US Poultry Vets Explore Control of Infectious Laryngotracheitis

Six papers were included in the session on Infectious Laryngotracheitis (ILT) at the 150th convention of the American Veterinary Medical Association (AVMA), held in Chicago in July 2013. The main focus was on improving disease control and characterising the causal virus.
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The National Poultry Improvement Plan Update

The National Poultry Improvement Plan (NPIP) held its 41st Biennial Conference for 2012 in New Orleans, Louisiana, reported the Plan's Senior Coordinator, Denise Brinson. The success of NPIP has been largely due to its ability to be modified relatively quickly in order to meet the changing needs of the poultry industry.

Twenty-eight proposed changes to the NPIP were presented at the conference for consideration, she explained. These proposals ranged from minor changes in testing methods to a major change in the way the NPIP regulations will be written in the future.

The results of delegate voting on these changes were discussed as well as the future of the NPIP in light of government cost-cutting, restructuring and federal advisory committee reorganisation.

Protective Efficacy of Complete and Fractionated Doses of Recombinant Herpesvirus-Vectored Vaccines against Infectious Laryngotracheitis in Commercial Broilers

The objective of this study at the University of Georgia was to evaluate the immunity against ILT provided by complete and fractionated doses of a recombinant HVT-vectored infectious laryngotracheitis (HVT-LT) vaccine in broilers, explained Yun-Ting Wang.

A total of 100 day-old commercial broilers were divided into four groups, three of which were subcutaneously vaccinated with either a full commercial dose (F) of HVT-LT vaccine; half dose (H); or one-fourth of a dose (Q). The non-vaccinated group was subdivided in challenged and non-challenged groups.

Challenge was done at 25 days old with a virulent ILTV isolate. Protection was assessed five days post-challenge by quantification of tracheal viral loads using qPCR and by clinical sign scoring. In-vivo replication of the recombinant vaccine was evaluated by qPCR at four, seven, 14 and 30 days post-vaccination in the spleen; and at 30 days of age in feather pulp.

Vaccine virus DNA was not detected in the spleens from any of the groups at four days post-vaccination. However, by 14 days post-vaccination, vaccine virus DNA was detected in 100 per cent, 80 per cent and 60 per cent of the spleens for F, H and Q groups, respectively.

Although no statistical differences were found in HVT load in feather pulp and spleen between the F and H groups at five days post-challenge, there were differences in the clinical sign scores and ILTV shedding between all experimental groups.

Dr Wang added that all tracheas (100 per cent) in the H and Q groups had detectable ILTV at five days post-challenge, whereas ILTV DNA was detected in only 70 per cent of the tracheas in the F group.

Assessment of In Ovo Vaccination of a FP-LT Vector Vaccine Followed by a Live LTV CEO Vaccination on Persistence of Clinical Signs, Virus Detection and Live Performance

In her paper, Kelli H. Jones of Mississippi State University reported that ILT vector vaccine (Vectormune FP-LT) and Chicken Embryo Origin (CEO) ILT vaccine were administered to commercial broilers. The vector vaccine was administered in-ovo at 18.5 days of incubation, and the CEO live vaccine was administered via spray at two weeks of age.

The objective of the study was to determine if using the FP-LT vector vaccine in front of the live ILT CEO vaccine would reduce the negative impacts often associated with live ILT vaccine reaction due to the spray administration.

Clinical signs were observed, and serology, histopathology, virus isolation and ILT virus identification by PCR method was collected on weekly basis, from five to nine weeks of age. The study was performed on six farms that were randomly selected (three farms per treatment). Live performance was evaluated on two groups of 17 farms vaccinated with FP-LT, and FP-LT followed by CEO vaccination treatments.

Experimental Long Term Immunity Against ILTV Induced by Inactivated, Recombinant and Live Modified Virus Vaccines in Commercial Layers

ILT is an infectious respiratory disease of chickens causing important economic losses worldwide and is preventable through vaccination with live-modified vaccines (CEO and TCO), according to Victor Palomino of the University of Georgia.

Recently, novel recombinant vaccines have been developed (HVT-LT and FPV-LT) for protection against ILT. The objective of this research was to study the immunity against ILT induced by various vaccines: a) recombinant; b) live-modified attenuated and c) inactivated virus vaccines.

Commercial layer pullets were vaccinated using one or more vaccines and challenged at four, nine, 35 or 74 weeks of age. Protection was assessed by scoring clinical signs; and by determining the challenge viral loads in tracheal swabs at five days post challenge.

The FPV-LT vaccinated birds showed no significant protection when challenged at four, nine or 35 weeks; the HVT-LT and TCO vaccines in combination provided protection similar to the groups of chickens vaccinated with HVT-LT or TCO at four and 35 weeks, whereas protection induced by HVT-LT and TCO co-administered was higher at 74 weeks.

The FPV-LT+TCO group showed less protection than the HVT-LT and TCO vaccinated group at nine weeks but was similar in the birds challenged at 35 weeks. (A 74-week challenge was not done).

Birds given the inactivated ILT vaccine had fewer clinical signs and/or viral shedding at 74 weeks of age when combined with TCO or HVT-LT, but not when given alone.

Finally, the CEO vaccinated birds had the greatest reduction of clinical signs and viral shedding when challenged at nine and 35 weeks of age, added Dr Palomino.

Adaptation and Attenuation of a Field Strain of ILTV in Primary Liver Cells and in Chicken Embryo Fibroblasts

In another paper, Dr Palomino explained that the objective of another study was to attenuate a virulent ILTV field strain (63140) isolated from an outbreak of ILT in broiler chickens by serial passages in cell cultures. The virulent ILTV was passaged eight times (P8) in chicken kidney cells (CK) before inoculation into various primary cell cultures and continuous cell lines, including Vero, LMH, chicken embryo fibroblasts (CEF), chicken embryo liver cells (CEL) and chicken embryo kidney cells (CEK). CEL and CEK were readily permissive to ILTV.

Stocks were prepared after each 10 passages in CEL and CEK until at least passage 90. Each virus stock was titrated in CK cells. One virus stock that was originally propagated twice in LMH cells was labelled as CEL(L) and then passaged 60 more times in CEL. This stock was selected for attenuation by serial passage due to its higher virus yield in cell cultures.

At P48, CEL(L) began inducing CPE in residual fibroblasts in the CEL cultures and thus propagation of CEL(L)P52 was attempted and demonstrated in CEF by immunofluorescence and electron microscopy. CEL(L)P52 was plaque-purified and further passaged in CEF, and its attenuation was examined in vivo by inoculation in broiler chickens.

Groups of broilers were inoculated with CEL(L)10 through 90 or with CEF-adapted ILTV pasaged six times or more. Positive (parental ILTV 63140) and negative control (not inoculated) groups were included.

Dr Palomino reported significant attenuation was observed with CEL(L) virus stocks passaged 60 times or more; and with CEF-adapted virus passaged six times or more in CEF.

Rapid Sequencing of a Field Isolate of Infectious Laryngotracheitis Virus

Calvin L. Keeler, Jr. of the University of Delaware explained that ILT virus is the causative agent of ILT, an acute poultry respiratory disease.

ILT is generally characterised by signs of acute respiratory distress in birds, including gasping, rales, and the expectoration of bloody exudate.

Field isolates exhibiting varying degrees of pathogenicity are routinely identified. A 2011 field isolate was selected for both molecular and biological characterisation based on its pathogenicity.

Recent technological advances in DNA sequencing have significantly reduced the time and costs associated with determining the sequence of an ILTV genome. These high-throughput sequencing approaches have been used to generate high quality full-length genomic ILTV sequence (Spatz et al., 2012).

He and his colleagues have extended the use of this approach to generate rapidly full-length genomic ILT virus sequence directly from egg-generated material. This eliminates the need to purify and grow the virus in tissue culture before sequencing.

They reported on the use of this relatively rapid and cost-effective method to characterise ILT virus isolates.

Further Reading

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

October 2013

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