What Do Bulk Tank Culture Results Mean?

by RUSS BEY and RALPH FARNSWORTH
University of Minnesota

How do I use bulk tank culture results to reduce my cell count?

Any contagious organisms (Staphylococcus aureus, Streptococcus agalactiae, or Mycoplasma) come from an infected quarter.
- When looking for contagious organisms, take multiple bulk tank samples over three to four days.
- There is usually an 85 to 90 percent correlation between the numbers of bacteria in the bulk tank and the number of cows infected - but not always. To know how many and which cows are infected, you may need to test individual cow milk samples.
Elevated levels of environmental pathogens (non-ag streps or coliforms) are usually not due to infection of the quarter. However, elevated levels of pathogens indicate an increased potential for infection with an environmental pathogen. There is a strong correlation between the number of organisms on the teat at the time of milking machine attachment and infection rates.
- Improve cow preparation. Target teat ends for cleaning.
- Improve bedding management. Groom sand stalls at each milking or clean and add fresh bedding to the back one-third of the stall daily.
- Reduce overcrowding of animals in a pen.
Staph species come from the teat skin.
- Improve postmilking teat dip coverage. Be sure to consistently cover 80 percent of the teat.

Bulk tank culture dos and don'ts:

Do use multiple-day samples. This increases accuracy of results due to intermittent shedding of bacteria by some cows.
Do have bulk tank cultures performed monthly.
Do use a high-quality laboratory. Ask your laboratory about quality control, personnel training, etc.
Do agitate the tank well before sampling.
Do take the sample from the top of the tank.
Do freeze the sample immediately after taking.
Do use a clean dipper.
Do make sure the lab uses a reliable bulk tank culturing technique.
Don't count on a single sample to tell you if you have contagious agents in your herd.
Don't take the sample from the outlet valve.
Don't assume all laboratories are equal. Some laboratories are more capable than others. You are going to make costly management decisions based on results. Therefore, be sure the lab can produce accurate, high-quality results.
Don't use bulk tank cultures only. Always use culture data in conjunction with your herd SCC and individual cow records as well.

TYPE OF BACTERIA	USUAL INFECTION SOURCE	MAJOR MEANS OF SPREAD	MASTITIS CONTROL MEASURES
Strep agalactiae	Infected udders of other cows in herd	Cow-to-cow by contaminated udder wash rag, teat cups, etc.	Use separate towels to wash/dry; use teat dip; treat dry cows; eradicate in special cases
Staph aureus	Infected udders of other cows; contaminated bedding from milk of infected cows	Cow-to-cow by contaminated udder wash rag, milkers hands, contaminated milking equipment, improperly functioning equipment	Use separate towels to wash/dry; use teat dip; treat dry cows; milk infected cows last; cull chronically infected cows
Mycoplasma	Infected udders of other cows, often purchased cows	Cow-to-cow by hands of milkers, equipment, and common towels; aerosol transmission from animals with respiratory signs; spread from a respiratory tract infection to the udder or joints	Use bulk tank and cow culture results to avoid buying infected cows; use separate towels to wash/dry; use teat dip; treat dry cows; milk infected cows last; cull any positive clinical cases
Non-ag streps	Environment of cow	Wet, dirty lots; contaminated bedding; milking wet cows; poor cow prep; milking machine air slips	Improve stall and lot sanitation; milk clean, dry cows; avoid air leaks and liner slips; change bedding frequently; keep cows standing after milking
Coliforms	Environment of cow	Wet, dirty lots; contaminated bedding; milking wet cows; poor cow prep; milking machine air slips; hot, humid weather	Improve stall and lot sanitation; milk clean, dry cows; avoid air leaks and liner slips; change bedding frequently; keep cows standing after milking
Staph species	Environment of cow	Poor teat dip coverage; poor cow prep; old bedding	Use teat dip consistently; adequately prep; change bedding more frequently