Sub-clinical mastitis

Milk from clinically mastitis infected, sub-clinically mastitis infected and healthy cows has been shown to differ in sodium, potassium, calcium, magnesium and phosphorus concentrations (El Zubeir et al., 2005). The study reported that milk from cows infected with sub-clinical mastitis had significantly lower potassium and higher sodium and phosphorus concentrations. Similarly, the milk from cows with the clinical form of the disease had a higher sodium and lower potassium, magnesium and calcium concentration. 

Sub-clinical mastitis, on the other hand, has no visible symptoms and can only be diagnosed with laboratory methods (Wendt et al., 1994) (see below). The diagnosis of sub-clinical mastitis depends on two parameters, the microbiological profile of sampled milk and the somatic cell count (Hamann and Fehlings, 2002). The different severity levels of mastitis and symptoms used in diagnosis are described in Table 11.3. 

Both clinical and sub-clinical mastitis are usually accompanied by an increase in the somatic cell count (SCC a measure of white blood cells (95%) such as macrophages, segmented neutrophil granulocytes or lymphocytes) in milk. SCCs above a certain level are an indication of immune-system activation and are used as an indicator of sub-clinical , latent or chronic udder infections/mastitis (Concha, 1986 Ostensson et al., 1988), which were shown to affect milk quality, composition and shelf life. Dairy companies therefore impose SCC thresholds or reduce payments to farmers if certain thresholds are exceeded (Hamann, 2001 Urech et al., 1999). 

Table 11.4 Definition of sub-clinical mastitis according to DVG guidelines3 (Hamann and Fehlings, 2002)...

The objective of mastitis treatments is to cure the infected udders from the infection, but cure is defined in very different ways. For example, in economic terms, the farmer needs to achieve a level of udder health that allows expected milk yields and quality parameters specified by processors/ national regulations to be achieved. On the other hand, cure with respect to antibiotic treatments, is often defined in terms of absence of bacterial pathogens in milk (bacterial cure), with the proportion of cows without detectable pathogen presence following treatment being defined as the bacterial cure rate (BCR). The main problem with using BCR as the main indicator of cure is that it was frequently shown to include a proportion of cows with drastically elevated SCC values (indicative of sub-clinical mastitis) after treatment, but without clinical (sensory) symptoms. 

Given the diversity of disease-causing organisms, the interactions between farm-specific environmental factors and the types of mastitis causing pathogens found, the potential efficacy of antibiotic treatments can only be assessed accurately in the context of the specific on-farm conditions. Furthermore, many recent investigations concluded that, except for some specific infections caused by streptococci, a prophylactic and or longer-term use of antibiotics for chronic and sub-clinical mastitis should not recommended in the future (Deluyker et al., 2005). 

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