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The method of infection in these cases is certainly direct, from contact with other rats dead from plague or suffering from plague, or plague infected material. It is a well known fact that the dead body of a rat is usually devoured as food by other rats.
The forms of plague as met with in rats may be divided into acute and chronic.
In the acute form, death occurs in a few days. There is loss of appetite, the hair becomes ruffled, and they usually sit in a corner of the cage curled up, pre- senting a very listless appearance. The post-mortem examination shows a marked inflammatory re-action in the spleen, lungs and liver. The spleen and liver are much enlarged, dark in colour, and contain enormous masses of bacilli. The lungs are hyperemic, the heart is full of fluid and tarry looking blood, which also has numerous bacilli. Frequently the peripheral lympathic glands are enlarged, and these may present all the appearances commonly met with in a Bubo in human subject.
In the chronic form, death may not occur for some considerable time, even for months. The post-mortem examination presents indurative swellings in the lungs, liver and spleen, some of which are usually in a condition of so called caseation. Plague bacilli are found in those areas which are capable of transmitting the disease.
In addition to rats, a number of other dead animals, as mice, dogs, cats and fowls, have been examined for the presence of B. pestis.
The number of fowls examined is small and the result was always negative. Four dogs were examined with a similar result. In the case of mice, a number have been examined, and in many distinct evidence of plague infection was found. So far as cats are concerned, the results obtained are highly interesting. Towards the end of the year one of the Godown Companies in Kowloon, who keep large num- bers of cats for destruction of rats, observed a higher death-rate than usual amongst their cats. Several of these were forwarded to me for examination, and in most cases the post-mortem examination showed unmistakable evidences of marked plague infection. Subsequently the cat mortality in these godowns greatly in- creased, and all the cats were forwarded to me. In almost all instances the cats died of plague. Further, several cats were forwarded to me immediately they showed signs of illness. They were kept under observation in isolation. Their chief symptoms was loss of appetite, ruffling of the hair, marked wasting, and in some cases paralytic phenomena showed themselves in the hinder extremities. In one instance the cat lived for over a week. In other two they died about 48 hours after I had them under observation. The post-mortem changes were mainly those of acute congestion with the presence of plague bacilli in the heart, blood and spleen. In one instance a small bubo was found in one groin, but did not present the extensive hæmorrhagic extravasation present in human beings.
The occurrence of plague infection in cats is one of the greatest importance from a domestic point of view. Undoubtedly the cats at present under considera- tion contracted plague from the plague infected rats in the godown. That the rats there were infected had conclusively been proved previous to the infection in the cats. The method of infection here had probably been in the majority of cases a direct one, per os, although of course other possible channels of infection cannot be completely disregarded.
THE GOVENMENT VACCINE INSTITUTE.
During the greater part of the year, Dr. CLARK, the Acting Colonial Veterinary Surgeon, had charge of this Institution.
In the month of October the duties of Director of the Vaccine Institute were taken over by the Government Bacteriologist.
The preparation of Small-pox Vaccine was at once commenced.
The total number of calves paid for the manufacture of vaccine was 23, an expenditure of $150.
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The total number of tubes of vaccine prepared during the year was 3,652, of which the Government Bacteriologist personally prepared 2,475.
The value of this lymph was $1,460.80.