Moisture

The moisture content of the skin is an important factor in human dermatology. If an excised piece of skin is dried out, it becomes hard and brittle and the application of oil or fat does not change this. Yet, if it is allowed to absorb some moisture, the skin becomes soft and pliable again. This simple experiment demonstrates the importance of moisture for the skin condition (BLANK 1952). The epidermis receives water from the sweat glands and from the underlying tissues by diffusion. It loses water to the environment by evaporation, yet the stratum corneum has a low water permeability (KLIGMAN 1964).

The loss of water to the atmosphere is of a much higher rate than the diffusion of water from the underlying tissue. In consequence, the stratum corneum is drying out under certain conditions (BLANK 1952).

Because of these findings, the existence of a thin barrier against water loss from the tissues underneath the stratum corneum was assumed (BLANK 1953). This hypothesis was tested by progressively stripping the stratum corneum of abdominal skin. Very little change in the rate of water loss was demonstrated until the final layer of stratum corneum or the upper layer of the underlying tissue was removed, and then a rapid increase in rate of water loss was observed (Figure 3). Thus, neither the lipid film on the surface of the skin, nor the stratum corneum itself represent the major barrier against water loss. A very thin barrier must exist near or at the base of the stratum corneum, which separates most of the water of the underlying tissues from the stratum corneum (BLANK 1953).

Fig. 3: Effect of stripping human abdominal skin with adhesive plaster on the rate of diffusion of water through the skin (BLANK 1953)

This barrier is of great importance for the body. It allows survival in dry environments by limiting the loss of water from the living tissues of the body to the atmosphere. It prevents water losses higher than 0.2 to 0.4 mg/h/cm² (TAGAMI 1982). On the other hand, the stratum corneum is able to lose water freely to the environment; therefore, it would dry out under most environmental conditions. Yet, it is also able to obtain moisture from the environment. Figure 4 demonstrates the concentration of water in human stratum corneum at 30°C as a function of the relative humidity of the environment.

An equilibrium state between evaporation and loss of moisture of the stratum corneum is reached at about 60 per cent. In this state, the stratum corneum contains approximately 10 per cent water of dry weight, and remains soft and pliable (BLANK 1952). If the water content drops below 10 per cent, the skin will become brittle and is in danger to chap. In this situation, the lipid content of the stratum corneum can not prevent it from becoming brittle (BLANK 1953).

Fig. 4: Concentration of water in human stratum corneum (g * cm^-3) in equilibrium with air at 30°C as a function of relative humidity (%) (BLANK et al.

1984)

Figure 4 shows that between 30 to 80 per cent relative humidity the concentration of water of the stratum corneum remains almost unaltered. Within this range, the stratum corneum is able to maintain a water concentration that keeps it in a flexible state (BLANK et al. 1984).

LLOYD and McEWAN JENKINSON (1980) mentioned the connection between bovine skin temperature and cutaneous moisture loss of the dorsal area, which was determined by the method of McLEAN (1963), described in section 2.3.1. These results can be found in Table 6.

Tab. 6: Mean skin water loss and skin temperature (±± standard deviation) (LLOYD and McEWAN JENKINSON 1980)

Temperature °C Rel. humidity % Water loss g/m²*h Skin temperature °C

15.2 37 8.2 ± 0.16 35.0 ± 0.12

29.7 41 64.7 ± 2.27 37.2 ± 0.09

15.4 85 9.9 ± 0.02 36.0 ± 0.12

30.5 78 47.9 ± 1.17 37.6 ± 0.71

The skin water loss was greatest in warm environments, the surface temperature increased for higher humidity.

The moisture of skin can be influenced by the application of various skin care products. In human medicine, the effects of ‘moisturisers’ (products that increase the moisture of the stratum corneum) on skin have been extensively studied. One application of a 10 per cent urea cream for example, resulted in an increase in skin moisture for at least 30 minutes, while petrolatum caused no initial moisturising effect, yet a slow increase in moisture over a longer period, probably because of its occlusive effect (TAGAMI et al. 1980; TAGAMI 1982).

BÜSCHER and LIPPOLD (1994) studied the moisturising effects of nine different emollients on human skin. It was found that propylene glycol, urea, sodium lactate and glycerol increased the moisture of the skin significantly between seven and 12 per cent. As in some experiments of this study, a capacitance device (Corneometer CM 820^, Courage and Khazaka electronic GmbH, Cologne) was used.

Very little information is available about the influence of the cow’s environment on teat skin moisture.

A smooth, healthy and moist teat skin is less likely to become chapped and therefore possibly carries fewer resident bacteria. This reduces the likelihood of contamination of the teat canal (Fox et al. 1990). Teat skin moisture is one factor of teat skin condition. The influence of this parameter on new intramammary infection (IMI) was emphasised frequently by different authors (SIEBER and FARNSWORTH 1981;

FRANCIS 1984; FOX 1992). Bad teat skin condition, for example eroded teat skin, is known to be particularly susceptible to colonisation by coagulase-negative staphylococci (SHARPE et al. 1962). Teat lesions were connected to increased subclinical mastitis (AGGER and WILLEBERG 1986), and teat skin chaps have been correlated to colonisation with S. aureus (FOX et al. 1991; BURMEISTER et al.

1998a). The severity of teat skin chapping has been associated to the degree of S.

aureus colonisation (FOX et al. 1991; FOX and CUMMING 1996).

Further information about the relationship between teat skin condition and the prevention of new IMI will be given in section 2.4.3.