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Blood Parasites of Mammals in New Zealand

Hepatozoon musculi (Porter, 1908) — (Plate 1, Figs. 1–10)

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Hepatozoon musculi (Porter, 1908)
(Plate 1, Figs. 1–10)

Miller (1908) established the genus Hepatozoon for an intraleucocytic parasite pathogenic for white rats which he named Hepatozoon perniciosum, a name which has since fallen as a synonym of Hepatozoon muris (Balfour). Members of the genus described at an earlier date had been considered to belong to Haemogregarina or Leucocytozoon, and the priority of Hepatozoon was not generally accepted for many years. Porter (1908) described as Leucocytozoon musculi a white cell parasite which she discovered in the blood of white mice in England. A year later Porter (1909) wrote: ". . . the structure and life history of avian Leucocytozoa are still subjects of controversy, and as the name Leucocytozoon was first applied to parasites of birds . . . the generic name Leucocytogregarina might be used for the highly specialized parasites of mamalian leucocytes, which have a different habitat from the strict Haemogregarines of red corpuscles." Sangiorgi (1912), in recording Leucocytogregarina musculi from Turin, was the first to use the name Leucocytogregarina in a generic sense, as Wenyon (1926) points out. Coles (1914) considered a parasite found free in the plasma of Mus sylvaticus to be closely allied to that described by Porter from Mus musculus, but suggested the name Haemogregarina sylvatici for his parasite in the event of its proving to be a new species. Yakimoff and Schokhor (1917) recorded Leucocytogregarina musculi from Petrograd (Leningrad), and Porter (1919) herself used Leucocytogregarina as the generic name of the mammalian haemogregarines. Wenyon (1926) finally established the priority of Miller's generic name Hepatozoon for these parasites.

Hepatozoon musculi has not previously been recorded from Australasia, although, as it probably transmitted through the agency of an ectoparasitic mite or louse (Porter, 1908; Sangiorgi, 1912), it is to be expected that its range will be found to correspond with that of its cosmopolitan host (c.f. Trypanosoma lewisi). Hepatozoon muris (Balfour), recorded from West Australia (Cleland, 1906) and. New South Wales (Johnston and Cleland, 1909), has not yet been found in New Zealand.*

A moderately heavy infection of H. musculi is recorded from a specimen of Mus musculus Linnaeus trapped in Wellington during March, 1948. Thirty-seven other mice examined during the survey were negative for haematozoa. The infected mouse had been dead for some twelve hours when a heart-blood smear was made. All but one of the parasites (Plate 1, Fig. 10) on the smear were free in the plasma. This is probably due to the fact that intraleucocytic forms had become free at some stage between the death of the host and the drying of the smear. Hepatozoon infections are usually characterized by a predominance of intraleucocytic parasites. Thus Yakimoff and Schokhor (1917) failed to find any free forms of H. musculi, while Porter (1908) and Sangiorgi (1912) found a predominance of intraleucocytic forms in fresh material. Coles (1914), who found only free forms of Hepatozoon in his preparations from Mus sylvaticus, failed to state how fresh his material was.

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Plate 1

Plate 1

Fig. 1: Erythrocytes of Mus musculus Linnaeus. Figs. 2–10: Hepatozoon musculi (Porter, 1908), from M. musculus. Fig. 11: Erythrocytes of Oryctolagus cuniculus (Linnaeus). Figs. 12–20: Yeast-like organisms resembling Hepatozoon cuniculi (Sangiorgi, 1914), from O. cuniculus.

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My average measurements for the free forms of H. musculi are 11.3μ by 4.0μ, while the size range is 10.6μ to 14.8μ by 3.8μ to 6.3μ. These figures compare favourably with those of Porter (1908), 10.9μ by 5.1μ (range 7.0μ to 17.0μ by 4.0μ to 5.9μ). Sangiorgi (1912) gives no averages, but his size range compares well with mine, 9.4μ to 14.2μ by 3.8μ to 6.3μ. Coles (1914) gives an average only, his figures of 15.5μ by 6.1μ coming towards the higher level of the size range of other investigators.

Free forms of H. musculi are reniform to crescentic bodies which occur in my material both with capsules (Plate 1, Figs. 4–9) and without them (Plate 1, Figs. 2 and 3). The ends are usually rounded, sometimes almost equally so (Plate 1, Figs. 4 and 7), but more frequently one end is markedly more pointed than the other (Plate 1, Figs. 3, 5, etc.). The close association of the members of the group of four parasites without capsules illustrated in Plate 1, Fig. 2, in a smear where three or four minutes' search was usually required for the discovery of a single parasite, points to their being recent products of schizogony. Non-encapsuled free forms are probably merozoites. These invade leucocytes of the host, and during the intracellular stage (Plate 1, Fig. 10) the capsule is developed. According to Porter (1908), intraleucocytic stages of H. musculi have average measurements of 8.0μ by 5.0μ, while Yakimoff and Schokhor (1917) give the size range as 8.0μ to 10.0μ by 4.0μ to 5.0μ. The single example on my slide measures 10.8μ by 4.4μ. Free, encapsuled forms are fully developed gametocytes.

In all stages of H. musculi encountered in my material, the finely vacuolated cytoplasm stains light blue with Giemsa. The cytoplasm is often markedly granular, and the small granules, which are particularly evident along the course of the myonemes (Plate 1, Figs. 3 and 5), take a deep blue stain and at times (Plate 1, Figs. 8 and 9) appear almost black. The nucleus is generally situated towards one end of the body, but it may be approximately central. This structure, which may be circular, oval, band-like, or irregular in shape—as will be seen from the figures—assumes a deep pink stain, its chromatin being aggregated into dense granules staining deep red. A prominent terminal cap of chromatic material may be present at one (Plate 1, Figs. 2–4, 8–10) or occasionally both extremities, as stated by Porter (1908). From five to seven myonemes (Plate 1, Figs. 3 and 5) may be traced in suitably stained parasites (Coles, 1914, mentions six or seven). As Coles also observes, the myonemes may give the nucleus a longitudinally segmented appearance (left-hand parasite, Plate 1, Fig. 9).

Broad (Plate 1, Fig. 7) and slender (Plate 1, Figs. 4–6) gametocytes may be distinguished, but it is doubtful whether there is any real sexual dimorphism. Such a hypothesis is favoured by the occurrence of cases of association between large and small gametocytes (Plate 1, Fig. 8), but there is by no means always a significant difference in the sizes of the gametocytes concerned. Porter (1908) and Sangiorgi (1912) also record and figure such association between the gametocytes of H. musculi. The process is possibly an artefact as seen in preparations of vertebrate blood. By analogy with the behaviour of H. muris in the gut of the mite Laelaps echidninus, noted by Miller (1908), such association between gametocytes in vertebrate blood cooling after the death of the host probably represents the commencement of the process leading to fertilization and zygote formation which normally takes place in the gut of the invertebrate host.

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It should be emphasized that on purely morphological grounds the parasite in my Mus musculus material fits available descriptions of Hepatozoon muris quite as well as it does those of H. musculi. The authenticity of many of the described species of Hepatozoon is questioned by Porter (1919) and Wenyon (1926), the former suggesting ". . . it is quite likely that the various leucocytogregarines now given separate specific names . . . are really only varieties of the first described species, L. canis (James, 1905)," while Wenyon postulates that Hepatozoon musculi and H. muris may well be one and the same species. Nevertheless, many mammalian blood parasites exhibit a close host specificity, and this fact necessitates the exercise of caution in determining their specific identity. At the same time, by analogy with the facts that H. muris occurs in several species of the genus Rattus, and that Trypanosoma lewisi and T. duttoni occur in more than one species of Rattus and Mus respectively, it seems unnecessary to regard the parasite recorded by Coles (1914) from Mus sylvaticus as distinct from Hepatozoon musculi on grounds of host specificity alone.

* Since this paper went to press, H. muris has been recorded from Rattus norvegicus in Raoul Island, Kermadec Group—Laird, M., 1950. A New Locality for Two Blood Parasites of Rats (Raoul Island, Kermadec Group). N.Z.Sci.Rev., 8 (9–10), 91–92.