An Experiment to Challenge the Resistance to Tuberculosis in B.C.G. Vaccinated Cattle in Malawi

flr. vel. J. (1972), 128, 54 1

AN EXPERIMENT TO CHALLENGE THE RESISTANCE TO TUBERCULOSIS IN B.C.G. VACCINATED CATTLE IN MALAWI By F. G.

'WADDINGTON* AND

D. C .

ELLWOOD

Veterinary R esearch Laborator y, K abe te, K enya, a nd Veterinary R esea rch Laboratory, Blantyre, Ma lawi

SUMMARY

An experiment to ascertain if Zebu cattle in Malawi vaccinated with E.C.G. vaccine developed resistance to bovine tuberculosis is described. The vaccinated cattle were mu ch more resistant to infection with Mycobacterium bovis by subcutaneous injection a nd oral administration than were unvaccinated controls, judged both by macroscopic lesions noted post mortem a nd subsequent labo ratory investigations. INTRODU CT IO N

Bovine tuberculosis is a disease with a high incidence in the indigenous cattle population of M alawi which is predominantly Zebu. It has been established by both m eat inspection findings and tuberculin testing that the disease is not uniformly distributed throughout Malawi but is restricted m ainly to the northern part of the country, where incidences of up to 50 per cent have been recorded in som e ar eas. The southern part of the country is virtually free of the disease. Economically the disease cau ses considerable loss through condemnations of carcases at the abattoirs . In 1970, out of 10,946 cattle slaughtered at the Cold Storage Company abattoir in Blantyre- which is the largest in Malawi and draws cattle from all R egions- 114. I carcases (10'4 per cent) had tubercul ous lesions; and whole and part carcases equivalent to 299 cattle (2·8 per cent) were condemned. The incalculable loss due to unthriftiness resulting from the disease is also economically important. The Veterinary D epartment d ecided in the early 1960's tha t a testing and slaughter policy to eradi cate the disease from th e northern parts of the country was impracticable and the possibility of B.C.G. vaccination to reduce the incidence of the disease was considered. This course was adopted at the suggestion ofMr. P. Ba nnister who was then the Director of Veterinary Services. Preliminary trials with B.C.G. vaccine were started on Government Farms as early as 1963, and th e resu lts indicated that the vaccine was safe to use in both allergic and non- allergic ca ttle; it produced a satisfactory conversion rate in non-allergic anim als; a nd vaccination did not cause any significant interferen ce with th e tuberculin test after a period of two years.

* F.A.O.

Veterinary Expert .

542

BRITISH VETERINARY JOURNAL, 128,

I I

It was decided as a result of these trials, to use B.C.G. vaccine* on a large scale in the infected areas. The campaign, involving the wholesale annual vaccination of calves with B.C.G., started in the Northern R egion in 1966, was extended to the Central Region, north of the Lilongwe River in 1967, and has continued annually since then. I t was realized that it would be some years before the efficacy of the vaccine in reducing the incidence of bovine tuberculosis could be ascertained by examination of carcases at the a battoirs. Cattle are usually over five years old when they are slaughtered. In 1967, in order to get an earlier indication of the effect of the vaccine, a challenge experiment was conducted by the authors, and this paper reports the results.

MATERIA LS AND METHODS

The eXjJerimental cattle Twenty-three Zebu cattle were selected from tuberculin-tested Government herds; all had histories of negative reactions to tuberculin tests. Fifteen had been vaccinated with B.C .G. in accordance with the manufacturer's directions and eight unvaccinated a nimals were used as controls. The vaccinates gave negative reactions to mamm alian tuberculin just prior to vaccination with B.C.G. and post-vaccination tests showed that they had converted satisfactorily to the vaccine. The eight control animals were also negative to tuberculin tests applied shortly before they participated in the experiment. The vaccinated cattle varied in age and vaccination history, as detailed in Appendix I. The cattle were assembled on a Government Quarantine Station and were isolated throughout the experiment. Tuberculin testing prior to challenge The cattle were tuberculin-tested twice with Weybridge P.P.D. tuberculins before challenge. Except for the animals which were vaccinated or revaccinated shortly before the tests, all gave negative reactions to the single intradermal comparative test. Challenge material Cultures of Mycobacterium bovis known to be virulent for cattle were obtained from the Government Veterinary Research Laboratories, Onderstepoort, South Africa, by courtesy of the Director. The bacilli were subcultured at the Veterinary Research Laboratory, Blantyre, so that young actively growing cultures were available at the time of preparation of the challenge material. Suspension of bacilli in normal saline were prepared by washing off the growth from solid media on the day that the experimental cattle were to be i~ec ted. The weight of bacilli in a known volume of suspension was estimated by a turbidimetric method and doses for administration to the cattle were calculated. * Glaxo.

B.C.C. VACCINATION IN ZEBU CATTLE

543

Method of challenge The experimental cattle were i~ ected on February 18th, 1968, as follows: Nine of the B.C.G. vaccinated ani mals and five of the unvaccinated controls each received o' I mg of bacilli by subcutaneous i~ ection on the left side of th e neck. Six of the vaccina tes and three of the controls were eac h given 1'0 mg of bacilli orally and this oral administration wa~ repeated ten d ays later. The viability and virulence of the suspension of bacilli were confirmed by the inoculation of guineapigs with I ml by intramuscular inj ection. All four guineapigs used died within a month and all showed generalized lesions of tuberculosis. Tuberculin testing after challenge The cattle were tuberculin-tested at one month and [our months after challenge. Post-mortem examinations The exp eri mental cattle were slaughtered at th e Cold Storage Company's abattoir in Blantyre during the period July 24th to August 7th, 1968. After a normal meat inspection routine on the carcases, the writers examined all the principal lymph nod es for macroscopic lesions by multiple slicing. The results of these examinations are detailed in Appendix 1. Laboratory investigations were carried out a t the Veterinary Research Laboratory, Blantyre, with the obj ect of d emonstrating the presence of acid fast bacilli in lymph nodes where lesions were noted macroscopically and to detect possible latent infection in nodes where no lesions were visible. These examinations included: (I ) The microscopic examination for acid-fast bacilli, of smears prepared from the infec tion site and associated prescapular lymph nodes of all su bcutaneously challenged cattle, and from all other lymph nodes which showed lesions, or were enlarged or indurated. (2) The cultural examination on Lowenstein-J ensen and Stonebrink m edia of th e lymph nodes from animals with and without visible lesions. The nod es selected where there were no visible lesions were the principal lymph nod es of the head and thorax in those a nimals infected by subcutaneous i~ection; and a selection of several m esenteric lymph nodes where infec tion was by th e oral route. (3) Guineapig inoculation. Material from all the bovine lymph nod es submitted to cultural examination was also inoculated into guineapigs by intramuscular i~ection of I ml of a suspension of the macerated tissue in sterile saline. All lymph node material was treated by maceration followed by incubation with freshly prepared 5 per cent oxalic acid at 37 °C for one hour, to eliminate contaminating organisms, before culture and guineapig inoculation. It can be seen in Appendix II that, of the six vaccinates challenged orally, material from only four animals was subj ected to laboratory examination.

544

BRITISH VETERINARY JOURNAL,

128, II

None of the six vaccinates had any signs of lesions at autopsy and a sample of four lots of material was considered sufficient. Note concerning one experimental animal. Before considering the results of this experim ent it should be noted that Animal No. B2 I 1/340 died in April as a result of intestinal impaction. The post-mortem findings are included in Appendix 1. The result of the first post-challenge tuberculin test is included in Appendix I , but is excluded from the comparison of test results given in Table I , as this animal was not subjected to a second test. Post-mortem findings are d etailed in T ables II and III. Cultural examinations a nd guineapig inoculations were not carried out with material from this animal but the evidence from lymph gland smears is included.

RESULTS

Post-challenge tuberculin testing The results obtained from these tests carried out at one a nd four months after challenge are recorded in Appendix I , fifth column; and the results in terms of allergy to mammalian tub erculin only are summarized in Table 1. TABLE I ALLERGY TO MAMMALIAN TUBERCULIN IN T H E FOUR CATEGO RI ES OF EXPE RIM ENTAL CAT TL E, SHO WN BY TUBERCULIN TESTS APPLIED AT ONE MONTH AND FOUR MO N TH S AFTER CHALLENGE I NFECTION WITH All. tuberculosis bo vis

Incidence of states of allergy T est atfour months Test at olle month Positive Doubtful Negative Positive Doubtful Negative

Categories of cattle B.C.C. Vaccinates: Total Subcutaneous chall enge Oral challenge Control Cattle: Not vaccinated, total Subcutaneous chall enge Oral challenge

2

6

3

5

nil

6

2

nil

2

nil

14

II

8

8

6

3

8

6

nil

2

7

nil

5

5

ni l

nil

5

nil

ni l

2

3

ni l

2

5

nil

ni l

It can be seen from Table I that the incidence of positive allergy was decreasing and tha t of negative allergy increasing in the B.C.G. vaccinated cattle. An opposite trend is apparent in the unvaccinated controls. This may imply that infection was becoming established in fewer of the vaccinated cattle than in the unvaccinated controls.

Tuberculous lesions seen at jJost-mortem examination The main object of the experiment was to compare th e degree of development of tuberculosis lesions in the groups of vaccinated and unvaccinated

B.C.G. VACCINATION IN ZEBU CAT T LE

545

cattle, to see if resistance to infection had resulted from the B.C.C. vaccination. The details of the post-mortem findings are given in Appendix I and condensed in Table II. The extent of infection in each category of cattle is indicated as None, Non-progressive and Progressive. The criteria used for judging th e extent of infection were as follows: (I ) None. Indicated absence of infection in both groups of cattle. (2) Non-progressive. Presence of lesions in cattle infected subcutaneously was expected at the injection site, and in the prescapular lymph node receiving drainage from it. The presence of lesions at these sites was regarded as indicating non-progressive infection. (3) Progressive. Lesions at sites other than those indicated in (2) were r egarded as evidence of progressive infection. A comparison of assessment of resistance to infection in the two groups is given in Table III. From this table it can be seen that whereas the unvaccinated controls had no resistance to infection with M.bo vis by the oral or subcutaneous route, the vaccinated cattle did show a resistance to infection by both routes.

Laboratory investigations The results are summarized in Appendix II, condensed in Table II and the two groups of cattle compared in Table III. TABLE II C OMPARISONS OF TYPES OF INFECTION IN CATTLE DET E RMI NE D FROM MA CROSCOP I C LESIONS AND FROM RES U LTS OF LABORATORY INVESTIGATIO NS ( SUMMARIZED FROM DATA GIVEN I N APPE N DIX II )

Categories of cattle

No. of cattle B.C.C. vaccinated Total 15 Subcutaneous challenge :

Extmt of i1ifection in lymph nodessome with no visible lesions- as determined by laboratory investigation

Extent of infection indicated by macroscopic lesions

2

6

Types of ilifection

None Nonprogressive Progressive

No . of cattle

N one

Type of ilifection Non-progressive Progressive

N.V .L.

2

I

6

4

2

9

5

3

Total

9

Oral challenge:

6

None

4

5

Progressive

5

5

3

Progressive

3

3

Control cattle, not vaccinated Total 8 Subcutaneous challenge: Oral challenge:

N.V.L. = No visible les ions

3

N. V.L.

BRITISH VETERiNARY JOURNAL,

128 , 11

A P P E NDIX I S U MM ARY OF

E X PE RIM ENT AL D A T A; E X CLU DI NG RESULTS OF LABORAT O R Y E XAM I N ATIO NS R ECO RD E D I N A PPEN DIX 11

Results of tuberculin

M ,tlwdof

Allimais'

A ges approxirruJleiy at

far tal

18.2.68

numbers

:/:;~~I?:B.

B .C.C. vaccinatio1l datu

bacilli

18.2.68

sIc

lests at 1 and 4 months after challtng~

R

i/c

years

1963 No v.

20 1/335

5 years

1963 No v.

R

R

X3 ~ /3 3 7

5 years

R R

R R

-454/345

JI

0"

mg

686/ 333

2!

years

196 3 No v. 1966 Ju ne

69 6 /3 30

21 years

1966 Ju ne

R

R

6 75/334

'2

1966 and

R

R

R

i/c

yea rs

revacc .

j U 3/33 8

'2

years

1967 Dec. 1966 a nd revacc.

1967 Dec. 1967 Nov.

R

R

R

R

i/c

Neg. i/c

N eg.

R

Neg.

Neg. It

Neg. Neg. R

il2 11 1343

6 m on ths

il2 '4/34 0

6 months

196i No \, .

235/ 326

5 yea rs

1963 Nov.

C I>368/344 689/328

1'2 '2

years years

1963 No v. 1966 and revacc. 1967

Not tes ted

died April

I mg o ra ll y re peated 28.2.68

I m g o;:all y repeated 28.2 .68

1966 and

N eg.

684/3 4 1

'2

years

M674/349 M673/342 j06/348

1

year

6 months 3 yea rs

3.)6:-

::s y('3rS

I{

It

~l ~lo l:n [

~

y ea rs

R

R

439 /3'16

I

R

i/c

R

R

N eg.

i/c

R

Neg.

N eg.

i/c

re vacc. 1967

1967 Nov. 196 7 Nov. Control not

R

vaccinated

~

yea rs

423 / 3'27

6 mon ths

33 7/3 29

'2

yea rs

I

m g ora ll y

re peated

28.2 .68 H8/33 2

l ~'

45 1/339

6 m o nths

H.

=

reacto r j

yf'ars

i/ c = jnconcl us ive.

B.C.G. vACCiNATION IN ZEBU CATILE

547

APPEND IX I SU MMARY OF EXPERIMENTAL DATA; EXCLUDING R ESULTS OF LABORATORY EXAMINATIONS RE CO RD ED IN APPENDIX II

Macroscopic lesions seen at post-mortem examinations of experimental cattle

Lymph Nodes

At sitt of injtction of T .B. bacilli

PreJcapuiar

Encapsulated caseous material Fibrous n odule no caseation No visible lesion Fibrous nodule no caseation

Tlwraci,

Htad

Othe,s

Caseous lesions (2) approx. 6 mm diam.

Encapsulated caseous material

Caseous lesion ( I ) 20 mm diam.

Encapsulated caseous material Encapsulated caseous material

Caseous lesion ( I ) appro x. 6 mm diam. Caseous lesion ( I ) approx. 5--6 mm diam.

Caseous abcess

Mediastinal small focal lesion

Retrophar: left foca l lesion

M ed. and bronc. and ind u,:a led . focal les io ;l in eac h ~1 ed. and bronc. enl a rged and indurated.

R e trophar. and parotid left sid e en larged a nd indu ra ted

Caseous lesion ( I J

Encapsulated caseous material

approx. 5 mm

Not applicable

Caseo us

lesion (4) 6- 10 mm diarn. CaSf'OUS a

Multiple small caseous lesions 2- 4 mm d iam.

bc('ss

c nl ar~ed

Med. focal Caseous a bscess

Caseous lesion ( I ) 3':\ mm diam.

Caseous abscess

I .arge caseous area with some calcifi cation developed

Ca...eous abeess

1\1ultiple caseo us lesions, 4- .'j mm

lesio n Med. en larged and indurated. Branda! caseo us les ions

Retrophal' bo th enlarged and indurated, foca l les ions

( 2) 10 - 1 2 mm

Bronc. caseous les ion 10 mm. Med. and bro nc. enlarged and indurated M ed. and bro nc. en larged and ind urated : focal les ion in each

Re tl'oph al'. left. Caseous les ion 10 mm Retropha r and parotid, hoth enlarged and indurated

r:aseo us

Not aJ)plic ablc

les ions 6- 7 mm. mesenteries of sma ll int(:;stinc One mesenteri c small intest. caseous lesion 6- 7 mm '[\VO

Retrophar and paratiels both enlarged and indurated. Focal lesion in each M ediastin a l en larged and indu rated focal les io n s (0)

- =

No visible les ions.

Rig ht parotid caseous les ion 1 0 mm Retrophar both enlarecd and indurated

1 'hl'ee mesente ries small intestin e caseous lesions 5- 6 mm

BRITISH VETERINARY JOURNAL, 128, APPENDI X Til E

B.C.G.

C HA LLENGE

OF

MA C ROSCOPIC

THE

OF PO ST-MORTEM EXPER IM ENTAL CA TTL E

LESIO NS

AND

DATA FROM

Anima/'s

Ear Tag numbers and da tes of

B.C.C. vaccination

II

EXPER I MENl' - IU;~ lJ LT S

ON SU MMARY

II

Melhod oj challenge wilh T.B. bacilli

0 '1

mg

sic

EXA MI NAT IO NS

LABORATORY I N VE STIGATIONS

E xtent 0/ infection as

indica ted by macroscopic [rsions

Non -progressive

20 I /~35

11)63 X 33/33 7 1963 686/333 1966

Non -progress ive

696/330 1966

N on-prog ressive

67'>/334 1966 and 1967

No n-prog ress ive

7 0 3/33 8 1966 and 1967

No n-prog ress ive

B211 / 343 1967

Prog ress ive . Died Apri l

B214/ 340 1967

Non- prog ress ive

235/3 26 1963

I

mg

None

ora ll y tw ice

G P368/ 344 1963

None

689/328 1966 and 67

No ne

68 4/341 1966 and 67

None

M674/ 349 1967

None

M673/ 342 1967

None

COlltrol not vaccillatf'd 706/34R

0 -1

m g sic

Prog ress ive

Prog ress ive

330/331

Progress ive

439/336

Prog ressive

Pwgress ive

337/329

1·0

mg

Prog ress ive

orally twice Progress ive

451 /339

Progressive

B.C.G. VAcciNATION IN

ZEBU CATTLE

549

APPENDIX II THE B.C.G . CHALLENGE

EXPERIMENT-RESULTS OF POST -M ORTEM EXAMINATIONS ON THE EXPER IM ENTAL CATTLE

SUMMARY OF MACROSCOPIC LESIO NS AND

DATA FROM

LABORATORY INVE STIG ATION S

r .B.

Evidence of infection with bacilli from the results 0/ laboratory lxamiTUltion of lymph nodes by smears. cultures and guineapig inoculations

Smears

Cultures

Extent of infection as indica ted by laboratory invutigations

Guintapigs

Inj. site only posi tive

Prescap. mediastinal and bronchial positive

All negative

Progressive

Negative

Prescaps. only positive

All negative

Non-progressive

Negative

Negative

All negative

None

Prescap. positive

Prescap. positive

Prescap. positive

Non-progressive

Prescap. positive

Prescap. med. and bronc. positive

Prescap. med. and bronc.

Progressive

Prescap. positive

Negative

Prescap. positive

Non-progressive

Pl'escap. JlO.'itive

Negative

Prescap. positive

Non.progressive

Ini. site prescap. and thoracic

Not done

Not done

Progressive

Prescap. positive

Presca p. positive

Prescap. positive

Non·progressive

Negative

Negative

Negative

None

Negative

Ne~ative

Negative

None

positive

•• • not examined •• •

Not assessed

. • • not examined •••

Not assessed

Negative

Negative

Negative

None

Negative

Retrophar. and mesenteric positive

Retrophar and mesen teries positive

Progressive

Prescap . med. and bronc. posi tive

Negative

Mediastinal and bronc. positive

Progressive

Prescap. positive

Negative

Med. and bronc. positive

Progressive

Prescap . and bronc. positive

Prescap. and bronc. positive

Prescap. med. and bronc. positive

Progressive

Prescap . retrophar. and bronc. positive.

Prescap. bronc. and Retrophar. positive

Prescap. and retrophar. positive

Progressive

Negative

Negative

Prescap. med. and bronc. positive

Progressive

Negative

Negative

Med. and bronc. positive

Progressive

Mesenteric positive

Retrophar and mesenteric positive

Mesenteric positive

Progressive

Mesenteric positive

Negative

Mesenteric positive

Progressive

55 0

BRITISH VETERI NARY JOUR NAL,

128, II

The criteria used for the classification of the degree of tuberculous infection in the experimental cattle wer e the same for the laboratory investigation, as those quoted for the findings p ost mortem, except tha t here the results are quoted on evidence of the p resence of bacilli and not on visible lesions (see T able II ). Cultures isolated from the infected a nimals were identified as M.bo vis. TABLE III COM P A R ISON OF ASSE SSMENT OF R ES ISTANCE TO I NFECTION I N B . C . G. VACCIN A TED CAT T LE AND U NVACCINATED CONTROLS DETE R MI NED F R OM THE DATA I N A P PENDIX II

Resistance to infection ill each category of cattle. Assessed from macroscopic lesiollS at autopsies Categories and No . of cattle

Fully resistant

Resistant progressive infection

Assessed f rom the result." of laboratory investigations

No resistance

Fully resistant

Resistant progressive infection

JVO

resistance

B.C.C. vaccinated

Subcut. challenge 9 Oral challenge 6

2

6

3

6

I

3 (2

not assessed)

Controls not vaccinated

Subcut. challenge 5 Oral challenge 3

5

5

3

3

It can be seen from Tables II and III tha t the vaccine was not so effective, judged by the result of the laboratory investiga tions, as it appeared to be from a consideration only of the m acroscopic lesions. Nevertheless the vaccine stood up to this more stringent laboratory assessment quite well for practical purposes. DISCUSSION

It has never been envisaged that the field use of B.C.G. vaccine in Malawi will eradicate bovine tuberculosis. The vaccine is being used with the expectation that it will produce greater resistance to the disease and by so doing should check the spread of infection among the herds and reduce the incidence of progressive lesions with consequent diminution of losses due to condemnation of tuberculous carcases at the abattoir. If therefore the vaccine produces a resistance to progressive infection in cattle, it can be regarded as satisfactory for the purpose for which it is intended. When considering the results of this experiment it should be noted that the doses of bacilli given to th e cattle subjected them to a far greater challenge than is ever likely to occur under natural conditions. It may be noted tha t the detection of viable tubercle bacilli by laboratory

B.C.G. VACCINATION IN ZEBU CATTLE

55 1

methods in lymph nodes with no visible lesions does not necessarily imply that these bacilli would have produced lesions in the future. One of the authors (Waddington) has had experience of experimental infection of cattle with Mycobacteria in K enya and, after subcutaneous inoculation of live bacilli in the neck, has found lesions in the prescapular lymph nodes and in the thoracic lymph nodes as soon as eight weeks after infection. In this Malawi experiment one animal, B2 I 1/343, died some eight weeks after infection and tubercular lesions were found in the lymph nodes of the thorax and head. It may also be noted that of the four B.C.G. vaccinated animals in which there was some evid ence of progressive infection, one animal, 454/345, was an adult when vaccinated in 1963, some four-and-a-half years prior to challenge, and possibly immunity had waned; two others, B21 1/343 and M673 /342, had been vaccinated some three months prior to challenge and possibly immunity had not developed fully at the time of the challenge. This challenge experiment has in the writers' opinion shown that the B.C.G. vaccine appears to produce a useful degree of resistance to progressive tuberculosis. ACKNOWLEDGEMENTS

The authors wish to express their gratitude to Mr R. G. Mares, Director of Veterinary Services, Malawi, and those members of his field and laboratory staff who assisted with this experiment; particularly Messrs P. Gascoyne and G. Stima, Meat Inspectors at the Blantyre abattoir and Mr R. Wheeler, Livestock Marketing Officer, in charge of the Quarantine Station. They record also their sincere thanks to Mr P. Bannister, Permanent Secretary, Ministry of Agriculture, for his permission to publish this paper. (Acceptedfor publication 7 July 197 2 )

Une experience testant la resistance de la tuberculose chez les betails vaccines par Ie B.C.G. au Malawi (Waddington et Ellwood) Resume. Une experience s'assurant que les betails de Zebus vaccines par Ie B.C.G. au Malawi d eveioppaient une resistance a la tuberculose bovine est decrite. Les b etails vaccines etaient plus resistants a l'infection par Mycobacterium bovis par injection sous-cutanee ou administration orale que ne l'etaient les b etails de reference non vaccines, ceci ayant ete apprecie d'apres les lesions microscopiques no tees en post-mortem et des investigations de laboratoire ulterieures. Bericht iiber einen Versuch in Malawi die Resistenz von nUt B.C.G.-geiInpften Rindern gegen Tuberkulose festzustellen (Waddington und Ellwood) ZusaDUnenfassung. Ein Experiment wird beschrieben, mit dessen Hilfe festgestellt werden sollte, ob Zebu-Rinder, die in Malawi mit B.C.G. Vakzine geimpft worden waren, Resistenz gegen Tuberkulose des bovinen Typs entwickelt hatten. Man fand, dass die geimpften Rinder widerstandfahiger gegen Mycobacterium bovis waren als ungeimpfte Kontrolltiere, und zwar sowohl bei Infektionen durch subkutane Injektion wie bei solchen durch orale Administration. Dies wurde anhand der makroskopischen Befunde bei der Sektion wie bei den darauf folgenden Laboratoriumsuntersuchungen bestatigt.

55 2

BRITISH VETERINARY JOURNAL, 128,

II

Un experinUento para probar Ia resistencia a Ia tuberculosis en el ganado vacuno vacunado con B.C.G. en Malawi (Waddington y Ellwood) ReS1UDen. Se describe un experimento realizado para determinar si el ganado vacuno Zebu en Malawi vacunado con vacuna B.C.G. desarrollaba resistencia a la tuberculosis bovina. El ganado vacuno vacunado se mostro mucho mas resistente a la infeccion con Mycobacterium bovis por inyeccion subcutanea y administracion oral que 10 hicieron los con troles no vacunados, a juzgar tanto por las lesiones macroscopicas notadas post mortem como por las subsiguientes investigaciones de laboratorio.