Propagation of an Avirulent Turkey Hemorrhagic Enteritis Virus Isolate in Chickens

Turkey Hemorrhagic Enteritis (THE): A Short Overview

Turkey Hemorrhagic Enteritis (THE) is an acute disease caused by a Siadenovirus that affects 4 week-aged and older turkeys, characterized by acute depression, bloody droppings, and a high mortality rate. The immunosuppressive attributes of THE can protract disease progression and create a predisposition in birds towards subsequent bacterial infectiodoralns involving Escherichia coli and Clostridium perfringens (necrotic enteritis). Turkey Hemorrhagic Enteritis Virus (THEV) predominantly affects turkeys and carries substantial economic implications for this industry. Macrophages and B lymphocytes are recognized as the predominant target cells for the virus, while the spleen is the principal site of viral replication. Infected cells have also been observed in various other tissues, including the intestines, bursa of Fabricius, cecal tonsils, thymus, liver, kidney, peripheral blood leukocytes, and lungs. The economic relevance of this disease is derived both from the high mortality rate, which can reach 60% depending on the virulence of the strain, and from subclinical disease responsible for poor performance in vaccinated animals. This review aims to provide a comprehensive overview of THE, spanning etiology, epidemiology clinical signs and gross lesions, prevention, and management.

Preliminary testing in turkeys of the safety and efficacy of a putative haemorrhagic enteritis virus vaccine

Haemorrhagic enteritis virus (HEV) causes clinical haemorrhagic enteritis in young poults and/or subclinical immunosuppression which is often associated with colibacillosis. This disease is controlled with live vaccines worldwide, however, importation of HEV vaccines or cells that support HEV propagation are not permitted in Australia. A major experiment in isolators was conducted to test the safety and efficacy of a putative HEV vaccine. The study had a factorial design with four factors namely vaccination age (28 and 42 days of age), vaccine dose (0, 10⁵ , 10⁶ , 10⁷ genomic copies of HEV vaccine), challenge with HEV (yes, no) and vaccination-challenge interval (7, 21 or 42 days). A total of 315 poults were used providing 6-8 birds per treatment combination. Turkey growth rate, mortality, pathological findings, anti-HEV antibodies and viral load were examined. Vaccination lead to significant increases in anti HEV antibody over the following 2-4 weeks. Overall, vaccination with 10⁶ and 10⁷ was protective against increase in relative splenic weight and splenic viral load in challenged birds. Clinical haemorrhagic enteritis was not induced by any treatment but there was an increased incidence of airsacculitis in groups receiving either HEV vaccine or challenge virus compared to the negative control birds (25.8-29.3% vs. 9.4%, P < 0.05). Growth rate, mortality and relative bursal weight were unaffected by vaccination. This laboratory level study indicates that the putative vaccine is safe and likely to be efficacious, but may cause elevated levels of airsacculitis. These findings require confirmation in larger scale field trials.

Turkey Hemorrhagic Enteritis Virus Can Be Titrated but Not Propagated in Chicken Embryos

This study aimed to investigate the feasibility of propagating and titrating hemorrhagic enteritis virus (HEV) in chicken embryos. A total of 308 embryonated eggs were used. At 10 days of embryonic age, eggs were inoculated via allantoic sac or chorioallantoic membrane routes with non-heat-treated (live) HEV or heat-treated (dead) HEV or served as negative controls. Allantoic fluid retrieved at 0, 1, 3, 5, and 7 days postinoculation (dpi) was tested for HEV by quantitative PCR. Inoculation with HEV did not cause visible growth impairment or lesions in the chicken embryos. Overall, there was no difference in postinoculation mortality rates among groups sham-inoculated (6/30, 20.0%) or inoculated with live (34/252, 13.4%) or dead (3/ 26, 6.9%) HEV (P = 0.58). The amount of HEV DNA detected in allantoic fluid at 7 dpi in eggs inoculated with live virus was similar to the inoculated dose, indicating that virus propagation in chicken embryos is not efficient. No HEV DNA was detected after 3 dpi in eggs inoculated with dead virus. Inoculation of chicken embryos combined with qualitative PCR can be used for titration of HEV virus stocks and presents a high correlation with in vivo titration using chickens (R² 0.98, P = 0.007). This method may be relevant in countries in which specific-pathogen-free turkeys are unavailable and in which the importation of RP19 cells, the only cell that supports effective propagation of HEV, is not permitted.