Controlling The Spread of ILT

Infectious laryngotracheitis (ILT) is a disease of economic significance for the poultry industry because it induces high mortality, delayed growth, increased feed conversion, occasionally increased condemnations, and drops in egg production and egg size because of temporary reduction in feed intake.  ILT is also a major concern because it is suspected to synergize the impact of pathogens that are normally of relatively little impact such as Mycoplasma synoviae.  ILT can also be the reason for banning poultry imports from affected countries. Imposing control measures against any disease, including ILT is costly but necessary. 

The disease.  ILT is caused by a virus classified as Gallid Herpes Virus I.  A particular characteristic of herpes viruses in general is that once they cause infection they never leave the host and can be reactivated at any moment, regardless of the immune status of the infected individual or flock.  The ILT virus (ILTV) infects chickens via the upper respiratory tract from a contaminated environment, infected pen mates, or people with contaminated footwear, clothes or tools or vehicles.  Because the virus is able to survive on inanimate objects for some hours or days, it is relatively easy to spread the virus unknowingly by a variety of mechanisms.  It is important to know that in the field the virus may have an incubation period of several days, typically 4-7, during which clinical signs of disease or mortality may not be noticed.  That is, birds can get infected and remain several days without expressing any clinical signs or mortality before the disease becomes evident.  Any personnel, vehicles and equipment that come in contact with infected birds can easily transport the virus mechanically to other farms, thereby spreading the infection.  Live attenuated vaccines can participate in a similar way by circulating in vaccinated chickens and by vaccine viruses being transported from vaccinates to non-vaccinates within the same flock or between flocks, thereby providing the opportunity for vaccine viruses to “back passage” and regain virulence if allowed to replicate uncontrollably.  An example could be a meat type chicken company that typically vaccinates its breeders with live attenuated vaccines but not the broilers because it does not have ILT in the broilers.  If by accident the vaccine virus used to immunize the breeders is mechanically transported to unvaccinated broilers, the latter will support back passaging of the vaccine viruses until they gain enough virulence to cause a full blown case of ILT (or vaccinal laryngotracheitis in the case, also known as VLT).  Biologically, as stated above, ILTV infects chickens via the upper respiratory tract, where it replicates actively for a period of several days using the epithelial lining of the nasal cavity, the conjunctiva, the larynx and the trachea.  ILTV does not typically infect the lower respiratory tract (lungs and air sacs) to any significant level and thus it does not typically result in increased condemnations, but it does cause severe respiratory disease characterized by overt dyspnea, loud respiratory noise, production of hemorrhagic exudate, severe conjunctivitis, and certainly high mortality.  A period of 4-7 days elapses from the moment of infection until the peak of clinical signs and mortality, but this period varies depending on many factors.  Clinical signs and mortality tend to disappear 10-14 days after the original infection, but the virus remains viable in the chicken barns for at least 3 weeks post-infection. ILTV has been detected in the chicken litter of affected barns days after all chickens have been removed from a contaminated farm. 

ILT control.  For decades, the control of ILT depended on biosecurity and vaccination using live attenuated vaccines. Additional critical control measures also contributed to the control of ILT, such as regulated movement of contaminated poultry litter, industry coordinated efforts, and organized vaccination efforts.  Some areas permitted the use of tissue culture origin (TCO) vaccines only and others allowed the use of both TCO and chicken embryo origin (CEO) vaccines.  The former are much safer vaccines, generally do not spread easily, and do not commonly revert to the virulent form, but have the enormous disadvantage of having to be administered individually by eye drop, a practice that the broiler industry simply will not follow anymore.  CEO vaccines are the most effective vaccines in existence against ILT, but they have very significant disadvantages that include residual virulence, horizontal spread, easy reversion to virulence, and their characteristic association with increased feed conversion and delayed growth in vaccinated broiler chickens.  A particular advantage of CEO vaccines is that they can be administered by either individual methods, or by mass application procedures such as spray or drinking water.  Even in the best of circumstances there is always a proportion of broiler flocks that will experience increased feed conversion and/or delayed growth and/or respiratory disease after the mass application of CEO vaccines.  Despite the disadvantages of live attenuated vaccines, industry used them successfully for many years in the control of ILT in the field.  In Northern latitudes, ILT typically appeared towards the end of the Winter or the beginning of the Spring, and after the implementation of control methodologies including CEO vaccination, ILT would be controlled almost invariably by the arrival of the hotter months.  Ever since the advent of recombinant vectored vaccines, the broiler industry discovered that it was no longer necessary to use CEO vaccines to control ILT, and thus it is not necessary any longer to suffer the consequences of increased feed conversion, delayed growth and increased mortality associated with widespread vaccination with CEO vaccines.  However, an interesting byproduct of this relatively recent change is that nowadays outbreaks of ILT or VLT tend to last more than just a season; indeed, some times years.  Part of the reason for this to occur is that currently available recombinant vaccines do not induce mortality and do not cause an increase in feed conversion or in mortality, but unfortunately they do allow field viruses to replicate in the trachea and conjunctiva, and in consequence, vaccinated-challenged chickens do get infected with field viruses, which replicate at similar levels as in completely naïve or unvaccinated unprotected flocks.  Paradoxically, when chickens vaccinated with recombinant vaccines are challenged with ILTV in the field they do express lower mortality and delayed growth than unvaccinated chickens, but they still experience very significant ILTV replication in the upper respiratory tract.  This is a critical epidemiological observation, because most outbreaks in broilers tend to occur at an age when broilers are typically sent to market.  That is, flocks vaccinated with recombinant vaccines that are challenged in the field often times will be shedding large quantities of virus particles at the time of transportation to the processing plants, thereby exposing other farms or flocks at risk of infection.  This in itself is a very important factor responsible for the perpetuation of ILT or VLT in the field. Despite this disadvantage, recombinant vaccines do have an important place in industry if used judiciously and realistically according to their actual potential for controlling disease. 

No vaccine, live attenuated or recombinant, or gene deleted, or gene modified is likely to ever be completely sufficient by itself to control ILT effectively.  Other very important procedures are as critical or more critical than vaccination alone.  Vaccination without biosecurity often fails; biosecurity without vaccination often succeeds in preventing or controlling ILT or VLT.  Although it is not possible to define in detail every biosecurity procedure required to control ILT, it is important to know at least that ILTV is typically spread by people, vehicles, equipment, contaminated litter, and infected chickens.  Every poultry production company should examine its own infrastructure, daily practices, equipment use, management practices, personnel traffic and its procedures for litter and chicken transportation.  The best biosecurity programs are the ones designed by committees involved in production and spanning every aspect of a poultry operation, from corporate, to production, to sales, involving breeders, hatchery, broilers, feed mill and processing personnel.  Whatever the adopted methods of control are, the highest risks are posed by personnel; vehicles and equipment that move between farms; vaccination crews; live haul crews and equipment; and manure movement and commercialization practices. Each poultry production company should examine in detail each of these risky areas and design a simple, practical biosecurity program that is tailored to the needs of the company.  In addition, it is critical that the local poultry industry is organized and informed timely of any developments in ILT outbreaks. Clear standardized methodologies for cleaning, disinfection and temporary quarantine should be spelled to the understanding of everyone in each company.  Also critical are the location of each documented case, its anticipated route to the processing plant, the type of vaccine (s) used in each case, and a clear policy regarding used litter treatment, withdrawal from the farm and transportation for commercialization purposes.  Finally, no outbreak can be managed appropriately without having the ability to rapidly and accurately diagnose ILT, or without having a centralized bank of information relative to the outbreak, typically a regional laboratory with some epidemiological analysis capabilities and with complete updated farm location data on geographic information systems. 

A look into the future.  Clearly, better vaccines are needed and academic institutions as well as vaccine companies are working hard on their development. They will have a cost and industry should be prepared to cover a higher cost for new generation vaccines against ILT. Not only better vaccines are needed, but also industry must have a better approach to controlling ILT.  There cannot be blatant disregard for the role of actively infected broilers being sent by the millions to processing plants thus exposing many more millions of broilers at risk; industry should not expect good results from improper mass application of CEO vaccines; as long as the poultry industry and/or integrated poultry growers continue to see poultry manure as a significant source of income; and as long as ILT vaccines continue being developed without having a significant understanding of truly immunogenic proteins of ILTV and of the immune responses against ILTV, industry will continue to experience outbreaks for years to come.