Source: University of Minnesota, Erin Royster, DVM, College of Veterinary Medicine
Quick facts
A good mastitis screening program will alert you that a problem is present before it has time to grow into a disaster. Having a program in place can protect the health of your herd as well as save time and money.
There are multiple types of screening tests and ways to combine them into a screening program to protect your farm.
Types of screening tests:
- Bulk tank somatic cell count (BTSCC)
- Bulk tank culture
- Individual cow culture
- PCR (Polymerase chain reaction)
The ultimate purpose of mastitis screening is to prevent a mastitis outbreak. An outbreak of contagious mastitis caused by bacterial infection is a serious problem that can result in significant losses due to decreased milk production, diagnosis and treatment costs, culling and death. An outbreak of environmental mastitis can be just as dire if left unchecked.
A farm’s specific risk factors dictate how aggressive the screening program should be. These main risk factors increase the chances of infection:
- Introducing purchased cows without testing for contagious pathogens.
- Commingling with young stock from other herds.
- Contact with animals from other herds, such as at county fairs.
- Adult animals and young stock housed under the same roof or shared airflow.
- No antibiotic treatment for clinical mastitis.
- No antibiotic dry cow therapy.
- Not using an effective, germicidal teat dip.
There are other risk factors, such as herd level stress and immunity, that can make cows more or less susceptible to new infections.
The most aggressive screening program would combine bulk tank surveillance with culture (or PCR) of individual fresh cows, cows with clinical mastitis or cows with high somatic cell counts (SCC).
The least aggressive but still acceptably risk-averse screening program would be monthly or quarterly bulk tank screening (appropriate for smaller herds), or screening cultures of individual cows (larger herds). The selection of which individual cows to test depends on the herd, but the most common strategy is fresh cows and cows with clinical mastitis.
Talk to your herd veterinarian if you don’t currently have a screening program in place, or if you want to review your program to make sure it is the most cost-effective strategy.
Types of screening
Bulk tank somatic cell counts (BTSCC)
BTSCC can be a good herd-level indicator of how things are going, but may not be the most sensitive alarm system to alert you of potential problems, particularly in a large herd. For example, in a 100-cow herd, one or two cows infected with Staph aureus can have a big impact on BTSCC. When BTSCC jumps up, that’s your cue to look for infected cows. But in a 3,000-cow herd, it takes more than a few infected cows to drastically increase BTSCC, and the change will likely be slower, meaning you will have more infected cows to deal with by the time you realize you have a problem.
Bulk tank culture
Bulk tank culture tells you if specific pathogens or pathogen groups are present in the bulk tank milk. For Mycoplasma spp. and Strep agalatiae, even a low count of those bacteria in the milk means you most likely have cows infected with those pathogens.
For Staph aureus, a low count in bulk tank milk could be from bacteria on teat skin, but moderate or high numbers mean you have cows infected with Staph aureus. However, bulk tank culture is not a perfect alarm system either. The larger the herd the less likely you are to be able to find a pathogen that is shed in low numbers, or from only a small number of infected cows.
There are two ways to increase your chances of finding a contagious pathogen if it is present.
- The first is repeated sampling. Bulk milk samples should be collected for 3 to 5 days in a row, frozen and then shipped for testing. Bulk tank cultures can be performed monthly or at least quarterly. The more often you test the more likely you are to find something if it’s there.
- The second approach in large herds is to decrease the number of cows in each bulk milk sample by string sampling.
Individual cow culture
Individual cow culture can be used a few different ways, depending on what tests are offered at the lab. The UMN Lab for Udder Health offers both diagnostic cultures and screening cultures.
- Diagnostic cultures (Full Mastitis Culture) tell you exactly what organisms are present in a sample from an individual quarter. This is most useful for cows with clinical mastitis or high SCC when you want to know exactly what is causing the infection, for treatment or culling decisions or to focus prevention efforts.
- Screening cultures cost less and only tell you if a contagious pathogen is present in a quarter or composite sample. This test may be done on cows with clinical mastitis, high SCC cows, fresh cows, or some other group that is either high risk or convenient for sampling.
Individual cow screening is more sensitive (better at find pathogens) than bulk tank screening, but is more costly and labor intensive.
PCR (Polymerase chain reaction)
PCR is a newer test that can be used on bulk milk or individual cow samples. While culture detects living, viable organisms grown on a culture plate, PCR detects DNA (genetic material). This makes interpretation of the results a little more challenging, particularly for environmental bacteria, as a small amount of DNA will result in a positive test. However, PCR is a good tool to detect contagious pathogens.
Because of the cost relative to culture, laboratory PCR is not typically used as a routine screening tool for individual cows. The one exception where PCR has a significant advantage over culture is in the detection of Mycoplasma spp. It takes about 5 to 10 days to culture Mycoplasma spp. because it grows slowly, but a PCR test takes just a few hours.
There is a new PCR test kit for on-farm use that may be a good option for some farms.
