EPIZOOTIOLOGY OF INFECTIOUS LARYNGOTRACHEITIS IN BRITISH COLUMBIA 1971-1973 L. B. Hayles, K. R. Macdonald, W. C. Newby, C. W. Wood, E. W. Gilchrist and A. C. MacNeill*

majority of ILT outbreaks occurred. For this reason, this smaller area is labelled the epizootic area. The location is shown in Figure 1. The poultry population of the epizootic area is comprised of about three million layers and replacement pullets, and close to 20 million broilers are marketed annually. There are about 400 producers in the area and about half of them belong to a closely knit social community in which there are frequent meetings between family members.

INTRODUCTION IN TH SPRING OF 1971 infectious laryngotra-

cheitis (ILT) was diagnosed in a flock of broiler chickens located in the Central Fraser Valley of British Columbia (BC). The disease spread rapidly and persisted within the area for more than three years. This resulted in an estimated annual loss of $250,000 to the poultry industry, as represented by vaccination costs and losses through morbidity, mortality and condemnations at slaughter. This report presents the chronological order of outbreaks, the further impact of the disease on the poultry industry, and the measures adopted to contain the epizootic. The relative values of the standard long-term control methods such as vaccination, and those introduced by the BC Department of Agriculture on a short-term basis, are discussed in the light of three years' experience.

Laboratory Methods Laboratory diagnosis was based on gross and histopathological findings supported where appropriate by virus isolation and identification (1). Virological confirmation was sought in early 1972 as a preliminary step in assessing the virulence of the epizootic strains of the virus. Assessment of virulence was considered necessary when it became apparent, ten months after the epizootic began, that an unusually large number of ILT diagnoses were being made in vaccinated flocks. Virus isolation was performed by inoculation of tracheal scrapings and lung suspensions on the dropped chorioallantoic membrane (CAM) of nine to ten day old chick embryos. Lesions on the CAM provided presumptive virus identification and confirmation was effected by virus neutralization or by immunofluorescence (1). For assessment of virulence, 12 isolates free of other poultry pathogens were compared in laboratory' experiments. Morphology of CAM lesions and ability to cause illness, eye and tracheal lesions, were assessed. These isolates behaved similarly and a representative named the Kliever virus was selected for further study. Aspects of this study which are of concern here, relate to protection and pathogenicity tests. In these tests the Kliever virus was used to challenge vaccinated birds of different ages in comparison with the standard laboratory challenge virus. The ability of the Kliever virus to produce tracheal lesions and illness in six week old birds following intraocular

MATERIALS AND METHODS Description of the Infected Area: Poultry Population at Risk The infected area is defined for purposes of regulation, as "that area of the province of

British Columbia which lies south of the 500 parallel of north latitude, and west from the 1210 meridian of west longitude to the Strait of Georgia". This area extends from Hope to Vancouver and is commonly known as the Lower Fraser Valley. In the infected area the greatest concentration of poultry lies within a ten-mile radius of Abbotsford. This is within the Central Fraser Valley where approximately 85% of the province's poultry are located, and where the

*Veterinary

Laboratory, British Columbia De-

partment of Agriculture, Abbotsford, British Columbia (Hayles, Macdonald and Gilchrist), Poultry Branch, British Columbia Department of Agriculture (Newby and Wood) and Animal Pathology Division, Health of Animals Branch,

Agriculture Canada, Vancouver, British Columbia (MacNeill). Present address of Dr. Hayles: Central Veterinary Laboratory, P.O. Box 50, Mazabuka, Zambia. Reprint requests to Dr. Macdonald.

'The details of these experiments, which were performed at Connaught Laboratories Ltd., Toronto, Ontario will be reported elsewhere. 01

CAN. VET. JOUR., VOl. 17, no. 4, April, 1976

CANADIAN VETERINARY JOURNAL

inoculation, was also assessed in relation to vaccine virus that had been passaged six times in six week old chickens. Accordingly, these tests also determined the capability of vaccine virus to cause disease under simulated field conditions. Regulatory and Control Methods: Disease Surveillance Vaccination offered the first major control method. Detailed advice on the availability and use of vaccines was given in the farm press and at meetings with poultry producers. The following vaccination regimen was recommended. A primary vaccine dose was to be given between six and eight weeks of age, a booster dose about 12 weeks later and thereafter, booster doses at six-monthly intervals. ILT was made a notifiable disease by regulations passed under the Contagious Diseases (Animals) Act of BC, six months after the outbreak began. These regulations stipulated compulsory vaccination of all hatchery supply flocks and of pullets which were moved from one farm to another. In the case of hatchery supply flocks, vaccine was provided free of charge by the BC Department of Agriculture and specified for use in flocks 19 to 21 weeks old. All suspect or confirmed ILT cases were to be reported to departmental officials. In turn, the department regularly notified feed suppliers, hatchery managers and others directly involved in servicing the industry, of the locations of infected premises. Fees for laboratory services with respect to cases of respiratory disease in poultry were suspended. This was expected to encourage producers to submit specimens from all suspect cases of ILT, and also, to aid in identifying enzootic foci of infection throughout the province. Special regulations were required to prevent ILT spread to the neighbouring province of Alberta. The most significant requirement was a total prohibition on shipment of pheasants and live chickens other than day old chicks, from any location in British Columbia to Alberta. Shipment of chicken eggs from the infected area as earlier defined, was also prohibited during the first 18 months of the epizootic. Subsequently, they were shipped under specified conditions which reduced the risk of introducing the disease to Alberta. Among them were the following. New cartons and crates were to be employed for each shipment and they should be destroyed, preferably by burning, as soon as possible after use. Records on composition and distribution of the contents of each shipment were to be maintained and provided to veterinary authori102

ties in both provinces. The same conditions applied to the shipment of day old chicks.

RESULTS

DISCUSSION A summary of the results is provided in Table I. The data are first presented in this manner in order to facilitate comparison between each of the three years under study. Significant findings and relationships are then discussed in further detail. AND

Chronology and Distribution of Outbreaks As far as we could ascertain, the epizootic began in April of 1971, but our efforts to determine the source of infection proved unsuccessful. The disease spread rapidly following the first appearance. Within three weeks, 37 cases were diagnosed among broiler and layer flocks all within a three mile radius of the first confirmed case (Figure 1). We considered that this initial rapid spread was assisted by the high susceptibility of flocks since vaccination had not been practised in the area. Four weeks after the introduction of vaccination the incidence of disease fell by about 50% and this rate of fall was repeated during the succeeding month. This gradual reduction in overall incidence continued throughout 1971 and is depicted in Figure 2. The disease occurred more frequently among broiler flocks, but, in view of the continuing decrease in incidence we believed that this was largely a reflection of the greater number of flocks in this category. However, the absence of vaccination among this group and the more frequent visits afforded such premises by servicemen, probably contributed. The figure also shows that in parallel with the reduced incidence in broilers there was a gradual increase among layers as the colder months approached, to complete the first year of the epizootic. This increase in incidence continued through 1972 and occurred as a slow lateral spread within the epizootic area. This pattern among layers culminated in a massive build-up of infection in this group in the last quarter of 1972, followed by widespread dissemination of the virus among both layer and broiler flocks throughout the succeeding year. The incidence was roughly parallel in both groups and sustained at a steady level throughout 1973 (Figure 2). Reference has already been made to the lateral spread within the epizootic area; this was in part a reflection of the delay with which the practice of vaccination expanded outwards from the main centre of initial ac-

ILT SCALE: miles

A

0

* = Infected premises

2

CC:rs

0~~~~~~~~~~~~~~~0~ 01

I-

c

\

>

BRADNI

ALDERG ROVE

13 9°oo'3l

I r

U S A -

-

- .

FicGuRE 1. Portion of Central Fraser Valley sh lowing ILT epizootic premises during first three weeks of epizootic.

tivity. It should be noted that the introduction of mandatory broiler-breeder vaccination in late 1971 met with considerable enthusiasm because of the protection that would be afforded to broilers, from hatching to marketing age. That was probably the reason for the low incidence of the disease among broilers in 1972 (Figure 2). Despite this early enthusiasm, the apparent success of vaccination, and the evident need for greater use of vaccine in a continually widening area, producers gradually lost confidence in the protective value of vaccination. Although a detailed analysis of this matter is given later, a few comments are appropriate here. In the latter part of 1972 when the epizootic was of about 18 months duration, producers ceased to vaccinate their flocks regularly. Booster vaccinations were often neglected and it became increasingly difficult to ensure a high level of vaccination among breeder flocks. This observation is supported by the high incidence of ILT cases among the over-20 week age group in 1972, as shown in the age distribution analysis presented in Table I. A marked change in age distribution of cases is depicted in the table for 1973, and 103

area

and distribution of infected

this is in keeping with the situation as shown for that year in Figure 2. By mid 1973 it became evident that mandatory vaccination of breeder flocks no longer served the purpose for which it was intended, and it was withdrawn as a statutory requirement. Morbidity and Mortality There was considerable variation in the nature and duration of illness and in morbidity and mortality rates among nonvaccinated flocks. Mild forms of the disease occasionally occurred. In these, respiratory rales and irregular nasal discharge were the main clinical signs; such cases lasted for five days or extended beyond three weeks. Usually, clinical signs were more acute consisting of conjunctivitis, ocular discharge, swelling of the infraorbital sinus, nasal exudation, rales, gasping, and in severe cases the coughing of bloodstained mucus. The course of the disease among most of these cases was about 15 days. The nature of the disease in a particular flock did not appear related to geographic location, to method of management or to season. Egg production loss on a flock basis was not a significant finding.

CANADIAN VETERINARY JOURNAL % OF TOTAL ANNUAL CASES 50

Layers Broilers

40

30

20

10

Jan.

Apr.

July

Oct.

Jan.

Apr.

July

Oct.

Jan.

Apr.

July

Oct.

Feb. Mar.

May June

Aug. Sept.

Nov. Dec.

Feb. Mar.

May

Aug. Sept.

Nov.

Feb.

May

June

Dec.

Mar.

June

Aug. Sept.

Dec.

1971

FIGURE 2. Seasonal 1971-1973.

1972 occurrence

1973

and relative distribution of ILT

cases among

layer and broiler flocks

TABLE I SUMMARY OF COMPARATIVE ILT DATA, 1971-1973

1971

1972

1973

37 41

62 13

46 39

78

75

85

5.6% Not done

4.2%

4.7%

11

44

Data

20% 25%

40% 21%

388,000 126,100 12,000 526,100 5,465,500

275,500 581,700 Nil 857,200 Data unavailable

ILT cases (No. of premises) Layers Broilers Total ILT cases as proportion of total laboratory chicken

submissions

No. ILT virus isolation Age distribution of ILT casesa

4-8 weeks 9-20 weeks >20 weeks Total ILT-affected flock size Layers Broilers Broiler breeders Total Doses ILT vaccine sold

unavailable

544,000 246,000 Nil 790,000 6,036,500

Nov.

55%

39%

ILT in vaccinated flocks Data in proportion to total cases unavailable 37.6% 60.5% aExpressed as proportion of total ILT cases per annum for all categories of flocks. 104

ILT

Producers' estimates of morbidity rates were usually highly subjective. Estimates at or close to 100% were often given but servicemen or departmental personnel invariably failed to substantiate these. Undoubtedly, morbidity rates exceeded 50% in several instances but no useful range or average figure can be given here, on the basis of information supplied. Flock mortality rates between five and ten percent were common with a peak between five and ten days after the start of the outbreak.

indicated that reversion to virulence was unlikely. Field strains of ILT virus have been known to differ in virulence. Jordan (2, 3) has reviewed several reports of such findings and of instances in which alteration in immunological and pathogenic properties occurred following laboratory manipulation. He cites the concept formed by Pulsford, which suggests that some epizootics of ILT might be caused by virus of altered virulence which is favoured in development and spread by the mass production methods of the poultry industry. Laboratory Findings Pulsford's concept was based in part on the Pathological lesions varied in type and inability to trace the source of epizootics and severity depending on the nature and stage on the appearance of acute and subclinical of clinical illness. No peculiar feature was dis- forms of the disease within the same area. cerned and no particular trend developed dur- These features were also present in British ing the period of observation. Variations Columbia. ranged from mild tracheitis with mucoid exudate, through severe hemorrhagic tracheitis to Backyard Flocks, Enzootic Foci, Intercurrent the presence of diphtheretic membranes, frothy Infection exudate and caseous casts within tracheal Noncommercial flocks consisting of 500 lumina. chickens or less were labelled backyard flocks. Eyes and bronchi were often affected and Seven infected backyard flocks were identified pulmonary congestion was occasionally ob- in 1973. It appeared that submission of birds served. All pathological diagnoses were based from these premises resulted from increased in part on the presence of the pathognomonic publicity given to the suspension of laboratory inclusion bodies in tracheal epithelium. Among charges for suspect respiratory disease. This several such cases routinely submitted for increased publicity occurred in early 1973 and virological examination, there was good corre- up to that time there had been no submissions lation between virological and pathological from such flocks. Three were chronic cases findings. For those which failed to present the requiring extended virological examination for classical histopathological features, virological confirmation, and in this manner demonstrated confirmation was mandatory. The most com- the potential to act as concealed sources from mon of these were mild clinical cases with which virus could be distributed over a proscanty tracheal exudate, and chronic cases tracted period. In this connection too, the in which tracheal epithelium had evidently disease was identified in a group of pheasants sloughed extensively and repair was well in July 1973. These were located 800 miles underway. A significant feature of some of from the epizootic area and we were unable these was the delay in development of lesions to determine the manner in which the virus on the CAM. Incubation for up to nine days had entered the premises. Under the aegis of was sometimes required and foci were in- the Contagious Diseases (Animals) Act of BC, variably small and few in number. At least these birds were destroyed and other precauone passage was necessary to secure sufficient tions taken to prevent virus spread from this material for virus neutralization tests. Table I source. The area has since remained free of lists the number of virus isolates obtained in infection. The disease occurred as sporadic 1972 and 1973. outbreaks on Vancouver Island and in the Reference has been made to the experi- Okanagan Valley in the fall of 1972, and in mental assessment of virulence of ILT iso- both locations, the source of infection was lates from the epizootic area (Materials and traced to the introduction of pullets from the Methods). Three commercial vaccines afforded epizootic area. The judicious use of vaccine only partial protection against the Kliever virus and the application of regulatory control when compared to the standard laboratory measures prevented extensive spread. The problem of multiple etiology (3) challenge strain. Comparative pathogenicity tests showed that the Kliever virus consistently among our ILT cases was considered during produced higher morbidity rates, although 1972. It was found that no agent was regupathological lesions remained within the usual larly associated with the primary pathogen, range. The tests with passaged vaccine virus but several were recovered on a sporadic basis. 105

CANADIAN VETERINARY JOURNAL

For example, infectious bronchitis virus was involved with ILT in five instances. Two unidentified chick embryo-lethal agents were recovered, and a mesogenic strain of Newcastle disease virus was isolated from a broiler flock on one occasion. This flock suffered a mortality of 30%. Pasteurella hemolytica was frequently isolated from tracheas of ILTaffected birds and serological evidence indicated exposure to Mycoplasma gallisepticum in several flocks. Pseudomonas aeruginosa infection occurred in one flock of 7,000 four week old birds, in which mortality exceeded 50% over a period of 15 days. Vaccine Coverage and Occurrence of ILT In

Vaccinated Flocks Three vaccines were commonly used up to the fall of 1973 when a fourth was introduced to the area. Apart from periodic variations in availability of a particular vaccine in quantity, it appears that all four products enjoyed wide acceptance among the local poultry producers. All the vaccines were of modified live virus for use by the intraocular method; three were of chick embryo origin and one was prepared in cell culture. Approximately six, and five and one-half million doses of vaccine were distributed through licensed outlets in 1971 and 1972 respectively (Table I). These were the first two years of vaccination. We estimated that the 1971 sales provided vaccine coverage for about half of the layer and breeder flocks and that in the following years, booster vaccinations and vaccination of replacement stock increased this coverage to about 70%. The majority of broiler flocks remained unvaccinated. The figures for vaccine sales in 1973 are unavailable but there was evidence that vaccination of young flocks (four to eight week age group) increased at least threefold in 1973. Broiler flocks represented the major part of this group. This leads to an examination of the occurrence of clinical ILT in vaccinated flocks; an event commonly labelled a vaccine break. Table I indicates that in 1972 and 1973, close to 60% and 40% respectively of all ILT cases diagnosed in our laboratory, occurred in vaccinated flocks. These percentages are based on a comparison between total cases on the one hand and cases in vaccinated and unvaccinated flocks on the other. As a measure of the prevalence of vaccine breaks in each year, these figures are misleading because they fail to take account of-significant epidemiological data related to vaccination. These concern the period of time between vaccination and the 106

laboratory diagnosis of ILT in a particular flock. Upon examination of individual flock histories and discussion with producers, a common factor became evident. Among those cases diagnosed within a 20 day period after vaccination, clinical signs of respiratory illness were generally mild. In addition, sick birds were few in number but usually well distributed throughout the flock and mortality was negligible, being within the acceptable limits for the age group concerned. We concluded that most of these cases represented relatively severe vaccine reactions rather than vaccine breaks. When these cases were excluded from further consideration, the approximate levels of possibly authentic vaccine breaks were 50% in 1972 and 25% in 1973. Mortality among vaccinated flocks rarely exceeded two percent over the seven to ten day period of illness generally observed in these flocks. The occurrence of vaccine breaks is examined in further detail in Table II. In each year, about one third of all vaccine breaks occurred among flocks nine to 20 weeks old. This represents a period between 24 and 38 days after primary vaccination, when adequate protection is normally expected (5). A considerably larger proportion of the breaks (60%70%) occurred among flocks older than 20 weeks. The majority were between 15 and 50 weeks after vaccination, so that apart from the increased stress of egg production, waning vaccine protection was likely to have been a significant factor. Among the 35 vaccinated flocks in this category, seven had been vaccinated twice. These findings suggest that vaccination as practised between 1971 and 1973, failed to provide adequate protection against the local epizootic strains of ILT virus. Reports of similar failures are rare, and all appear related to cloacal vaccination (2, 4). Failures have been attributed to insufficient virus in the vaccine, to poor vaccination practices, and to intercurrent infection. We believe that the last two factors played a role in British Columbia but that the major elements have been the highly virulent epizootic strains of field virus as represented by the Kliever virus, and the waning immunity in flocks which commonly failed to receive booster vaccinations. The cumulative effects of these factors were easily discerned in 1972 when 50% of our cases were from vaccinated flocks. Through the following year as more experience was gained in the use of vaccines and as the practice of revaccination increased, this rate was reduced to 25%. We doubt that this had

ILT

AGE DISTRIBUTION

OF

TABLE I I ILT CASES AMONG VACCINATED FLOCKS (VACCINE BREAKS) 1972a

Age Group of Flocks

No. Flocks Affected

1973B

% of Total Breaks

No. Flocks Affected

% of Total Breaks

4-8 weeks 1 (4) (9) 1 (9) (27) 9-20 weeks 10 (18) 28.5 (38) 5 (10) 29 (30) 24 (25) >20 weeks 68.5 (53) 11 (14) 65 (43) aThe naked figures in each column represent the authentic vaccine breaks, i.e. cases occurring later than 20 days after vaccination. These breaks represented approximately 50% and 25% of the total number of ILT cases diagnosed in 1972 and 1973 respectively. The bracketed figures include the vaccine reactions (see text) as well as the authentic vaccine breaks.

been influenced by attenuation in virulence among field strains of virus, for throughout 1973 we found no changes in clinical or pathological features among our cases, nor did we note any alterations in the range of pock morphologies among our virus isolates. Reports have been made on the value of vaccination in the face of an outbreak among unvaccinated birds (2). Producers in BC reported that such a practice was rarely of benefit in shortening the duration of illness or in reducing mortality within a flock. This provides further support for the virulence of the Kliever virus. SUMMARY

From 1971 to 1973 an epizootic of infectious laryngotracheitis caused an annual loss of approximately $250,000 to the poultry industry of British Columbia. The outbreaks began in a small area within the Central Fraser Valley, where 85% of the province's poultrv are located. Within this local area the disease spread rapidly, but the early introduction of vaccination coincided with a substantial fall in incidence within three months. As part of a comprehensive control program which included vaccination and other control measures at the farm level, the British Columbia Department of Agriculture introduced a number of regulations under the Contagious Diseases (Animals) Act. These regulations served in part to restrict the movement of poultry and poultry products from the main epizootic area. In this manner, spread of the disease to the neighbouring province of Alberta was prevented, but sporadic outbreaks occurred on Vancouver Island and in the Okanagan Valley where the source of infection was traced to the introduction of pullets from the Central Fraser Valley. Analysis of data showed that during the first year of the epizootic, there was a gradual fall 107

in incidence of the disease among broiler flocks. A coincident increase among layers culminated in an explosive series of outbreaks among this group when the epizootic was about 20 months old, and this was related to inadequate vaccination among adult flocks. This situation was succeeded, in the third year of the epizootic, by a widespread dissemination of the virus among both layer and broiler flocks. At the same time, chronic virus carriers were identified among small, noncommercial (backyard) flocks. Producers failed to maintain an adequate programme of vaccination and this was complicated by the occurrence of vaccine reactions and authentic vaccine breaks. In 1972, 50% of all laboratory confirmed cases occurred in vaccinated flocks, and in the following year the proportion was reduced to 25%. Although these breaks occurred primarily among flocks which failed to receive booster vaccinations, the presence of highly virulent field strains of infectious laryngotracheitis virus was partly responsible. REsuME De 1971 a 1973, une epizootie de laryngotracheite infectieuse causa une perte annuelle

d'environ $250,000.00 a l'industrie avicole de la Colombie Britannique. Les eruptions de la maladie debuterent dans une region restreinte de la vallee centrale du Fraser, oiu on eleve 85% des volailles de la province. La maladie se propagea rapidement a l'interieur de cette region; un prompt recours 'a la vaccination se traduisit cependant par une diminution appreciable du nombre de cas, en l'espace de trois mois. Comme partie d'un programme d'eradication rationnel qui incluait la vaccination et d'autres mesures de repression sur les fermes,

CANADIAN VETERINARY JOURNAL

le ministere de l'agriculture de la Colombie

troupeaux vaccines. L'annee suivante, cette

IDritannique int6gra un certain nombre de proportion descendit A 25%. Meme si les bris reglements speciaux a sa loi sur les maladies de vaccination survinrent surtout dans des contagieuses animales. Ces reglements contri- troupeaux qui n'avaient pas requ de vaccina-

buerent entre autres a restreindre la sortie de volailles et de produits avicoles en dehors des principaux foyers d'infection. On empecha ainsi la maladie de se propager a la province voisine d'Alberta; des eruptions sporadiques firent cependant leur apparition sur l'ile de Vancouver et dans la vall6e d'Okanagan, suite a l'introduction de poulets provenant de la vall6e centrale du Fraser. L'analyse des donnees revela qu'il se produisit une baisse graduelle de l'incidence de la maladie dans les troupeaux de poulets de gril, au cours de la premiere annee de l'epizootie. En meme temps, la maladie sevissait de plus en plus au sein des troupeaux de poules pondeuses et atteignait son paroxysme sous la forme d'une serie explosive d'6ruptions dans ces troupeaux, vers le 20e mois de l'epizootie; on realisa que ce phenomene resultait d'une vaccination fautive des troupeaux de volailles adultes. A cette situation succ6da, dans la troisieme annee de l'epizootie, une dissemination generale du virus au sein des troupeaux de pondeuses et de poulets de gril. On decela en meme temps des porteurs chroniques du virus, dans des petits troupeaux non commerciaux "d'arri'ere cour". Les eleveurs negligerent de maintenir un programme adequat de vaccination; a cela s'ajouterent des reactions vaccinales et d'authentiques bris de vaccination. En 1972, 50% des cas confirmes au laboratoire provenaient de

tion de rappel, la presence de souches du virus de la laryngo-tracheite infectieuse particulierement virulentes etait en partie responsable des difficultes auxquelles il fallut faire face. ACKNOWLEDGMENTS

We gratefully acknowledge the cooperation of colleagues in the B.C. Poultry Station and Veterinary Laboratory, and of numerous poultry producers who provided information for this report. Mrs. Linda Brown's patience in typing the manuscript is appreciated and we thank the Provincial Veterinarian, Dr. A. B. Kidd, for his assistance. REFERENCES 1. COMMITTEE ON ANIMAL HEALTH. Methods for Examining Poultry Biologics and for Identifying and Quantifying Avian Pathogens. pp. 109-125. Washington: National Academy of Sciences. 1971. 2. JORDAN, F. T. W. A review of the literature on infectious laryngotracheitis (ILT). Avian Dis. 10: 1-26. 1966. 3. JORDAN, F. T. W. The epidemiology of disease of multiple aetiology: The avian respiratory disease complex. Vet. Rec. 90:

556-562. 1972. 4. RAGGI, L. G. and G. G. LEE. Infectious laryngotracheitis outbreaks following vaccination. Avian Dis. 9: 559-565. 1965. 5. RAGGI, L. G. and G. G. LEE. Duration of immunity to infectious laryngotracheitis. Poult. Sci. 44: 509-514. 1965.

LETTER TO THE EDITOR SHARE YOUR EXPERIENCE DEAR SIR:

It is sad in this day and age to have a drug representative come into the office and inform you that so and so was using this product in such and such a way and getting marvellous results. This happened today and referred to an anti-inflammatory product originally used on horses, but which is now finding some application in some diseases of dogs and cats. Why are these veterinarians not reporting 108

their innovative use so that we can all share. One veterinarian said he had "saved" an eyeball with its use. Think how many eyeballs might have been saved if he had reported it in such a way that we could all benefit. Yours sincerely, J. A. HUrcMisoN, D.V.M. 1814 Bank Street Ottawa, Ontario KIV 7Y6 Editor's note: See Practice Tip, page 115.

Epizootiology of infectious laryngotracheitis in British Columbia 1971-1973.

EPIZOOTIOLOGY OF INFECTIOUS LARYNGOTRACHEITIS IN BRITISH COLUMBIA 1971-1973 L. B. Hayles, K. R. Macdonald, W. C. Newby, C. W. Wood, E. W. Gilchrist an...
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