The early development of infection with Trichinella spiralis in the host and its relation to treatment

THE EARL Y DEVELOPMENT OF INFECTION WITH TRICHINELLA SPTRALIS IN THE HOST AND ITS RELA TION TO TREATMENT

BY

R. F. PHILLIPSON*, W. E. KER.SHAW

Department of Parasitology and Entomology, Liverpool School of Tropical Medicine, England

Experimental Results

Experiments were carried out in some 800 mice with standard doses of infective larvae obtained by artificial acid-pepsin digestion of infected muscle, and, after muscle-teasing in the early stages of infection or after digestion in the later stages, the numbers of larvae deposited were counted. The following results were obtained.

1. Production and Deposition of Larvae

a) No larvae were recovered from the muscles during the first five of days infection (i. e., after the ingestion of infective larvae).

b) Larvae were recovered from the mu scles on the sixth day.

* Seconded by May - Baker Ltd., of D agenham, Essex, England.

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c) The number of larvae recovered increased steadily from the sixth to the 13th day, after which few further larvae were found.

d) Individual larvae are believed to remain in the blood-stream for only a very short time, since they are very difficult to find in the blood.

2. Growth of Larvae

a) Larvae recovered from the muscle in the earliest stage ofinfection are the same size as those expelled by the adult female worms and as those found in the blood.

b) The rate of growth of the larvae during the first few days is slow; it accele- rates between the 12th and 20th days , and then slows down again until the maxi- mum length is reached about the 24th day.

c) On the 11th day larvae are able to live in norma! saline, and on the 17th day they become coiled in the muscle, arc encased in a cyst, and resist digestion in acid-pepsin; they become coiled after extraction, and on death remain curved instead of straight. They are also, after the 17th day, infective to new hosts.

d) The length of fully grown larvae varies in individual mice; in generał, female larvae are larger than the males.

Significance in Chemotherapy

In mice, any drug intended to reduce the number of larvae deposited must be given before the 13th day, since by that time most of the larvae will have been deposited. In infections in man, the earliest time at which trichinosis can be sus- pected is 10-14 days after infection, when insomnia and oedema of the face begin to occur. If the infections in mice and in man develop at the same rate , then , in our opinion, the search for a drug to reduce the production of larvae by the fornale worms is hardly worth pursuing.

It is questionable whether attempts to kill the larvae already deposited will

assist the host, since the most serious clinical manifestations are associated with

lesions in the central nervous and cardiovascular systems, and no larvae are found

in these tissues, though there is widespread focal necrosis and an intense eosino-

philic cel\ular reaction. It must be assumed that, altough larvae are deposited in

these tissues by the blood, they do not survive to develop. That the reaction of the

host to the parasite, rather than the direct effect of the parasite on the host, is

responsible for the clinical manifestations is supported by the markedly beneficial

effect of cortisone.