Infectious laryngotracheitis in poultry

Introduction

The ILT disease was officially reported in Peru in August 2008, after the emergence of several outbreaks which began in the second quarter of that year and was characterized by a high morbidity and mortality. The disease was reported for the first time in poultry farms in the Department of Lima. Later on, the disease was described in other areas, including the departments of Arequipa, Ancash, Ica, La Libertad and Tacna (OIE WAHID Interface, 2010).

The purpose of this study is the to quantify the ILT impact in terms of production and profitability in poultry companies in Peru, before and after the use of a vectored vaccine against infectious laryngotracheitis using the smallpox virus as a vector.

Materials & Methods

Survey
A survey aimed at the technical directors of nine poultry companies in Peru was conducted. The required information was divided into three parts. The first part included the productive parameters of the batches that were not affected by the infectious laryngotracheitis virus with normal historical results. The second part of the survey incorporated historical production data of the batches that faced a field challenge posed by ILT virus and presented clinical symptoms. Finally, the third part incorporated production and sanitary data of batches immunized with the vectored vaccine, located in high ILT challenge areas.

Results and Discussion

Locations
During this study 9 companies were contacted, which then collected production data on their operations. The companies surveyed with their lines of production and the genetic lines used are summarized in Table 1.

Table 1. Production Line of the companies surveyed.

Company	Location	Production Line	Genetic Lines
1	Chincha	Comm. Layers	ISA HISEX Brown and white, Bovans black
2	Chincha	Comm. Layers	HISEX Brown, Hy Line Brown, Lohmann Brown
3	Trujillo	Comm. Layers	Hy Line Brown
4	Chincha	Comm. Layers	Lohmann Brown
5	Huaral	Broilers	Cobb 500
6	Lima	Broilers	Cobb 500
7	Lima	Broilers	Cobb 500
8	Chincha	Broilers	Cobb 500
9	Lima	Breeders	Cobb 500

Reported ILT Outbreaks

The diagnosis of ILT outbreaks was confirmed through clinical signs and lesions, ELISA, PCR or both (Table 2).

Table 2. Diagnosis methods for ILT and date of first ILT outbreak.

Company	Diagnosis Method	Date of Outbreak
1	PCR	July 2008
2	Clinical signs and lesions	July 2008
3	ELISA	December 2008
4	Clinical signs and lesions	May 2008
5	PCR - ELISA	October 2008
6	PCR	No Outbreak
7	PCR, ELISA	No Outbreak
8	PCR	August 2008
9	Clinical signs and lesions	No Outbreak

Production parameters in batches of birds affected during ILT outbreaks

The information collected made it possible to quantify the impact of ILT outbreaks in production, compared to historical and normal data of the surveyed companies. In broilers, the mortality during the ILT outbreak was 8.2% to 12.51% higher than the normal parameters, while the average daily weight gain (ADWG) decreased 6.65 to 8.28 g. The data collected on layers farms affected by ILT showed that during the disease mortality increased 5.46 to 16.32% in egg laying and 35.22% in a breeder farm. Furthermore, the number of eggs per hosted bird (ENHH) and the mass of eggs per hosted hen (EMHH) suffered a decrease in the range of 10.7 to 74.4 eggs and 0.7 to 4.65 Kg, respectively, compared to the performance expected, according to each genetic line's reference tables.

Economic losses caused by ILT

The companies surveyed reported a worsening in the physical conditions of birds during the onset of the ILT symptoms. At the end of the production cycle, producers of broiler chickens affected with ILT reported losses in the range of 21.74% to 25.68% of the expected sale value. In commercial laying hens, these losses ranged from 6.16% to 38% of the expected sale value, taking into account the increase in mortality, both in breeding and egg laying. In contrast, the implementation of the vectored vaccine represented only 1.05% of the expected value of broiler chickens, whereas in layers this figure ranged from 0.18% to 0.36%.

Persistence of the ILT field virus in the studied areas

Different vectored vaccine immunization programs were used in the poultry companies surveyed (Table 3). In order to confirm ILT field challenges on batches of vectored vaccine immunized birds in company no. 5, 20 blood samples were taken from broiler chickens at the age of 40 days for ELISA in a local laboratory. In addition to this, two trachea swabs and two conjunctive samples were prepared on FTA cards. Later on, 4 more trachea swabs of 46 day old birds were prepared on FTA cards. All the trachea and conjunctiva samples were sent to the University of Georgia for real-time PCR (qPCR) to be performed.

Table 3. Immunization programs with vectored vaccines.

Company	Production Line	Age at Vaccination
1	Comm. Layers	Day 28 via SC
2	Comm. Layers	Week 3 application in wing crease
3	Comm. Layers	Week 6 application in wing crease
4	Comm. Layers	Week 3 application in wing crease
5	Broiler Chickens	First day via SC
6	Broiler Chickens	First day via SC
7	Broiler Chickens	First day via SC
8	Broiler Chickens	First day via SC
9	Breeders	Week 7 application in wing crease

For the ELISA test a commercial kit (Synbiótics^®, Kansas, MO) was used. The result of this test was negative to the identification of ILT antibodies. qPCR analysis showed positive results for the infectious laryngotracheitis virus in 7 of the 8 analyzed samples, indicating a recent field challenge by the ILT virus, still without sero-conversion in the birds of the affected batches.

This study shows the severe economic impact of the ILT virus when present in its acute form, as well as the proper way to prevent this infection by using effective and safe vaccines. The data obtained showed that all production parameters in chickens and laying hens were severely affected during ILT outbreaks. It should also be noted that during the first clinical signs in ILT outbreaks, antibiotics, expectorants and disinfectants were used with poor results in improving the health status of the flocks, which also increased the costs of these operations (data not presented).

From the results shown in this study it is clear that the field Challenger posed by ILT may not always be diagnosed accurately by the ELISA test. This hypothesis is supported by the fact that no ILT antibodies were identified in the company no. 5 samples during the March 2010 outbreak. In addition to this, only 37.5% of the samples had low titles in the company no.7 samples (birds vaccinated against ILT - data not presented) when the ILT virus field challenge was identified by PCR. Furthermore, several authors have mentioned the lack of correlation between the ELISA titles and protection against the disease (Bauer et al., 1999;) Fuchs et al., 2007). By contrast, qPCR proved to be a very sensitive test to detect the viral DNA of ILT, even in samples with a low number of genome copies, which is in agreement with the publication by Callison et al. (2007). These observations are supported by Crespo et al. (2007), who consider that serology is not useful in ILT diagnosis and consider qPCR as the most sensitive test for viral ILT DNA identification.

There was ILT virus field challenges in flocks previously vaccinated with the vectored vaccine. The reason for this to happen may be the high density of poultry farms and backyard poultry production in these areas. Small poultry operations in Peru not always vaccinate to prevent ILT and generally do not apply protocols to properly remove organic materials such as organic bed, mortality and trash. All these aspects represent a risk factor for poultry operations that may be challenged by diseases such as ILT. Taking into account the ability of the ILT virus to survive in organic matter (Bagust & Johnson, 1995), priority must be given to the treatment and proper disposal of carcasses and organic matter.

The use of the vectored vaccine had a clear positive impact and benefit on ILT control through adequate protection against the ILT virus, as it has been previously demonstrated by Davison et al. (2006). Furthermore, the use of the vectored vaccine avoids the risk of viral transmission to exposed birds as it is the case with the TCO or CEO vaccines, which over time may turn into virulent strains (Rodriguez-Avila et al., 2008).

Conclusions

Vaccination with the vectored vaccine is an effective and practical way to meet the field challenge posed by the ILT virus in areas of highly dense poultry activity, especially when biosecurity measures and organic waste management protocols are as heterogeneous as those found in Peru.

Bibliography

Bagust TJ & Johnson MA. 1995. Avian infectious laryngotracheitis: virus-host interactions in relation to prospects for eradication. Avian Pathol. 24: 373-391.

Bauer B, Lohr JE, Kaleta EF. 1999. Comparison of commercial ELISA test kits from Australia and the USA with the serum neutralization test in cell cultures for the detection of antibodies to the infectious laryngotracheitis virus of chickens. Avian Pathol. 28: 65-72.

Callison SA, Riblet SM, Oldoni I, Sun S, Zavala G, Williams S, Resurreccion RS, Spackman E, Garcia M. 2007. Development and validation of a real-time Taqman PCR assay for the detection and quantitation of infectious laryngotracheitis virus in poultry. J. Virol. Methods 139: 31-38.

Crespo R, Woolcock PR, Chin RP, Shivaprasad HL, Garcia M. 2007. Comparison of diagnostics techniques in an outbreak of infectious laryngotracheitis from meat chickens. Avian Dis. 51: 858-862.

Davison S, Gingerich EN, Casavant S, Eckroade RJ. 2006. Evaluation of the efficacy of a live fowlpox-vectored infectious laryngotracheitis/avian encephalomyelitis vaccine against LTI viral challenge. Avian Dis. 50: 50-54.

Fuchs W, Veits J, Helferich D, Granzow H, Teifke JP, Mettenleiter TC. 2007. Molecular biology of avian infectious laryngotracheitis virus. Vet. Res. 38: 261-279.

OIE, World Animal Health Information Database (WAHID) Interface. 2010. http://web.oie.int/wahis/
public.php?page=disease_timelines

Rodriguez-Avila A, Oldoni I, Riblet S, Garcia M. 2008. Evaluation of the protection elicited by direct and indirect exposure to live attenuated infectious laryngotracheitis virus vaccines against a recent challenge strain from the United States. Avian Pathol. 37: 287-292.