The susceptibility of breast-fed and cow's milk formula-fed infant guinea pigs to upper respiratory tract infection with influenza virus.

Breast-fed infant guinea pigs from immune mothers were partially protected against infection with influenza virus when compared to those from nonimmune mothers. Virus titres in nasal washes at 24 h post-infection were reduced and virus clearance from the upper respiratory tract accelerated. When infants of immune mothers were deprived of colostrum and hand-reared on a formula-feed their ability to reduce virus yields at 24 h post-infection was lost. Infants partially breast-fed and partially formula-fed gave total virus yields similar to their fully breast-fed peers. Infants of immune mothers possessed high titres of serum IgG antibody to the virus prior to infection. Post-infection, IgG antibodies appeared on the mucosal surface of breast-fed seropositive infants earlier than for seronegative infants of nonimmune mothers but IgM and IgA responses of seropositive infants were less vigorous than those of seronegative infants. There was little evidence that antibody present in a mother's milk was transmitted to the nasal mucosa of her offspring. Fully and partly formula-fed seropositive infants showed enhanced transudation of serum IgG antibody on to the mucosal surface and this effect was most marked in the partly formula-fed group which showed greater protection. In both formula-fed groups serum and nasal IgM and IgA responses were completely suppressed.