Pathogenesis of measles virus infection

Introduction
Measles is a highly contagious disease caused by an enveloped RNA virus of the genus Morbillivirus in the family of Paramyxoviridae (Griffin et al, 1994). It is a major cause of child morbidity and mortality, particularly in developing countries, despite the introduction of attenuated measles virus vaccines which have greatly reduced the incidences since the 1960s (WHO, 2009). The window period of infection for infants lies between the disappearing maternal antibody protection and vaccine administration (Manchester and Rall, 2001). In 2008, 164,000 measles deaths were reported and majority was children under five years old (WHO, 2009).
Affected individuals combat measles by generating cell mediated immunity to clear the virus and humoral immunity to provide long-term protection (Manchester and Rall, 2001). However, measles virus (MV) induces immunosuppression during infection and for weeks after recovery, rendering infected individuals susceptible to secondary infections (Griffin et al, 1994). The evidence of immunosuppression caused was first recognized in 1908 when von Pirquet reported that children lost positive skin test for tuberculin antigen during MV infection (von Pirquet, 1908).

Research has been carried in vitro and in vivo in order to define the pathogenesis pathways of MV. Immune responses to MV have been described on transgenic mice and cynomolgus monkeys models (Sato et al, 2007) suggesting that multiple potential mechanisms are linked to the virus-induced immunosuppression (Schneider-Schaulies et al, 2002).
Infection
Measles is transmitted via airborne exposure from coughing and sneezing or close contact with nasal and throat secretions. MV remains active in the air for up to two hours. It enters the body through the respiratory system and spread systemically by infecting lymphoid cells. Infection and spread is a complex process. The structure and proteins of MV are important determinants of virus tropism and pathogenesis (Yanagi et al, 2006).
Measles virus consists of a non-segmented single negative-strand RNA genome (16,000 ribonucleotides) with a diameter of 150 to 300 nm. The outer envelope comprises with the inner matrix protein to form a lipid bilayer surrounding the viral genome. It encodes six structural proteins and two nonstructural proteins which are important for attachment of the virus to the host, replication and spreading of the virus in the body (Horikami et al, 1995).
Table 1 briefly describes the functions and locations of structural components and
Figure 1 illustrates the structure of a measles virus.
Table 1: Locations and functions of Measles virus structural proteins
Structural proteins