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Transmission of Ranikhet disease of fowls
Virus in water. One hundred cubic centimetres water into which 2.0 gm. fresh infected spleen
had been stirred was put into the drinking pots of each of the seven birds. Each bird took appro-
ximately 17 c.c. of the suspension within half an hour and was then segregated Another fowl was
given 0.5 gm. of tissue suspended in 5 c.c. water, the mixture being pipetted on to the back of the
throat. All the eight birds contracted the disease and died within 4-7 days after the infective drink.
EXPERIMENTS ON INHALATION
Two wooden boxes were so constructed that through an opening on one side the neck of the
fowl would protrude, the base of the neck being held in position by means of a cardboard collar.
The two boxes were so placed that there was a distance of two feet between the beaks of the two
birds, one infected 48 hours previously with Ranikhet disease virus and the other healthy. The
entire structure was covered with a piece of muslin cloth for ventilation. After three hours' exposure
the healthy bird was removed; four days later, it developed symptoms of the disease and died on
the fifth day. The day after removal of the first healthy bird, another healthy one was exposed
under identical conditions to the infected bird now in its third day from inoculation. In this case
also the disease was conveyed through the expired air to the healthy bird which died on the seventh
day. Another healthy bird was similarly exposed to a fowl infected four days previously showing
signs of acute illness. In this case infection was not transmitted and the bird was later found sus-
ceptible. In three subsequent trials the distance was increased to three feet, the time of exposure
and other conditions remaining unchanged. In all these, however, the results were negative.
In a second series of experiments carried out under similar conditions except that the muslin
was replaced by thick cloth of close texture so as to minimise air currents, a healthy bird was ex-
posed for three hours to an infected fowl at a distance of two feet. Fifteen days later the exposed
bird proved susceptible to Ranikhet virus. This experiment was repeated thrice with similar results.
In another experiment a healthy bird was exposed under the same conditions but for a duration of
six hours with negative result. In a third experiment of this second series involving three infected
and three healthy fowls the distance was reduced to one foot. However, in only one case did the healthy
bird contract the disease, dying on the 7th day.
DISCUSSION
It appears that Ranikhet disease virus is rendered inert in the digestive tract. The above
results may be explained by the fact that, whereas solid food is quickly swallowed, during the process
of drinking fowls fill the mouth, raise the head and swallow each gulp gradually so that the mouth,
pharynx and larynx become contaminated. It seems likely therefore that infection takes place
via. the nasopharynx and before the virus enters the crop, since swabbing the throat with the virus
easily produces infection. There is little doubt however, that under natural conditions droplet
infection is the most common method of transmission. The experience of Farinas [1930] that infec-
tion is not readily transmitted between cages may be explained by the lack of air currents favourable
for spread. In droplet infection the chances of transmission by coughing or breathing largely depend
on the distance between the infected and susceptible. If the distance is too great, the virus may be
diluted beyond its infective potentiality. After expulsion the virus may be carried by air currents
some distance from its source and contaminate the surrounding areas. This explains how, as some-
times noted in practice, an outbreak of Ranikhet disease starting at one end of a poultry farm may
spread in a few days to the interior and even to the farthest corner of the farm.
SUMMARY
1. Ranikhet disease virus, enclosed in a gelatin capsule, within wheat dough or mixed with
grains, when introduced directly into the digestive tract does not produce the disease in susceptible
birds.
2. Susceptible birds contract the disease when allowed to drink water containing Ranikhet
disease virus in the form of tissue suspension from infected birds.