By Y. Quadir. University of Virginia.
How do the coupled dynamics of specic immune cell populations and matching parasite variants together determine the total length of infec- tion and the uctuating density of parasites available for transmission? What determines the order in which parasite variants rise in successive parasitemias? Dierent par- asite surface molecules may cause infection of dierent body compart- ments buy amoxil canada. The surface molecules that aect tissue tropism may also be strong antigenic determinants purchase amoxil 250mg mastercard. I mentioned that diversifying tissue tro- pisms during the course of an infectioncandiversifyantigenic variation within the host 500mg amoxil otc. Thus, variants with certain tropisms may sequester themselves in refuges from immune pressure. These protected sites may provide a source of chronic infection or generate relapses after ap- parent clearance of the initial infection. Host variability aects the relative success of dierent parasite epitopes and the distribution of antigenic variants. By contrast, limited genetic variability occurs in the germline genes that encode the antibody and T cell binding regions. Instead, vari- able antibody and T cell binding sites arise by somatic recombination. Somatic mechanisms to generate variation may buer the need for hosts to vary genetically. This variation leads to dierences in the thresholds that trigger immunity and in the intensity of particular immune eectors deployed against parasitic attack. Quantitative dierences in immune regulation can aect the intensity of selection on antigenic variants and the im- munodominance of host responses against dierent variants. Immu- nodominance, in turn, denes the selective pressures that shape the distribution of antigenic variants. Afewmajorpolymorphisms have been found in the promoters of cytokines, molecules that regulate key aspects of the immune system. Dierent promoter genotypes correlate with better or worse success in combating certain pathogens. Regulatory polymorphisms may be main- tained by trade-os, in which a more intense immune response clears parasites more eectively but also causes more collateral tissue damage to the host. Major regulatory polymorphisms have dierent alleles at high fre- quencies, each allele with a signicantly dierent eect on immune re- sponse. Each individual probably carries several minor regula- tory variants, causing signicant quantitative genetic variability between hosts in the regulation of the immune response. Strong challenge by a particular para- site could lead to selection favoring or disfavoring specic patterns of proteolysis. The intensity of direct selection on germline polymorphisms may be rather weak because specic recognition of antigens depends primarily on somatic mechanisms to create variability. However, the germline alleles do set the initial conditions on which somatic processes build, so it is certainly possible that germline polymorphisms inuence individual tendencies to react to particular antigens. Dierences between species do not directly inuence antigenic variation in parasites unless the parasites infect dierent species. Hill (1998) reviews cases in which variations in the hosts vitamin D and other cellular receptors are associated with susceptibility to various diseases. It is not clearwhetherminor variants of cellular receptors occur suciently frequently to favor matching variation of parasites for attachment to those receptors. Linkage studies of mice have begun to map locations of genes that in- uence quantitative variability in components of immunity (Puel et al. Many studies of humans report nucleotide poly- morphisms in promoters of cytokinesandother immune regulatory loci (Daser et al. Some human polymorphisms are associated with dierential response to particular diseases (Hill 1998; Foster et al. Various transcription factors and steroid hormones interact with the promoter region of this gene to produce synergistic combinations of positive and negative stim- ulifor transcription (Terry et al. These promoter polymorphisms inuenced expression level in a nonadditive way a single nucleotide change may have been associated with higher or lower levels of expres- sion depending on other variable sites in the haplotype. The three single nucleotide polymorphisms are separated by 25 and 398 nucleotides. I consider afewpossibilities in the remainder of this section and in the following sections. Thus, positive interactions and linkage between promoters and coding regions seem unlikely in this case. Alternatively, polymorphisms that aect phenotype are often main- tained by a balance between the rate at which deleterious mutation adds variability and the rate at which selection can remove deleterious mu- tants. Mutation-selection balance probably explains a signicant por- tion of the total quantitative geneticvariability observed in populations (Barton and Turelli 1987). Mutation-selection balance usually matches a high-frequency allele maintained by selection against a distribution of low-frequency mutant variants. Natural selection culls those lower-tness variants, but mu- tation maintains a constant ow of new variants. Mutation-selection balance proba- bly does explain the two rare trinucleotide haplotypes at frequencies of less than 1% observed by Terry et al. En- hanced exibility could occur in heterozygotes by increasing the range of inputs that control the response kinetics of the cytokine. However, by this scenario of heterozygote advantage, many individuals would carry lower-tness homozygote genotypes. Giv- en the three haplotype frequencies listed above, the expected frequency of homozygotes would be the sum of the squared haplotype frequencies, or 45%. The frequency of heterozygotes would be increased by a larger num- ber of promoterhaplotypes. Butsuchdiversity would mean thatany individual carried two randomly chosen haplotype patterns of regula- tory control among the possible haplotypes. It is dicult to image how complementarity between diverse regulatory haplotypes would occur. Variable transcription rates were associated with nucleotide polymorphisms in the promoters of these alleles. Thus, variable regulatory genotypes can inuence important aspectsofthehosts immune responses. Tissue-specic expres- sion or intermediate expression does not require heterozygosity with the associated cost of frequent, disadvantaged homozygotes. Continuous divergence of promoters as a function of phylogenetic distance suggests drifting changes constrained by the balance between mutational input and selection to maintain functional integrity. There may also be a tendency for compensatory nucleotide changes, in which one slightly deleterious substitution is compensated by a second substi- tution at a dierent site (Hartl and Taubes 1996; Burch and Chao 1999). For example, functionally synergistic associ- ations may exist between nucleotides in promoter and structural regions that cannot be explained by common phylogeny.
After fungal penetration generic amoxil 500 mg, the principal alteration observed was the lyses of intestine and leakage of intestinal content to hemocoel buy discount amoxil 500mg on-line. Mortality was observed after 96 and 120 h post-infection discount 250mg amoxil mastercard, and sporulation was detected after 120 144 h (Garcia et al. The penetration process of the fungi through the cuticle involves secretion of enzymes such as proteases and chitinases, and is assisted by mechanical processes of the appressorium infection peg (Charnley and St. After penetration, the fungus invades the internal organs, produces mycotoxins, and kills the host (Kaaya et al. However, tick species may display differential susceptibility to entomopathogenic fungi due to fungistatic compounds present in the epicuticle of certain tick species (Kirkland et al. Possibly, there is cooperation (mass action) between neighboring germi- nating conidia on the arthropod cuticle (Zhioua et al. Formulation Several entomopathogenic organisms need to be ingested to infect their arthropod host; however, the penetration of pathogens via oral ingestion is not feasible for arthropods that are exclusively hematophagous. Conidial suspension formulation may improve eld performance of conidia under adverse envi- ronmental conditions. Oil formulations have been used with ultra low volume spray technology, which proved effective probably due to improved promotion of conidial adhesion to the hydrophobic surfaces (Prior et al. Other studies have also demonstrated satisfactory results when conidia are formulated in a polymerized cellulose gel (Bittencourt et al. Formulation of conidial suspensions also may protect conidia against desiccation and ultraviolet radiation. Therefore, correct formulation may greatly enhance the effectiveness of entomopathogenic fungi as biocontrol agents. Immune system of ticks As with vertebrates, the immune system of invertebrates recognizes invading foreign bodies as non-self, responding with cellular (hemocytic) and acellular (humoral) reactions (Ratcliffe et al. However, spherulocytes and oenocytoids seem to be related to the granular cell complex (Tanada and Kaya 1993), and possibly store antimicrobial peptides that are secreted following contact with foreign organisms (Goodman et al. A well developed capability for phagocytizing microbes and other foreign organisms appears to exist in ticks, similar to many other arthropods (Sonenshine 1991). A similar phenomenon may occur in ticks infected with a virulent form of entomopathogenic fungi, such as B. Humoral factors facilitate self/non-self recognition and activate defense reactions. It is important to note that no globulins similar to those of vertebrates have been detected in insects (Tanada and Kaya 1993) or ticks. Besides lectins, defensins, lysozyme and possibly other antimicrobial peptides are secreted into the hemolymph plasma to combat invasion by foreign organisms (Goodman et al. More studies are necessary to understand the role of hemolymph contents in humoral immunity. Diseases of Mites and Ticks 87 Many studies on the tick immune system are limited in identifying and quantifying hemocytes, or in characterizing antimicrobial peptides (Sonenshine 1991; Carneiro and Daemon 1996; Pereira et al. The expression of four forms of defensin was observed in the intestine, fat body and reproductive tract of Orni- thodorus moubata (Nakajima et al. It is not known, however, if expression of these peptides increases after infection. Greater understanding of the immune responses of ticks to infection by entomopatho- genic fungi would help elucidate the infection process, and this information might be useful in choosing appropriate entomopathogenic fungal isolates for biological control of ticks. Also, knowledge on the effects of formulation products may prove important in devising superior formulations for eld use. Biocontrol strategies Based on the results reviewed in this study and the ecological conditions in South America, the best use of entomopathogenic fungi to control ticks would probably be to apply formulated conidia directly onto tick-infested animals. Although the spraying of conidial suspensions on pasture plants has reduced the population of tick larvae (see Table 5), the concentration of fungal inoculum used to control ticks in the eld still remains high in comparison to those used for agriculture arthropod pests (Maniania et al. On the other hand, a system with pheromones and carbon dioxide delivery in the eld could possibly attract ticks to a localized fungus-treated spot in the vegetation; however, further investigation is needed to improve this system (Maranga et al. Combinations of chemical acaricides and entomopathogenic fungi also have been studied, aiming for compatibility and synergism between them. It is important to note that most of the studies evaluated the possible negative effects of chemical products on entomopathogenic fungi; however, Paiao et al. The results suggest that the combination of chemical acaricides with entomopathogenic fungi is potentially an important tool for integrated management of ticks. Conclusion Entomopathogenic fungi are the most promising of the currently available alternatives to chemical acaricides for tick control; especially since these organisms penetrate directly 88 J. In addition, fungi might initiate natural epizootic outbreaks (Alves 1998), and where environmental conditions (e. The genetic variability among fungal isolates is another advantage, in that simple assays may detect the most virulent isolates, level of host specicity, and tolerance to eld conditions. Unfortunately, most eld trials performed to date have reported rather low efcacy of fungi for the control of tick populations in South America, with the exception of A. Only a few eld trials have been conducted in South America, and in most cases, a simple aqueous conidial suspension was used. In Brazil, an acaricide needs to achieve 95% efcacy to be commercialized (Ministerio da Agricultura, Pecuaria e Abas- tecimento 1997). Therefore, studies on formulation are required to improve conidial performance under environmental conditions. These programs may vary with tick species due to different biological behavior and geographic region. Tick biological control offers several advantages over currently available chemical acaricides, including lower costs; and therefore, there are increased efforts in South America to develop new biological products. Unfortunately, regulations for microbial pesticides in many South American countries are poorly developed and/or virtually not enforced, and thus have impeded the development of quality biological control products. However, while more research is necessary, entomopathogenic fungi have great promise as alternatives to current tick control methods, and they could alleviate many of the current environmental and health concerns that come with the present-day methods. Am J Trop Med 19:103 108 Dutra V, Nakazato L, Broetto L et al (2004) Application of representational difference analysis to identify sequence tags expressed by Metarhizium anisopliae during the infection process of the tick Boophi- lus microplus. Rev Bras Parasitol 15:157 162 Ministerio da Agricultura Pecuaria e Abastecimento (1997) Regulamento tecnico para licenciamento e/ou renovacao de licenca de produtos antiparasitarios de uso veterinario. Cienc Rural 35:855 861 Prior C, Jollands P, Le Patourel G et al (1988) Infectivity of oil and water formulation of Beauveria bas- siana (Deuteromycotina: Hyphomycetes) to the cocoa weevil pest Pantorhytes plutus (Coleoptera: Curculionidae). Academic, New York Samish M, Rehacek J (1999) Pathogens and predators of ticks and their potential in biological control. J Parasitol 87:1355 1359 Samish M, Ginsberg H, Glazer I (2004) Biological control of ticks. Losson Originally published in the journal Experimental and Applied Acarology, Volume 46, Nos 1 4, 95 104. Biological control is considered as a realistic alternative to chemotherapeutic control. Laboratory experiments were carried out to evaluate the pathogenicity and the thermotolerance of twelve isolates of entomopathogenic fungi from four genera (Beauveria Vuillemin, Metarhizium Sorokin, Paecilomyces Bainier and Verticillium Nees). At this concentration the entire mite population became infected with all isolates but B. The thermotolerance of each isolate was evaluated by measuring its growth on an articial medium kept between 25 and 37.
Ivermectin in this setting was equally effective whether given at 3-monthly order amoxil 500 mg overnight delivery, 6-monthly purchase amoxil 250 mg online, or annual intervals buy amoxil 500mg without a prescription. For treatment of individuals outside of endemic areas, a single dose of ivermectin, 150 g/kg, may be given every 3 6 months until asymp- tomatic. In contrast to mass treatment programs in Africa where reactions are rare, 17/33 (55%) of a series of patients in London had reactions, so it is advisable that the rst dose of ivermectin is given in hospital. Treatment should be repeated if there is recurrence of pruritus, rash, or eosinophilia [26,27]. Doxycycline Wolbachia symbiotic endobacteria have been identied as essential for larial worms fertility and these offer new targets for treatment. Addi- tional treatment with doxycycline to sterilize worms enhances ivermectin- induced suppression of microlaridermia. A combination of doxycycline, 200 mg/day, for 6 weeks plus ivermectin is now advisable for individ- uals with imported Onchocerciasis . Doxycycline is given prior to ivermectin to deplete Wolbachia, thereby reducing inammatory reactions induced by ivermectin. Ivermectin may be effective if given immediately at the end of doxycycline treatment but in the studies to date it was given after 4 6 months. Future treatments A macrolaricide that can kill adult worms is desirable so that a single course of treatment could be potentially curative. Pro- tective clothing such as long-sleeved shirts and long trousers and an insect-repellent are advisable especially in the early morning and late evening, which are peak biting times. Caution is needed in areas coendemic with loiasis because ivermectin can cause serious encephalopathic adverse effects in patients with high L. Integrated Control Programs Integrated control programs for onchocerciasis and lymphatic laria- sis employ annual mass treatment with ivermectin and albendazole. Onchocerciasis/Filariasis 217 Lymphatic lariasis Introduction Of the 120 million people infected worldwide, 40 million are seri- ously incapacitated and disgured. The prevalence of the disease is increasing as unplanned growth of cities creates numerous breeding sites of the mosquito vector. Travelers to endemic areas for extended periods of time and extensively exposed to mosquitoes may become infected. Most cases seen in developed countries are in immi- grants from endemic countries. Newcomers to endemic areas develop acute and chronic manifestations of the disease much more rapidly than residents who have been exposed since birth, and lymphedema may develop within 6 months and elephantiasis within 1 year of arrival. Residents of communities endemic for lymphatic lariasis may have asymptomatic infections or develop acute or chronic presentations. Approximately half of all individuals in endemic areas appear clinically normal but have microiariae circulating in their blood and have covert lymphatic and renal damage (microscopic hematuria and proteinuria). This state of asymptomatic microlaria is associated with downregulation of the immune system. Acute manifestations The most common acute problem is larial fevers affecting the limbs or scrotum, which is caused by bacterial or fungal superinfection of tissues with already-compromised lymphatic function. Less frequently, larial fevers are triggered by an inammatory response, which starts in the lymph node with retrograde extension down the lymphatic tract and an accompanying cold edema. Tropical pulmonary eosinophilia is a rare syndrome, mainly in Asia, characterized by nocturnal cough and wheezing and fever. Chronic changes The most common chronic manifestation is hydrocele, which increases in prevalence with age. The folds, crevices, and warty protuber- ances of an elephantine limb harbor bacteria and fungi that intermittently breach the epidermis and cause local and systemic infection. It has excellent specicity and greater sensitivity than previous parasite-detection meth- ods and can be used on nger-prick blood samples collected at any time of the day. Detection of microlariae by microscopic examination of blood sample Microlariae only circulate in the blood at or near the peak biting time of the vector, so it is important to time blood samples with the known peri- odicity of the microlariae. The vectors in Africa are all night-biting, so blood samples in these areas have to be taken within a few hours either side of midnight. A thick blood smear should be made and stained with Giemsa or hematoxylin and eosin. Yields may be increased by passing heparinized blood through a Millipore lter, which retains the micro- lariae that can then be easily visualized on microscopy. Treatment Doxycycline For individual patients living outside a transmission area, treatment regi- mens including doxycycline 200 mg/day for 4 6 weeks are emerging as the optimal treatment. Doxycycline is usually followed by single dose treat- ment with ivermectin 150 g/kg or albendazole 400 mg, 3 4 months after the onset of doxycycline treatment . Doxycycline 200 mg/day for 6 weeks plus a single dose of ivermectin has been shown to render infected Ghanian residents of an endemic area completely amicrolaremic after 12 months . Treatment is best initiated with smaller doses for 2 3 days and antihistamines or corticos- teroids may be required to reduce allergic reactions due to disintegration of microlariae. Surgical treatment, including nodovenous shunts and excision of redun- dant tissue can improve elephantiasis. One of the most signicant advances in treatment has been the recog- nition that much of the progressive pathology is due to bacterial and fun- gal superinfection of tissues with impaired lymphatic function. Rigorous attention to hygiene of affected limbs and measures to improve lymphatic drainage reduce the frequency of larial fevers and even slowly improve lymphedema and elephantiasis. Prevention Protection from mosquito bites through use of personal repellents, bed- nets, or insecticide-impregnated materials is prudent. Fifty-three endemic countries have started mass drug administration to interrupt transmission, representing the largest mass drug administra- tion program ever conceived. Single doses of the two drugs are given together at annual intervals and this needs to be repeated for at least 5 years. Loiasis Clinical picture Travelers to at-risk areas who stay for long periods of time are more likely to become infected than short-term travelers. Loiasis is often asymp- tomatic, though nonspecic symptoms of pruritus, pain or swelling of a limb, and urticaria can occur over several months as the fourth-stage lar- vae develop into adult worms. These are more common in expatriate Europeans and are thought to represent local hypersensitivity reactions to the subcutaneous passage of an adult worm. The adult worm can sometimes be seen temporarily as it passes across the eye underneath the conjunctiva causing irritation and unilateral palpebral edema, but eye-worm is more common in residents of endemic areas. Diagnostic procedures Parasitological methods Microlariae may be demonstrated in a Giemsa-stianed smear of daytime blood, with the sensitivity increased by passing the blood through a Milli- pore lter. Microlariae are not found in the blood before about 5 months after the onset of Calabar swellings and expatriates often have negative peripheral blood examination. The adult worm may be sur- gically extracted from the subcutaneous tissue or conjunctiva as a minor procedure.