Viral respiratory diseases (ILT, aMPV infections, IB): are they ever under control?

Abstract

1. The use of vaccines is the main approach to control of the economically important poultry viral respiratory diseases infectious laryngotracheitis (ILT), avian metapneumovirus (aMPV) infections and infectious bronchitis (IB). This paper appraises the current methods of vaccine control in the light of the nature of each virus and epidemiological factors associated with each disease. 2. Infectious laryngotracheitis virus (ILTV) exists as a single type with a wide range of disease severity. It is a serious disease in certain regions of the world. Recent work has distinguished molecular differences between vaccine and field strains and vaccine virus can be a cause of disease. Vaccines have remained unaltered for many years but new ones are being developed to counter vaccine side effects and reversion and reactivation of latent virus. 3. Avian metapneumoviruses, the cause of turkey rhinotracheitis and respiratory disease in chickens exists as 4 subtypes, A, B, C and D. A and B are widespread and vaccines work well provided that accurate doses are given. Newer vaccine developments are designed to eliminate reversion and possibly counter the appearance of newer field strains which may break through established vaccine coverage. 4. IB presents the biggest problem of the three. Being an unstable RNA virus, part of the viral genome that codes for the S1 spike gene can undergo mutation and recombination so that important antigenic variants can appear irregularly which may evade existing vaccine protection. While conventional vaccines work well against homologous types, new strategies are needed to counter this instability. Molecular approaches involving tailoring viruses to suit field challenges are in progress. However, the simple use of two genetically different vaccines to protect against a wide range of heterologous types is now a widespread practice that is very effective. 5. None of the three diseases described can claim to be satisfactorily controlled and it remains to be seen whether the newer generations of vaccines will be more efficacious and cost effective. The importance of constant surveillance is emphasised and the testing of novel vaccines cannot be achieved without the use of vaccine-challenge experiments in poultry.

Figure 1.. Electron micrograph of the herpesvirus of infectious laryngotracheitis. The icosahedral virion measuring 100 nm diameter is surrounded by a lipoprotein envelope. [Negative staining].

Figure 2.. The pleomorphic appearance of avian metapneumovirus particles under the electron microscope. The surface is covered by a fringe of even-width spikes (peplomers). The large particle measures approximately 100 nm across. [Negative staining]

Figure 3.. Infectious bronchitis virus particles, each measuring 100–140 nm diameter. The surface of each particle is covered in club-shaped spikes, which are important in cell attachment, virus neutralisation and genotyping. [Negative staining].

Figure 4.. In uncomplicated infections, avian pneumoviruses principally affect the upper respiratory tracts in the chicken and the turkey. Immunofluorescence staining shows the presence of virus (white) in the epithelial cells of a section of turkey turbinate.

Figure 5.. Prevalence of the main infectious bronchitis genotypes in Western Europe (UK, Belgium, France, Holland, Germany and Spain) between 2002 and 2006 detected by RT-PCR and sequencing of the S1 spike gene. 793B variant (including 4/91 and IB88 vaccines) and Massachusetts types predominate. Of the new genotypes, Italy 02 appears to be in decline, while QX is in the ascendancy. Different countries show distinct differences in prevalence of types (from Worthington et al., 2008).

Figure 6.. An example of good protection offered by one genotype of live infectious bronchitis vaccine (D274) against an unrelated genotype (793B). Chicken were vaccinated at one-day-old with D274 vaccine and challenged at 21 d of age with D274, 793B or Massachusetts types. Five days later chicks were killed and protection was assessed according to the ciliary activity in tracheal sections. Protection against another different strain (Mass) was poor (from Dhinakar Raj & Jones, 1996).

Figure 7.. The use of two heterologous vaccines (Massachusetts H120) and 4/91 gives good protection against a strain of IBV unrelated to either (here A1121). Protection against this virus by either vaccine alone is poor. Protection was assessed as in Figure 6 (from Cook et al., 1999).

Figure 8.. A further example of the use of combinations of two live IBV vaccines at day old giving good protection, this time against the recent European variant Italy 02. IBMM+Ark comprises a Massachusetts vaccine given simultaneously with Arkansas vaccine, while the IB primer comprises Massachusetts vaccine in combination with D274. The protocol was again assessed as in Figure 6 (from Jones et al., 2005).