Genetic characterization of wild-type measles viruses circulating in Kenya
Mbugua, Francis Muturi
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Measles is an acute illness caused by measles virus from the genus Morbillivirus, a member of the family paramyxoviridae. The virus has a single stranded negative-sense RNA genome. Measles is one of the most infectious human diseases. The virus can be transmitted in the air, in respiratory droplets, or by direct contact with nasal and throat secretions of infected persons. Complications of measles include otitis media, pneumonia, diarrhoea, blindness and encephalitis. Although a vaccine-preventable disease, the World Health Organization (WHO) estimates that worldwide in 2004, there were 20-30 million cases of measles and 453,000 deaths, one-third of all vaccinepreventable childhood deaths. Molecular genotyping of measles virus strains is based on analyzing the genetic variability of nucleoprotein (N) and haemagglutinin (H) genes. Nomenclature for wild-type measles viruses has been standardized by WHO. WHO has designated two measles strain banks; the measles virus section of the Centres for Disease Control and Prevention (CDC),USA and the Health Protection Agency (HPA), to acquire,analyse,store and dispense representative strains. In 1998, WHO published guidelines for uniform nomenclature for designating wild type measles viruses and describing genotypes. The sequence of the 450 nucleotides that code for the 000Hterminal 150 amino acids of the nucleoprotein (N) is the minimum amount of data required for determining the genotype of a measles virus. If a new genotype is suspected, a viral isolate and the entire 1855 nucleotide haemagglutinin (H) gene sequence should be obtained in addition to the456 nucleotide-carboxyl terminal coding region of the gene. The terms Clade and genotype are used to describe the genetic characteristics of wild-type measles viruses. For molecular epidemiological purposes, the genotype designations are the operational taxonomic unit, while the clades are used to indicate the genetic relationship between the various genotypes. The WHO has recognized 8 clades (A to H) and 23 measles genotypes (A, B1-B3, C1-C2, D1-D10, E, F, Gl-G3 and Hl-H2.The study was based on 300 samples comprising 92(30.7%) nasopharyngeal swabs (NPS) and 208 (69.3%) urine samples. Serum samples numbering 294 (98%) were taken for IgM test by ELISA technique. Measles viruses were recovered from 36 out of 92 NPS (39.1%) NPS and from 97 out of 208 (46.6%) urine samples in Vero/SLAM cells. Serum samples numbering 117 out of 294 (39.8%) were ELISA positive. All the 133 measles virus isolates were RT-PCR positive. The alignment and phylogenetic analysis of the sequences of nucleoprotein (N) and haemagglutinin (H) genes of the isolates were performed using the p-distance neighbour joining algorithm in the MEGA version 3.1 software packages. Measles genotypes D4, B2 and 133 and were detected. Genotype D4 was detected in 2005 and 2005.Genotypes B3 and 132 were detected for the first time in 2005. More measles viruses were recovered from children who had received one dose of measles vaccine. This study has observed the interruption of transmission of D4 measles strain by mass immunization campaign against measles that took place in Kenya in 2002 and subsequent high routine immunization coverage that followed. Measles viruses isolated from Somali refugees who had camped in Eastleigh in Nairobi and Dadaab Camp in North Eastern Province indicated the likelihood of the B3 strain having been imported into the country from Somalia. The data was analyzed by using EpiInfo Programme.