1.The object of this work was to follow the course of infections and pathological effects of a relatively non-virulent species of avian malaria caused by P. rouxi. These observations were used to make a comparison with other types of malaria in birds, monkey, and man. Twenty-four adult female canaries were used in this work. Over 1000 slides of tissue sections and blood smears form the basis of the parasitological and pathological studies.
2.The spleen responded rapidly to infections with P. rouxi. The resultant splenomegaly was due mainly to an increase of the mononuclear cells in the red pulp. The mononuclear cells were phagocytic, Deposition of pigment in the spleen was heavy and in a band-like pattern.
3.Very active phagocytosis occurred in the Küpffer cells of the liver. One case of vacuolation of the hepatic cells was seen during the peak of the infection but as no additional ones were seen it was concluded that this response was not a constant accompaniment of this infection. In general, the infection caused no disorientation nor necrosis of the liver parenchyma.
4.The bone-marrow became hyperplastic during the acute stage losing some but not all of the fat cells.
5.Both the bone-marrow and spleen showed a hyperplasia even in old chronic stages. The spleen remained enlarged and soft, and microscopically showed a continued hyperplasia of the red pulp. Much pigment was present in the spleen even in a six-months old infection.
6.The Küpffer cells of the liver remained large and actively phagocytic in the chronic patent cases.
7.A comparison of the pathology of infections caused by P. rouxi, P. cathemerium (birds), P. brasilianum (monkeys), and the plasmodia of man was made. In general, the human plasmodia had the most widespread and severe effects and P. rouxi the least.
8.P. rouxi exerted its greatest effects on the spleen, bone-marrow, and liver. There was less initial activation and hyperplasia of these organs than that found in P. cathemerium.
9.Activation of the spleen, bone-marrow, and liver persisted at a high rate and for a period of time (at least six months) much longer than that found in P. cathemerium (one to two months).
10.The immunological responses of infections with P. rouxi and P. cathemerium were reviewed. The type of host reaction elicited by P. rouxi (low) correlated with its immunity to superinfections (relatively inefficient) suggests that the severity of the primary attack of avian malaria is an important factor in determining the immune response; the more intense the attack, the greater the immunity conferred.