Properties of the virus
Morphology and Biologic properties Measles virus is a typical paramyxovirus, related to canine distemper and bovine rinderpest. All 3 lack neuraminidase activity. Measles agglutinates monkey erythrocytes at 37 degrees Celsius but does not elute, and it interacts with a distinct cell receptor. Measles virus also causes hemolysis and this activity can be separated from that of the hemagglutinin.
Animal susceptibility and growth of virus The experimental disease has been produced in monkeys. They develop fever, catarrh, Koplik's spots, and a discrete popular rash. The virus has been grown in chick embryos; in cell cultures of human, monkey, and dog kidney tissue; and in human continuous cell lines. In cell cultures, multinucleate syncytial giant cells form by fusion of mononucleated ones, and other cells become spindle-shaped in the course of their degeneration. Nuclear changes consist of margination of the chromatin and its replacement centrally with an acidophilic inclusion body. Measles virus is reliably unstable after it is released from cells. During the culture of the virus, the intracellular virus liter is 10 or more times the extra-cellular liter.
Pathogenesis and pathology The virus enters the respiratory tract, enter cells and multiplications there. During the prodome, the virus is present in the blood, through the respiratory tract, and in nasopharyngeal, tracheo-bronchial, and conjunctival secretions. It persists in the blood and nasopharyngeal secretions for 2 days after the appearance of the rash. Trans-placental transmission of the virus can occur.
Koplik's spots are vesicles in the mouth formed by focal exudations of serum and endothelial cells, followed by focal necrosis. In the skin, the superifical capillaries of the corium are first involved, and it is here that the rash makes its appearance. Generalized lymphoid tissue hyperplasia occurs. Multinucleate giant cells are found in lymph nodes, tonsils, adenoids, spleen, appendix, and skin. In encephalomyelitis, there are petechial hemorrahes, lymphocytic infiltration and later, patchy demyelination in the brain and spinal cord.
Measles nucleoprotein antigens have been identified by immunofluorescence within inclusion bodies in nerve cells of the brain in subacute sclerosing pan-encephalitis (SSPE). The virus has been grown by co-cultivating HeLa cells with brain biopsy material or lymph node material from patients. The presence of latent intracellular measles virus in these specimens suggests a tolerant infection with defective cell-mediated immunity.
If measles antibody is added to cells infected with measles virus, the viral antigens on the cell surface are altered. By expressing fewer viral antigens on the surface, cells may avoid being killed by antibody-or cell-mediated cytotoxic reactions, yet may retain viral genetic information. This may lead to persistent infection as found in SSPE patients.