50^th Anniversary Celebration—University of California, Davis

Eleventh Annual Fall Symposium on Advances in Clinical Veterinary Medicine

It is clear from past experiences that dairy herds can be nearly completely free of contagious mastitis pathogens. The NMC recommended procedures work! Many herds in California are completely free of Streptococcus agalactiae, have only occa sional cases of Staphylococcus aureus or mycoplasma. These herds have extremely low bulk tank SCC (<150,000 cells/ml) on a consistent basis. Regarding mastitis, they have optimize production. And it is possible to keep them contagious mastitis free.

Biosecurity is a combination of two main words. First, "bio" indicates that something alive is involved and that something in the dairy context are the cows on the dairy ranch. Security implies concern for safety or protection. The combined word, biosecurity, indicates concern for the safety and protection of dairy cows. Moreover, biosecurity usually refers to those management practices, which protect the herd from new diseases, or at the very least reduce the risk of new disease introduction. Th ese practices also minimize disease and economic impacts when new diseases are inadvertently introduced into a dairy herd. The threat is real as indicated by the 1996 NAHMS Dairy study. They reported that on larger dairies (>200 head) which are common in California, nearly 50% purchase bred heifers and almost 25% purchased lactating cows or bulls. It has been said that California actually only has two herd, one in the San Joaquin and the other in Chino.

To apply the principles of biosecurity to maintain a contagious-mastitis free dairy herd, it is important to focus on the common diseases which might be introduced; the current disease status of the herd; the ways new diseases are introduced onto the dairy; how these diseases spread from cow to cow; and management practices which could provide some risk reduction and protection. The questions are - is all this possible, or practical or even profitable?

The contagious mastitis pathogens which are of greatest significance are Streptococcus agalactiae, Staphylococcus aureus, and the mycoplasma species. These are all considered contagious as they move from the infected cows to the non-infecte d cows. The critical carrier unit to be considered is the infected cow.

The extent to which these pathogens pose a threat to a dairy depends in large part on their prevalence or occurrence that already exist on the dairy. And the prevalence is determined in large part by the dairy managers concern about mastitis. As a n example, consider a dairy experiencing a serious, continuing mastitis problem. The herd has a consistent bulk tank somatic cell count of >500,000 cell/ml and regular bulk tank isolation reports of Staphylococcus aureus and Mycoplasma bovis . This dairy is not under any threat from the outside by these contagious mastitis pathogens as it already has the problem. Therefore an important part of biosecurity is knowing what diseases are already present on the dairy.

Some form of a mastitis-monitoring program needs to be operational on the dairy to provide this information. As the herd veterinarian, you can help develop a monitoring program specific for each of your dairies. A mastitis monitoring program might inc lude a review of the monthly bulk tank somatic cell count, weekly to monthly bulk tank microbiological analysis depending on herd size, culture of fresh cows, culture of new herd entries and culture of milk from all clinical cases of mastitis. The abilit y of the monitoring program to detect early changes in mastitis herd status will depend on how many of the possible tests are being routinely done and the sensitivity of the tests. Relying on the monthly cooperative bulk test as the sole herd monitor may allow a mastitis outbreak a running start before it is detected. With multiple testing scheme, early detection is assured.

In the future it may become important to even take a more in-depth assessment of the herd mastitis status. It may become important to look at the antibiotic sensitivity of the mastitis pathogens within the herd. Consider a herd with some Staph aure us infections. It is entirely probable that another Staph aureus might be introduced into the herd which is much more resistant to antimicrobials than the current Staph aureus. This would complicate the mastitis control situation. A sim ilar situation might arise when a more pathogenic form of mycoplasma is introduced into an already mycoplasma-positive herd.

Next, it is important to give some thought to the high-risk situations, which might lead to introduction of new diseases. For the most part, the high risk comes from movement of animals onto the dairy. These in-coming animals include introduction of milk cows by purchase, return of heifers from a calf-rearing ranch, cattle returning from shows or fairs. These animals pose two risks. One, they might be bringing in a pathogen which do not exist on the ranch or for which a control program does not c urrently exit. Or, they may themselves not have immunity to pathogens already on the ranch. In either case, a disease outbreak may result.

It is also important to consider how disease spreads between cows. Disease may spread by direct contact between cattle, spread by fluids or secretions such as blood , milk or manure. Disease may also spread by breathing air containing infectious o rganisms. Finally it may spread by contact with items that have been in contact with cows (milking machines, water, feed and other equipment). The most obvious means of spread for contagious mastitis is by organism-laden milk from infected cows carried t o another cow during the milking process. It might be carried by the milking machine, milker's hand or common use towels or rags. Less obvious might be the spread via aerosol. This may happen with mycoplasma as mastitis outbreaks have been associated wit h respiratory outbreaks in calves.

Many management options are available for the dairymen to protect the health and production of their dairy herds. Each dairymen should be aware of all the potential options. However, not all options may be feasible for use on each dairy. Since mos t of the bacterial diseases have relatively short incubation periods, new arrivals can be quarantined for 3 weeks. This will allow any introduced disease to become apparent before the newly arrived cattle are mixed with resident populations. Quarantine w orks particularly well when only occasional herd additions take place. During this time, new cattle can also be tested for diseases of concern and cultured for mastitis pathogens. It is also advisable to check new milking cows for antibiotic residues as soon as possible after arrival on the dairy. They may have been recently treated and still shedding antibiotic at detectable level in the milk.

Our attitudes about pre-purchase examinations may need to change in the future. Right now very few in-coming animals have been tested for mastitis prior to entering the milking herd as is generally suggested. Future constraints may make this more appe aling. While this is probably the best procedure to keep new pathogens away, it may not be readily adopted in many herds. So, it will probably continue to be the practice to buy cows sight-unseen and bring them directly onto the dairy.

Non-tested cows should be kept separate on the dairy until their mastitis status is known. New cows certainly should not be placed in the hospital pen. If possible they should be penned separately for other resident cows. Or at the very least they sho uld be milked last just before the milking equipment is to be cleaned. As soon as possible all these cows should be cultured to determine their status and a decision made on their future.

Don't forget the animals which have gone to exhibits or shows. They were probably milked with a common milking machine. Test them when they return. Also, beware of relief milkers. They can bring new pathogens into the herd on their hands. Insist on th orough hand washing with soap prior to milking.

The only reasons for a mastitis biosecurity program are to protect the market for the dairy's milk and preclude the need to cull excessive numbers of mastitis infected cows. By developing a mastitis biosecurity program, the profitability of the he rd can be sustained. The cost of milk culture is probably around $5.00 per composite sample. If a 1000 cow herd is bringing in enough cows to maintain the herd size, it would take 350 new additions at a 35% yearly cull rate. At $5.00@, that would amount to $1750 per year to know that status of the in-coming cattle. A monitoring program would cost about $6200 ($5000 for fresh cows, $1000 for clinical cases (20% of herd) and $200 for bulk tank culture (2x per month). The total cost of the biosecurity prog ram for mastitis would be about $7950.

In contrast, a herd without biosecurity and monitoring programs might sustain a mastitis outbreak. If Staph aureus or M. bovis came into the herd, many animals might become infected before the invasion was detected. As an example, if 25 M. bovis infected cows were purchased and another 75 resident cows became infected, the cost could be as much as $86,000. This would include $60,000 in lost value of the cows ($600@ lost when sold), $5000 to culture herd, $1000 veterinary fees (10 hrs@ $100), $20,000 in lost bonus for milk quality ($0.10 per 100 cwt). An additional cost could be added to all the failed mastitis treatments used on the mycoplasma until the presence of mycoplasma was discovered.

It does not take a rocket scientist to see that the insurance buffer provided by a mastitis biosecurity program is very cheap in comparison with a cost of a typical outbreak of mycoplasma mastitis.

In summary, it is possible to maintain dairy herds with extremely low prevalence of contagious-mastitis, pathogens. It is also possible, practical and profitable to keep them out. Each dairy should have a biosecurity plan to deal with newly arrive d cattle. They are the critical control point for preventing entry of new contagious pathogens. The plan should be based on knowledge of the current diseases on the dairy and those diseases that the dairyman and their veterinarian feel are a significant risk to the current dairy population and the incoming cattle. The plans must be flexible and specific to each dairy. To continue without a biosecurity plan is to risk a devastating disease outbreak with severe economic consequences. Biosecurity is an ins urance plan against severe economic loss.