Somatic Cell Counting in Goat Milk
Somatic cells are primarily white blood cells. They are the first defence against bacteria that enter the teat canal. High somatic cell counts (SCCs) may indicate infection in the udder which affects milk production as well as the quality and quantity of dairy products into which milk is made (i.e. cheese). To fully understand the significance of somatic cell counts in goat milk, a quick review of some facts may be helpful.
Comparing Cow and Goat Milk SCCs
Goats produce milk differently than cows. The way milk is produced in the healthy udder of the goat naturally results in a greater number of skin cells and cell fragments containing DNA being present in the milk as compared to milk from healthy cows. These skin cells and cell fragments are counted as somatic cells when samples are tested using automated counting methods such as those used at the University of Guelph, Agriculture and Food Laboratory (AFL) and milk testing labs worldwide. As a result, goat milk SCCs are higher than those found in cow milk. Goat milk SCC levels also tend to be elevated towards the end of lactation, in does with a higher number of lactations, and in herds with caprine arthritis encephalitis (CAE). All of these factors contribute to a higher 'normal' SCC level in goat milk than in cow milk. Nevertheless, routine bulk tank SCC test results should be monitored from month to month to be aware of any upward trends which can indicate udder infection.
In Ontario the regulatory standard for SCCs in cow milk is less than 500,000 cells/mL and will be amended to 400,000 cells/mL as of August 1st, 2012, similar to several other provinces (British Columbia, Alberta, New Brunswick). A bulk tank somatic cell count of 1 million per millilitre in cow milk would indicate that the herd is suffering from major udder health issues. However the same result in a sample of goat milk would not raise concerns. In fact a SCC level as high as 1.5 million cells per millilitre would meet the United States' regulatory standard for goat milk and therefore be acceptable. Ontario does not have a regulatory SCC standard developed specifically for goat milk however many Ontario producers are able to maintain SCCs in the range of 600,000 - 800,000 cells per millilitre.
Currently there are two methods commonly used for counting somatic cells in goat milk.
Direct Microscopic Somatic Cell Count (DMSCC) using pyronin y-methyl green dye (PYMG) or 'Manual method'
In the United States, the DMSCC PYMG method is the approved method for counting somatic cells. A sample of goat milk is manually placed on a microscopic slide and then treated with a special dye (PYMG) that only stains the DNA in the various cells contained in the goat milk. This method requires highly qualified technicians who are trained to distinguish somatic cells from other cells and cell fragments by size, shape and colour. One significant problem with this manual test is that the level of training and experience of the technician directly influences the final count. For this reason, the somatic cell count on the same sample can vary significantly from one technician to another. As well, this method can suffer from poor accuracy and precision due to the variable thickness of the milk layer on the slide and poor distribution of the cells in that milk layer. The test procedure is also time consuming and therefore expensive to perform.
In Europe, Quebec and Ontario a rapid and more automated method is used to count somatic cells in goat milk. The Fossomatic FC used at AFL counts somatic cells in a milk sample by using a process called flow cytometry. In this process, a mixture of milk and staining solution is passed through a flow cell in which the stained cells containing DNA are exposed to light of a specific wavelength. When exposed to this light, the stained cells then emit fluorescent light pulses which are counted and recorded. The Fossomatic provides greater precision, faster turnarounds and reduced costs when compared to the manual method.
Comparison of Methods
Each method has its advantages and disadvantages yet both provide reliable estimates of somatic cell counts in goat milk.
Researchers in the United States compared the somatic cell counts in goat milk from different stages of lactation using the DMSCC PYMG and Fossomatics calibrated with either goat or cow milk standards.
Both methods gave comparable results when calibrated with goat milk standards. Fossomatics calibrated with cow milk standards tended to give results about 25% higher than a Fossomatic calibrated with goat milk standards.
Since the bulk of the testing performed on the Fossomatics operating at AFL is performed on cow milk, the instruments are calibrated using cow milk standards. At this time it is not economically feasible to dedicate one Fossomatic instrument to testing only goat milk samples. This then explains why the somatic cell count from AFL's Fossomatic is slightly higher than that reported by US regulatory agencies. Nonetheless the Fossomatic still provides a cost effective farm management tool for the goat milk producer by indicating upward trends in SCCs which may point to mastitis.
Tips on Avoiding High SCCs
The following key farm management practices will help producers avoid high bulk tank SCCs and the spread of mastitis in the herd.
A good source of information on the management of SCCs and mastitis in goat herds has been issued by the Vermont Agency of Agriculture, Food and Markets and can be found at: http://www.uvm.edu/newfarmer/production/livestock/SCCgoats2010.pdf
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