Biomedical subjects
Publications and source records attributed to D M Robinson.
An epidemiological study in a mature oil palm estate in Peninsular Malaysia has demonstrated a low prevalence of R. tsutsugamushi infection in small mammals. The direct fluorescent antibody technique for assaying infections in chiggers proved more sensitive than mouse inoculation. Most infections in both chiggers and rodents were caused by the Karp strain.
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Most silvered leaf monkeys inoculated with selected strains of Rickettsia tsutsugamushi developed rickettsemia. The time of onset and the duration of rickettsemia were related to the infecting strains, but these parameters could not be correlated with the virulence of the strain. The rickettsemia was not terminated by the appearance of humoral antibody, as detected by the indirect fluorescent antibody test. The specificities of the antibody responses were related to the antigens present in the infecting strains.
Numbers of L. (L.) deliense larvae were determined in adjacent habitats over a 16 month period. Both R. argentiventer and R. tiomanicus were highly efficient hosts for L. (L.) deliense. R. argentiventer was host to significantly greater numbers of chiggers per rat than was R. tiomanicus. The 2 habitats were similar in numbers of chiggers collected. No consistent correlation was apparent between numbers of chiggers and any single weather factor, but the chigger population seemed to be adversely affected by a 2 month period during which total evaporation greatly exceeded total rainfall. Direct fluorescent antibody examination of tissues from unfed L. (L.) deliense showed that 2 of 420 larvae (0.5%) contained organisms morphologically resembling R. tsutsugamushi. Considering the vector load and numbers of chiggers being returned to the ground by a given host, a rate of 0.5% appeared adequate to account for the prevalence rate of R. tsutsugamushi observed in the 2 host species.
Over 1,200 isolations of R. tsutsugamushi were obtained over 7 generations of L. (L.) arenicola and no reproducible differences in virulence or antigenic markers could be detected among these isolations. The F8 generation yielded rickettsia with the same antigenic makeup and virulence properties as the original female parent mite. When these isolates were inoculated into mice they elicited the same predominant antibody specificities, i.e. they all produced antibody which reacted with the TA 763 and Karp strains. These were the same as the major antigens detected by direct fluorescent antibody staining of the isolates.
Dog sera, collected from different communities throughout Selangor, Peninsular Malaysia, were investigated for the presence of antibodies to R. tsutsugamushi and R. typhi. Scrub typhus antibodies were present in animals from the rural areas only, whereas murine typhus antibodies were observed in equal numbers of dogs from both rural and metropolitan areas. Greater percentage of dogs from suburban areas had demonstrable antibody titers to murine typhus than from the urban area.
An explanation was sought for the disparity between the low reported incidence of scrub typhus and the high prevalence of antibody to Rickettsia tsutsugamushi in the rural population of Malaysia. A combination of isolation of the organism, titration of antibody by indirect immunofluorescence, and the Weil-Felix test was used to confirm infections. Scrub typhus was found to be very common, causing 23% of all febrile illnesses at one hospital. The infection was particularly prevalent in oil-palm workers, causing an estimated 400 cases annually in a population of 10,000 people living on one plantation. The clinical syndrome, whether mild or severe, was difficult to distinguish from that due to other infections. Eschars, rashes and adenopathy were uncommon. When used to examine early sera, the Weil-Felix test failed to confirm the diagnosis in most infections.20
Silvered leaf monkeys (Presbytis cristatus) inoculated with avirulent strains of Rickettsia tsutsugamushi showed little or no signs of disease, whereas those inoculated with moderately or fully virulent strains developed severe disease. Infection with the virulent strains produced early leukocytosis followed by leukopenia and significant decreases in packed-cell volumes. In animals infected with avirulent strains, these parameters were normal throughout the experimental period.
Primary chicken embryo cell cultures were evaluated a s an alternate cell system for the production of attenuated Venezuelan equine encephalomyelitis (TC-83 strain) vaccine. The TC-83 strain virus was shown to remain stable during 10 serial passages in chicken embryo cell culture with regard to plaque size and morphology, virus yield, potency, and virulence for mice and hamsters.
A microimmunofluorescence (micro-IFA) test has been adapted to the study of naturally occurring antibody to Rickettsia tsutsugamushi in humans. The micro-IFA test was comparable to the present immlnofluorescent assay in sensitivity and reproducibility, but offered distinct advantages in the quantity of reagents necessary. Also the micro-IFA could be used to simultaneously titer a serum to as many as nine strains of the organism resulting in a great saving of time.
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Based on the prevalence of antibody, an estimated 3% of the population of rural Malaysia is infected with Rickettsia tsutsugamushi each year, resulting in positive antibody rates in focal areas of 6 to 69%. Most of these infections do not appear to produce clinical scrub typhus. A wide range of seropositivity rates was found in areas otherwise resembling each other in predominant occupation, terrain, and nearby habitat. The prevalence rates however were significantly higher in people who worked in forested areas and significantly lower in people with urban occupations.
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The passive transfer of convalescent sera did not protect the majority of mice against challenge with the homologous strain and was completely ineffective against challenge with strains unrelated by fluorescent antibody techniques. When the immune sera was incubated with the rickettsia in vitro and then inoculated into the mice a dramatic increase occurred in the number of surviving mice. The importance of these data in relation to published results with other species of rickettsia is discussed.
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The attenuated M-44 strain of Coxiella burneti was heated to free it of adventitious agents that may have been present. Parallel titrations of the strain before and after heating indicated that the virulence for guinea pigs was markedly decreased. Guinea pigs immunized with the strain derived after heat treatment were protected from the effects of challenge with C. burneti in either phase I or II.
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