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BREEDING FOR RESISTANCE TO MAREK 'S DISEASE

Dr. R. K. Cole

Dept. of Poultry ScienceCornell Univ.

Ithaca, New York

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"BREEDING FOR RESISTANCE TO MAREK 'S DISEASE

The effect of heredity upon susceptibility to Marek's disease, or fowl

paralysis as it was formerly designated, has been known ever since Asmundson

and Biely (1932) first demonstrated significant differences among families,

and it has been obvious to any one who has analyzed data on pedigreed chicks

from a population in which the disease has been present. Biely etal (1933)

further demonstrated that the disease could be transmitted by the incoulation

of l- to 7-day old chicks and that it spread to a similar level (30%) among

the pen-contact controls, with mortality occurring as early as 37 days of

age. Isolated controls had only a few cases, none of which occurred before

? months of age. One-fifth of the pullets had tumorous ovaries while a

smaller percentage had similar-type tumors in other organs. It is perfectly

clear that they were working with the same disease that we now know as Marekls

and possibly with the so-called acute type. They further suggested that

resistance might be due to a simple dominant gene.

Exploitation of the demonstrated differences in levels of susceptibility

to the disease (considered by most people in the past as the avian leukosis

complex) by breeding for resistance seemed a logical approach to the problem

of control. It was attempted at several Agricultural Experiment Stations

(Bearse eta l, 1963; Hutt and Cole; 1957, Marble 1939; and others). Varying

degrees of success were obtained. In retrospect it now appears that one of

the major impediments was the inclusion of two etiologically unrelated

neoplastic diseases under the one name, that of the leukosis complex. The

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apparent need to rely on natural exposure to demonstrate genetic differences

in resistance further complicated the procedure because of (1) the variation

and uncertainty in level of actual exposure so provided and (2) the now-known

differences in methods of natural spread of the two diseases. Until recently

there has been no inoculation technique that produced results which were of

practical value to a selection program designed to improve viability under

field conditions.

In spite of these difficulties, at Cornell we have been able to develop

two strains of White Leghorns (K and C) that are quite resistant to the com-

plex and one (S) that is extremely susceptible. It is obvious that in the

resistant strains there has been simultaneous selection for resistance to

both major components of the complex, now considered as Marek's disease and

lymphoid leukosis. The selection has been effective because of two major

factors. One has been the purposeful exposure of the newly hatched chicks

of all three strains to indirect contact with adult birds for the first two

weeks of life. Recently the method of exposure has been improved by the use

of a wind-tunnel by which all air for ventilation during the initial brooding

period originates in a house of adult birds. Secon@ly, the extremely

susceptible S-strain not only provides a monitor system for the level of actual

exposure but also provides for a significant level of infected birds,

presumably discharging infectious material into the environment, within the

population. Resistance expressed under such conditions cannot be assigned to

a lack of exposure. Data for the last three years reveal a mortality from

all types of neoplasia in the resistant strains that averages a little over

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3% v__sover62% for the susceptible strain. The difference for the current

year (1966-67) will be even greater_ A major portion of the losses in the

S-strain is due to Marek's disease, but this strain also shows a significantly

higher loss from presumably lymphoid leukosis than do either of the two

resistant strains.

Sevoian et al (1962) found that the Cornell S strain was extremely

susceptible to his JMvirus (Marek's disease). Further comparative tests

indicated that the Cornell resistant strains required 700 to lO00 times more

of the agent to produce the same frequency of lesions, following inoculation.

Reciprocal crosses between the S and K strains were intermediate in sus-

ceptibility to the JM agent.

The nice parallel between susceptibility to the JM-virus preparation

and susceptibility expressed under our conditions of rearing and exposurei

at Cornell suggested that both procedures were probably measuring the same

type of resistance or susceptibility. If so - why not speed up the process,

ensure a known level of exposure, and identify resistant and susceptible

families in a short 6-to-8-week period rather than the long one required

when we rely on natural exposure to induce the disease.

RESISTANCE UNDER I_v_BORATORYVS FIELD EXPOSURE

In 1963 I turned to the use of the JM virus as a means of controlling

level of exposure. We naturally thought first of the many pedigreed cockerel

chicks that were discarded after hatching and sought to use them to evaluate

, the sire for his ability to beget resistant progeny. The question to bel

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asked was whether the results obtained with a sample of a breeder's progeny

(cockerel chicks) inoculated with the JM virus would be indicative of per-

formance of the pullet chicks from the same sire when reared under normal

field conditions.

The results of tests on 38 sires (Cole, 196h) indicated extreme differences

among the strains, and some significant differences within the resistant

strains in susceptibility to the JM inoculation technique. Within strain,

where selection would have to be practiced, there were only relative minor

differences in susceptibility among the pullet progenies, hence correlations

between the two measures of resistmnce were not meaningful. However there

was some indication of a positive relationship between the two measures of

resistance. The technique was then applied to cockerel chicks from 32 different

strains of White Leghorns, 27 of which were in the New York RandomSample

Test. The Cornell K strain was the most resistant (18%), the Cornell S strain

the most susceptible (96%), while the commercial strains ranged from 28 to 78%.

Because of low levels of losses among the pullets that constituted the entries

in the RS Test, there was no opportunity to correlate susceptibility to the

JMvirus, in the shor_-term laboratory test, with mortality among the pullets

under field conditions of exposure. A repeat of the same procedure on lO

commercial strains in 1965 again revealed significant strain differenoes in

susceptibility to the JMvirus and also differences among the pullets, as entities

in the RS Test. This time there was some indication of a positive relationship

between the two measures of resistance.)

We had also looked at the problem from the opposite direction. We

reasoned that if there actually was a relationship that would be of value in

the development of resistant lines why not select for such resistance in

the laboratory first and then test the resulting stocks under field conditions.

SELECTION FOR RESISTANCE TO THE JM VIRUS

The Regional Cornell Randombred stock, obtained from the Federal Laboratory

at Purdue University, was used as a base from which we hoped to develop, by

testing and appropriate selection, sub-lines that would differ significantly

in their susceptibility to the JM virus. Previous tests of this RB stock

revealed a susceptibility of 54 - 56%. I have now completed two generations

of progeny-test selection and breeding for response to a standardized

procedure of exposure to the JM virus and the results will provide the basis

for the present report. The testing of the 3rd generations for resistance to

the JM virus in the laboratory and the testing of the derived lines under

field conditions of natural exposure will be Completed in the next 18 months,

Testing procedure:

\\ Fully-pedigreed chicks are inoculated i.p. on day 2 with 1/_ ml of a

2-1/2% tumor-tissue suspension (gonads) in saline to which 100,OO0 units of

penicillin and 0.05 g of streptomycin are added per lO0 ml of inoculum.

Tumor material is taken from 12 or more donors which had been inoculated 6

to 8 weeks previously with JM material. Chicks are randomized and brooded Sn

batteries for 8 weeks. Birds that show well-developed symptoms and those

_ that die are posted at that time. The locations of gross lesions are recorded.

At the completion of the 8-week test period all remaining birds, including

those that show no symptoms, are posted and the presence or absence of gross

lesions determined and so recorded, Testing facilities are in separate

isolated quarters_

Basis for selection of breeders:

Birds chosen to produce the next generation of potential breeders for

testing are selected entirely on the basis of the response of a good sample

of their progeny to the inoculation test with JMvirus. Birds used to

produce chicks for testing or for the production of the next generation of

breeders are never intentionally exposed to Marek's disease. They are raised

under conditions of management that result in a low mortality from all causes

and actually only a very few cases of any form of the leukosis complex.

Where possible, some selection for rate of egg production or semen production

is practiced to ensure maximum reproduction during the 28-day period during

which hatching egg_ are saved_

Breeding schedule:

Programming for the breeding and testing procedure is such that it is

feasible to produce adequate numbers of progeny, test them for resistance to

the JM virus, and thereby select potential breeders on the basis of their

progeny test and then to produce the next generation of breeders for testing -

all within one calendar year. Birds are first used for production of hatching

eggs when they are 6-1/2 months old. The system would have to be modified

} to allow for selection for traits of economic importance to _hich every breeder

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of commercial stock must direct his attention. However, it is entirely

feasible to progeny-test cockerels for resistance to the Marek's disease

agent prior to the time decisions have to be made as to which sires will

be assigned to pure-line single-sire matings.

Scope of the selection program:

It was decided to select equally in two directions, starting with a stock

in which there had been no intentional selection for resistance to either

Marek's disease or lymphoid leukosis. Selection pressure was to be moderately

heavy and limited to resistance or susceptibility to the JMvirus. Choice

of the Regional CURB stock was based on its known and intermediate reaction

to the JM virus and the fact that it would be available later in the project

as a control strain. All matings were made by artificial insemination and

the breeders were housed in individual cages.

Matings of 25 sires, each to _ dams (obtained as hatching eggs from the

Federal Laboratory), provided the original population for testing. From among

the 25 sires and 95 dams with progeny that had been tested equal pressure

of selection (5 extreme sires and 20 extreme dams) was used to provide parents

for first generation within the JM-N and JM-P lines. Equal numbers of first

generation JM-N and JM-P breeders (15 sires and 60 dams per line) were tested

for their ability to produce resistant or susceptible progenies. As in the

initial generation these tested breeders were reduced to the 5 best sires

and 20 best dams, within line, and then used to produce the second-generation

i . parents for subsequent testing. From these last matings 15 sires and 75 dams per

line were used to produce the 2nd-selected generation of chicks for testing

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with the JMvirus. In all the matings for testing, as well as those for

reproduction of the lines, close inbreeding has been avoided.

Results:ml

The combined results of tests of the first three generations were as

follows:

Chicks Susceptible to JM virus

tested Total mange for Sire familiesGeneration (no.) _ (%)

GO 1618 51.1 16 to 89

G 2340 51.3 9 to 99I

G2 2547 52.6 4 to I00

There was considerable variability among the families from the initial

25 sires and 95 dams that were tested, (Go). Sire familiesp averaging 65

chicks each, ranged from 16 to 89% susceptible while the smaller-sized

(aver. 17 chicks) dam families ranged from 0 to 100% (Cole, 1966).

The breeders selected to produce the Ist selected generation of the JM-N

line averaged 25 and 17% susceptibility for the sires and dams respectively.

For the JM-P line the corresponding susceptibilities were 75 and 92%. There

was, therefore, initially a considerable difference in the progeny-test

performance of the breeders used to establish the two new lines, (Table i).

When the subsequent un-exposed progeny from these selected sires and

dams were evaluated for their ability to transmit resistance or susceptibility

(GI chicks) greater variation among sire families (9 to 99%) existed than

in the previous test of non-pedigreed breeders. The 1186 chicks from the 15

sires and 60 dams within the JM-N line had a susceptibility of 31% vs 72%

for the 1154 chicks from the 15 sires and 61 dams within the JM-P line. After

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only one generation of selection the t_ lines were quite different in their

response to the JM virus, under the testing procedures employed.

Matings to produce second generation chicks for testing, employing

15 sires and 75 dams per line, produced 2_47 chicks with an average of 83

and 87 per sire family and 17 per dam family, within the two lines. Although

the mean susceptibility for the entire population was essentially the same

as that for the GO and G1 populations, progeny from the JM-N line breeders

had a susceptibility of only 12.9% v__s90.7% for progeny from the JM-P line

breeders.

Among the sire families that constituted the G1 population of chicks

for testing, only two JM-N families exceeded the population mean in susceptibility

and likewise only two JM-P families were less susceptible than the mean for

_he entire population. For the G2 generation there was no overlapping of

sire families and, in fact the most susceptible one in the JM-N line was 26%

v__sa minimum of 84%, for the least susceptible sire family of the JM-P line.

Among the dam families of the G2 generations, 75 per line, only one

family (60%) within the JM-N line exceeded 50% susceptibility while among

the JM-P line dam families only one (57%) was _ ss than 60% susceptible.

The sharp distinction between the two lines, after only two generations of

progeny-test breeding, suggests that under appropriate testing and selection

programs it should be possible to develop a high degree of resistance to the

JM virus of Marek's disease.

)

SOME PRACTICAL CONSIDERATIONS

Short cuts in testing and breeding procedures are warranted when results

of comparable value and accuracy are obtainable with less effort or in

shorter time. One of the most tedious tasks required for the testing program

we have outlined is the post-mortem examination of all the test bi_ds, especially

at the termination of the 8-week test period. At such a time there is also

the pressure of getting all the results tabulated and appropriate decisions

made as to which birds qualify for use as breeders. One might question if

the autopsy work is really necessary.

Dead and paralyzed birds:

Starting with the hth week there is an increasing number of such birds

that are available for autopsy each day. Among those removed because of

typical symptoms of paralysis there is only an occasional one in which gross

lesions cannot be readily recognized° It would seem, therefore, that such

birds might be recorded as susceptible and discarded without autopsy. For

birds which die without showing symptoms of paralysis, practically all have

proved to have gross lesions of some vital organ or nerve trunk. Only rarely

have I found among such birds, under our conditions of testing, any that

have to be recorded as negative to the JMtest. Such sporadic cases as do

occur are at random and, even if considered as positive for susceptibility

to the JM virus, would not materially change one's evaluation of a sire family.

I am not recommending that no atuposy be done, but only that enough be done

to ensure that some other disease is not present to confound the mortality picture.

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S_ptom-free birds:

At the completion of the 8-_eek test period there remains a large number

of birds which are part of the test population but for which the presence

or absence of the disease can be determined only by an autopsy. The

proportion of such birds that are affected with gross lesions of Marek's

disease will vary with the stock being tested, and of course being less in

those groups which have higher levels of resistance. Is it really necessary

to autopsy all these birds?

The basic objective of the testing is to identify the most resistant sire

families and if this can be done with a reasonable degree of accuracy without

all the post mortem examinations - so much the better. To answer this

question I have analyzed 7 different groups of strains or sire families to

correlate the rank of the group or family on the basis of the frequency of

mortality and symptoms during the test period with the ranking when post-

mortem data on all of the progeny are considered. The rank-order correlations

_s values) are all quite high, being significant at the P =_.O1 level for

the G2N group and at P =<.OO1 for the other 6 groups, (Table 2).

When selecting for resistance at the 20% level, 20 of the 25 most

desirable sires or groups have been chosen and none selected would have been

as low as the median. If the best third were to be selected then 33 of such

_2 sires or strains would have been chosen and only two would have been below

the level of the median. The greatest discrepancy occurred among the 15 G2N

sire families of which 5 had had no mortality nor symptoms but a few cases

of gross lesions among the surviving birds. With the mean level of symptoms

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at 3.8% and of total incidence at 12.5% it takes only a very few susceptible

birds to disrupt the rank-order correlation. However among the 7 most

resistant families 5 would have been identified on the basis of the frequency

of symptoms or mortality during the test period.

Influence of sex of the test chicks:

In most tests that have been reported there has been a higher susceptibility

to sgents of Marek,s disease among pullet chicks than among cockerel chicks.

What bearing, if any, does this have on a testing program designed to improve

genetic resistance to Marek,s disease?

One might argue that under commercial conditions the breeder of egg-

production stock is more concerned with the resistance among the pullets.

Should he, therefore, do his testing of sire families by using pullet chicks?

On the other hand, the large number of pedigreed cockerel chicks that are

normally destroyed could be used instead for testing of the families for

resistance to Marek,s disease providing the results obtained with them would

indicate comparative resistance among the groups or sire families of pullets.

My initial use of the JM virus was to test cockerel chicks, on the theory

that they would serve the purpose adequat&ly. However, for the populations

within the JM-N and JM-P lines chicks of both sexes have been used because

it was convenient to do so. When the results of those tests are analyzed on

the basis of sex it is clear that the females are more susceptible than are

the males, (Table 3). The sex difference is greatest when the mortality level

is neither very high nor very low. When susceptibility is relatively high

the sex difference is more pronounced during the test period than at the

end when all data are complete.

Rank-order correlations were calculated for various groups of sires or

strains, comparing the rankings based on male progeny with those based on

female progeny. When the data are analyzed on this basis the number of test

animals and therefore reliability is lower than when all the data are con-

sidered. Some rankings were based on as few as 13-1h progeny (of one sex),

but the averages exceeded 30 birds of each sex in every comparison. In all

but two cases (G2 populations) when sex differences were quite small, thews

values were significant at the .O1 level of probability, (Table _).

DISCUSSION

The influence of heredity upon resistance or susceptibility to the JM

virus of Marek's disease is clearly indicated, not only by highly significant

differences among existing strains of chickensand among sire families within

strains, but also by the very great difference that was obtained in this

experiment following only two generations of progeny-test breeding. Selection

appeared to be equally effective for resistance and for susceptibility. For-

tunately there was considerable genetic variability within the Randombred

stock from which the unselected breeders were taken for the initial test.

There is a certain amount of evidence which suggests that resistance

to the JM strain of Marek's disease in the laboratory is indicative of

resistance to the field strains of Marek's virus. Mills (1966) has stated,

but supporting evidence was not given, that "resistance to natural and

artificial exposure to Marek's disease can be taken as one". I presume that

he was referring to the Biggs B-lh strain of Marek's virus. It remains

to be proven that stocks selected specifically for resistance following

artificial exposure by inoculation with one of the laboratory strains of

Marek's disease virus will prove to be equally resistant when tested under

commercial conditions of natural exposure to field strains of the virus (s).

There is also the very important question of which of the several available

strains of Marek's disease agent should be used in such a testing program.

These viruses do vary in their p_tency and in the lesions produced and hence

resistance to one might not indicate resistance to another one nor to the

field strains. Perhaps the JM-N and JM-P lines can be used to answer some

of these questions. They will be tested in 1968 (as pullets) and raised at

Cornell along with chicks from the K, C, and S strains, under conditions that

provide for intentional natural exposure. It should, therefore, be possible

to determine whether the demonstrated resistance to the JM virus by the JM-N

line will be matched by good resistance under field conditions of natural

exposure.

The testing program employed has made use of very good numbers of progeny

from each sire. The number of chicks probably exceeds that required to obtain

a good indication of relative resistance. It would, no doubt, be better to

test more sires, each with fewer progeny, for a sample of 20 to 50 should be

adequate. However, if one were to test dams few would produce fa_lies

large enough to measure an all-or-none trait. Under commercial conditions of

poultry breeding the testing of sires would seem to offer the best opportunity.

These could be tested early in life, by mating to a random sample of

appropriate females. Results of the tests would be known about the same

time as the initial evaluation of the female sibs for egg production, egg

size, etc. Such a procedure would permit prior selection among potential

sires for resistance to Marek's disease. An alternative would be to use the

cockerel chicks, normally discarded. Obviously some entire sire families would

then need to be eliminated on the basis of high susceptibility to Marek's

disease among the inoculated male sibs. It seems apparent that although there

is a sex difference in resistance, chicks of either sex, or a mixed group,

can be used equally well to evaluate the sires.

Evaluation of sire families on the basis of the proportion of the progeny

that show symptoms of paralysis or die during the test period will identify

the most resistant families. To do so eliminates the need to autopsy any

of the birds, save for occasional examinations to rule out mortality from some

other disease. Accuracy might be improved by the use of larger-sized families

and perhaps by extending the test period.

SUMMARY

Strains of White Leghorns that differ in resistance to Marek's disease

under conditions of natural exposure tend to react in a corresponding way to

the JM virus, following inoculation and observation over an 8-week test period.

By selection on a basis of progeny-test only (of both sires and dams)

it has been possible in two generations to establish a JM-N line which has

the low level of susceptibility of only 12%. From the same base (Regional CU)

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Rnado_breds) and at the same time a JM-P line was derived which is

characterized by a high susceptibility of over 90%. The abilltyof these

lines to survive under field conditions of exposure remains to be determined.

Although the sex of the test chicks influences the level of susceptibility,

it does not interfere with the identification of the more resistant sire

families. The testing of sire families for relative resistance can be accomplished

without the need for post-mortem examination of the test animals.

Analysis of existing data suggests that selection for resistance or for

susceptibility on a family basis will be equally effective.

REFERENCES

Asmundson, V. S. and J. Biely. 1932. Inheritance of resistance to fowlparalysis (Neurolymphomatosis gallinarum). I. Difference in sus-ceptibility. Can. J. Res. 6: 1Tl-175.

Bearse , G. E., _I.A. Becket, and C. M. Hamilton. 1963. Resistance and

susceptibility to the avian leukosis complex in chickens. PoultrySci. 42:110-121.

Biely, J., V. E. Palmer, and E. A. Lloyd. 1933. Fowl paralysis (Neurel_pho-matosis gallinarum). Proc. Vth World's Roultry Congress IIIi 143-150,

Cole, R. K. 1964. Strain difference in response to the JM leucosis virus.Poultry Sci. 43: 1308-1309.

Cole, R. K. 1966. Genetic resistance to the JM-leucosis virus (Marek,sdisease). Poultry Sci. 45:1077.

Hutt, F. B. and R. K. Cole. 1957. Control of leukosis in fowl. J.A.V.M_A.131:491-495.

Marble, D. R. 1939. Breeding Poultry for Viability. Penn.Agric. Exper.Sta. Bull. 377.

J

Mills, R. F. M. 1966. Lymphoid leukosis and Marek's disease in a practicalpoultry breeding programme. The Genetic Control of Leukosis and

Marek's Disease. One-day Scientific Conference, London, Oct.l_, 1966.(Organized by Houghton Poultry Res. Lab., Ranks Hovis McDougall Ltd.,and F & G Sykes Ltd.)

Sevoian, H. D., D. M. Chamberlain, and F. Counter. 1%2. Avian lympho-matosis. Experimental reproduction of the neural and _isceral forms.Vet. Med. 57: 500-501.

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Table i. - Testing and selection of breeders used to establish and tomaintain the JM-N and JM-P lines.

Breeders tested Breeders selected

Progen_ Suscept. Progen_

Gen. Sex No. Aver. _ Aver. ___ No. Aver. _

JM-N Line

GO o" 25 65 27-91 51 16-89 5 25 16-37

95 17 1-29 51 0-i00 20 17 0-26

GI o" 15 79 62 -95 31 9-60 5 15 9-22

60 20 7-30 31 0-i00 20 II 0-21

G2 d 15 83 61-102 12 4-26 5 7 4-9

9 75 17 5-25 13 0-60 20 4 0-Ii

JM-P Line

GO o_ 25 65 27-91 51 16-89 5 75 66-89

95 17 1-29 51 o-Ioo 20 92 75-1o0

GI o_ 15 77 58-88 72 37-99. 5 87 80-99

9 61 19 7-26 73 19-100 20 93 81-100

G2 o_ 15 87 62-I01 91 84-100 5 97 95-100

9 75 17 3-28 91 53-100 20 98 92-100

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Table 2. - Rank-order _orrelations, among strains or sire families, forsusceptibility to JM virus based on total incidence vs frequencyof symptoms and mortality.

Strains Total Symptoms &or incidence mortality

Stock Sires Aver. Range Avel_U-----N_ge rs P-value

(no.) -_ _ _ "

RS Tests 32 52 18-95 43 14-91 +.943 .00_

Strains 9 58 35-98 31 13-92 +.933 .OO1

C 25 51 16-89 31 2-51 +.892 .O01O

G N 15 31 9-60 12 0-30 +.904 001I

GIP 15 72 37-99 42 9-88 +.782 .001

G2N 15 12 4-26 4 0-15 +.716 .01

G2P 15 91 84-100 68 43-86 +.872 .001

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Table 3. - Sex difference in susceptibility to the JM virus.

In Perscent._6b_ end of week no.Gen____t. Sex _-- 7 L8 Final

G2N _ O.2 O.2 I.4 i.9 3.7 12.3dif.* -0.2 0.0 -0.4 0.2 1.2 1.2

G N _ 0.2 I.O 3.6 4.5 9.1 28._I dif. 0.3 0.2 1.2 3.1 5.7 5.3

G o" 1.3 4.8 9.4 16.2 26.1 48.10 dif. 0.8 4.5 7.0 9.7 ii.0 7.0

GIP o" 0.2 4.5 8.9 14.3 33.9 67.5dif. i.I 4.1 9.3 13.7 16.2 8.7

G2P o" 0.9 6.5 16.0 30.6 59.1 88.3dif. 0.0 4.6 12.3 22.9 18.5 4.8

*- Per cent. for oo++ less that for _.

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Table 4. - Effect of sex of chicks on the ranking of sires forsusceptibility to the JM virus

Strains oo progeny oo progeny

or Number Suscept. Number A_Stock Sires Av. Range Av. Range Av.-'-_ rs-- -VJ?.)

G N 15 43 28-54 12 2-29 40 33-53 14 2-36 +.2652

GIN 15 38 33-46 29 11-62 40 26-50 33 6-66 +.909

GO 25 33 14-48 47 7-86 32 13-48 54 20-92 +,867

GIP 15 37 28-47 69 36-97 40 29-54 76 38-100 +.829

G2P 15 43 26-55 89 79-100 43 36-51 93 81-100 +.583

Misc. 9 52 46-60 53 30-96 52 46-61 63 38-100 +.750

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DR° R. K. COLE - "BREEDING FOR RESISTANCE TO MAREK'S DISEASE

D. V_ ZANDER: Have you tested cells from your Marek's Resistant line

for resistance to RIFA & B?

COLE: No_ but embryos that wolfef_.llsibs to G1 generation breeders,

produced for testing,_have been examined by Dr. Crittenden. Additional embryos

will be examined by klm next spring and these will come from sires and dams

to be selected as parents from among the G3_generation breeders that will be

tested next winter.

A. W. NORDSKOG: What is the origin of the JM virus and where can it

be obtained?

COLE: The JM virus was isolated by Mr. Martin Sevoian, University of

Massachusetts, from a field-case of the disease. You would have to contact

Dr, Sevoian for a stock of the agent_ which he has been willing to release

under appropriate conditions_ which now include supervision by a veterinarian.

D. HARRIS: %_]_atwould you say about two possible modifications of your

procedure? l° ) Inject "spreaders" and a]so get a measure of resistance

to contagion°

2°) Inje,_-ta _J__:tureof virus strains rather than just JM.

COLE: (1) The data indicate that resistance to the injection is probably

the same as resistance to infection by contact. It takes only a short time

to inject each individual chick and thus to ensure known and uniform

level of exposure. I would prefer to rely on the injection of each chick.

j There is the further advantage that the length of the test period can be

kept quite short relative to that required following exposure by contact,

especially when the level of resistance is high.

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(2) If resistance to each of the several strains of the virus is

independent then very few birds would likely survive such a test procmdure.

From a practical viewpoint, if the mixture of viruses is restricted to those

prevalent under field conditions then such a modification should be satisfactory.

J. H. QUISENBERRY: Couldn't commercial breeders use approximately the

same selection pressure with one or two generations (one half or one year)

delay for multiplication assuming that resistance stands up under field

pressures?

EOLE: Yes, It is a question of relative importance of the several

traits for which selection is being practiced. Considerable selection pressure

could be exerted on sires by testing them for resistance to Marek's disease

prior to the normal breeding season and then assigning only the most resistant

ones to the single-sire breeding pens. I do not believe it is feasible

for the commercial breeder to test individual dams for resistance to Marek's

disease.

W. HECKER: Doesn,t your rapid response to selection indicate selection

for a single gene?

COLE: I prefer to consider resistance as a polygenic trait. Of course

at this stage I have no way of knowing the number of genes involved.

K. GOODWIN: Would the use of another measure of response, such as

days of survival, be'useful in a procedure of this sort?

COLE: Yes. In general, as the data on one of the slides showed, the

most resistant families had a delay in appearance of symptoms and mortality.

- lO3 -

In the G N generation 5 of the 15 sires had no cases of symptoms or mortality2

during the 8-week test period while among the most susceptible sire families

most of the chicks died during the test period, and hence at an earlier age.

R. N. SHOFFNER: Because of the tremendous divergence of the up and down

lines, major genes are suggestive for the "resistance-susceptibility,,response.

Do you think that selection for egg and semen production would result in a

correlated response?

C_JE: No. The selection practiced was simply among several available

full sibs and covered a production period of only 4 weeks. The same type of

selection was applied in both lines.

G. E. DICKERSGN: Have you tested for presence of RAV 1 or RAV 2 anti-

body in your exposure?

COLE: No.

G. E. DICKERSON: What evidence is there that JM is a virus transmitted

in cell free material?

COLE: Dr. Sevoian has transmitted the disease with cell-free material

and has produced a relatively high level of infection, with all types of

lesions being produced° The disease is also easily transmitted by air and

presumably in such cases the transfer of cells does not occur.

DAVID: Is the inoculum fresh suspension? Can there be differences in

virulence of the agent? Is Marek's Disease caused by a virus?

COLE: The inoculum I use is a suspension of fresh tumorous gonads.

, Marek's agents do vary in virulence as the various strains used by Dr. Peter

- IO_-

Biggs so nicely illustratsd_ Personally I consider the sgent of M_rek's

disease to be a virus although, to the best of my knowledge, no one has seen

it under the electron microscope.

* Dr. Crittenden presented evidence on the lack of correlation of

resistance to _ and Lymphoid Leukosis. Dr. Crittenden pointed out that in

the East Lansing lines they have been unable to show that there is any

correlation between resistance to Lymphoid Leukosis and resistance to Marekts

Disease.

-_. Since Marek's disease and lymphoid leukosis are caused by distinct and

unrelated agents I would not expect that resistance to one would be associated

with resistance to the other. There is considerable evidence from other

sources to indicate a lack of a common resistance. This does not mean that

a given stock cannot be resistant to both diseases. Our K and C strains at

Cornell are, we consider, relatively resistant to both.

* Comment

_ COLE'S answer to comment°