There is general consensus that regional laryngotracheitis (LT) outbreaks are occurring with greater frequency and heightened virulence resulting in greater financial losses. Transport of pullets, spent breeders and mature broiler flocks as well as movement of personnel, feed delivery, improper disposal of dead birds, and mishandling of litter all serve as mechanisms to spread LT virus.
The disease is caused by Gallid herpesvirus 1, resulting in a permanent carrier state following infection of a susceptible flock. The virus can also be shed for prolonged periods by flocks vaccinated with chick embryo origin (CEO) vaccine. The realities of modern broiler production which involves the concentration of breeder and broiler farms, frequently operated by different integrators in a localized region, contributes to the spread and persistence of infection.
Advances in the epidemiology of LT
Despite structured field observations, laboratory studies and anecdotal observations, the epidemiology of LT under commercial conditions is poorly defined. Last year, the University of Georgia performed genetic characterization on 46 laryngotracheitis virus (LTV) isolates collected from five states from the beginning of January 2006 through the spring of 2007. These samples yielded nine genotypes which varied in their relatedness. Significantly, the study confirms that many of the field isolates are either CEO vaccine or a closely related genotype. Interestingly, isolates derived from backyard flocks are clustered among genotypes which are rarely encountered in commercial flocks. This contradicts the frequently repeated assertion that backyard flocks represent the reservoir of LT infection.
In all likelihood, long-lived breeders and commercial egg production flocks vaccinated with CEO serve as the source of infection from season to season within a region. Laboratory studies confirm that passage of CEO vaccine in susceptible birds under controlled conditions results in increased virulence.
It is reasonable to assume that administration of CEO on an inconsistent basis within a poultry population will result in a mosaic of immunity and susceptibility. This is in large measure due to difference in vaccination programs carried out by individual integrators and inherent efficiency in administration. Maintaining a variable level of susceptible flocks will provide an opportunity for infection to persist in a region given low levels of biosecurity relative to the infectivity of LTV. Increasing virulence can occur within a complex unless flocks are effectively immunized by application of potent vaccine over successive cycles.
A study conducted on the Eastern Shore indicated that the number of houses on a growing unit was correlated with the risks and consequences of LT infection. It is evident that broilers less than 6 pounds in live weight are relatively short lived and if only two or three rounds of vaccination are followed, a population in a region can move from immune to susceptible within a short time following cessation of vaccine protection. This creates the situation where the proportion of susceptible flocks in the potentially exposed population rises to the outbreak threshold. Introduction of either CEO vaccine or a related field genotype of moderate to severe virulence will then result in clinical cases.
Factors in virus dissemination
LTV can survive up to 20 days in deep litter and retains viability in carcasses for at least three weeks. Although LTV is destroyed by heat from either gas brooders or composting, infectivity will be retained at low ambient temperatures or when exposed to the outside of houses under cold cloudy conditions. Studies have shown that wind dispersal is a factor in disseminating virus associated with particles of litter removed from houses. LTV adhering to the contaminated clothing of service personnel or dust on vehicles can also transmit infection.
The second factor in relation to LT concerns the prolonged incubation period which may extend up to 10 days. Infected flocks will shed virus two to four days before showing clinical signs. Service persons and work crews may potentially be involved in the spread of infection.
The mildest of the available live LT vaccines comprises tissue culture origin (TCO) products which were originally promoted as “non-spreading.” Molecular biological techniques point to occasional low-level spread of TCO vaccine, but the detection of viral DNA has not been associated with a measurable immunologic response indicating an ability to cause disease. Generally, TCO vaccine can be used to prime immunity in long-lived birds when administered by the intraocular (eye drop) route with a second CEO vaccine as a booster.
CEO vaccine is currently the mainstay of the poultry industry world-wide. Classified as a Genotype IV, commercial CEO vaccines are administered in drinking water providing varying degrees of protection. Long-lived birds receive CEO vaccine by the eye-drop route which provides adequate protection but contributes to prolonged shedding from the trachea and persistence in the trigeminal ganglia of the head. Under conditions of stress, vaccinated long-lived flocks can shed LT virus.
Use of CEO vaccine to protect flocks from ILT is the quintessential double-edged sword. Traditionally, state authorities have limited the use of the vaccine frequently until too late in the progress of a regional outbreak. Due to the indirect costs associated with a depression in production, application of CEO vaccine, even if allowed, is delayed in the hope that individual outbreaks will be contained by biosecurity or the administration of vaccine by neighboring complexes. It is now generally accepted that if CEO vaccine is to be used in an area, then application should be universal among all complexes, and it should be initiated rapidly along with heightened biosecurity. Vaccination with CEO should be maintained for at least two cycles after the last reported clinical case. Regional eradication of LT can only be achieved if the proportion of susceptible flocks falls below the endemic threshold. Basically, this situation deprives the virus of susceptible hosts, resulting in the inability of the LTV to persist and hence cessation of clinical cases. Total eradication probably does not occur since the long-lived birds in the area outlast the vaccination program and given both climatic and operational factors, subsequent outbreaks will occur as virus extends seasonally from reservoirs to susceptible flocks.
Recombinant vector LT vaccines have been under development for at least two decades. Advances in molecular biology have resulted in two major categories of recombinant vaccines. The first commercial LT vaccine to be marketed in the USA was based on an avipox vector modified to express genes for specific antigenic LT glycoproteins. Although effective under laboratory conditions, field experience has been extremely variable. This is due to the presence of maternal antibodies to pox or exposure to LT prior to the administration of the vaccine, usually at 14 to 18 days of age. Some beneficial results have been obtained combining the pox vectored LT vaccine as a primer followed by CEO products later in the growing cycle.
The recently introduced HVT vector LT vaccine offers greater potential for protection against clinical outbreaks, but cost is a major deterrent to adoption of this vaccine as a routine measure in broilers. Industry veterinarians are divided on the advantages of rHVT-LT vaccine. Most companies evaluating the product have reported acceptable suppression of the clinical signs and erosive effects of ILT. Reports of a three to six point improvement in feed conversion efficiency between the relatively more pathogenic CEO vaccine and the rHVT-LT product apparently compensate for the differential in cost. There is a consensus that to effectively suppress LT in a given area, application of rHVT-LT will require prolonged periods of use administered either subcutaneously to long-lived birds or in ovo to broilers. Unlike CEO, the recombinant vaccine does not stimulate circulating LT antibody as detected by ELISA. This is a function of the need for rHVT vector to proliferate in feather follicles and indirectly stimulate immunity through the expression of LT glycoproteins. Generally, regulatory veterinarians prefer recombinant vaccines over traditional CEO vaccine and it is possible that artificial trade barriers might be erected against states allowing the use of CEO in the future.
The right timing
Veterinarians at a May meeting of stakeholders called by the Poultry Regulatory Veterinarian of a mid-Atlantic state accepted that a combination of rHVT-LT vaccine followed by CEO, although expensive to administer, was most effective in suppressing clinical cases. The important question was when vaccination could cease in a year and when vaccination should resume. Veterinarians at complexes growing birds to over 6.5 pounds maintained that retention of rHVT-LT for two cycles after the last reported case was prudent.
The take-home message is that irrespective of current control programs involving biosecurity and selection of vaccines, it is generally accepted that LT, whether ILT or VLT, can only be suppressed but never eradicated in the context of commercial broiler production in endemic areas. A secondary consideration is that ILT serves as a surrogate for low-path avian influenza. Failure to control LT bodes ill for the future management of H7N2 infection should this pathogen extend from live-bird and backyard flocks into the commercial broiler industry.