Measles - Causes and Pathogenesis

Causes of measles

The cause of measles is a virus that was isolated by scientists D. Enders and T. Peebles from the body of a sick person in 1954. The measles virus is an enveloped single-stranded virus with a negative RNA genome, the genus Morbilivirus, the family Paramyxoviridae, has a special affinity for mucopolysaccharides and glycoproteins, in particular for cellular receptors containing sialic acid. The place of RNA synthesis of paramyxoviruses is the cytoplasm of affected cells; there is no need for a primer to initiate transcription. The viral particle is pleiomorphic, has a round shape, a membrane shell and a helical nucleocapsid formed by three proteins of the virus and RNA. The nucleocapsid is surrounded by an outer membrane of matrix protein, which carries surface glycoproteins that form protrusions (peplomers): conical (hemagglutinin H) and dumbbell-shaped (fusion protein F), due to which the virus has hemagglutinating and hemolytic activity. When reproducing, the measles virus causes the formation of multinucleated giant cells-symplasts and eosinophilic inclusions. Multinucleated cells are formed by the fusion of membranes of nearby cells. The formation of daughter measles virus occurs by "budding" on the surface of infected cells.

In a dried state at a temperature of -20 °C, the measles virus does not lose activity for a year. At a temperature of 37 °C, 50% of the virus population is inactivated after 2 hours, at 56 °C the virus dies after 30 minutes, at 60 °C instantly. It is inactivated by a 0.00025% formalin solution, sensitive to ether, an acidic environment (pH <4.5).

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Pathogenesis of measles

The entry point for infection is the mucous membrane of the upper respiratory tract. The measles virus multiplies in epithelial cells, particularly in the epithelium of the respiratory tract. Electron microscopy of material taken from Filatov-Belsky-Koplik spots and skin rashes reveals clusters of the virus. From the last days of incubation for 1-2 days after the rash appears, the virus can be isolated from the blood. The measles virus is carried hematogenously throughout the body, is fixed in the organs of the reticuloendothelial system, where it multiplies and accumulates. At the end of the incubation period, a second, more intense wave of viremia is observed. The pathogen has pronounced epitheliotropism and affects the skin, conjunctiva, mucous membranes of the respiratory tract, oral cavity (Filatov-Belsky-Koplik spots) and intestines. The measles virus can also be found in the mucous membrane of the trachea, bronchi, and sometimes in the urine.

In some cases, the virus can be carried to the brain, causing specific measles encephalitis. In hyperplastic lymphoid tissues, in particular in the lymph nodes, tonsils, spleen, thymus gland, giant reticuloendotheliocytes (Warthin-Finkelday cells) can be found. Destroyed chromosomes are detected in many leukocytes. The epithelium of the respiratory tract can become necrotic, which contributes to the addition of a secondary bacterial infection. From the 3rd day of the rash, viremia decreases sharply, and from the 4th day the virus is usually not detected, from this time virus-neutralizing antibodies begin to be detected in the blood. With measles, a specific allergic restructuring of the body develops, which persists for a long time. In vaccinated people, over time, antibody titers to the measles virus decrease sharply, while allergization persists for a long time, which causes an atypical course of the disease 5-7 years after vaccination. Measles leads to a state of anergy, which is manifested by the disappearance of allergic reactions (to tuberculin, toxoplasmin) in infected individuals, as well as an exacerbation of chronic bacterial infections.