Current Epidemiology

Clinical Infectious Laryngotracheitis (ILT) is a sporadic disease in most parts of the US, with locally widespread epornitics [outbreaks of disease in a bird population] at irregular intervals, mainly in areas of concentrated production. These epornitics of ILT tend to occur where there are large populations of naïve, unvaccinated birds, i.e. in concentrated areas of broiler production. ILT seems to be an increasing problem in many concentrated broiler producing areas. In just the last 2-3 years, there have been major outbreaks in Delmarva, North Carolina, South Carolina, Georgia, Alabama, Arkansas, and California.

In many areas, and typically in Georgia, ILT epornitics in broilers appear to occur in a 7-10 year cycle. Most outbreaks begin in the fall or winter months through early spring. The appearance and disappearance of outbreaks is frequently difficult to explain and the question of their origin is a difficult one. Outbreaks appear to be more common in areas where breeders, layers and broilers are all present in the same geographical area and where long-lived birds are vaccinated with chick embryo origin (CEO) vaccines. Management and housing types appear to have little influence. It has been observed that areas that prohibit the use of CEO vaccine (Texas, Mississippi) seem to have fewer broiler outbreaks. In 2001, a survey was conducted of colleagues in broiler and layer production about vaccination practices for ILT, opinions on the origin of outbreaks and means of spread and control. Most respondents agreed with the statement that the majority of sporadic outbreaks in breeders and layers could be attributed to lapses in vaccination programs and biosecurity.

* "ILTV may be subclinically endemic or latent in the broiler population itself, and unknown factors lead to the emergence of clinical epornitics from this source"

A variety of hypotheses were advanced about the origin of broiler epornitics. High on the list is the use of CEO ILTV vaccines in the area, with several mentioning Leghorns specifically. Backyard birds were also frequently suspected as the source, and there is certainly evidence to support this. A case control study on Delmarva found that case flocks were 36 times more likely to be located within a mile of a backyard flock than control flocks¹.

Interestingly, four respondents advanced the hypothesis that ILTV may be subclinically endemic or latent in the broiler population itself, and unknown factors lead to the emergence of clinical epornitics from this source. Recent findings of occult cases via polymerase chain rreaction (PCR) techniques and studies suggesting survival of ILT in biofilms in water lines and other locations within broiler houses lend some credence to this hypothesis.

The spread of epornitics is almost universally attributed to lapses in biosecurity and spread via people, litter, dead bird disposal, equipment, vehicles and along live haul routes. There appears to be plenty of circumstantial evidence to suggest that spread by the wind is possible.

A risk analysis on Delmarva found that case farms were 9.9 times more likely than control to be located within the wind vector of a clinical flock during the 14 days prior to signs². In the recent outbreaks in Georgia, vaccinated layers and breeders have been affected, which was not formerly a feature of the disease. The strain associated with the recent outbreaks in Georgia appears to be quite virulent in unprotected birds, but the various vaccines (CEO, Pox-LT and HVT-LT) have provided adequate protection in different operations.

Control Programs

Experience has demonstrated that cooperative programs in which industry, government, academia, laboratories, and allied industries work together to control the disease are the most successful approach currently available to minimize cases and stop outbreaks. These programs should consider the known epizootiology of the disease, and should contain provisions for rapid diagnosis; the use of Geographical Information Systems (GIS) to design appropriate zones for quarantine, vaccination, and movement routes to processing plants; and open communication among all players.

* "In the recent outbreaks in Georgia, vaccinated layers and breeders have been affected, which was not formerly a feature of the disease."

Design of a cogent control program requires some knowledge of the biological behaviour of the virus. ILTV should be regarded as highly infectious. The chicken is the only natural host and wild birds, rodents, pets, etc. can transmit it only as mechanical vectors. The experimental incubation period, i.e. the time from inoculation until the appearance of signs, is 2-4 days but under natural exposure the incubation period probably ranges from 6-12 days. Shedding of the virus may occur 4 days prior to the onset of signs. This fact means one could visit a flock that appears clinically healthy and unknowingly become contaminated with the virus, and emphasizes the importance of continual attention to biosecurity and the maintenance of log books by production personnel.

Another important characteristic of ILTV is the development of a latent carrier state after either natural infection or vaccination with CEO live vaccine. Stress can reactivate shedding and result in spread to susceptible birds. The virus is not vertically shed, and spread by day-old chicks is probably negligible. Infections in young birds are rarely seen before 18 days of age but they are susceptible and can be infected.

Virus Survival Period

The question always arises of “How long does it survive?” Survivability of the virus in the environment is subject to debate, and many of the quoted references are quite old, dating back to the 1930’s to 1950’s.

Survivability varies depending on how the virus is stored, in what substance, at what temperature, whether it is exposed to light, and so forth. On dry swabs, it will survive for 14 days at 0-5°C (32-41° F) but only 3 days at room temperature (22°C, 72°F). When stored in suitable diluents at refrigerator (or winter) temperatures of 4-10°C (39-50°F), the virus may survive for one to several months. ILTV is reported to survive for 30 days in the tracheas of carcasses at 4-10°C (39-50°F), while one old reference suggested survival for 10-100 days in exudates or carcasses at 13-23°C (55-73°F).

As the temperature increases, survivability drops. At 37-38°C (98.6-100°F), survivability is usually about 24-72 hours depending on the substance in which the virus is found, including litter. At 55-56°C (131-133°F) survivability is in terms of minutes to hours. Survival periods of 3-20 days have been reported for litter and manure. Composting for 5 days is reported to reduce the virus below detectable levels.

The key is the combination of time and temperature, with an eye toward the amount of protective organic debris present. It is apparent that the current recommendation such as the classic “100°F for 100 hours” is probably in line with the available information. How much overkill to apply depends on the degree of confidence desired. The author suggests that at the typical target of 100°F for at least 72 hours is needed, and more is better, particularly when one considers that many facilities may not maintain a constant 100°F or higher. If one is relying on time alone, three weeks probably is reasonable in most cases. The virus is inactivated by most common disinfectants. It should come as no surprise that the efficacy of disinfectants for ILTV is affected by the presence of organic matter.

Controlling Epornitics

In the 2001 survey of broiler and layer veterinarians, there was nearly universal agreement on the use of zone vaccination, biosecurity measures and traffic control to control epornitics. Several stressed the importance of cooperation among operators in designing zones, vaccinating all susceptible birds, and coordinating vaccination start and stop dates. The control programs used in Georgia have been reasonably successful in most recent outbreaks, although the present outbreak is in its second year. Some of the major features of this program are reviewed here.

Basic biosecurity is obviously important:

• Growers must be educated to separate farm activities and all other off-farm activities
• Dedicated clothing and footwear should be used exclusively on the farm, and showering after off-farm activities and prior to attending to the chickens is encouraged
• Equipment sharing is discouraged and cleaning and disinfection of equipment that must be shared prior to entry and departure is necessary
• Company biosecurity for servicemen, feed deliveries, chick deliveries, and catch crews is stressed
• During an outbreak, zoning of these activities between the affected and unaffected zones is practiced to the extent possible.

Georgia uses a Poultry Technical Advisory Committee (PTAC) that operates from the Georgia Poultry Laboratory (GPL) System. The GPL is a quasi-governmental entity that is funded by the Georgia Department of Agriculture but operated by the Poultry Federation. The GIS system is owned by the Federation and shielded from public disclosure. The PTAC includes a representative from every commercial company in the state, including layers, primary breeders and broilers, plus a representative of the game fowl association, the GPL, the Georgia Poultry Improvement Association (the official state agency for the NPIP) and the Poultry Diagnostic and Research Center (PDRC) at the University of Georgia. (This Committee also functions for other disease emergencies such as AI or NDV.)

All cases of ILT are reported to the PTAC, with a GIS map of the case and all surrounding farms. Suspect ILT cases are to be submitted to a GPL lab or PDRC for rapid diagnosis. Six to7 recently killed birds are to be triple-bagged, with the outer two bags sprayed with disinfectant. The access area to the lab is to be disinfected, the submitting vehicle disinfected, and the courier is to go home for the day and shower and change before returning to work. GPL uses direct fluorescent antibody testing (DFAT), backed up by histopathology and virus isolation (VI). PDRC uses histopathology backed up by polymerase chain reaction (PCR), VI, and sometime immunohistochemistry. This system has worked quite well for early detection.

The first positive case is immediately quarantined. In Georgia, this is a voluntary industry quarantine enforced only by the GPL. This is done to avoid the political repercussions of a state-enforced quarantine, and has also worked very well.

• Servicing is discontinued, feed deliveries are the last of the shift for that driver, and the truck is disinfected.
• The flock is sent to slaughter as soon as possible, and the route is designed to avoid as many poultry farms as possible, approved by the GPL, and reported to the PTAC using a GIS map.
• Most companies preemptively quarantine farms within one mile of the live haul route of an affected farm.
• Catch of a case is to be the only catch for that crew, the crew vehicles and fork lifts are to be cleaned and disinfected, and crews are instructed to go directly home, shower and change all clothing.
• Cleaning and disinfection of live haul trucks is encouraged but often is not practical or possible for many companies.
• After slaughter, clean-out of affected farms is not allowed.
• The houses are to be closed immediately.
• The house must be heated to 100°F for 100 hours prior to placement of the next flock. The heating is encouraged immediately but may be delayed until pre-placement.
• The affected farm has a down time of 3 weeks.

In the author's opinion, the heating and down time have been effective in preventing recurrences on the affected farms, and that any additional safety for that farm gained by allowing cleaning out, washing and disinfection (even if the litter is restricted to the farm, piled for composting, and covered) is negated by the danger to adjacent farms created by the dust and equipment movement attendant to cleaning out.

We establish a 5-mile Biosecurity Zone around the first case. Enhanced biosecurity in this zone includes a minimum down time of 2 weeks for all farms regardless of signs, with 3 weeks encouraged when possible; limiting servicing to essential functions only, and stopping all clean out and litter spreading. Companies also institute their own enhanced biosecurity, which usually includes zoning of personnel movements, feed deliveries, chick placements, and catch crews. The GPL prepares a letter for distribution by the companies to utility services (power companies, gas companies, equipment installers and service persons, etc) requesting increased biosecurity, such as estimated billing without reading meters until the outbreak is over.

We will generally handle the first few cases by expanding this biosecurity zone as needed. As you are all aware, institution of CEO vaccination in broilers is not taken lightly. The vaccine extracts an economic penalty and back passage of CEO vaccine, hauling birds shedding back passaged vaccine to slaughter, and creation of latent carriers can extend the outbreak.

Factors influencing the decision to start broiler vaccination include:

• subsequent cases with no epidemiological connections
• cases jumping the boundaries of the biosecurity zones
• cases in vaccinated long-lived birds (layers and breeders)
• season (holding off on cases late in the spring is more successful than cases in late fall)
• the frequency of diagnoses (cases occurring in rapid succession spur the decision to vaccinate).

Zones for vaccination are also established by vote of the PTAC using the GIS system. Experience suggests that the zones need to be large fairly early on. In past years, establishment of small zones has inevitably led to a leap-frogging pattern. Factors considered in establishing the zones besides size include natural breaks, known familial or business relationships among growers, prevailing winds, live haul routes out of the zone, etc. Clean out and litter spreading is stopped in the vaccination zone until all farms in the zone are vaccinated. Clean out and spreading is then allowed again, but litter from vaccinate farms must stay within the vaccination zone. Our reasoning is that this practice is less of a threat to vaccinated populations, and is necessary to maintain bird health during use of such an aggressive vaccine. If litter is to be removed from the zone, it must be heated to 100°F for 100 hours or composted for 5 days prior to movement. Careful segregation of activities between the vaccination zone and unvaccinated areas is practiced by each company. Service routes may be realigned so that service persons have completely vaccinated or unvaccinated routes and feed trucks, chick deliveries, and catch crews may be zoned.

Zones are expanded as needed. Vaccination is usually continued until cases have stopped for at least 4-5 weeks (3 incubation periods) and the season is judged favourable (usually late spring or early summer). A date is set to stop vaccination, and clean out and litter spreading is again stopped. The phasing-out period is critical, and service schedules of going strictly from younger to older birds become imperative. Some flexibility is needed in accommodating familial and geographical exigencies but at some point, attempts to segregate vaccinated and unvaccinated birds have to be stopped. With the advent of the vectored vaccines, some areas are considering a phase-out program in which those using CEO vaccine will be asked to switch to a vectored vaccine prior to complete cessation. The extreme expense of the vectored vaccines and variable success will be barriers to this approach. The outbreak is declared over when the last vaccinated farm has successfully grown an unvaccinated flock with no signs of ILT. Clean out and litter spreading is allowed again at that point.

Vaccination

There are four classes of vaccines available for use in the US:

• There is one source for a tissue culture origin (TCO) live ILTV vaccine. This vaccine should be given only by eye drop, and it is not suitable for mass application. Properly applied, it gives reasonably good protection (although less than the CEO vaccines) and has the advantages of mild reactivity and stability; it does not tend to spread rapidly or revert to virulence.
• There are a number of sources of CEO vaccine. Most are cleared for eye drop administration, some for drinking water application, and a few by coarse spray. These vaccines tend to be highly reactive in broilers, will spread and revert to virulence, and can induce latent carriers. They afford the most solid protection.

Careful attention to detail is needed for mass application to broilers. Ensure that the serials purchased for use in broilers have an adequate titer (preferably >10^3.8, and do not cut the dose. Although some have successfully used spray application for broilers, the author does not recommend it, and has had the most success with a “double-pump” technique.

Make sure the drinker lines are clean and free of any disinfectants or other substances that could harm the vaccine. Temporarily remove any water filters downstream of the medicator bibs. The birds should be sufficiently water-starved so that they are thirsty but not famished for water. The author uses about 4 hours in winter, less in summer, and prefers to turn off the water but leave the lines down for about an hour so the birds can drain the lines, then raise the lines to ensure adequate water deprivation. It also helps to open a hose bib downstream of the shut-off so that any overhead lines will drain.

When the time arrives to vaccinate, mix one-half of the total dose in barrels and use dye to colour the water. Be sure to use appropriate stabilizers to neutralize any chlorine and protect the vaccine titer. Table 1 gives the suggested number of barrels to use depending on house and bird size. The follwing procedure is successful:

1. Open the by-pass valves on the regulators and the ends of the drinker lines
2. Pump the dyed vaccine from the barrels until all lines are full
3. Close the by-pass valves and ends and lower the lines
4. Walk the birds frequently to ensure that all birds get a drink
5. As the last barrel is running, mix the second half of the dose in a medicator bucket (Table 2 gives the suggested volume to use in the medicator bucket at 1:128)
6. As the last barrel runs out, turn on the medicator and administer the rest of the dose.

When the medicator is empty, turn the water back on. It should take about 20 minutes to charge the system with vaccine, 20-25 minutes to pump the remaining barrels, and 1-1.5 hours to empty the medicator bucket, for a total exposure to the vaccine of about 2 hours.

Newcastle-Bronchitis Boost during LT Vaccination

The question always arises about how to manage the Newcastle-Bronchitis boost during an LT vaccination program, and there is no good answer. Reducing the NDV-IBV boost to the bare minimum is helpful but often not possible. One option is to split the two, usually giving the LT first at 12-14 days and attempting to wait as late as we feel comfortable (+21-22 days) for the NDV-IBV boost, or they may be given together.

The author generally gives them all together at 14-16 days of age. It is generally held that mass application of CEO LT vaccine prior to about 10 days of age results in insufficient immunity, while mass application to broilers much after about 21 days of age is prone to harsher reactions. Consequently, most companies attempt to stay within a window of about 12-21 days of age.

A pox-vectored ILT vaccine is also available. It is intended for wing-web application to pullets 8 weeks of age or older. This vaccine provides reasonable immunity to pullets when properly administered and has the distinct advantage that it contains no live ILTV. It cannot shed, spread or induce latent carriers. This vaccine has been used extra-label by the in-ovo route, usually at a reduced dose, typically at 1/3 to ½ dose.

Reports from the field have varied greatly. Some respondents have indicated very poor success while other report quite good results, especially in smaller birds and/or in areas with more limited challenge. It appears to the author that the 'therapeutic index; or the ratio between the dose needed to provide sufficient LT protection and the dose that is injurious is fairly narrow. The vaccine does result in grossly visible lymphocytic nodules in the lungs and can result in an increased incidence of wry necks, paralyzed chicks in dorsal recumbency and elevated early mortality. These problems appear to peak about 5-6 days of age and then resolve. The problem appears to be more likely in transfers at or before 18 days, and less in transfers at 19 days.

There is also an HVT-vectored ILT vaccine available. It is intended for subcutaneous injection at one day of age but has also been used extra-label in-ovo in broilers. It appears to give good protection when administered properly at full dose according to label directions. Full immunity does not develop until about 4 weeks of age. This vaccine also has the advantage of containing no live ILTV so it also does not shed, spread or induce latent carrier states. Due to the expense, it is usually cut for application to broilers, typically at ½ dose.

Experience with this vaccine has also been variable in the northeast Georgia outbreak. One large-bird company had massive, catastrophic failures and quickly returned to CEO vaccine. The reason for these failures remains unknown but early challenge has been proposed as one likely cause. Several other companies had reasonably good success. The author used millions of doses during the winter of 2007-08, vaccinating roughly 75% of a 3.2-million bird complex between December and the presentation of this paper in mid-April. There were 17 known breaks to date, and all were relatively mild, usually reaching a peak mortality of 60-100 for several days and then declining. The farm managers felt this vaccine was preferable to the problems usually experienced with the CEO vaccines.

References

1. Johnson Y.J., Colby M.M., Tablante N.L., Hegngi F.N., Salem M., Gedamu N. and Pope C. 2004. Application of commercial and backyard poultry geographic information system databases for the identification of risk factors for clinical infectious laryngotracheitis in a cluster of cases on the Delmarva Peninsula. Int. J. Poult. Sci. 3:201-205.
   
2. Johnson Y.J., Gedamu N., Colby M.M., Myint M.S., Steele S.E., Salem M. and Tablante N. 2005. Wind-borne transmission of infectious laryngotracheitis between commercial poultry operations. Int. J. Poult. Sci. 4:263-267.

October 2008