Rhoptry proteins of Plasmodium sp. participate in host cell invasion and intracellular parasite development. In this review, the major rhoptry proteins of P. falciparum are discussed with respect to their importance in the biology of Plasmodium species and as malaria vaccine candidates. The morphology and organization of the rhoptries in Plasmodium species are compared with those of other apicomplexans, and the contributions of apical complex proteins to invasion in ookinetes and sporozoites are discussed. Furthermore, the significance of host cell binding by apical complex proteins, and their role in host cell invasion among the different invasive stages is also reviewed.

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The 110 kDa Rhop-3 rhoptry protein of Plasmodium falciparum is secreted into the erythrocyte membrane during invasion. It is an erythrocyte binding protein that is non-covalently associated with two other proteins, the 140 kDa Rhop-1 and the 130 kDa Rhop-2. We identified the Rhop-3 gene homologue in P. yoelii and demonstrated that the C-terminus is highly conserved. In order to identify the Rhop-3 gene homologue in P. berghei and P. chabaudi, a set of primers were designed based on the cDNA sequence of clone Y 1412 of P. yoelii and used to amplify genomic DNA from P. berghei and P. chabaudi by polymerase chain reaction (PCR). Analysis of the DNA and deduced amino acid sequence demonstrated sequence homology to P. falciparum Rhop-3. We examined the distribution of Rhop-3 epitopes within isolated rhoptries of the three rodent Plasmodium species and P. falciparum, and investigated the erythrocyte binding property of rodent Plasmodium Rhop-3. We also evaluated the immunogenicity of isolated rhoptries from the rodent Plasmodium species following treatment of the organelles with 100 mM sodium carbonate, pH 11.5 and 0.5 % SDS, to stimulate antibodies to proteins not accessible when untreated organelles are used for immunization. We show that the integrity of the isolated organelles was stable and the organelles were reactive with Rhop-3 specific antisera. In addition, the rodent Plasmodium Rhop-3 protein bound to mouse erythrocytes and was recognized by Rhop-3 specific antibody. Furthermore, treated organelles stimulated antisera that recognized additional rhoptry proteins not seen when antisera prepared against whole untreated organelles were examined by western blotting. Taken together, these data suggest that malaria vaccine studies using the Rhop-3 protein can be performed directly in vivo using the rodent Plasmodium models.

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The precocious line, Nn-P125, was assayed on its characteristics relevant to an attenuated live vaccine. The Nn-P125 line was attenuated to such an extent that all chickens did not exhibit any clinical signs by oral inoculation with 9 x 10⁴ oocysts/bird. In contrast, the same number of oocysts of the parent strain killed all the birds inoculated. The prepatent period of the line was shortened about one day as compared with that of the parent Nn strain. These characteristics of Nn-P125 were stable throughout repeated relaxed passages in chickens. The line has an immunological potential to the extent of that chickens received 25 or more oocysts/bird resisted the challenge conducted 4 weeks after vaccination. Chicks aged 3 days or more were found to be suitable for vaccination. Based on these results, the Nn-P125 line was recognized to be an appropriate seed of an attenuated live vaccine against Eimeria necatrix infection.

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Ticks were collected from cattle and their attachment preferences determined. The preferred site of attachment for Boophilus decoloratus was the neck, abdomen and inguineum. Rhipicephalus evertsi evertsi ticks attached predominantly to the perineal and inguinal areas. The other three tick species, Rhipicephalus follis, Rhipicephalus gertrudae and Rhipicephalus punctatus showed very similar sites of attachment, with most ticks preferring the neck and inguineum. Rhipicephalus follis and R. gertrudae was also found to prefer the tail area. The differences in attachment preferences should be considered when resource-poor farmers intend using acaricides sparingly in tick management and control.

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