Clinical mastitis can present itself in a wide degree of severity of symptoms which can range from mild to moderate to severe. The degree of illness and the symptoms present will depend on many factors, such as the nutritional or immune status of the cow, which pathogen is responsible for the inflammation, and a range of environmental factors such as cleanliness, humidity and ambient temperature. Moderate to severe clinical cases can be very painful and unpleasant for the cow.
The most obvious symptoms of clinical mastitis are abnormalities in:
- The udder such as swelling, heat, hardness, redness, or pain; and
- The milk such as a watery appearance, flakes, clots, or pus.
Other symptoms, depending upon the severity of the illness and how systemic it has become, can also include:
- A reduction in milk yield.
- An increase in body temperature.
- The lack of appetite.
- Sunken eyes.
- Signs of diarrhoea and dehydration.
- A reduction in mobility, due to the pain of a swollen udder or simply due to feeling unwell.
In severe cases of acute, clinical mastitis - in many instances caused by E. Coliinfections - the cow may appear very ill indeed. In contrast, subclinical mastitis can result in few symptoms and may only be detectable in a higher than normal Somatic Cell Count.
Most of the indicative symptoms, such as the swelling, heat, redness and the milk abnormalities are a result of an immune response in the cow, the changes in milk constituents in particular caused by infection-fighting white blood cells attempting to eliminate the infective organisms which may further be responsible for producing toxins which damage the milk-producing glands within the udder, and can be responsible for permanent udder damage in some cases.
In some instances the cow's immune response is sufficient to efficiently generate a self-cure for the illness, usually in mild cases of the disease where the cow is strong and has a good immune response. Other cases can result in more severe illness, perhaps even leading to loss of a quarter or more of the udder, the loss of body tissue due to gangrene and in worse case situations, death.
Changes in milk composition even in cows with subclinical mastitis can result in significant changes in the protein composition in milk. While overall protein content may be unaffected, changes in the types of protein present may be affected by the leaching of (low-quality) blood serum proteins into milk; casein, an important protein found in healthy milk can be significantly reduced in sub-mastitic cows, and a further complication is that casein is closely linked with calcium levels in milk production.
An overall effect of the chemical alterations in milk mean that the pH of milk, normally around 6.6, can increase to 6.8 or 6.9 in mastitic cows. The presence of certain blood enzymes in milk from mastitic cows can affect the taste of milk and its ability to be made into other dairy products such as cheese or yoghurt.
Clinical cases of mastitis are those where the cow displays definitive symptoms of the disease. These may be acute, where the disease flares up relatively suddenly in a formerly healthy cow; these cases may be further defined as per-acute, where the rapid onset of severe inflammation, pain and systemic symptoms results in a severely ill cow within a short period of time, or sub-acute mastitis, the most frequently seen instance of the disease, where the few symptoms tend to be mild inflammation in the udder and visible changes to the milk, such as small clots.
Long-term recurring persistent cases of the disease are termed chronic. These cases may show few symptoms between repeated occasional flare-ups of the disease where symptoms are visible and can continue over periods of several months. Often with chronic mastitis, irreversible damage is caused to the udder from the repeated clinical occurrences of the illness, and often these cows are culled.
Many incidences of mastitis are directly indicated only by a high individual cow Somatic Cell Count through milk recording schemes or via California Mastitis Testing; the cow herself displaying no obvious clinical symptoms of the illness and no visible changes to the composition of her milk. These cases of mastitis are termed sub-clinical, and can be up to 40 times more common than clinical cases of the illness.
Subclinical infection is more likely to be caused by contagious pathogens. The presence of a causative pathogen is done via a bacteriological culture and indirect indications of subclinical mastitis can be given via electro-conductivity testing of milk, which is performed in an automated form in many modern parlours and robotic milking systems.
A subclinical cow, while appearing unaffected by the illness, may experience a reduction in yield potential due to the high SCC, and certainly represents a possible source of infection for other cows, who can become subclinical sufferers themselves, or may go on to show clinical signs of the illness, due to differences in immune status between cows.
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