Plasmodium relictum (Grassi and Feletti, 1891)
Plate 2, Figs. 1–12

Doré's (1920) generic record of Plasmodium from Turdus ericetorum is now expanded to Plasmodium relictum on the basis of an examination of one of his preparations in the Victoria College collection and of fresh material (three of eight thrushes shot at Mangere in July, 1947, one of these birds also being infected page 8 with Haemoproteus danilewsky as mentioned earlier). P. relictum is also recorded from another of Doré's hosts, Turdus merula, two birds shot at Mangere in July, 1947, being infected, out of a total of 14 examined. Passer domesticus is now listed as a host for Plasmodium relictum for the first time from New Zealand (two out of 11 birds infected, one collected at Mangere in October, 1944, by a member of the staff of the Royal New Zealand Air Force medical laboratory, and one at Wellington in October, 1948). The English sparrow is well known as a host for P. relictum in other countries (Coatney and Roudabush, 1936). The parasite rates for all three New Zealand hosts were rather light, averaging fewer than 20 plasmodia per 10,000 erythrocytes.

In addition, an extremely light infection of Plasmodium sp? is recorded from one of 210 specimens of Anas poicilorhyncha. The parasitized bird was shot near Lake Wairarapa, Wellington, in May, 1947. Anas poicilorhyncha is a new host for Plasmodium. During these studies no material was examined from Anthus novaeseelandiae and Alauda arvensis. Doré's description of Plasmodium from these hosts are too general to be of much assistance in specific identification, and the slides which he deposited in the Dominion Museum, Wellington, are too badly preserved to be of use in this respect.

Trophozoites of P. relictum in my material have rather alveolar cytoplasm staining sky blue with Giemsa, and a small chromatin mass staining bright pink. Ring-forms are some 2μ in diameter, and amoeboid forms (Pl. 2, Fig. 2) measure from 2.8 to 5.6μ by 2.0 to 4.0μ in their greatest dimensions. Three trophozoites only have been found in a smear of the peripheral blood of the specimen of Anas poicilorhyncha referred to above. These are elongate reniform bodies (Pl. 2, Fig. 3) of more regular outline than is usual in trophozoites of P. relictum. As neither schizonts nor gametocytes are present in the preparation, specific identification cannot be made. Wolfson (1939) inoculated P. relictum into ducks (Anas boschas domestica), and found that in the first transfer parasites could only be demonstrated by subinoculation of the blood of infected ducks into canaries. At a later stage, gametocytes of P. relictum found in the blood of the ducks were atypical, being elongate instead of round. Wolfson suggests that the differences in morphology between P. relictum in ducks and other hosts may be due to physico-chemical differences between the red blood cells concerned. From the facts that only one of 210 specimens of Anas poicilorhyncha examined for haematozoa was positive for Plasmodium, and that the infection is so extremely light and the trophozoites atypical, it appears likely that this duck is not normally a host for Plasmodium. The presence of all but the youngest trophozoites of Plasmodium relictum, also those of Plasmodium sp? of the grey duck, within a red corpuscle, usually causes displacement of the host-cell nucleus to a greater or lesser extent.

Schizonts of P. relictum encountered in heart-blood smears from the song thrush (Pl. 2, Figs. 5–9), blackbird, and sparrow (Pl. 2, Fig. 4) have alveolar cytoplasm page 9 which stains sky-blue and from eight to sixteen chromatin masses staining light to deep pink. A varying amount of brownish black pigment is present. The young red cell containing a schizont with 12 nuclei seen in Pl. 2, Fig. 6, has been engulfed by a large mononuclear leucocyte. This was found in Doré's smear from a song thrush heavily infected with Plasmodium relictum and Haemoproteus danilewsky. Schizonts vary in size from 4.9 to 7.9μ by 4.0 to 6.0μ. Host cells containing them are often distorted and slightly hypertrophied, and their nuclei are markedly displaced.

Macrogametocytes (Pl. 2, Figs. 10 and 12) have rather dense cytoplasm which stains deep blue and an irregularly shaped nucleus staining deep pink. The pigment granules, which are scattered throughout the cytoplasm, are round or ovoid and variable in size. They are never elongate or rod-like as are those of Plasmodium cathemerium (Manwell, 1938). Macrogametocytes are usually round or ovoid in shape, and dimensions range up to 7.0 by 6.5μ. Microgametocytes (Pl. 2, Fig. 11) are somewhat smaller than macrogametocytes. They have rather hyaline cytoplasm which stains light blue, and a diffuse nucleus rather larger than that of the macrogametocytes and staining light pink. The pigment granules resemble those of the macrogametocytes but are fewer in number. Host cells containing gametocytes have their nuclei markedly displaced, and they are usually distorted in outline and slightly hypertrophied.

The Plasmodium recorded from Turdus ericetorum, T. merula and Passer domesticus during the present survey is identified as P. relictum from the morphological features seen in Giemsa-stained blood smears. Its gametocytes are round, oval, or irregular in shape, a feature shared by only five well-established species of Plasmodium from birds, three of these each being restricted to a single host species (Manwell, 1938). The other two species, P. relictum and P. cathemerium. may be distinguished by the shape of the pigment granules of the gametocytes, those of the former species being round or ovoid and those of the latter elongate or rod-like. As the pigment granules of the gametocytes of the species under discussion are round or ovoid, this parasite is identified as P. relictum.