26
VOLE
VACCINATION
[Vol. XXIII, Part I,
The vole bacillus possesses certain characters, which distinguish it from the
human, bovine or avian types of tubercle bacilli. One of the important distinguishing
features is its shape. The majority of the bacilli are bent into various forms, which
have been likened by Griffith [1942] to a shepherd's crook, a sickel, a spiral or the
letter S. It is not uncommon to see the bacillus even forming a complete circle.
There are, however, characteristics of the vole bacillus, which make it impossible
to classify this organism otherwise than as a type of tubercle bacillus. The vole
bacillus has, therefore, been recognised as a third mammalian type of tubercle
bacillus and is named as the 'murine type'. It occurs naturally, mostly in wild
voles (Microtus agrestis) and also in bank vole (Clethrionomys glareolus), the wood
mouse (Apodemus sylvaticus) and shrew (Sorex araneus). Wells [1937] found it in.
wild tuberculous voles in aforested parts surrounding Lake Vyrnwy Estate in Wales.
Such naturally infected voles were found in other places in England as well.
Vole bacillus was found to be incapable of producing progressive disease in
laboratory animals, except when administered in very large doses. This observation
suggested the possibility that a passing infection with this organism might confer
some protection against subsequent infection with human or bovine tubercle bacilli.
Wells and Brooke [1940] studied the resistance produced in guinea-pigs on
vaccination with vole bacilli to subsequent infection with virulent tuberculous
organisms.
The guinea-pigs were killed after eleven weeks of the test dose. On,
post-mortem examination it was observed that the animals which had received an
immunising dose of vole bacilli showed notably less tuberculous infection than the
controls or those which had been injected with B.C.G. Similar experiments were
also carried out on guinea-pigs by Griffith and Dalling [1940]. Ten animals were
given subcutaneously 10 mg. of a culture of vole bacilli divided in nine doses, eight of
1 mg. each and the last of 2 mg. The interval between consecutive injections was one
week. Four weeks after the last dose the immunised animals and the five controls
were each injected subcutaneously with 0.001 mg. of virulent bovine-tubercle bacilli.
All the controls died of severe generalised tuberculosis in an average of 87 days,
whereas the vaccinated animals died in an average of 152 days also of severe genera-
ralized tuberculosis. They found in another experiment that controls died in an
average of 152 days, also of severe generalized tuberculosis. They found in another
experiment that controls died in an average of 71 days and the vaccinated animals
which were killed 94 to 95 days after infection, showed lesions of the disease of
a moderate degree. They concluded that all the animals vaccinated with vole
bacilli were more resistant to tuberculosis than the controls. Living tubercle bacilli
were, however, present in the lesions of the vaccinated animals, which suggests
that these bacilli would have caused progressive fatal disease when the immunising
effect of the vole acid-fast bacilli had faded. Similar experiments were repeated
on calves. The results of their preliminary experiment on calves [Griffith and
Dalling, 1940] are given below:
Nine calves were given varying doses (total quantity of the culture received by
different calves ranged from 1.1 mg. to 125 mg.) of vole bacilli, by the intravenous
or subcutaneous routes or by the combination of both and two animals were kept as
controls. These animals were killed 208 to 223 days after infection with a dose of
virulent bovine tubercle bacilli, which was sufficient to produce progressive