QUALITY OF COLOSTRAL PASSIVE IMMUNITY AND PATTERN OF SERUM PROTEIN FLUCTUATION IN NEWBORN CALVES

Immunity acquired by newborn animals is known as passive immunity, and for ruminants, antibody acquisition depends on the ingestion and absorption of adequate amounts of immunoglobulins from colostrum. This study relates different initial levels of acquired passive protection and serum total protein (TP) and immunoglobulin G (IgG). Serum immunoglobulin concentration and total protein were evaluated for female Holstein calves in the first sixty days of life. Animals were separated into three groups according to their initial level of passive immunity: group 1animals with a low level of passive immunity (below 20 mg mL); group 2animals with a medium level (between 20 and 30 mg mL), and group 3animals with a high level (above 30 mg mL). Serum total protein was determined through the biuret method and IgG was determined by radial immunodiffusion. Data were analyzed as a completely randomized, split-plot statistical design. Fluctuation of the variables along the experimental period was determined through non-linear regression by the DUD method (PROC NLIN  Non Linear SAS). Animals with low antibody acquisition started to produce antibodies earlier, reflecting a compensatory synthesis. On the other hand, animals having adequate levels exhibited an extended period of immunoglobulin catabolism and the beginning of the endogenous phase was delayed. Regardless initial levels, the fluctuations in IgG contents occurred around adequate physiological concentrations, ranging from 20 to 25 mg mL.


INTRODUCTION
Survival and healthiness of newborn animals directly depend upon the acquisition of maternal antibodies.In the case of ruminants, the newborn relies on the ingestion of colostrum to obtain immunoglobulins (Brambell, 1958;Jeffcott, 1972).
The serum immunoglobulin content found in calves after receiving colostrum is quite variable.
After the absorptive period, the animal undergoes a transitional stage during which stops benefiting from mother-acquired antibodies and starts to respond actively to environment challenges.However, animals presenting different initial levels of antibodies acquired from colostrum respond differently to the endogenous production (Husband & Lascelles, 1975;Machado Neto & Packer, 1986;Baracat et al., 1997;Aldridge et al., 1998).
The objective of this work was to establish a relation between different initial levels of acquired passive protection and variables serum total protein (TP) and immunoglobulin G (IgG), and to define behavior patterns for their curves, during the first 60 days of life, aiming to facilitate the development of newborn calf management alternatives.

MATERIAL AND METHODS
Fifty nine female Holstein calves were utilized in this study, distributed in three groups according to their initial concentration of acquired serum immunoglobulins at 24 hours after birth: group 1-animals with low level of passive immunity (up to 20 mg mL -1 IgG); group 2medium level (between 20 and 30 mg mL -1 IgG); and group 3-high level (above 30 mg mL -1 IgG).
A completely randomized experimental design with split-plots in time was adopted, and the effect of the initial immunoglobulin levels was attributed to plots, with the measurements repeated in time considered as subplots.Correlation, Pearson and Sperman analyses were carried out to verify associations of interest between the TP and IgG variables.Non-linear regressions were plotted for the groups to determine the behavior of variables TP and IgG during the experimental period; these regressions proved to be more suitable for this type of variable; one curve was obtained for each group.The chosen model was: Y=L+U*(R-ID)+V*(ID-R), where Y= variable's concentration, ID= animal age (days), L= point of minimum concentration, U= slope during the period that preceded the point of minimum concentration, V= slope during the period after the point of minimum concentration, R= age at the point of minimum concentration.
The DUD method, an iterative process to reach the convergence criterion, was then applied by means of the PROC NLIN (NonLinear) SAS (1999) procedure.The general curve model selected corresponded to that which best fitted the serum variable data.Since it is a non-linear model, it was necessary to utilize an iterative process to solve the equations.It requires the selection of initial values as starting point to iterate according to some preestablished criterion (Diaz, 1992).However, no algorithm exists to determine adequate initial values, and the body of knowledge suggested by previous research had to be resorted to.There were no values available in the literature for the proposed model, so the initial values for the parameters were selected based on the mean values obtained for each group.

RESULTS AND DISCUSSION
Animals presenting adequate concentrations of passive immunoglobulins modulate the immune response by delaying the production of endogenous immunoglobulin, while those having low contents will speed up their active production as a response to environmental challenges (Husband & Lascelles, 1975;Logan et al., 1974;Ribeiro et al., 1983;Machado Neto & Packer, 1986).The decrease in IgG concentration during the first month after birth for groups 2 and 3 (Table 1; Figure 1), resulted from the catabolism of proteins passively acquired from colostrum.The age at the point of minimum concentration (R) was 22.290 and 26.019 day for groups 2 and 3, respectively; the confidence interval analysis indicated that the groups did not differ in relation to age at the point of minimum concentration for the variable IgG.Many studies report that the endogenous production of antibodies does not contribute towards increasing the amounts of serum antibodies until two weeks after birth, since this fraction is lower than the rate of catabolism of maternal antibodies (Husband et al., 1972;Daniele et al., 1994a;Rajala & Castren, 1995;Baracat et al., 1997).
For group 3, parameter V was non significant by the confidence interval analysis.This result indicates the occurrence of a plateau between the date of minimum concentration R=26.019 and the end of the experimental period.This fact is a consequence of the high initial concentration of acquired immunoglobulin, thus delaying the endogenous production of antibodies.For group 1, however, there was no initial peak of IgG followed by a reduction, and this can be justified by the initial condition, which determined a reduced catabolic activity and an early endogenous production of antibodies.Animals of high serum IgG contents, correlated with high concentrations of total serum protein, would delay the endogenous synthesis of immunoglobulins, while animals of low initial values would accelerate their production (Logan et al., 1974;Husband & Lascelles, 1975;Machado Neto & Packer, 1986).

Figure 1 -
Figure 1 -Average fluctuation of seric IgG concentration, with adjusted values. e

Figure 2 -
Figure 2 -Average fluctuation of total protein concentration, with adjusted values.

Table 2 -
Mean predicted values of serum total protein concentration in the experimental groups.