Relationship between mortality, clinical signs and tracheal pathology in infectious laryngotracheitis

Previous studies in our laboratory using a combination of polymerase chain reaction and restriction fragment length polymorphism have identified at least five different genotypes of infectious laryngotracheitis virus (ILTV). However, the virulence of these classes of ILTV was not investigated. In this study, five groups (16 birds each) of 3-week-old specific pathogen free chickens were inoculated via the intratracheal route with 10(3) median embryo infected dose of five different strains of ILTV. Three further groups of chickens were inoculated similarly with the vaccine strains SA2 and A20 or with sterile phosphate-buffered saline (PBS) for comparison. Four days post-inoculation, clinical signs were monitored for scoring, and eight chickens from each group were subsequently euthanized, weighed and subjected to pathological and histopathological examinations. The remaining birds were monitored for clinical signs and mortality until 21 days post-inoculation. All groups inoculated with ILTV strains showed moderate to severe clinical signs 4 days after inoculation. The strain Q1-96 caused only minimal breathing symptoms with a median score that was not significantly different to that of the group inoculated with PBS, but was significantly different to those of the groups inoculated with other ILTV strains. The strain Q1-96 caused severe photophobia and conjunctivitis with a median score that was significantly higher than those of all other groups except for the group inoculated with the strain N3-04. All ILTV strains caused a significant reduction in weight gain when compared with the group inoculated with PBS. The strain Q1-96 caused an average weigh loss of 14% that was significantly higher than those of other ILTV strains. The strains S2-04 and Q1-96 induced only minor microscopic tracheal lesions while all the other ILTV strains, including the vaccine strains A20 and SA2, induced moderate to severe microscopic tracheal lesions. Median scores for microscopic tracheal lesions were well correlated with the number of viral genomes detected in trachea. The results revealed that there is considerable variation among ILTV strains in their tropism for trachea or conjunctiva. In addition it was revealed that ILTV strains with high affinity for conjunctiva can severely affect weigh gain. The ILTV numbers and microscopic lesions in trachea were not found to be reliable indicators of virulence since they are not necessarily correlated with mortality rate in ILT.