Establishing the bacterial cause of clinical mastitis through questioning
When a herd has a problem with clinical mastitis it is very helpful to have accurate mastitis records and bacteriology results to establish the cause of the infection. Computerised records that allow easy mastitis analysis are invaluable. In most developed dairy countries farmers are required to keep accurate mastitis records, however, in other dairy areas records might be missing or inaccurate. Some farmers carry out regular bacteriology sampling from clinical cases while others might have none. An accurate diagnosis of the bacteria causing mastitis allows targeted steps to resolve the problem. It is helpful to be able to breakdown mastitis infections into contagious and/or environmental, lactation and/or dry period infections and the specific bacteria causing mastitis.
This article describes ways of trying to establish bacterial causation through a series of questions. While not fool proof, it has helped the author to establish a presumptive diagnosis and to offer practical control measures while bacteriology samples are collected.
What is the level of mastitis?
It is important to know if the level of clinical mastitis is above or below the target. This author defines a case of mastitis as one quarter being infected once. A cow that calves down with mastitis in all four quarters counts as four cases. This author’s target is to have a mastitis rate (clinical cases per 100 cows per year) of between 20 and 25. The mastitis rate is useful as this allows comparison between herds irrespective of herd size.
In the absence of clinical records, a farmer can be asked about the average number of clinical cases per week or month, depending on the size of the herd. A herd of 240 cows that has an average of 10 clinical cases per month or 120 cases for the year. This gives a mastitis rate of 50 (120 cases/240 cows x 100), which is more than twice the target level indicating problems with clinical mastitis.
A herd of 600 cows has an average of three cases per week, or 156 cases per year which is equivalent to a mastitis rate of 26 (156 cases/600 cows x 100).
What is the herd cell count?
A low herd cell count, under 150,000/ml, indicates a low level of contagious mastitis. It is highly likely that clinical mastitis in a low cell count herd is going to be due to Gram negative infections, such as Escherichia coli, Klebsiella etc.
If a herd has a high cell count, over 250,000/ml, then clinical mastitis could be due to either contagious and/or environmental infections. A high cell count only tells you that there is contagious mastitis in the herd, it does not give any indication about the level of environmental infection.
What proportion of the herd has had mastitis?
The target is to have at most 20% of cows with a case of clinical mastitis. If over 25% of the herd. this suggests problems with environmental mastitis which randomly infects any cow at any stage of lactation. The risk of environmental mastitis rises when cows are kept in dirty conditions.
Be aware that a high percentage of the herd with clinical mastitis confirms the presence of environmental infections but there could also be problems with contagious mastitis.
How clean are the cows?
The risk of environmental mastitis infections during lactation should be lower when udders and teats are clean. If you get a rise in mastitis when cows are dirty, then this is most likely going to be due to environmental bacteria. This is a question that most farmers will be able to answer easily. Photo 2 is a dirty cow from Africa at the start of the rainy season. Cows were very dirty resulting in a large outbreak of coliform clinical mastitis.
What is the incidence of mastitis in the month after calving?
Most farmers will know whether they have clinical mastitis problems in the first 30 days after calving. Dry period infections are suspected when more than 8% of cows have clinical mastitis within the first 30 days of lactation, that’s more than one in twelve cows. Figure 1 shows 37 mastitis cases in the first week of calving and a total of 62 cases in 40 cows within 30 days of calving. As this herd has only 300 cows, this is equivalent to 13% of cows, well above the 8% target, indicating problems with dry period infections or very poor conditions at calving.
Is mastitis seasonal?
Figure 2 shows the majority of cases are occurring between November and April when cows were housed. This suggests problems with environmental infections at housing.
Figure 3 shows seasonality of mastitis in an autumn calving herd. The majority of cases occur around the calving period between July and September and when cows were at pasture. This suggests problems with environmental infections which could be related to either dry period infections or cows kept in overcrowded and dirty fields around calving and early lactation. Mastitis levels between November and April, when cows were housed, are below target which indicates that the winter housing is not a significant risk factor for mastitis.
What type of bedding is used?
Different bedding materials can be associated with different bacteria.
Straw
Cows bedded on straw, in particular straw yards, have a much higher risk of Streptococcus uberis mastitis. One of the problems with straw yards is that cows can pass manure anywhere contaminating the bedding. A cow can lie down so that her teats and udder and lying directly on contaminated straw bedding.
Photo 4. Overstocked and dirty straw yards are a Streptococcus uberis risk
Sawdust and wood shavings
Klebsiella is associated with the use of bedding materials from wood products such as sawdust and shavings. The risk is low when the bedding has been kiln dried and where the dry matter level is around 95%. Use of fresh sawdust, with a lower dry matter level of around 60-70% is commonly associated with Klebsiella infections. Fresh sawdust ferments allowing the Klebsiella to multiply exponentially. You can often feel the heat from fermentation when you put your arm into a pile of fresh sawdust it.
Recycled manure solids
Klebsiella can thrive in recycled manure solids which contain about 30-35% of solids, so they are mainly liquid. The challenge is to remove as much of the liquid as possible. Management of recycled manure solids can be very difficult especially in high humidity climates which allows rapid bacterial growth.
Photo 6. Klebsiella risk increases when using recycled manure solids especially in a high humidity climate
How clean is the milk filter after milking?
If the milk filter is consistently dirty with a lot of faecal matter present, this suggests poor premilking teat preparation which will increase the risk of environmental mastitis.
Are teats washed with water from a bore hole?
Some farmers have their own water supply which can become contaminated with Pseudomonas bacteria, especially tanks that store water for use in the parlour for washing teats before milking. Water samples can be collected and analysed for the presence of Pseudomonas.
How severe are clinical cases?
Acute mastitis (Grade 3 mastitis)
Most acute mastitis occurs after calving and is often caused by E. coli. These cows typically have a watery serous discharge. Many cows will have raised temperatures. Some have subnormal temperatures, and are likely to die as they are not mounting an immune response to infection.
Cows with high temperatures (Grade 2 mastitis)
These can be due to any of the Gram-negative infections, such as E. coli and Klebsiella, as well as Strep. uberis.
Gangrenous mastitis
Gangrenous mastitis can be caused by Staphylococcus aureus and Bacillus cereus.
What is the response to treatment?
A poor response to treatment is seen when cases recur in the same quarter in the same lactation and/or if there is no response to treatment. A poor response to clinical mastitis treatment could be due organisms that respond poorly to treatment (for example Staph. aureus and Strep. uberis), delayed mastitis detection and/or a poor treatment protocol. Most farmers will be aware if clinical mastitis cases recur. Computerised records are invaluable.
Figure 4 shows clinical mastitis records for a herd over the first 300 days in milk. The red bars are the first cases, and the other colours are for repeat cases. This graph shows a very high recurrence rate indicating problems that need investigation.
Figure 4. High level of recurrent mastitis
Sometimes clinical quarters become hard and there is no response to treatment. This could be due to yeast infection, a severe coliform infection or the more unusual bacteria such as Mycoplasma or Prototheca.
Have there been any mastitis deaths?
Lactating cows.
Death from clinical mastitis in lactating cows is most commonly due to acute E. coli infection around the time of calving. Deaths can also occur from Klebsiella, E. coli or other mastitis pathogens at other stages of lactation.
Gangrenous mastitis can cause death. Staphylococcus aureus or Bacillus cereus are causes of gangrenous mastitis. If the herd has a low cell count, under 150,000/ml, then the likelihood of gangrenous mastitis due to Staph. aureus is likely to be lower.
Dry cows
Death after dry off is commonly associated with acute Pseudomonas infection. These can occur from contaminated wipes or cloths used to prepare teats prior to infusing antibiotic dry cow therapy and/or an internal teat sealant. Pseudomonas are introduced into the teat on the intramammary tube. Death is most likely to be caused by poor hygiene at infusion.
Which barns get most mastitis?
It is helpful to ask about mastitis incidence between cow barns. Figure 5 shows the mastitis rates for four different robot groups on one farm. This shows that the high yielding and straw yard groups have the highest incidence. The straw yard group was very difficult to keep clean and the barn was changed to cubicles. The herd had overstocked the high yielding group. When the stocking density was reduced the number of milkings per cow per day and yield increased and the clinical mastitis decreased back to target levels.
Summary
This article shows that you can find out a lot about the likely cause of mastitis through good questioning. This can be very helpful in herds when there are no or inaccurate mastitis records and where no bacteriology has been carried out.
Text and illustrations: Peter Edmondson
