Plasmodium undergoes one round of multiplication in the liver prior to invading erythrocytes and initiating the symptomatic blood phase of the malaria infection. Productive hepatocyte infection by sporozoites leads to the generation of thousands of merozoites capable of erythrocyte invasion. Merozoites are released from infected hepatocytes as merosomes, packets of hundreds of parasites surrounded by host cell membrane. Intravital microscopy of green fluorescent protein–expressing P. yoelii parasites showed that the majority of merosomes exit the liver intact, adapt a relatively uniform size of 12–18 μm, and contain 100–200 merozoites. Merosomes survived the subsequent passage through the right heart undamaged and accumulated in the lungs. Merosomes were absent from blood harvested from the left ventricle and from tail vein blood, indicating that the lungs effectively cleared the blood from all large parasite aggregates. Accordingly, merosomes were not detectable in major organs such as brain, kidney, and spleen. The failure of annexin V to label merosomes collected from hepatic effluent indicates that phosphatidylserine is not exposed on the surface of the merosome membrane suggesting the infected hepatocyte did not undergo apoptosis prior to merosome release. Merosomal merozoites continued to express green fluorescent protein and did not incorporate propidium iodide or YO-PRO-1 indicating parasite viability and an intact merosome membrane. Evidence of merosomal merozoite infectivity was provided by hepatic effluent containing merosomes being significantly more infective than blood with an identical low-level parasitemia. Ex vivo analysis showed that merosomes eventually disintegrate inside pulmonary capillaries, thus liberating merozoites into the bloodstream. We conclude that merosome packaging protects hepatic merozoites from phagocytic attack by sinusoidal Kupffer cells, and that release into the lung microvasculature enhances the chance of successful erythrocyte invasion. We believe this previously unknown part of the plasmodial life cycle ensures an effective transition from the liver to the blood phase of the malaria infection.

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To get an overview of the current state of endoparasite prevalences in stray and not well-cared dogs and cats, faecal samples of 445 stray and foster dogs and 837 stray and foster cats were collected at their arrival at animal shelters in Lower Saxony (Germany). They were investigated for infections with endoparasites by the use of sedimentation-flotation method. Additionally, 341 canine and 584 feline samples were investigated by IDEXX SNAP® Giardia test. Stages of endoparasites were found coproscopically in 9.4 % (n = 42) of the canine samples, 4.0 % were positive for Toxocara canis, 0.9 % for hookworms, 0.4 % for Toxascaris leonina and 0.2 % for Hammondia-like oocysts. Giardia-coproantigen was detected in 11.4 % of the canine samples. In cats, 33.6 % (n = 281) were coproscopically positive for helminths and/or protozoa. Toxocara cati was found in 27.1 %, Isospora spp. in 7.5 %, Capillaria spp. 5.0 %, Taeniidae in 2.0 %, hookworms in 1.1 %, Giardia sp. in 0.7 %, Aelurostrongylus abstrusus in 1.0 % and Toxoplasma-like oocysts in 0.1 %. Coproantigen specific for Giardia sp. was detected in 6.8 % of the feline samples. Dogs and cats up to 1 year of age were more frequently infected with endoparasites than animals over 1 year of age (p < 0.001). Toxocara spp. and Isospora spp. were detected significantly more often in younger dogs and cats, respectively (p < 0.05 and p < 0.001). Stray dogs or cats older than 1 year were significantly more frequently infected with endoparasites than dropped off animals of the same age group (p < 0.05). Using the faecal egg count reduction test, the therapeutic efficacy of some anthelmintics was tested. All tested anthelmintics showed high efficacy and no suspected anthelmintic resistance was found. However, endoparasite-infected stray and free-roaming cats and dogs may contribute considerably to the contamination of public parks, playgrounds and sandpits with zoonotic parasites and therefore have to be considered a public health problem.

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SummaryPrevious studies suggested Plasmodium sporozoites infect hepatocytes after passing through Kupffer cells, but proof has been elusive. Here we present new information strengthening that hypothesis. We used homozygous op/op mice known to have few Kupffer cells because they lack macrophage colony stimulating factor 1 required for macrophage maturation due to a deactivating point mutation in the osteopetrosis gene. We found these mice to have 77% fewer Kupffer cells and to exhibit reduced clearance of colloidal carbon particles compared with heterozygous phenotypically normal littermates. Using a novel quantitative reverse transcription polymerase chain reaction assay for P. yoelii 18S rRNA, we found liver infection of op/op mice to be decreased by 84% compared with controls. However, using another way of limiting Kupffer cells, treatment with liposomeencapsulated clodronate, infection of normal mice was enhanced seven-to 15-fold. This was explained by electron microscopy showing temporary gaps in the sinusoidal cell layer caused by this treatment. Thus, Kupffer cell deficiency in op/op mice decreases sporozoite infection by reducing the number of portals to the liver parenchyma, whereas clodronate increases sporozoite infection by opening portals and providing direct access to hepatocytes. Together these data provide strong support for the hypothesis that Kupffer cells are the portal for sporozoites to hepatocytes and critical for the onset of a malaria infection.