Page 247 - Avian Virology: Current Research and Future Trends

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provide a better immuno-protection when combined with adju- hens without antibody. Chicks were hatched from those that did
vant CpG or white oil adjuvant YF01 (Li et al., 2017). A potential not transmit virus to their embryos, based on tests on at least
recombinant Marek’s disease virus (rMDV) vaccine expressing three embryos per hen. (2) Non-immune, non-virus shedders.
ALV-J antigenic genes, either env or gag+env, has also been shown Hens without antibody were selected on the assumption that they
to be effective in the prevention of ALV-J (Liu, Y. et al., 2016). had not ever been infected and were less likely to become inter-
Both multi-epitope subunit vaccine and DNA vaccine induced mittent shedders. (3) Non-viraemic hens regardless of immune
significant humoral and cellular immune responses in chickens status. These were used to provide replacements; however, up
against ALV-J infection (Xu et al., 2015, 2016). An inactivated to four generations of testing were needed to establish freedom,
ALV-B vaccine has been developed that could induce high anti- although freedom from infection cannot be guaranteed (Zander
body titres that protect from experimental ALV-B infections in et al., 1975).
chickens (Li et al., 2013). Despite these reports of experimental Application of ALV eradication programs in commercial
positive results, adoption of vaccination as a major strategy for flocks has depended on the different types of infection status in
ALV control is less likely. It is also worth noting that congenitally hens, eggs, embryos and chicks as outlined below (Spencer et
infected chicks are immunologically tolerant and, thus, cannot be al., 1977): (1) egg albumen may contain exogenous ALV and
immunized even if a suitable vaccine was available. These chick- gs antigen, and both are usually present together; (2) a strong
ens are the major source of transmission and are the most likely to association exists between ALV or gs antigen in egg albumen and
develop neoplasms. ALV in vaginal swabs; (3) an association exists between ALV in
vaginal swabs or egg albumen and ALV in chicken embryos and
Treatment newly hatched chicks. Consequently, hens with a low probability
In general, all attempts to treat virus-induced neoplasia have of producing infected embryos are hens negative for virus (or
resulted in negative or non-reproducible results. RNAi (RNA gs antigen) by the vaginal swab test, or hens that produce eggs
interference)-based methods of inhibiting ALV replication have with albumen free from virus or gs antigen. Commonly, virus
been demonstrated experimentally (Chen et al., 2007; Meng et al., in vaginal or cloacal swabs and in egg albumen can be detected
2011) although its value in future treatment cannot be predicted. by ELISA. It is unlikely that a single test will detect all potential
The COP9 signalosome subunit 6 (CSN6) has been identified as shedder hens, warranting the need for repeated tests. A problem
a novel ALV integrase binding protein which inhibited integrase that arises in applying the ELISA test to albumen or swabs is the
activity in vitro and may be a negative regulator of ALV replica- need to differentiate positive reactions due to the presence of gs
tion by binding to and inhibiting integrase (Wang et al., 2014). antigen derived from endogenous ALV and the exogenous ALV
RNA-Seq analysis of liver tissue from the ALV-J resistant and infection (Ignjatovic, 1986). Reactions due to the latter are usu-
susceptible chickens identified GADD45β, an anti-tumour gene, ally markedly higher, but the setting up the accurate cut off value
inhibiting ALV-J replication in chickens (Zhang et al., 2016). Dai is difficult. High reactions due to exogenous virus are clearer with
et al. (2016) showed that recombinant chicken interferon-alpha albumen samples than with swabs (Crittenden and Smith, 1984).
inhibits ALV replication in DF-1 cells. The use of monoclonal antibodies and PCR tests that can dif-
ferentiate between endogenous and exogenous infections could
prove useful in the future.
Prevention and control procedures A procedure for eradication of ALV involves (1) selection
of fertile eggs from hens negative in the egg albumen or vaginal
Eradication swab test reviewed in (de Boer, 1987; Payne and Howes, 1991;
Eradication of ALV from primary breeding stocks is the most Payne and Venugopal, 2000; Nair and Fadly, 2013); (2) hatch-
effective means for controlling ALV infection in chickens. Primary ing of chicks in isolation in small groups (25–50) in wire-floored
breeding companies of layer-type and meat-type stock have made cages, avoidance of manual vent sexing (Fadly et al., 1981b), and
significant progress in reducing or eradicating ALV of subgroups of vaccination with a common needle (de Boer et al., 1980) to
A, B, and J from their elite breeding lines (Payne and Nair, 2012). prevent mechanical spread of any residual infection; (3) testing
Programs for eradication of ALV infection depends on breaking of chicks for ALV, and discarding reactors and contact chicks
the vertical transmission of virus from dam to progeny. Breeder (Okazaki et al., 1979; Fadly et al., 1981a,b); and (4) rearing ALV-
hens are tested by various methods for the presence of ALV, and free groups in isolation (Fadly et al., 1981b; Witter and Fadly,
those that test positive are discarded, breaking the transmission 2001). In practice, the selection of hens with a low shedding
cycle. In order to establish an ALV-free flock, it is necessary to rate is a simpler requirement to fulfil than the subsequent chick
hatch, rear, and maintain in isolation a group of chickens free from testing and isolation rearing needed to achieve complete eradica-
congenital infection. To achieve this, embryos must be obtained tion. Consequently, some commercial breeder organizations are
from dams that are not transmitting virus to their progeny. In concentrating only on reduction of infection rate by hen testing.
earlier work on development of ALV-free flocks, several methods Small group hatching, and rearing procedures allowed identifica-
for selecting dams were used. The dams selected to produce the tion and removal of groups containing chickens infected prior
next generation and hoped to be a virus-free generation were (1) to hatching and prevented horizontal transmission of ALV-A in
immune, non-virus shedders. Hens with antibody were selected egg-type chickens and ALV-J in meat-type chickens (Witter and
on the assumption that they were less likely to shed virus than Fadly, 2001).