Antibiotic Susceptibility Testing as an Aid to Mastitis Treatments: How Can Test Results Be Used To Make Good Decisions?

What is Antibiotic Susceptibility Testing?

Antibiotic Susceptibility Testing is commonly used as an aid to determine which drug (antibiotic) has the greatest chance of killing or stopping growth of mastitis causing bacteria. The information can be used to improve the odds of eliminating a mastitis infection if the recommended antibiotic is then used to treat the infected quarter of the cow whose milk sample was analyzed. It can also be used to establish a herd pattern for mastitis causing bacteria and determining what mastitis antibiotic protocols work and don’t work in YOUR herd over time.

Culture testing is done on the same milk sample as susceptibility testing to identify the type of bacteria (e.g., Environmental Streptococci, Staph. aureus, E. coli, etc.) present in the cow’s quarter milk sample and most likely causing the mastitis infection. Based on the results of both tests (culture and Antibiotic Susceptibility Testing), YOU and YOUR veterinarian can determine which, if any, approved antibiotic will have the best chance for success toward getting rid of a mastitis infection in a specific cow and specific quarter.

In many cases, due to the time delay in getting results back, you will have already treated the cow’s quarter for mastitis before you get results in the mail. So is it worth your money to ask the laboratory to do Antibiotic Susceptibility Testing when you are going to treat before you even get the results? What do you do with the information when it comes back? Do you file it as interesting but another unusable piece of an endless stream of information that will help you make good decisions if only you knew how to use it and it was available when YOU needed it?

Many producers have expressed frustration with Antibiotic Susceptibility Testing. The test result often comes back to the dairy suggesting that a particular antibiotic should work according to the lab tests, when in fact the antibiotic when used does not get rid of the cow’s infection. How can that happen? Should Antibiotic Susceptibility Testing always be requested along with culture? If so, how do you gain good knowledge from every test, and ensure good decisions are made with the test results? Is it best to request the test only for certain types of mastitis (e.g., only specific bacteria, or new vs. chronic infections)? Are there records that can be kept to maximize use of Antibiotic Susceptibility Test results?

This article will hopefully provide you with answers to some of these questions and assure you that Antibiotic Susceptibility Testing is worthwhile when the test is requested appropriately and information from test results is used with culture results and good on-farm records of prior antibiotic effectiveness and cow history of treatments for mastitis.

Understanding the Basics of Antibiotic Susceptibility Tests

The Basics on YOUR End: Collecting the Sample Just like with culture, “good” milk samples must be collected to ensure “good” Antibiotic Susceptibility Test results. “Good” sampling means to use clean techniques. This minimizes non-mastitis causing bacteria and other junk included in the milk sample. You don’t get any discounts on contaminated samples – tests are run, costs are incurred and results come back as contaminated. A lot of effort and cost for no new information.

Collection of a “good” sample can be done by cleaning the teat end carefully with alcohol before sampling and wearing nitrile gloves to reduce the chance of sample contamination. The sample should be collected early in the week, refrigerated and sent overnight (Monday through Wednesday) to the laboratory. These are the basics of collecting “good samples”. More detailed procedures for sample collection can be found on the National Mastitis Council Web Site (NMC) or by asking your Veterinarian for NMC procedures for milk sample collection.

Antibiotic Susceptibility Tests are not accurate on composite samples (milk from all 4 quarters put into one vial) or bulk tank samples. That means that requesting Antibiotic Susceptibility Tests on either of these types of samples will be a waste. Collect milk only from individual quarters when requesting an Antibiotic Susceptibility Test.

The Basics on the Laboratory’s End: Testing Procedures and Need for Good Standardized Lab Procedures An organization called the National Committee for Clinical Laboratory Standards (NCCLS) has recently established specific guidelines to be used by lab technicians when analyzing and interpreting results of Antibiotic Susceptibility Tests. Until recently, Antibiotic Susceptibility Tests from human blood studies were used to predict performance of antibiotics toward cow mastitis-causing pathogens. Just in the last decade, the Veterinary Committee of the NCCLS has established and continues to establish criteria to predict antibiotic effectiveness against mastitis organisms when the antibiotics are given in the infected quarter and then have to travel through milk, milk components, and blood to the site of infection.

Test criteria are based on response of bacteria to discs impregnated with specific antibiotics on a culture plate. The level (dose) of antibiotic used on the discs is similar to the legal dose that is given to the cow for treating mastitis infections. In addition, information on how the antibiotic circulates and acts within the mammary gland to kill or inhibit growth of specific mastitis bacteria is included in the evaluation of effectiveness. Results given back to the veterinarian or to the farm are usually scored as ‘Susceptible’, ‘Intermediate’, and ‘Resistant’. This information must then be interpreted by the veterinarian or producer based on specific pathogen causing the infection, nature of the mastitis (chronic, or new infection), and characteristics of the cow infected (# times previously treated and history of cures) to determine if the recommended antibiotic will or would have worked to cure the infection.

Laboratory techniques need to be highly accurate because things like ‘amount of bacteria plated on the culture plate’ and ‘how the distance between the antibiotic disc and bacteria is read by the technician’ can impact the results. Choose veterinary labs that have experience and demonstrate quality techniques in doing mastitis diagnostic work.

Bacteria that are ‘SUSCEPTIBLE’ to a particular level of antibiotic are expected to be killed or otherwise inhibited so they no longer cause mastitis. The expectation, based on lots of good data and extrapolation, is that the drug will also work against this bacteria on your farm. The predicted outcome of using that particular antibiotic according to recommended dose, and assuming “good” sanitation during treatment, would be a cured infection. On the culture plate (in the lab), there is a small or no ‘zone of inhibition’ meaning the bacteria is unable to repel the antibiotic and the antibiotic will likely work to get rid of the bacteria in real life.

Bacteria that are ‘INTERMEDIATE’ in terms of susceptibility to a particular antibiotic may be inhibited or not when the antibiotic is used to treat an infection. In other words, your chances of it working are greater than if the bacteria is ‘RESISTANT’ to the antibiotic, but not as good as an antibiotic to which the bacteria is ‘SUSCEPTIBLE’. If you have a choice between choosing an antibiotic from the ‘SUSCEPTIBLE’ column or ‘INTERMEDIATE’ column, your odds are better with the ‘SUSCEPTIBLE’ column.

Bacteria that are ‘RESISTANT’ in terms of susceptibility to a particular antibiotic will rarely be killed (or growth inhibited) when that antibiotic is used against that particular bacteria in your herd. This is probably your surest bet within Antibiotic Susceptibility Testing. Any antibiotic listed as bacterial ‘RESISTANT’ will rarely work to cure a case of mastitis (for this particular case). This knowledge can be as important in making good decisions as information on ‘SUSCEPTIBLE’ antibiotics.

Why Lab Results Don’t Always Match Performance Results in the Cow

As mentioned previously, Antibiotic Susceptibility Testing uses a lab procedure and additional interpretation to predict how antibiotics will work in the mammary gland to cure a mastitis infection. Although the predictions are becoming more accurate, there are conditions that affect the ability of a given ‘SUSCEPTIBLE’ antibiotic to work its’ magic. A few of the factors are listed below (See Table 1).

Several characteristics of bacteria can affect the way a given antibiotic works when actually used to treat a mastitis infection. Use of ‘SUSCEPTIBLE’ antibiotics on chronic Staph. aureus cases are an example. Often Staph. aureus bacteria, especially in cows with chronic and recurrent infections, are able to hide inside immune cells. In doing so, the bacteria protect themselves from being killed or inhibited by antibiotics. YOUR knowledge of presence of Staph. aureus in the herd, and cases of chronic Staph. aureus may alert you to this being a potential factor for poor antibiotic success with such infections in your herd. If a particular cow has a history of chronic Staph. aureus infections, the chance of cure during lactation, despite results from Antibiotic Susceptibility Testing, will probably be poor. Hence utility of having an Antibiotic Susceptibility Test run more than once or twice for this type of case will be low.

Most experts also agree that the value of Antibiotic Susceptibility Testing for coliform infections is relatively poor (e.g., E. coli and Klebsiella). Since treatment options are limited, utility of Antibiotic Susceptibility Testing for these types of infections will also be poor.

It is certainly worthwhile to have both culture and Antibiotic Susceptibility Tests run on cows with a history of mastitis if neither test has been done in the past. Test results may help to explain why treatments have not worked in the past and may point you in a new direction. On the other hand, if a cow has a history of mastitis, and culture and Antibiotic Susceptibility Tests have been done and results implemented and found to be ineffective, it probably doesn’t pay to run tests again. The cow can be classified as a non-responder and permanent mastitis cow. Her mastitis status will be integrated with other indicators to determine her short term profitability in the herd.

Using Records to Make Decisions with Antibiotic Susceptibility Test Results

Herd mastitis records can help enormously to determine when and how Antibiotic Susceptibility Testing should be used. As mentioned previously, cow records on number of cases of mastitis, and number of previous treatments can be used to determine whether Antibiotic Susceptibility Testing should be requested. In addition, culture results can be used to determine the utility of Antibiotic Susceptibility Testing. If culture results indicate coliform bacteria are causing the mastitis infection it probably does not make sense to pay for the additional test. Possibility of antibiotic cure for a cow with multiple cases of S. aureus mastitis will be low regardless of whether the lab results come back listing ‘SUSCEPTIBLE’ antibiotics. This is especially true when treatment will be given during lactation. Antibiotic Susceptibility Testing of a milk sample from a cow before dry-off, regardless of chronic status, may be worthwhile, but only for cows that have not gone through more than one dry period with a mastitis infection.

Records of Antibiotic Susceptibility Test results for specific cows, specific bacteria, and for specific types of infections can and should be summarized within your herd. It can be used to establish a pattern of what antibiotics work and don’t work for different types of infections. Such information should be used to reduce number of treatments and increase the cure rate of antibiotic treatments protocols used in your herd. See Table 2 for an example of possible information that may be monitored to establish a herd pattern and monitor the effectiveness of Antibiotic Susceptibility Testing.

Summary

Antibiotic Susceptibility Testing has become more accurate than it was a decade ago. Knowledge of what bacteria are causing a mastitis infection in your herd can help to determine the utility of Antibiotic Susceptibility Testing. In addition, utility of Antibiotic Susceptibility Testing should be improved when the information is used to treat new infections compared to chronic or recurrent infections. Even if the cow is treated prior to receiving your list of antibiotic options, the information when received can assist in validating whether the proper antibiotic was used for treatment. If a ‘RESISTANT’ antibiotic was used and the cow becomes re-infected in the near future, the correct antibiotic treatment protocol can be initiated more quickly.

Table 1. Factors Contributing to Inability of Lab Antibiotic Susceptibility Tests to Accurately Predict Cure of a Mastitis Infection

Table 2. Worksheet Example – Developing a Herd Pattern from Culture and Antibiotic Susceptibility Test Results – What Works and Does not Work?

Sandy Costello, Penn State Dairy Alliance, Dairy Production & Milk Management