed through a machine which counts the coloured particles. Bactoscan figures give higher results than standard cultural methods of TBC, because Bactoscan includes all bacteria present, whereas by culture only those organ­isms which grow at a specific temperature on a particular growth medium are counted.

As of January 1998 in the UK, a TBC of over 20,000 bacteria per millilitre and a Bactoscan count of over 100,000 incur penalties, and some dairy companies pay an additional premium for milk with a TBC below 10,000/ml. It is likely that stricter limits will be imposed in the future.

ANTIBIOTIC RESIDUES IN MILK

One of the more expensive aspects of mastitis is the milk which has to be discarded from cows under treatment. There are several reasons why milk contaminated with antibiotics should not be sold. These are:

1. Public health. Some people are allergic to antibiotics, especially the penicillins, and even fatalities have been known to occur. If, in this health-conscious age, milk gains a public reputation for containing antibiotics, liquid sales could decline quite rapidly.

2. Interference with manufacturing. Antibiotics can destroy the bacterial cultures used in yoghurt and cheese manufacture.

3. Legality. In the UK it is a contravention of both your contract with the dairy company and of Public Health (Trading Standards) Regulations to sell milk contaminated with antibiotics.

Since 1997 the maximum permissible level of antibiotic in milk in the EU has been 0.004 i.u. per millil­itre. The test is based on the addition of penicillin-sensitive bacteria to milk and monitoring their growth. Other types of antibiotics (e.g. neomycin) and certain sulphonamides are less easily detected. Most test failures are simply due to not discarding the milk for the recommended length of time following intra­mammary antibiotic treatment. This may be deliberate or accidental, e.g.

the rest of the herd or using a separate cluster fitted onto a bucket or a churn which is kept in the pit. If cows under treatment have to be milked into the jars, the jars should be drained and then rinsed with clean water before continuing onto the next cow. Even then, leaking fittings can allow enough antibiotic milk into the bulk tank to lead to a test failure.

The other common reason sus­pected for test failures stems from dry cow therapy. The contract with most dairy companies states that milk should be withheld for the first four days after calving, and if this is not done there is a risk of antibiotic conta­mination. Cows which calve early pose a particular problem especially if dry cow therapy has been administered in the preceding three to four weeks.

Table 7.5. Reasons suggested for antibiotic test failures.

Table 7.5 shows a list of reasons suggested by farmers as to why their milk had failed the test. The fig­ures add up to more than 100% because several farmers gave more than one reason for a test failure. Poor records and inadequate withholding periods seem to be the main causes. Most dairy company con­tracts state that all milk should be discarded from a cow under any form of antibiotic or oestrogen ther­apy. However, very occasionally individual cows produce natural inhibitors usually in association with udder injury, colostrum or mastitis. They then fail the test, but no antibiotic is present.