Systematic Veterinary Virology

PATHOTYPING

• Newcastle disease virus are classified into Velogenic, mesogenic and lentogenic based on following three tests:
• Intracerebral pathogenicity index (ICPI) in day old chickens. This involves the inoculation of virus derived from fresh infective allantoic fluid into the brain of ten day-old chicks from specific pathogen-free parents. Each bird is examined at 24-hour intervals for eight days and graded zero if normal, one if sick and two if dead. The index is the mean score per bird per observation over the 8-day period. The most virulent viruses give ICPI values approaching the maximum score of 2.0, while lentogenic viruses give values of, or close to, 0.0
• Intravenous pathogenicty index (IVPI) in 6 weeks old specific pathogen free (SPF) chickens. 0.1 ml of the diluted virus is injected intravenously into each of ten 6-week-old SPF chickens. Birds are examined at 24-hour intervals for 10 days and scored at each observation - 0 if normal, 1 if sick, 2 if paralysed or showing other nervous signs, and 3 if dead. The intravenous pathogenicity index (IVPI) is the mean score per bird per observation over the 10-day period. Lentogenic strains and some mesogenic strains will have IVPI values of 0, whereas the indices for virulent strains will approach 3.0.
• Mean death time (MDT) in embryonated eggs. The mean death time (MDT) in eggs means time in hours for the minimum lethal dose to kill all the inoculated embryos. The MDT has been used to classify ND virus strains into velogenic (taking under 60 hours to kill); mesogenic (taking 60 to 90 hours to kill); and lentogenic (taking more than 90 hours to kill).
• Molecular typing by studying the aminoacid between 112-119^th position in F gene.
• Panels of MAbs are also used to establish antigenic profiles of ND virus isolates based on whether or not they react with the viruses.