Submitted to: Avian Diseases
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/28/2010
Publication Date: 9/1/2010
Citation: Dunn, J.R., Witter, R.L., Silva, R.F., Lee, L.F., Finlay, J., Marker, B.A., Kaneene, J.B., Fulton, R.M., Fitzgerald, S.D. 2010. The Effect of the Time Interval Between Exposures on the Susceptibility of Chickens to Superinfection with Marek Disease Virus. Avian Diseases. 54(3):1038-1049. Interpretive Summary: The evolution (changing of the virus to become more deadly) of Marek's disease virus (MDV) is a major concern to the poultry industry. The ability for more than one type of MDV to infect the same bird is likely for a new more evolved virus to become established within a bird and flock. Until now, demonstrating more than one virus in the same bird has been difficult. This study used new procedures to demonstrate more than one virus in the same bird. When birds were infected with two viruses following a short interval (24 hours) both viruses were seen in many birds. When birds were infected with two viruses following a long interval (13 days), the second virus was rarely seen. Protecting animals from disease by using a vaccine had no effect on this relationship, suggesting these findings may be relevant on commercial poultry farms. These studies demonstrated more than one MDV in the same bird for the first time and suggest that the short time between infections is important for both viruses to be seen which may lead to evolution of the virus. The information is essential for any strategy to improve control of this economically important disease of chickens.
Technical Abstract: Marek’s disease virus (MDV) is ubiquitous within commercial poultry flocks since current vaccines do not prevent MDV infection or transmission. In order for newly evolved MDV strains to become established within a flock, it seems inevitable that any new strain would need to infect and replicate in chickens previously infected with resident MDV strains. This phenomenon is difficult to detect and there is no clear evidence that it is even possible. Four experiments were performed to demonstrate superinfection and evaluate the effect of time between challenges on the effect of superinfection using two pairs of fully virulent MDV strains that could be discriminated by novel technology: 1) JM/102W and rMd5//38CVI , and 2) rMd5 and rMd5//38CVI. Feather follicle epithelium (FFE), spleen, and tumor samples were collected at single or multiple time points from the same bird to determine the frequency and distribution of each virus present following superinfection using pyrosequencing and immunohistochemistry. Superinfection was observed in 82 of 149 (55%) FFE samples following short interval challenge (24 hours) compared to only 6 of 121 (5%) samples following long interval challenge (13 days), indicating a strong influence of challenge interval. In cases where the first inoculated virus was weak or delayed, the second inoculated virus was detected in 42 of 95 (44%) birds. In tumors from dually challenged birds, the second virus was again present much higher following short interval challenge (68%) compared to long interval challenge (11%). Virus mixtures were less common compared to FFE samples. Vaccination with HVT had no significant effect on the virus frequency for either virus pair or challenge time interval, suggesting these conclusions may be applicable to vaccinated chickens in the field. These studies demonstrated superinfection for the first time with two fully virulent MDV strains and suggest that short interval challenge exposure and/or weak initial exposures may be important factors leading to superinfection, a prerequisite for the establishment of a second virus strain in the population. This model system should be useful to further elucidate this important phenomenon.