Systematic review of the scrub typhus treatment landscape: Assessing the feasibility of an individual participant-level data (IPD) platform

Background

Scrub typhus is an acute febrile illness caused by intracellular bacteria from the genus Orientia. It is estimated that one billion people are at risk, with one million cases annually mainly affecting rural areas in Asia-Oceania. Relative to its burden, scrub typhus is understudied, and treatment recommendations vary with poor evidence base. These knowledge gaps could be addressed by establishing an individual participant-level data (IPD) platform, which would enable pooled, more detailed and statistically powered analyses to be conducted. This study aims to assess the characteristics of scrub typhus treatment studies and explore the feasibility and potential value of developing a scrub typhus IPD platform to address unanswered research questions.

Methodology/principal findings

We conducted a systematic literature review looking for prospective scrub typhus clinical treatment studies published from 1998 to 2020. Six electronic databases (Ovid Embase, Ovid Medline, Ovid Global Health, Cochrane Library, Scopus, Global Index Medicus), ClinicalTrials.gov, and WHO ICTRP were searched. We extracted data on study design, treatment tested, patient characteristics, diagnostic methods, geographical location, outcome measures, and statistical methodology.

Among 3,100 articles screened, 127 were included in the analysis. 12,079 participants from 12 countries were enrolled in the identified studies. ELISA, PCR, and eschar presence were the most commonly used diagnostic methods. Doxycycline, azithromycin, and chloramphenicol were the most commonly administered antibiotics. Mortality, complications, adverse events, and clinical response were assessed in most studies. There was substantial heterogeneity in the diagnostic methods used, treatment administered (including dosing and duration), and outcome assessed across studies. There were few interventional studies and limited data collected on specific groups such as children and pregnant women.

Conclusions/significance

There were a limited number of interventional trials, highlighting that scrub typhus remains a neglected disease. The heterogeneous nature of the available data reflects the absence of consensus in treatment and research methodologies and poses a significant barrier to aggregating information across available published data without access to the underlying IPD. There is likely to be a substantial amount of data available to address knowledge gaps. Therefore, there is value for an IPD platform that will facilitate pooling and harmonisation of currently scattered data and enable in-depth investigation of priority research questions that can, ultimately, inform clinical practice and improve health outcomes for scrub typhus patients.

Scrub typhus is a febrile illness most commonly found in rural tropical areas. It is caused by a Gram-negative bacteria belonging to the family Rickettsiaceae and transmitted by mites when they feed on vertebrates. There is an estimate of one million cases annually, with an estimated one billion people at risk, mostly in Asia-Oceania. But relative to the scale of the problem, scrub typhus is largely understudied. Evidence-based treatment recommendations by policymakers vary or are non-existent. We searched databases and registries for prospective scrub typhus clinical treatment studies published from 1998 to 2020 and reviewed them. Data from clinical trials and particularly for specific groups, such as pregnant women and children, were minimal. The methods used to measure treatment efficacy were heterogeneous, making it difficult to directly compare or conduct a meta-analysis based on aggregated data. One way to improve the current level of evidence would be by pooling and analysing individual participant-level data (IPD), i.e. the raw data from individual participants in completed studies. This review demonstrated that there is scope for developing a database for individual participant data to enable more detailed analyses. IPD meta-analyses could be a way to address knowledge gaps such as optimum dosing for children and pregnant women.

Dual Genotype Orientia tsutsugamushi Infection in Patient with Rash and Eschar, Vietnam, 2016

We report a dual genotype Orientia tsutsugamushi infection in Vietnam in 2016. The patient had fever, rash, and an eschar. The Kawasaki genotype was identified in the eschar specimen and Karp genotype in the whole blood specimen. The genotype co-infection rate for scrub typhus is unknown and should be further evaluated.

In Vitro Activity of Tigecycline Against Orientia tsutsugamushi

Scrub typhus is a zoonosis caused by Orientia tsutsugamushi (O. tsutsugamushi) occurring mainly in autumn in Korea. The need of new antibiotics has arisen with a report on strains resistant to antibiotics and chronic infection. This study aims to identify susceptibility of tigecycline in-vitro as a new therapeutic option for O. tsutsugamushi. Antibacterial activity of tigecycline against the O. tsutsugamushi was compared with doxycycline using flow cytometry assay. The inhibitory concentration 50 (IC₅₀) was 3.59×10⁻³ μg/mL in doxycycline-treated group. Whereas in 0.71×10⁻³ μg/mL tigecycline-treated group. These findings indicate that tigecycline may be a therapeutic option for the treatment of scrub typhus.

In vitro bacteriostatic effects of rifampin on Orientia tsutsugamushi

We performed an in vitro cell culture experiment to ascertain whether rifampin exhibits bactericidal effects against Orientia tsutsugamushi, the causative agent of scrub typhus. ECV304 cells were infected with the Boryong or AFSC-4 strain of O. tsutsugamushi and then, the cultures were maintained in media with increasing concentrations of rifampin, azithromycin, doxycycline, or chloramphenicol for 4 days. On day 5, the media were replaced with fresh antibiotic-free medium and the cultures were maintained until day 28. On days 5, 13, and 28, immunofluorescence (IF) staining of O. tsutsugamushi was performed. IF staining on days 13 and 28 revealed increasing numbers of IF-positive foci in all cultures, even in cultures initially exposed to the highest concentration of rifampin (80 µg/mL), azithromycin (80 µg/mL), doxycycline (20 µg/mL), or chloramphenicol (100 µg/mL). The present study reveals that rifampin has no bactericidal effect against O. tsutsugamushi as observed for azithromycin, doxycycline, and chloramphenicol. A subpopulation of the bacteria that are not killed by high concentrations of the antibiotics may explain the persistence of O. tsutsugamushi in humans even after complete recovery from scrub typhus with antibiotic therapy.

Differences in clinical features according to Boryoung and Karp genotypes of Orientia tsutsugamushi

Background

Scrub typhus is an infectious disease caused by Orientia tsutsugamushi. The differences in virulence of O. tsutsugamushi prototypes in humans are still unknown. We investigated whether there are any differences in the clinical features of the Boryoung and Karp genotypes.

Methodology/Principal Findings

Patients infected with O. tsutsugamushi, as Boryoung and Karp clusters, who had visited 6 different hospitals in southwestern Korea were prospectively compared for clinical features, complications, laboratory parameters, and treatment responses. Infected patients in the Boryoung cluster had significantly more generalized weakness, eschars, skin rashes, conjunctival injection, high albumin levels, and greater ESR and fibrinogen levels compared to the Karp cluster. The treatment response to current antibiotics was significantly slower in the Karp cluster as compared to the Boryoung cluster.

Conclusion

The frequency of occurrence of eschars and rashes may depend on the genotype of O. tsutsugamushi.

Lack of effect of dexamethasone on growth of Orientia tsutsugamushi Gilliam in mouse L929 cells

PURPOSE

Previous studies and our own clinical experience suggest that concurrent corticosteroid treatment for severe rickettsial disease with multiorgan failure may improve the clinical course or reduce mortality. However, the use of corticosteroids as adjunctive treatment for rickettsial diseases is controversial. We attempted to determine the influences of corticosteroid on the growth of Orientia tsutsugamushi in vitro to justify and evaluate the clinical applicability of corticosteroid in rickettsial disease.

MATERIALS AND METHODS

L929 cells were infected with Orientia tsutsugamushi Gilliam. Dexamethasone was added to the cells at final concentrations of 10¹ and 10⁷ pg/mL. Cultures were incubated at 35°C and processed for flow cytometry on the 6th day after addition of dexamethasone.

RESULTS

Observation on the 6th day after treatment with dexamethasone in infected cultures revealed that there was no difference in fluorescence intensity among the treatment wells. Treatment of the cells with dexamethasone at concentrations of 10¹ and 10⁷ pg/mL showed no influence on the growth of Orientia tsutsugamushi.

CONCLUSION

Our results to show that isolated corticosteroid does not enhance the replication of Orientia tsutsugamushi in vitro. Concurrent use of anti-inflammatory or immunosuppressive doses of corticosteroids in conjunction with antibiotics may not have detrimental effects on the course of scrub typhus.

Inactivation of Orientia tsutsugamushi in red blood cells, plasma, and platelets with riboflavin and light, as demonstrated in an animal model

BACKGROUND

Treatment of blood products with riboflavin and light has been used to reduce the number of certain pathogens. Orientia (formerly Rickettsia) tsutsugamushi, the scrub typhus agent, is an obligate intracellular bacterium that grows free in the cytoplasm of infected cells. This study evaluated the capability of riboflavin and light to inactivate O. tsutsugamushi in red blood cells (RBCs), platelets (PLTs), and plasma, as measured by mouse infectivity.

STUDY DESIGN AND METHODS

A total of 108 mice, equally divided into groups receiving RBCs, plasma, and PLTs, received untreated products infected with 10(0) to 10(5) organisms. Eighteen mice received products infected with 10(5) organisms and were subsequently treated with riboflavin and light. Mice were monitored daily for up to 17 days for signs and symptoms of infection (e.g., lethargy, labored breathing, rough coat) and killed upon appearance of symptoms or on Day 17 after infection. Real-time polymerase chain reaction (PCR) on blood and Giemsa stains from peritoneal exudates were performed.

RESULTS

A total of 102 of 108 mice receiving the untreated products developed signs and symptoms of infection and had positive PCR and Giemsa stain results. None of the 18 animals receiving riboflavin and light-treated blood products exhibited signs or symptoms of infection, nor was infection observed by PCR testing or Giemsa staining.

CONCLUSIONS

Riboflavin and light are effective in reducing O. tsutsugamushi. Mice injected with blood products inoculated with 10(5) organisms and treated with riboflavin and light did not experience any signs or symptoms of infection, 17 days after inoculation. A 5-log reduction of this organism in blood was achieved as assayed in an animal model.

Improved antibiotic susceptibility test of Orientia tsutsugamushi by flow cytometry using monoclonal antibody

Orientia tsutsugamushi causes scrub typhus, which is endemic in many countries in the Asia-Pacific region including Korea. Recent emergence of doxycycline-resistant strains from Thailand has underlined the importance of the susceptibility tests of O. tsutsugamushi to antibiotics. To improve the flow cytometric technique for the susceptibility test, we applied a monoclonal antibody (MAb) in the quantification of O. tsutsugamushi. With using MAb FS15, we determined the doxycycline susceptibility of two strains, Boryong and AFSC-4 strain which is reported to be doxycycline-sensitive and resistant, respectively. The growth of both strains was inhibited to below 10% of the control in the presence of 0.1 microg/mL or higher concentrations of doxycycline. We suggest that our approach is more quantitative and reproducible than the conventional microscopic methods.

Long-term egg-yolk adaptation of the Orientia tsutsugamushi for preparation of a formalinized immunogen

To assess the feasibility of preparing a killed rickettsial vaccine against the scrub typhus, the Karp and Gilliam strains of the Orientia tsutsugamushi were adapted through sequential passages in eggs for more than 100 times over 9 years to produce approximately five times more antigen than the unadapted rickettsia. The egg-grown rickettsia was purified by differential centrifugation and batch-type ion-exchange chromatography, and inactivated by formalin treatment. Strong protective immunity was acquired against lethal challenges of the homologous strains and lasted fully for longer than 8 months in the C3H/He mice immunized with either of the single-strain immunogen or in combination of the two strain immuinogens. However, neither immunogen protected animals from the challenges with the two heterologous strains or Boryong, the prevalent strain in Korea, despite that three antigenic proteins of 47, 56, and 110 kDa were eminent in both preparations. IgM, IgG, and neutralizing antibody were induced in the immunized mice in a level and pattern comparable to that in animals infected with live rickettsiae.

Application of monoclonal antibody, specific for intracellular Orientia tsutsugamushi, to immunofluorescent antibody test for determining antibiotic susceptibility

The simple quantification of viable intracellular bacteria is important for the study of an obligate intracellular bacterium, Orientia tsutsugamushi. We applied a novel monoclonal antibody (M686-13)--specific for intracellular Orientia--to an immunofluorescent antibody (IFA) test for determining antibiotic susceptibility of O. tsutsugamushi. M686-13 did not react with Orientia that was inhibited by doxycycline, although bacterial particles still remained in the cells. This preferential staining of proliferating bacteria made the IFA test rapid and precise. Using this method, we could successfully measure the minimal inhibitory concentration (MIC) of a Korean strain of O. tsutsugamushi to doxycycline and clindamycin. This method may be used in other procedures to evaluate the growth of Orientia.

Scrub typhus during pregnancy: a case report and review of the literature

Scrub typhus is a rickettsial disease that is uncommon during pregnancy. We report a case of a 33-year-old woman, G1P0, 29 weeks pregnancy who presented to hospital with high fever, chill and headache for two weeks. Her diagnosis of scrub typhus was confirmed by serum immunofluorescent assay. She was successfully treated with chloramphenicol, but preterm delivery occurred. Her infant died from respiratory distress syndrome. No vertical transmission was demonstrated in this case. Scrub typhus should be listed in the differential diagnosis of acute febrile illness in pregnant women, who either live in, or return from, endemic areas. Chloramphenicol can be used safely during pregnancy if it is not circulating at the time of delivery.

Orientia tsutsugamushi suppresses the production of inflammatory cytokines induced by its own heat-stable component in murine macrophages

Orientia tsutsugamushi is a Gram-negative obligate intracellular bacterium, which causes scrub typhus. To understand the pathogenesis of scrub typhus, we have investigated the induction of tumor necrosis alpha (TNF-alpha) and interleukin-6 (IL-6) by O. tsutsugamushi in two murine macrophage cell lines. Both live and heat-killed orientia stimulated the production of cytokines in J774A.1 cells. Polymyxin B does not affect the secretion of cytokines. These together with the fact that the immature macrophage cell line, P388D1, did not produce TNF-alpha when induced by either live or heat-killed O. tsutsugamushi strongly argue against any roles of lipopolysaccharide (LPS) in cytokine production. Furthermore, the result that the cytokine responses were more brisk when macrophage cell lines had been induced by heat-killed O. tsutsugamushi than by live organisms strongly suggest that a heat-stable molecule might be responsible for the induction of cytokine production and O. tsutsugamushi might have mechanisms suppressing the production of inflammatory cytokines induced by its own heat-stable molecule.

Psoralen photochemical inactivation of Orientia tsutsugamushi in platelet concentrates

BACKGROUND

The risk of transfusion transmission of disease has been reduced by the combination of predonation questions and improved transfusion-transmitted disease assays, but the risk is still present. This study was conducted to determine if psoralen photochemistry could inactivate an obligate intracellular bacterium, with documented potential for transfusion, in PCs to further improve safety.

STUDY DESIGN AND METHODS

PCs were inoculated with MNCs infected with Orientia tsutsugamushi. The concentrates were treated with amounts ranging from 0.86 to 138 micromol per L of 4'-(aminomethyl)-4,5',8-trimethylpsoralen hydrochloride (AMT) combined with a constant long-wave UVA light (320-400 nm) exposure of 5 J per cm(2). The effects of photochemical treatment were analyzed by using a mouse infectivity assay along with in vitro testing by PCR, indirect fluorescence antibody, direct fluorescence antibody, and Giemsa staining.

RESULTS

AMT, at 0.86 micromol per L or more, combined with UVA light of 5 J per cm(2), inactivated O. tsutsugamushi that contaminated PCs. The PCs that did not receive the combined treatment caused infection.

CONCLUSIONS

The psoralen AMT, in conjunction with UVA light exposure, effectively abolished the infectivity of PCs deliberately contaminated with the scrub typhus organism O. tsutsugamushi, as tested in a mouse infectivity assay.

Azithromycin activities against Orientia tsutsugamushi strains isolated in cases of scrub typhus in Northern Thailand

Azithromycin was given to mice and humans infected with strains of Orientia tsutsugamushi from northern Thailand, where drug-resistant scrub typhus occurs. Azithromycin and doxycycline yielded comparable mouse survival rates (73 and 79%, respectively; P > 0.5). Symptoms, signs, and fever in two pregnant women abated rapidly with azithromycin. Prospective human trials are needed.

A new ecology for scrub typhus associated with a focus of antibiotic resistance in rice farmers in Thailand

Following the documentation of chloramphenicol-resistant and doxycycline-resistant strains of Orientia tsutsugamushi (Hyashi) in northern Thailand, we conducted ecological and epidemiological studies near the houses of patients hospitalized with antibiotic-resistant infections. New associations between chiggers, rodents, and O. tsutsugamushi in active rice agriculture areas, an ecological habitat not described previously, are reported. Rattus rattus (L.) was the most common species (representing 85.8% of the 1,433 rodents processed), followed by Rattus losea (Swinhoe) (9.4%), Bandicota indica (Bechstein) (3.6%), and Rattus argentiventer (Robinson and Kloss) (1.3%). O. tsutsugamushi was isolated from 30% of the R. rattus and R. losea, 29% of the B. indica, and 33% of the R. argentiventer collected. Mean minimum infection rates were 0.03 in Leptotrombidium chiangraiensis Tanskul & Linthicum, a new species of chigger, and 0.002 in Leptotrombidium imphalum (Vercammen-Grandjean & Langston), a chigger species not previously associated with scrub typhus transmission. Efficient vertical and horizontal transmission of O. tsutsugamushi by L. chiangraiensis and L. imphalum was demonstrated. During a 19-mo period from October 1993 to April 1995, the overall prevalence of human IgM and IgG antibody to O. tsutsugamushi was 25.5 and 47.3%, respectively. L. chiangraiensis and L. imphalum are incriminated as vectors of O. tsutsugamushi in a rice field habitat associated with a focus of antibiotic resistance.

Occurrence of high ratio of males after introduction of minocycline in a colony of Leptotrombidium fletcheri infected with Orientia tsutsugamushi

In colonies of Leptotrombidium fletcheri mites infected with Orientia tsutsugamushi (Ot), the agent of scrub typhus, males rarely appear. In the present study, the development of a high ratio of males was observed after introduction of minocycline (MC). A high dose of MC was injected subcutaneously into a mouse, and by feeding unfed larvae from an infected mite colony on this mouse, the Ot in the mites were successfully killed. Of a total of 130 unfed larvae attached to the mouse, 29 developed into females; of these, 9 laid an average of 112.4 eggs/female. Unfed larvae in the succeeding generations were attached to untreated mice. All adults in the P and F1 generation were females, and males started to appear at the F2 generation. The ratio of males to females was 332:7, 108:13, 263:61 and 71:9 at the F2, F3, F4 and F5 generations, respectively. These data suggest that Ot in the ovary or gonad may suppress the development of males.

Scrub typhus infections poorly responsive to antibiotics in northern Thailand

BACKGROUND

Rickettsia tsutsugamushi, the aetiological agent of scrub typhus, is common in Asia and readily infects visitors to areas where disease transmission occurs. Rapid defervescence after antibiotic treatment is so characteristic that it is used as a diagnostic test for R tsutsugamushi infection. Reports from local physicians that patients with scrub typhus in Chiangrai, northern Thailand responded badly to appropriate antibiotic therapy prompted us to do a prospective clinical evaluation and antibiotic susceptibility testing of human rickettsial isolates.

METHODS

The clinical response to doxycycline treatment in patients with early, mild scrub typhus in northern Thailand was compared with the results of treatment in Mae Sod, western Thailand. Prototype and naturally occurring strains of R tsutsugamushi were tested for susceptibility to chloramphenicol and doxycycline in mice and in cell culture.

FINDINGS

By the third day of treatment, fever had cleared in all seven patients from Mae Sod, but in only five of the 12 (40%) from Chiangrai (p < 0.01). Median fever clearance time in Chiangrai (80 h; range 15-190) was significantly longer than in Mae Sod (30 h; range 4-58; p < 0.005). Conjunctival suffusion resolved significantly more slowly in Chiangrai (p < 0.05). Antibiotics prevented death in mice infected by Chiangrai strains of R tsutsugamushi less often than after infection by the prototype strain (p < 0.05). Only one of three Chiangrai strains tested in cell culture was fully susceptible to doxycycline.

INTERPRETATION

Chloramphenicol-resistant and doxycycline-resistant strains of R tsutsugamushi occur in Chiangrai, Thailand. This is the first evidence of naturally occurring antimicrobial resistance in the genus Rickettsia.

Rickettsia tsutsugamushi infection in cell culture: antibiotic susceptibility determined by flow cytometry

Recent unpublished reports from northern Thailand of severe and sometimes fatal cases of scrub typhus, despite appropriate antibiotic therapy, suggest that resistance may occur. Current antibiotic susceptibility methods that use direct microscopic counts of Giemsa-stained cells or mouse protection assays are slow, labor-intensive, and expensive. We explored the use of flow cytometry to measure rickettsial infection in vitro in L-929 cells treated with and without doxycycline, ciprofloxacin, erythromycin, and chloramphenicol. It was possible to detect the rickettsiae down to a level of 83% of the cells infected, mean of 37 rickettsiae per cell, and 40% of cells with too many rickettsiae to count. This level of sensitivity was sufficient to determine the inhibitory effect of all four drugs at standard screening concentrations. At lower concentrations of doxycycline, flow cytometry detected inhibition of rickettsial growth at a concentration of 6.25 x 10(-2) micrograms/ml but not at 6.25 x 10(-3) micrograms/ml, suggesting that the minimum inhibitory concentration is somewhere between these two values. The data from this study show that flow cytometry permits the rapid screening of numerous rickettsial isolates for their susceptibility to a variety of antibiotics, but that visual counts of infected cells provide a more precise indication of rickettsial growth.

In vitro effectiveness of azithromycin against doxycycline-resistant and -susceptible strains of Rickettsia tsutsugamushi, etiologic agent of scrub typhus

In an effort to find a potential alternative treatment for scrub typhus, we evaluated the effectiveness of the standard drug doxycycline and the new macrolide azithromycin against a doxycycline-susceptible strain (Karp) and a doxycycline-resistant strain (AFSC-4) of Rickettsia tsutsugamushi. The antibiotics were tested in an in vitro assay system in which infected mouse fibroblast cells (L929) were incubated for 3 days in various concentrations of the drugs. Rickettsial growth was evaluated by direct visual counts of rickettsiae in Giemsastained cells or by flow cytometry. Initial tests were conducted at the concentration of each antibiotic considered to be the upper breakpoint for susceptibility (16 micrograms/ml for doxycycline and 8 micrograms/ml for azithromycin). Growth of both Karp and AFSC-4 was strongly inhibited with both antibiotics, as measured by visual counts, although the percentage of cells infected with AFSC-4 in the presence of doxycycline was three times greater than the percentage of cells infected with Karp but was only 60% as great as the percentage of cells infected with Karp in the presence of azithromycin. Flow cytometry confirmed that rickettsial growth occurred in the absence of antibiotics, but it failed to detect it in the presence of high concentrations of either drug. Visual counts of rickettsial growth at lower concentrations of the antibiotics (0.25 to 0.0078 microgram/ml) showed that the Karp strain was 16 times more susceptible that the AFSC-4 strain to doxycycline. Azithromycin was much more effective than doxycycline against AFSC-4, inhibiting rickettsial growth at 0.0156 microgram/ml to levels below that achieved by 0.25 microgram of doxycycline per ml. Azithromycin was also more effective than doxycycline against the Karp strain, causing greater reductions in the number of rickettsiae per cell at lower concentrations. If in vivo testing confirms the in vitro effectiveness of azithromycin, it may prove to be the drug of choice for the treatment of scrub typhus in children and pregnant women, who should not take doxycycline, and in patients with refractory disease from locations where doxycycline-resistant strains of R. tsutsugamushi have been found. When tested in an in vitro assay system, azithromycin was more effective than doxycycline against doxycycline-susceptible and -resistant strains of R. tsutsugamushi.

Classification of Rickettsia tsutsugamushi in a new genus, Orientia gen. nov., as Orientia tsutsugamushi comb. nov

Recent studies of Rickettsia tsutsugamushi have demonstrated clearly the phenotypic and genotypic differences between this microorganism and other species belonging to the genus Rickettsia. Therefore, classification of R. tsutsugamushi in a new genus, Orientia gen. nov., is proposed.

The role of tumor necrosis factor in host defense against scrub typhus rickettsiae. I. Inhibition of growth of Rickettsia tsutsugamushi, Karp strain, in cultured murine embryonic cells and macrophages by recombinant tumor necrosis factor-alpha

Recombinant murine tumor necrosis factor-alpha (TNF-alpha) inhibited intracellular growth of Rickettsia tsutsugamushi, Karp strain, in the mouse embryo cell line C3H/10T1/2 clone 8 at doses of 100 to 10 U/ml. The growth inhibitory effect of TNF-alpha was also evident when peritoneal exudate macrophages or bone marrow-derived macrophages were used as the host cell for rickettsial growth. Interferon-gamma (IFN-gamma), at doses up to 1,000 U/ml, did not affect the growth of this strain of rickettsiae in the mouse embryo cell line but, as expected, profoundly inhibited rickettsial growth in peritoneal exudate macrophages and bone marrow-derived macrophages. The effect of TNF-alpha on rickettsial growth in the mouse embryo cell line was not reproducibly enhanced by IFN-gamma. Treatment of the cell line with TNF-alpha delayed rickettsial cytopathic effects, but the rickettsiae ultimately grew to high numbers in the cells and caused cell death. These findings show that, at least in our system, R. tsutsugamushi is resistant to IFN-gamma-mediated antirickettsial effects in cells other than macrophages. The results of this study support the suggestion that TNF-alpha may inhibit rickettsial growth in cells other than macrophages.

[Comparison of in vitro susceptibilities of Rickettsia prowazekii, R. rickettsii, R. sibirica and R. tsutsugamushi to antimicrobial agents]

The in vitro activities of 16 antimicrobial agents against Rickettsia prowazekii (Breinl strain), R. rickettsii (Bitterroot strain), R. sibirica (ATCC No. VR151) and R. tsutsugamushi (Gilliam, Karp, Kato, Shimokoshi, Kawasaki and Kuroki strains) were determined by the cell culture method. Tetracycline, demethylchlortetracycline, doxycycline, minocycline, chloramphenicol, kitasamycin and rifampicin were generally effective (MIC, 0.005-0.78 micrograms/ml) to all strains tested. Quinolones such as norfloxacin, ciprofloxacin and ofloxacin were moderately active, but they were less active against R. tsutsugamushi than other rickettsial species. Penicillins and cephems showed low activity against most of the strains tested, but high concentrations of benzylpenicillin (MIC, 25-50 micrograms/ml) inhibited R. prowazekii, R. rickettsii and R. sibirica. These findings may be applicable for differentiation of species of genus Rickettsia.

Factors influencing Rickettsia tsutsugamushi infection of cultured cells

Several factors which influence Rickettsia tsutsugamushi infectivity of, and replication in, cultured cells have been investigated in order to establish standard laboratory conditions for reliable quantitation and maximization of rickettsial yield and to begin to explore the nature of the relationship between the rickettsia and its host cell. The extent of scrub typhus rickettsial association with host cells was dependent on the rickettsial and target cell concentrations and on the medium in which the infection took place. Brain heart infusion broth, a standard rickettsial diluent for infectivity, was markedly inhibitory. Both rickettsial and host cell replication were dependent on a component(s) supplied by serum to the tissue culture medium; rickettsial multiplication was less affected by small temperature variation (optimum 34 to 37 degrees C) than was host cell growth (optimum 37 degrees C). The antibiotic daunomycin was very useful in selectively inhibiting host cell replication and permitting attainment of somewhat greater rickettsial yields than in uninhibited cells. These findings have underscored the close relationship between parasite and infected cell and have led to more predictable, higher yields of rickettsiae in vitro.

Penetration of Rickettsia tsutsugamushi into cultured mouse fibroblasts (L cells): an electron microscopic observation

The mechanism of penetration of purified Rickettsia tsutsugamushi (Gilliam strain) into cultured mouse fibroblasts (L cells) was examined by electron microscopy. After 10-40 min of infection, rickettsiae in the process of being phagocytized were often seen on the cell surface. These were restricted to the rickettsiae which seemed to be intact in morphology, while heavy plasmolyzed ones were never phagocytized. Additionally, rickettsiae were taken up individually into a phagosome, and phagocytosis of several rickettsiae together was rarely observed, except in the case of heat-inactivated microorganisms. In the cells, phagosomes whose membranes enclosed rickettsiae either tightly or loosely were seen. Rickettsiae in the loose phagosomes often showed signs of plasmolysis and were rarely released into the cell cytoplasm. Partial disintegration of phagosomal membranes and the escape of rickettsiae from the phagosomes were seen only in tight phagosomes. Large phagosomes containing a clump of several rickettsiae were observed occasionally, in which case the microorganisms were deformed and seemed to be denatured. From the above observations and the frequency of appearance of these different penetration stages in the specimens 10, 20, and 40 min after infection, it was concluded that the rickettsiae enter initially into a tight phagosome by phagocytosis and are then released into the cell cytoplasm by disruption of the phagosomal membrane. No other mechanisms of penetration were found. On the other hand, rickettsiae inactivated by trypsin did not attach to host cells. Inactivation by heat or UV irradiation resulted in reduction of phagocytosis, and rickettsiae treated with rifamycin could penetrate into the host cell cytoplasm to the same extent as in the case of infection with intact rickettsiae.

Penicillin-induced unstable intracellular formation of spheroplasts by rickettsiae

Penicillin G (greater than or equal to 20 micrograms/ml) is rapidly rickettsiacidal for intracellular Rickettsia prowazekii. Light and electron microscopic examinations revealed that penicillin G in culture medium induced a predictable transformation into typical enlarging spheroplasts deficient in the internal, putative peptidoglycan layer of the outer membrane. Under certain conditions, spheroplasts ruptured to discharge contents into host cell cytoplasm and to leave empty shells of defective outer membrane and diffuse amorphous intracytoplasmic antigen. Host cell destruction often accompanied spheroplast rupture. Penicillin G (100 micrograms/ml) caused similar spheroplast formation by Rickettsia rickettsii, but 1,000 micrograms/ml caused neither growth inhibition nor spheroplast formation in Rickettsia tsutsugamushi. The clinical and epidemiological significance of a practical rickettsiacidal drug for the treatment of louse-borne typhus fever is discussed. Practical pharmacologic considerations preclude the use of penicillin for the treatment of typhus or spotted fever.

Experimental scrub typhus immunogens: gamma-irradiated and formalinized rickettsiae

Scrub typhus immunogens were prepared by exposing infected yolk sac suspensions of Rickettsia tsutsugamushi to various doses of gamma radiation. Mouse lethality was abolished at doses greater than 200 krads, whereas immunogenicity of the suspensions, as shown by mouse protection tests, was diminished relatively little by radiation doses in the 200- to 400-krad range. Using a 300-krad gamma dose to provide a safety factor, immunogens were prepared and their protective capacity was contrasted with formalinized scrub typhus immunogens prepared by conventional techniques. Formalinized suspensions afforded mice only partial protection against intraperitoneal challenge with 1,000 50% mouse lethal doses of the virulent homologous strain and no significant protection against similar challenge with an equally virulent heterologous strain. Using the same strains, radiation-inactivated preparations provided 100% protection against 10,000 50% mouse lethal doses of the homologous strain and 70% protection against challenge with the same doses of a heterologous strain. Neither immunogen was a potent stimulator of antibody production as measured by the complement-fixation test. Cell-transfer studies using inbred mice indicated a role for cell-mediated immunity after vaccination with gamma-irradiated immunogens, but no cell-mediated protection could be demonstrated after vaccination with formalin-inactivated rickettsiae.