Dr Guillermo Zavala, DVM, amd colleagues at the Poultry Diagnostic and Research Center in the Department of Population Health at the University of Georgia have investigated the factors interfering with the control of infectious laryngotracheitis (ILT), focussing on vaccine interference, persistence of ILTV in the Field, and relative virulence of recent ILT virus (ILTV) isolates. The research was supported by the US Poultry & Egg Association.

The objectives of this research were:

1. possible interference of Newcastle disease virus (NDV) and/or infectious bronchitis virus (IBV) vaccines with protection against ILT
2. virulence of recent field ILTV isolates relative to a reference ILTV isolate (USDA standard challenge strain), and
3. permanence and circulation of ILTV in areas where ILT vaccines were used recently or where an ILT outbreak occurred recently; and in vaccination zone buffer areas.

Regarding the first objective, NDV and IBV interfered with live attenuated ILT TCO (tissue culture origin) and ILT CEO (chicken embryo origin) vaccine replication in the trachea. However, protection against ILT was not compromised after multivalent vaccination against ND and/or IB, and ILT when the ILT vaccine involved was of chicken embryo origin (CEO), but did to a moderate extent when the ILT vaccine involved was TCO. Antibody levels against IBV and NDV were less in chickens receiving CEO or TCO ILT vaccines along with NDV and/or IBV vaccines.

With respect to the second objective, the USDA standard challenge strain of ILTV appeared to be less virulent than a field strain of ILTV circulating in North Georgia (63140). When given at a low dose, the field strain of ILTV 63140 persisted longer in the trachea of infected chickens than the USDA standard challenge strain. Also, the incubation period for ILTV 63140 was only two to three days – much shorter than that reported for ILTV in previous experiments and in the scientific literature.

For objective 3, antibodies against ILTV were detected only in poultry operations situated within the so-called 'vaccination zone' or 'ILT zone' in a serologic survey involving the entire state of Georgia.

These results are important for the poultry industry since multi-valent vaccinations (NDV and IBV) along with simultaneous ILT vaccinations are routinely practised in areas experiencing outbreaks of ILT. This research suggests that one of the factors possibly compromising control of ILT may be sub-optimal immunisation against ILT resulting from multivalent vaccinations. Reducing the number and diversity of live virus vaccines given at the same time as ILT vaccines may optimize protection against ILTV and possibly against the other viral respiratory diseases.

Also, it was demonstrated that the field strain ILTV (GA 63140) may induce earlier and more severe clinical signs than standard challenge ILTV strains. The greater virulence of this field strain may explain the severity of the most recent outbreak in Georgia.

In addition, this research showed that when field ILTV infects the trachea at a low dose – in contrast with higher doses – the virus may persist longer in the tissues of infected chickens before being neutralised. This finding is relevant because some in the broiler industry frequently lower ILT vaccine doses on the premise that vaccine reactions may be lessened. It is known that a high titre of ILTV vaccine is required to get a prompt neutralising immune response. Thus, over dilution or 'cutting' of vaccine would seem to be counter-productive.

Finally, contrary to previous suggestions, ILT serology may be used as a screening tool for epidemiological studies since the present research suggested that ILTV antibodies may be found in significant levels only in geographical areas involved in an outbreak of ILT or in flocks vaccinated with live, attenuated ILT vaccines.

Further Reading

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Find out more information on infectious laryngotracheitis (ILT) by clicking here.

March 2010