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This article was published in 1969-70
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Recent Findings in Infectious Laryngotracheitis
DR. B. SINKOVIC, Department of Veterinary Medicine, The University of Sydney, Camden, N.S.W. 2570
Until recently the accepted features of Infectious Laryngotracheitis (ILT) were as follows:
- (1) The immunity following vaccination lasts for at least 12 months.
- (2) Age, breed and sex do not influence the severity of the disease.
- (3) Type of management and seasonal factors do not play any role on the incidence and severity of ILT.
- (4) ILT virus is fairly labile.
- (5) Vaccination by cloacal route, preferably at the age of six weeks and over, gives a satisfactory control.
Over the last few years the above aspects have been investigated and the following results, confirmed in some instances in overseas countries, have been obtained.
DURATION OF IMMUNITY
Under the laboratory conditions the duration of immunity in chickens vaccinated by cloacal route has been shown to only last between 8 and 16 weeks. Under field conditions, however, it is most uncommon to experience any breakdown in immunity, even in birds vaccinated 18 to 20 months previously. This could be demonstrated either by the challenge of a small proportion of the flock, or more commonly, by the absence of ILT outbreaks in vaccinated flocks at a certain time and in a certain area where numerous severe outbreaks occurred in unvaccinated birds. The question arises—why the discrepancy between the laboratory and field findings? A feasible explanation is that virus-carriers may occur following cloacal vaccination in spite of the fact that ILT virus was found to remain at the site of vaccination only for a period of nine days. It is possible that during the process of vaccination, or during the post-vaccinal multiplication of the virus in the cloacal mucous membrane, some birds, for one reason or other, may become tracheal carriers. It is a well-established fact that after a natural outbreak of ILT a small number of chickens remain tracheal carriers for as long as 16 months. If the assumption is correct that carriers may occur after vaccination, then it could be expected that their proportion would be even smaller than following the natural infection, and that the chances of carrier-birds among the vaccinates would be much higher under field than under laboratory conditions. The experimental groups involve, namely, relatively small numbers of birds, and the groups are often well isolated from each other which is usually not the case in the field. Once some carriers are present in the flock further immunisation of partially protected birds can easily take place.
INFLUENCE OF AGE, BREED AND SEX
It has been shown that younger birds after an artificial infection with ILT experience significantly higher mortality than older birds. In respect to the breed, it has been found that Australorps, followed by their crosses with White Leghorn, and White Leghorns are more susceptible than meat-crosses. Male birds have proved more susceptible than females to the disease.
INFLUENCE OF ENVIRONMENT
Chickens exposed to cold or heat stress (40°F. and 100°F.) have shown significantly higher mortality than those kept at room temperature (70°F.). The effect of cold stress has been, however, more marked than heat stress.
STABILITY OF VIRUS
The virus of ILT has been found to be of adequate stability when suspended in tap water. Over a period of six hours at an environmental temperature of 75°F. there was a drop in titre of about 1 log10.
NEW ROUTES OF VACCINATION
It has been found that vaccination by conjunctival instillation offers several advantages over the cloacal route, such as:
- (a) It is more hygienic and more rapid.
- (b) It gives an even vaccinating dose.
- (c) It can be used in birds of any age.
- (d) It does not require any special skill; and
- (e) It results in a better immune response.
It has been further shown that even the most virulent isolates of ILT applied by ocular route will cause insignificant mortality, and this finding can prove valuable in case of a reversion to virulence of the mild vaccine strain presently used in Australia.
Chickens as young as day-old have successfully been vaccinated by the eye-drop method, and these days approximately 10 to 12 million broilers are vaccinated annually by this method in Australia. The vaccination is carried out usually at the age of four to seven days, simultaneously with debeaking and fowl pox vaccination.
Recent laboratory and field trials have revealed that vaccination against ILT via the drinking water, at the age of six days and over, results in immunity comparable to that achieved by eye-drop method.
The development of immunity following either eye-drop or drinking water vaccination has been proved to be as rapid as after the cloacal route of vaccination. Partial protection has been demonstrated by the second or third day and full protection by the fourth or fifth day post vaccination.
In the light of the above findings some of the traditional thinking on ILT would require modification to incorporate the new knowledge.