Metaphylactic treatment at arrival, gradual decline of maternal immunity, incomplete maturation of the immune system and the slowly progressive nature of the dominant pathogens in European veal production, namely Mycoplasma bovis and BVDV, might have influenced the occurrence of the peak incidence at the age of 1-1.5 months instead of at arrival [ 24, 53- 55].
To examine the key factors in age-related functional decline of innate immunity, we employed a genetic model organism, the fruit fly, Drosophila melanogaster.
Several mechanisms, including the decline of host immunity with inbreeding, may underlie HFC patterns (O'Brien and Evermann 1988; Coltman et al. 1999; Keller and Waller 2002; Altizer et al. 2003).
In mice, SkQ1 doubled median lifespan arrested development of such traits of the senescence process as involution of thymus and decline of other immunity mechanisms; osteoporosis; disappearance of regular estrous cycles in females, cataract, retinopathies, balding, catinies, hypothermia, chromosome aberrations, peroxidation of lipids and proteins, etc. [ 10, 14- 20].
Recent study of Elbers et al. [ 24] have shown the decay of maternal antibodies in calves 5 6 months after their birth, however the decline of passive immunity can vary depending individual species of wild ruminants and it may proceed at a different pace than in cattle.
A more rapid decline of specific immunity following vaccination may also account for observations on the questionable efficacy of influenza vaccine in older compared to younger individuals [ 26, 27], as well as shifts in age distribution towards older individuals following a pandemic, which we already observed in later epidemics of A(H1N1 pdm09 infection in Singapore [ 6] and elsewhere [ 7].
In parallel, rinderpest virus control and eradication may have favored the decline of cross-immunity in small ruminants and their increased risk for PPRV as predicted earlier by Taylor (38 ).
Nevertheless, none of these findings alone can explain satisfactorily the decline of cell-mediated immunity with age.
Decline of cell-mediated immunity is often attributed to decaying T-cell numbers and their distribution in peripheral organs.
Loss of virus-specific T cells from the airways may be a critical factor underlying the decline of T-cell immunity to respiratory infections 6.
