A vaccine reaction in young broilers
SUMMARY
This report describes an unusual occurrence of severe neck
lesions and poor growth in broiler chickens.
PRESENTING PROBLEM
Flocks of chicks appeared to develop well during their first
few days but then a proportion became dull and lethargic, and
in the second week this was associated with a dramatic
increase in mortality.
HISTORY
Year:1981 Country: Brazil Case : 6215
Bird Species: Chicken Bird Type : Broiler
Breed: Ross Age: 7-15 days
Clinical signs: Lethargy, chicks huddling together, reduced
feed consumption, stiff necks.
Birds on farm: 60.000
Birds in flock: 10.500
Birds affected: 25%
Total Mortality: 800
Mortality last 3 days: 800
Age when problem first noted: Day 3
Feed type: Commercial broiler starter feed (Purina)
Water source: Well
Water treatment: Chlorinated by manual dosing
Litter type: New wood shavings, condition normal
Management: Experienced grower, considered good
Vaccination History:
Marek : <DELETED> s.c. in neck at 1 day (hatchery)
Newcastle : (Salsbury Eye drop) Due at day 12
Pox : <DELETED> s.c. in neck at 1 day (hatchery)
Bronchitis: Not used
Treatment already performed : Chloramphenicol and polyvitamin
solution for 3 days in drinking water (Days 5-8).
CLINICAL AND PATHOLOGICAL INVESTIGATION
Clinical examination:
About 30 chicks were presented in the original submission.
Chicks were small for age, and remained huddled together and
immobile. Some had visible thickening of the neck. In a few
this was very marked and was associated with dyspnoea.
Palpation of the neck confirmed this visual impression. As
well as the thickening there was marked reduction in the
mobility of the neck. Chicks were sacrificed by CO2 gas to
avoid damaging the neck area.
Post-mortem examination:
The thickened area in the neck was seen to involve the
sub-cutaneous tissues and the muscles. The muscles were pale,
swollen and easily separated by virtue of inter-muscular and
sub-cutaneous exudation. In a few localised areas the exudate
was becoming caseous. Samples were taken aseptically for
bacteriological examination. Additional samples of muscle
were homogenised in buffered Marek's disease vaccine diluent
to which
antibiotics were added. A few small lesions typical of
fowl-pox were also seen in the area of the neck. Examination
of all other organs did not reveal marked abnormalities except
for the small body size, little food in the intestine, and
bile stasis in some chicks.
Initial action:
The presumptive diagnosis was bacterial infection due to a
contaminated Mark’s disease vaccine. The flock was medicated
with a soluble polivitamin preparation while awaiting the
bacteriological examination. Specific guidance was given to
the farmer with respect to the following aspects of management
:
Maintenance of brooder temperature appropriate to the size
rather than the age of the chicks.
Maintenance of easy access to feed (trays) and drinkers (leave
the fountains in and service them).
The hatchery was contacted and it was confirmed that similar
problems were occurring in other flocks. On questioning about
any changes in vaccination procedures it was established that,
while combined fowl-pox and Marek's disease vaccination had
been in regular use during the summer months (peak fowl-pox
challenge) for some years, the affected flocks coincided with
a change of vaccine manufacturer. Bacterial contamination was
still suspected and the vaccine manufacturers were informed.
Use of the product was suspended pending the outcome of tests.
Results of laboratory tests:
Bacteriological culture of affected tissues on blood agar,
McConkey agar and Brilliant Green agar were uniformly
negative. Bacteriology of the vaccine and diluent was
performed by inoculation in nutrient broth in other
laboratories and was also negative. This caused us to consider
the possibility of viral aetiology. Triturated muscle tissue
from lesions was inoculated on to chorio-alantoic membranes
(CAM's) of SPF chick embryos. This resulted in a heavy
concentration of lesions typical of pox-virus after 3-5 days.
Cytoplasmic inclusion bodies were confirmed in the CAM's. To
test the hypothesis that this particular pox-virus was
invading the tissues and causing the observed reaction, some
CAM's were homogenised in vaccine diluent and injected
sub-cutaneously (0.1 ml per bird) in the neck region of 10
conventional broiler chicks. In-contact control chicks were
injected with distilled water. All inoculated chicks
presented the same pathological and clinical features as were
found in the natural disease. Control chicks remained normal
during the 10 day observation period. Unfortunately neither
the original affected tissue, the vaccine or the CAM's used to
reproduce the disease were titrated to determine the number of
infective particles.
Outcome : This and other affected flocks tended to reach a
peak daily mortality between 10 and 20 days of age, the total
often reaching a total mortality of 15% or more. Although the
original evaluation suggested that 25% of the birds were
affected, it became obvious that a much higher percentage of
birds were or became at least mildly affected. Weight gain was
very poor, the flocks became very uneven and many birds were
not suitable for marketing. The vaccine manufacturer admitted
blame and compensated affected growers. The total cost of
this compensation was said to be in excess of US$50.000.
Discussion : A similar problem has been seen in broiler flocks
in Georgia (Dr Caswell Eidsen, pers. comm.). Workers there
attribute its occurrence to the use of a vaccine strain which
is highly chick-embryo-adapted. When it is administered into
day-old chick tissues (which are not that different to those
of the embryo) it manages to replicate in tissues in which
fowl-pox virus would not normally replicate. In areas where
broilers are grown in open-sided houses (ex. South-Eastern
U.S. and South America) it is quite common to apply fowl-pox
vaccine mixed with the Marek's disease vaccine, especially
during the summer months when mosquitoes tend to transmit
fowl-pox. It is normally recommended that the Marek's vaccine
be diluted first, that the fowl-pox be diluted with the
Marek's diluent prior to mixing the two, and that normal
aseptic precautions be observed. Good protection against both
diseases can be achieved without any adverse effects. It
transpired that the vaccine manufacturer had offered a package
of vaccine (Marek's + "mild" fowlpox) in competition to the
brand which had been in use in the hatchery for a number of
years. However this was done without the knowledge of
technical staff at the vaccine laboratory, and, in fact, this
particular fowl-pox vaccine had never been tested for
innocuity in day-old chicks using the sub-cutaneous route. It
had been tested using a standard technique (wing-web stab) in
older birds. Subsequent testing proved that it was easy to
reproduce the clinical condition seen in the field by s.c.
injection of this "mild" vaccine in day-old birds. This case
is a dramatic illustration of how an iatrogenic disease can
cause large financial loss, and emphasises the need to take
great care in selection of vaccine strains and their mode of
application.
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Paul Mc Mullin MVB CertPMP MRCVS
MSD AGvet
Hertford Road
Hoddesdon
Herts
Current (10/98) Contact Details:
E-mail:PaulMcMullin@Compuserve.com
Note for Certificate Candidates:
This copy of the case-histories were prepared for the RCVS library after
the examination. The author is identified. Materials to be presented for
examination should however, be anonymous.
Acknowledgments should be provided separately
(see examination regulations for further details).
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