| The milk secretory process of mammary gland epithelial cells of cows naturally infected with foot-and-mouth disease (FMD) was examined for synthetic events which may play a role in acquired virus stability.;The virus survived in milk and milk components after high temperature-short time (HTST) pasteurization (a minimum temperature of 71.7C for at least 15 sec). Skim milk casein and the milk fat globules (MFG) of cream were considered candidates for conferring insulative protection against thermal and pH inactivation of FMD virus in milk. Treatment of skim milk with sequestering agents EDTA and EGTA resulted in partial dissolution of casein, but was without effect on virus titer. However, treatment of skim milk with sodium dodecyl sulfate (SDS), a protein denaturant, increased the infectivity titer by 0.8 to 1.0 log(,10).;Fractionation of skim milk on a ficoll sucrose gradient produced a non-infectious visible band which contained vesicle-like structures when examined by electron microscopy (EM).;The infectivity titer of the buttermilk fraction of butter was 1.0 to 1.5 log(,10) pfu/ml higher than that of the original cream sample. The membranous nature of this fraction was confirmed by EM observation of membranous strands of varying lengths containing adsorbed virus-like particles. The pelleted aqueous fraction contained 0.007% of the original infectivity of the cream sample. The butter and butter-oil fractions contained intermediate concentrations of infectivity.;A natural route of infection was achieved by contact exposure of susceptible cattle to FMD-infected donor animals. The onset of infection, as determined by the observation of clinical signs of FMD or the initial detection of a virolactia (presence of virus in milk) varied in occurrence with each cow. The virus was recovered from milk and components: skim milk, cream and reconstituted pelleted debris, for periods up to 9 days after the onset of virolactia. Virolactias were detected before, during and after the appearance of clinical signs of FMD. Infectivity titers were slightly higher in cream than in the skim milk component.;Milk from cows before infection contained multi-sized milk fat globules (0.1 to 1% containing cytoplasmic fragments (signets)), anucleated membrane limited vesicle-like structures and a few lymphocyte and basophiles. After the appearance of clinical signs of FMD, the concentration of white blood cells, cellular fragments, debris and on occasion gram positive cocci in the milk, increased significantly.;The hematogenous spread of FMD virus in cattle infected by contact exposure was evidenced by histopathological changes in mammary gland secretory epithelial cells in hematoxylin and eosin (H&E) preparations, as well as by immunofluorescent and Protein-A horseradish peroxidase (PrAPo) labeling of FMD viral antigens.;The synthesis and release of casein and milk fat globules by FMD-infected cells was demonstrated by the respective labeling or staining of these constituents with anti-casein gammaglobulin-PrAPo or oil Red O.;Based upon EM examination, at least four mechanisms for viral exocytosis from infected cells exist: (1) release contained in membrane limited vesicles; (2) release associated with lipid droplet within MFG membrane; (3) merocrinal release associated with casein into the lumen and (4) release through lysis of the infected cell.;The association of FMD virus with milk components was observed throughout the course of infection and has the potential to enhance the stability of FMD virus in milk. The internal entrapment of virus particles by milk components in FMD infected cells would provide an adequate protective environment against thermal and pH inactivation. |
