by Winston Ingalls, Ph.D.
West Agro, Inc., Kansas City, MO
Mastitis discussions inevitably split the issue into sub-clinical and clinical mastitis and generally the emphasis then goes towards explaining how sub-clinical mastitis is the major cause of reduced production, elevated SCC, inferior milk quality, etc, etc. All of this is well documented and true!
However, few issues are more upsetting to dairymen than fighting case after case of clinical mastitis with more and more cows in the sick pen. It represents extra time to properly handle such cows, lost production, vet calls, treatment products, concern about contaminated milk and an occasional dead or culled cow. Clinical mastitis is a highly visible problem and generally produces the impression something has gone wrong. It causes dairies to question everybody and everything including milking equipment, milking procedures, teat dips, housing and the weather.
Clinical Mastitis Symptoms
You generally will know it when you see it. Typically during the pre-milking routine, visual observation of noticeable changes in milk such as flakes or clots in foremilk samples is how it is spotted. Fore-milking can provide valuable information about each cow. Milk may also appear watery or have an off-white color. Often changes in milk characteristics are associated with more general observations such as a hot, hard quarter, sensitive to touch. In other cases these symptoms are absent.
Examination of milk filters after milking also can be an indicator. The presence of slime and clots on filters indicates clinical cows were milked but that milk is now in the tank.
The relationship between any of these visual characteristics and the specific type of bacteria responsible is generally weak. Culturing milk samples from infected cows is the best tool to determine the species of organisms involved in a clinical case but be cautious with results. Always involve a veterinarian familiar with interpreting milk culture reports to help assess the situation and determine an appropriate course of action.
Clinical Samples -- No Growth Culture Indication
Culture results may produce confusing information. Various studies have indicated 25-40% of clinical case milk samples result in a bacteriological finding of “No Growth." This doesn’t seem possible but it happens and there are explanations. Milk samples may contain such low concentrations of bacteria that culture procedures cannot detect them due to lack of test sensitivity. Increasing the sample volume tested may help. The cow may have been treated with antibiotics before sampling and the bacteria are dead. Possibly the animal’s system eliminated most of the bacteria prior to sampling or sample handling may have been inappropriate and killed the bugs. Regardless, it is possible and quite common for milk samples, from cows suffering obvious clinical mastitis, to produce a “No Growth” culture result.
Clinical Mastitis -- Related Factors
Cows and first calf heifers develop more clinical mastitis at dry-off, around calving and early lactation so there is a stage of lactation affect.
Certain bacteria species are more likely to be associated with clinical outcomes than others and there also appears to be an association between season, climate and parity and clinical problems. The hot weather of late spring, summer and early fall tends to be associated with more clinical cases.
Benchmarks -- What’s Normal
Clinical mastitis may be inevitable, the key is to minimize the incidence rate. Written guidelines frequently suggest clinical cases should represent less than 2% of the milking herd/month or 2 cows/100 cows milked per month. Despite this guideline, analysis of numerous published reports indicates a rate of 3-3.5% is quite common.
Be careful with the numbers. Clinically infected cows may be repeat offenders and a single cow may account for multiple clinical cases. Such cows require special consideration. Looking only at cows developing new clinical infections, in a defined timeframe, helps eliminate the bias of repeat offenders.
Mastitis causing bacteria produce very different rates of clinical mastitis. Coliform and environmental Strep infections tend to be associated with relatively high incidence rates of clinical mastitis. It is estimated 40-50% of all environmental Strep infections result in clinical mastitis so this particular organism group can be a major headache in terms of the clinical consequences. Staph aureus on the other hand is more likely to produce chronic, sub-clinical mastitis with occasional clinical flare-ups, often associated with periods of stress.
When a number of these infections occur in a short period it can cause everyone to think somebody or something has changed and is causing the problem. Teat dips and liners are often switched in an attempt to stop the problem but the actual cause may be something totally different.
Factors Influencing Clinical Mastitis
Well managed dairies, focused on controlling mastitis, generally do an excellent job of minimizing contagious mastitis problems due to Staph aureus and Strep ag but they may still get stung by clinicals caused by other bacterial species. The reason may be quite straightforward.
Contagious bacteria reside primarily in the udders of infected cows and on the teat skin. Minimizing or eliminating mastitis caused by contagious organisms is best accomplished by a rigid program of post milking teat dipping, treatment of cows as they are dried off and culling of chronically infected cows. As contagious mastitis problems are minimized, environmental bacteria become the primary mastitis concern and unfortunately a high percentage of infections caused by this group are likely to cause clinical symptoms.
Based on results of numerous studies it is clear that around dry-off, calving and early lactation, cows are more likely to develop new infections and a high percentage of the new infections are due to environmental organisms.
Why? Many reasons have been offered, all likely play some role. Dry cow treatments are ineffective against coliforms. Also, dry cow products do not persist through until calving. Suppressed immunity, cleanliness of dry cow housing areas and physical characteristics such as leaking milk and swollen udders may play a role. Milk fever, ketosis, lameness, fat cows, displaced abomasums also occur more frequently around calving and during early lactation. These disorders likely pre-dispose cows to mastitis problems even though they may not be the primary cause.
What’s The Cost?
There is no absolute answer. It depends on the specifics of each situation. If treatment involves antibiotics there will be milk loss. This may mean several days milk production down the drain plus a reduction in yield for that lactation. Many clinicals occur during peak lactation, so milk loss is maximized. If it involves a good cow producing 100 pounds/day or more, losses quickly become significant. In addition there is the treatment cost itself. Literature estimates indicate that a case of clinical mastitis may result in costs in excess of $100/episode with the greatest percentage attributable to discarded milk and lost production, but each case is a bit different.
How Do You Prevent It?
It’s tough to eliminate but management must do everything possible to minimize the occurrence rate. Effective dry cow management is key, knowing that this is a critical period. Clean dry-cow housing and calving areas are critical. Vaccinating cows with a coliform vaccine is a relatively new tool that helps reduce the severity of problems. Dry cow diets need to provide adequate selenium and vitamin E, both of which function to strengthen the infection defense systems of the cow.
Body condition scoring is helpful in assuring cattle are not too fat or thin. Fat cows often have difficulty transitioning to lactation and can suffer from fatty livers and downer cow problems. Early lactation cows may not be able to handle the transition period when they have to use body condition to supplement dietary energy intakes and may suffer ketosis.
Each of these factors can pre-dispose cows to other health problems including clinical mastitis.
Evaluate the contagious mastitis profile in the herd. If it is minimal most mastitis problems are likely to be associated with environmental bacteria and clinical cases. Determine if there is a pattern to the occurrence of clinical cases.
Evaluate pre-milking udder prep procedures. Milking clean dry teats is one of the most effective ways of preventing environmental mastitis and everyone periodically has to be reminded of this. Dirty teats, wet teats and long udder hair all contribute to potential clinical problems due to an increased presence of environmental bacteria.
Prepare for seasonal problems. Coliforms and environmental Streps thrive in bedding and where manure buildups occur. Under warm, moist conditions numbers can dramatically increase exposing teat ends to very high bacteria concentrations. Such conditions are common during spring through late summer in most areas. Maintenance of stall conditions during these periods is critical to minimize problems but it is not only stalls. Each of these conditions causes increased exposure to high levels of environmental bacteria even under conditions that would appear ideal.
Heat can add to problems and force cattle to congregate in shady areas seeking relief. The end result may be buildup of very high levels of bacteria in places that are not normally a concern. These include areas under corral shades, shade trees and similar locations. These situations have to be properly managed to limit problems.
Clinical mastitis is no fun and can be the cause of serious infections and milk loss. Understanding potential causes and proactively taking action to minimize them will help limit the problem.