Análisis del contexto

A question to be addressed is whether the piglets in the present study were growing and maturing over the two week experimental period in a

manner comparable to piglets

raised naturally by their

dam

or under other artificial rearing conditions. A study of the impact of gastroenteritis on development requires that the severity of the disease be as uniform as possible, which dictated that both the infected and control animals were reared artificially, and that the piglets had to be deprived of colostrum. They also may

not have acquired the normal gut flora by not contacting their

dam

. It is therefore necessary to compare the growth and development of the control piglets with others naturally-reared before it can be considered valid to extrapolate from the present model to animals with a naturally-acquired infection.

For evaluation of the effect of the infection on body and gut development, control piglets must be prevented from contamination with

Y. enterocolitica

and all piglets should ideally be kept free of other pathogens. This necessitated prevention of contact of the piglet with the skin of its

dam

after birth, and probably resulted in the piglets failing to acquire some of the normal gut flora. Colostrum deprivation would appear to be an important condition in setting up a repeatable model of

Y. enterocolitica

enteritis in the newborn piglet, since earlier studies (Schiemann, 1 988) were less successful when maternal antibodies were available to the piglets. Because of their lack of protective maternal antibodies, strict hygiene was required, making artificial feeding necessary, and a commercial milk formula the practical solution. Because many infected piglets developed anorexia for a short period, some control animals were subjected to a short period of restricted feed intake (Appendix

O.

It is, therefore, likely that the artificial rearing conditions would make growth different from that in suckled piglets.

The body weight gain during the flrst 24 hours in the present study (8%) was the same as that in piglets nursed by sows (Xu

et ai.,

1992a) but lower than the 12.6% reponed by Widdowson and Crabb ( 1 976). By days 3 and 14, there were only 12.2% and 5 1 % increases over birth weight in the present piglets, whereas increases of 23% by day 3 (Xu

et aI.,

1 992a) and 100% by day 10 (Widdowson and Crabb, 1976) have been reported in suckled piglets.

Along with the slower body growth, organ and gut growth in the present piglets was also less than in suckled piglets over the fIrst three days after birth in the study by Xu

et al.

(l992a,b): the stomach weight was 85% less, the small and the large intestinal weights 1 4% and 89% less, the small intestinal length 12% less, and the weight of mucosa and muscle of the small intestine 17% and 6% less. The effect would appear to be less in the small intestine than in the stomach and large intestine. Similar comparisons in rodents have produced conflicting results. Newborn rats, fed for 40 hours on an artiflcial formula isocaloric with expressed rat milk, had signiflcantly lighter stomachs than those fed natural milk (Berseth, 1987b), whereas isocaloric milk formula fed rats and guinea pigs had greater stomach and caecal weight (Tonkiss

et al.,

1985;

Weaver et al., 1991).

The weight of most organs examined was much lower than that in naturally-suckled

piglets except for the pancreas, which had almost the same weight. The liver, spleen

and kidneys were 72 g, 3.5 g, and 14.6 g respectively in the present control group at 14

days compared with 129 g, 7 g and 23.7 g in suckled piglets (Widdowson and Crabb,

1976).

Whilst recognising that artificial feeding could impact on growth and development of

both the control and infected piglets, it is not possible to quantify the relative importance

of the different factors. Nutrient value of the diet, quantity of milk consumed and its

digestibility should all be considered. Colostrum, in addition to its nutrient value, is an

important source of growth factors. Beagle puppies fed artificial bitch milk gained as

much body weight as those that were suckled, but there was no appreciable growth of

the intestinal mucosa (Heird et al., 1984). Epidermal growth factor (EGF) is a major

growth-promoting agent in human

milk: (Carpenter, 1980), stimulating cellular

proliferation in intestinal mucosa (Klein and McKenzie, 1980; Al-Nafussi and Wright,

1982). EGF-fed animals have heavier and longer intestines and their intestines contain

more DNA and RNA than animals not fed EGF (Berseth, 1987a). EGF also influences

the maturation and the proliferation of enterocytes in the suckled mouse (Malo and

Menard, 1982), Insulin is also believed to influence small intestinal growth and

development. The concentration of insulin in human and pig colostrum is 3- to 30-fold

that in serum (Kulski and Hartmann, 1983; Slebodzinski et a!., 1986; Westrom et al.,

1987). Oral insulin increases the weight of the mucosa and the protein and RNA

content in the mucosa of the small intestine in newborn miniature pigs (Shulman, 1990).

These valuable growth factors were absent from the milk: formula used in the present

study.

The slower growth in body and in internal organs in the present piglets than in the

suckled piglets is likely to be due in large part to the lower nutrient value in milk

formula than in sow milk:, particularly in protein concentration. Sow colostrum contains

about 17.8% protein and mature milk: 5.8%. The piglets were deprived of colostrum,

and fed

milk:

formula (containing only 1 .68% protein) during the period of the

experiment. A low protein concentration in the milk: reduced the growth rate of the

gastrointestinal tract in piglets (Simmen et al., 1 990) and rabbits (Butzner and Gall,

1988a). It was decided not to add extra protein to raise it to the level in porcine milk

,

because a very high protein concentration in the milk: formula may have exacerbated the diarrhoea in infected piglets. since dipeptidase activities in the small intestinal mucosa

may have decreased. as occurs in human patients with diarrhoea (Berg

et al

.• 1973).

The presence of the bacterial flora in the intestine is believed to influence gut morphology, since both germ-free rearing and the administration of antibiotics alter structural features of the gut. Germ-free rats and pigs have a significantly lower weight

of the wall of the intestinal canal (Gordon

et aI.,

1 966; Miniats

et aI.,

1973), mainly due

to

a reduction in scattered reticuloendothelial cells, Peyer's patches and connective tissue

in the mucosa and submucosa (Gordon and Bruckner-Kardoss, 1961). Renewal rates of

the intestinal epithelium were lower in germ-free mice (Abrams

et aI.,

1963). It has

been reported that antibiotic treatment reduces the thickness of the intestinal wall (pond

and Houpt, 1978). Antibiotics were administered

to

a group of infected piglets in the

present study to examine the extent of recovery of the gastrointestinal tract from bacterial gastroenteritis, however, when the infected piglets were administrated antibiotic

from day

5

to days 9 or 13, all the control piglets were treated in the same way.

It is clear that piglets reared colostrum-deprived and fed human

milk

formula grew more

slowly than piglets nursed by sows, the greatest difference being in the weight of the body, the stomach and large intestine. However, as a model for biomedical research, overall growth rate is not important as long as the animals are healthy and their internal

organs grow in proportion to the body.

In

the present study, all control piglets were

healthy and growth of almost all internal organs was proportional to the body as a whole during the period of the experiment. The growth ratio was comparable with, or slightly higher than, that in piglets suckled by sows. The weight of the small intestine relative to body weight was similar to that in piglets nursed by sows (Cranwell and Moughan,

1989). The lengths of the small and large intestines in the 14-day-old piglets

(respectively 268 and 53 cm/kg) were much greater than in suckled piglets ( 1 86 and 36

liver

cm/kg) (Widdowson and Crabb, 1976; Widdowson

et aI.,

1976). Stomach'Jand kidney

weights relative to body weight

(5.0,

32. 1 and 6.52 g/kg respectively) were almost the

same as those in sow-nursed piglets (4.8, 35.5 and 6.5 g/kg respectively) (Widdowson

and Crabb, 1976; Widdowson

et al.,

1976; Braude

et al.

1981). The relative pancreatic

weight was greater than in suckled animals (Cranwell and Moughan, 1989).

The villus epithelium of the small intestine in the control piglets appeared less mature than in piglets nursed by sows. The apical position of the nuclei in the jejunum in 3-

day-old piglets was different from that in suckled piglets at the same age, as discussed earlier. The villus length and goblet cell number were comparable with those reported in suckled, neonatal piglets. Artificial feeding did not affect the height of the longest

villi of the small intestine. The average height of the longest villi

(782

Jlm) from six

segments of the jejunum and ileum on day 3 in the present piglets was no shorter than

that

(7 1 8 Jlm) in suckled piglets aged 2 days studied by Cera et al. (1988), using similar

methods of measurement. The crypt depth in control animals was similar at all ages. Apart from the differences in some morphological characteristics of the enterocytes, the microstructure of the intestine was comparable in control piglets and naturally-suckled animals.

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