Blood Infections

The genus Plasmodium includes at least 172 named species of intraerythrocytic parasites infecting a wide range of mammals, birds, reptiles, and amphibians. Malaria imposes an enormous burden of illness and substantial mortality on the tropical world and in many subtropical regions. There are an estimated 515 (95% confidence interval, 330 to 660) million clinical episodes of malaria due to Plasmodium falciparum, the most common cause of malaria, and 1.5 million to 2.7 million deaths due to malaria each year. Gastrointestinal complaints, nausea, vomiting, and diarrhea, which may be bloody, are also not uncommon and should not distract the clinician from the diagnosis of malaria. In the face of rapidly developing drug resistance, development of new antimalarial drugs and, ultimately, an effective malaria vaccine are high priorities. The genus Babesia includes approximately 100 species that are transmitted by ticks of the genus Ixodes and infect a variety of wild and domestic animals. The diagnosis of babesiosis should be considered for a patient with the appropriate clinical symptoms and a history of travel to areas where the disease is endemic, exposure to ticks, or recent blood transfusion. Examination of Giemsastained thin blood smears is the most direct approach to diagnosis.

From 1947 to 1951, monkeys were used in the search for a safe and effective radical-cure drug, one superior to pamaquine, pentaquine, and isopentaquine. Thirty-four derivatives with diverse origins were screened, and one in particular, SN-13272, prepared by Robert C. Elderfield was especially interesting. Elderfield’s compound was named primaquine. Gram for gram, primaquine was about four times as effective as pamaquine. In the early stages of the American involvement in the war in Vietnam, the U.S. military was faced with increasing numbers of casualties from chloroquine-resistant P. falciparum. Tafenoquine was shown to eliminate the dormant or “sleeping” hypnozoite stages in the livers of P. cynomolgi-infected rhesus monkeys. The pathway from methylene blue to tafenoquine is shown. Tafenoquine is under development jointly by WRAIR and Glaxo-SmithKline Pharmaceuticals as a replacement for primaquine. The mechanism of therapeutic action and toxicity of this 8-aminoquinoline, like those of primaquine, remain incompletely understood despite more than five decades of study. In vitro tests showed that tafenoquine was 5- to 10-fold better at killing asexual stages of P. falciparum than was primaquine. Orally administered tafenoquine was slowly absorbed and, in marked contrast to primaquine, slowly metabolized, with an elimination half-life of 14 days. Hopefully, with Food and Drug Administration (FDA) approval, tafenoquine will be marketed possibly for use by travelers as a single weekly dose.

The oldest and most effective public health measures have been interventions that prevented or reduced the transmission of malaria. This chapter presents a discussion on the control of malaria by blocking its transmission. In August 1899, Ronald Ross identified the Anopheles mosquitoes that were the main transmitters and set about to eliminate their breeding sites. In 1947, an eradication campaign was sold to the nations of the world based on the near-miraculous blow that dichlorodiphenyltrichloroethane (DDT) had dealt to malaria. By 1950 many nations had antimalaria programs dependent on routine annual spraying with DDT. Another approach to blocking transmission involved reducing the infectivity of the human population itself to mosquitoes by widespread distribution and use of antimalarial drugs. An alternative to the use of antimalarial drugs to suppress human infectivity for mosquitoes is to develop and deploy vaccines designed to limit the transmission of human malaria infections to mosquitoes – transmission-blocking vaccine (TBV). One of the great potential strengths of proteins (Pfs48/45 and Pfs230) used for the malaria TBV is that these proteins are expressed in the human host so that there can be a boosting effect. Another vaccine, RTS,S, showed a 34% reduction in the first appearance of parasites in the blood over a 16-week period. Bed nets (mosquito nets) and sporozoite vaccines can also be effective transmission-blocking agents for malaria.

This section focuses on diagnostic parasitology. Specimen collection and processing procedures constitute the ova and parasite examination (O&P exam). The direct wet smear, concentration, and permanent stained smear constitute the routine O&P exam on fresh stool specimens. The section provides some of the immunoassay options available for stool protozoa. Currently, immunoassays are available for Giardia lamblia, Cryptosporidium spp., the Entamoeba histolytica/E. dispar group, and E. histolytica. Most effective techniques for the identification of the intestinal protozoa, the wet preparation smear and the permanent stained smear are recommended as the most relevant and accurate procedures for identification. Some helminth eggs are quite heavy and will not float, even when zinc sulfate with a specific gravity of 1.20 is used. The scientific (genus and species) names should be used on the final report that goes to the physician and on the patient’s chart. It is also recommended that the stage of the organism be included (trophozoite, cyst, oocyst, spore, egg, larvae, adult worm); various stages of the malaria parasites affect the outcome of therapy.

The human malarias, Plasmodium falciparum, P. vivax, P. ovale, and P. malariae, are transmitted through the bite of an infected female anopheline mosquito when she injects sporozoites from her salivary glands during blood feeding. The long-term consequences of malaria infections are an enlarged spleen and liver and organ dysfunction. In pregnant females, falciparum malaria may result in stillbirth, lower than normal birth weight, or abortion. On the basis of clinical patterns supplemented by the stained blood film, the human malaria parasites can clearly be distinguished from one another. By using a light microscope, as few as 5 to 10 parasites in a small drop of blood can be seen. Qinghaosu (artemisinin), a Chinese herbal medicine, is both the newest and the oldest in the arsenal of antimalarials. It has been used in China for 2,000 years to reduce fever. It is derived from the leaves of the wormwood Artemisia annua. Today, malaria still ravages many countries, targeting the indigenous people and slowly and inexorably killing them. Malnourishment increases the susceptibility to malaria and a variety of other diseases, leads to lower productivity, and puts a further strain on the already fragile health care system. Sadly, in Africa the Horsemen of the Apocalypse have as their principal target the children; one-third of infected (and untreated) children will die from malaria. Thus, it is a certainty that malaria will continue to devastate nations unless something is done to control it.

The various life cycle stages of Plasmodium provide potential targets for immune (and drug) interventions. Apart from the use of malaria to treat the symptoms of late-stage syphilis, the early research on protective vaccines and immune mechanisms was carried out using birds, monkeys, and rodents as surrogates for the disease as it occurs in humans. This chapter discusses fundamental findings of malaria parasite. Only 4 years separated Laveran’s discovery of the human malaria parasite P. falciparum and the discovery of avian malaria parasites by Basil Danilewsky in 1884. Danilewsky (1852 to 1939), a physician from Kharkov, Russia, examined 300 birds from the Ukraine and found malaria-like parasites in their blood. Longenecker et al. provided additional support for the findings of antibody-mediated immunity as had been described by the Taliaferros. Weidanz’s contributions to the understanding of malaria immunology have involved the immunosuppressive effect of experimental infection, the function of antibody-independent T-cell-mediated immunity in resistance to malaria, and the use of genetic dissection to study the roles of cells and molecules in the immune response to experimental malaria. In 1999, Biswas confirmed the 60-year-old findings of Coggeshall and Eaton by repeatedly infecting rhesus monkeys with P. knowlesi-infected blood and measuring the immunoglobulin levels by a sensitive and specific assay. The conclusions to be drawn from using bird and monkey malarias are inescapable: protection could be achieved after vaccination with the various stages of the parasite, although in most instances this required the use of an adjuvant.

Identification of the surface antigens of Plasmodium falciparum can be traced to studies carried out by Monroe Eaton in the 1930s. Eaton showed that serum from monkeys infected with P. knowlesi could agglutinate schizontinfected red cells. This schizont-infected cell agglutination (SICA) reaction was a clear indication that the surface of the malaria infected red cell had been altered. Sequestration and immune evasion are linked to the finding that P. falciparum erythrocyte membrane protein 1 (PfEMP1) has three different adhesive domains: DBL (Duffy ligand binding), a CIDR (cysteine-rich interdomain region), and an acidic region, called the ATS (acidic terminal sequence), that is presumed to anchor the molecule to the red cell surface. Each year over 50 million women are exposed to the risk of malaria during pregnancy. Pregnancy malaria (PM) results in substantial maternal and especially fetal and infant morbidity and mortality, causing 75,000 to 200,000 infant deaths annually. Rosetting has also been observed in P. vivax, P. ovale, P. malariae, P. coatneyi in the rhesus monkey, and P. fragile in the toque monkey. Rosetting in some of these hosts may be benign, but in others, i.e., humans infected with P. falciparum, it may contribute to pathogenesis leading to severe malaria. The substances to which the rosetting red blood cells bind are heparan sulfate, blood group antigens, and complement receptor 1.

Pneumococcal disease results when colonizing pneumococci in the nasopharynx successfully invade sterile sites. The first half of this chapter focuses on individual disease models and provides general descriptions of the commonly used models. It also discusses considerations important to the application of these models to studies of vaccine antigens and some of the pros and cons of each model. The latter half of the chapter presents experimental considerations that relate to animal models in general: the need for multiple models, inclusion criteria for experiments, and statistics. Bacteremia is a potential outcome in virtually all models of pneumococcal infection and is easily evaluated. The classic mouse model for assessing protective immunity has been the use of otherwise fatal infections following intraperitoneally (i.p.) challenge to detect the effects of active immunization or passive antibodies. As neonates and infants are a major target for pneumococcal vaccination, the murine model of intranasal challenge with aspiration has been adapted to 1-week-old mice, whose immune system corresponds to that of the human neonate, and 3-week-old mice, whose immune system corresponds to that of the human infant. Otitis media models in several animal species have been developed to evaluate vaccine effects. The classic model of otitis media was developed using the chinchilla and has been well reviewed by Giebink. Advantages and limitations of the various animal models of invasive pneumococcal diseases emphasize the importance of a careful selection of animal models to address different scientific questions.