The American Journal of Tropical Medicine and Hygiene - Volume 51, Issue 2, 1994

The Dielmo project, initiated in 1990, consisted of long-term investigations on host-parasite relationships and the mechanisms of protective immunity in the 247 residents of a Senegalese village in which malaria is holoendemic. Anopheles gambiae s.l. and An. funestus constituted more than 98% of 11,685 anophelines collected and were present all year round. Inoculation rates of Plasmodium falciparum, P. malariae, and P. ovale averaged respectively 0.51, 0.10, and 0.04 infective bites per person per night. During a four-month period of intensive parasitologic and clinical monitoring, Plasmodium falciparum, P. malariae, and P. ovale were observed in 72.0%, 21.1% and 6.0%, respectively, of the 8,539 thick smears examined. Individual longitudinal data revealed that 98.6% of the villagers harbored trophozoites of P. falciparum at least once during the period of the study. Infections by P. malariae and P. ovale were both observed in individuals of all age groups and their cumulative prevalences reached 50.5% and 40.3%, respectively. Malaria was responsible for 162 (60.9%) of 266 febrile episodes; 159 of these attacks were due to P. falciparum, three to P. ovale, and none to P. malariae. The incidence of malaria attacks was 40 times higher in children 0–4 years of age than in adults more than 40 years old. Our findings suggest that sterile immunity and clinical protection are never fully achieved in humans continuously exposed since birth to intense transmission.

When 155 ticks collected in different regions of Switzerland were tested by the hemolymph test, 10.3% were found to contain spotted fever group rickettsiae. Six rickettsial isolates were made from Dermacentor marginatus ticks and three were made from Ixodes ricinus ticks. The polymerase chain reaction followed by restriction fragment length polymorphism analysis showed that the Dermacentor ticks were infected with Rickettsia slovaca and the Ixodes ticks were infected with a spotted fever group rickettsia. Microimmunofluorescence serologic typing, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins, and Western blot assay with polyclonal mouse antisera confirmed the results and determined that the Ixodes were infected with R. helvetica, the only previously described Swiss rickettsia. However, an additional new strain that could not be isolated was detected in one I. ricinus by hemolymph test and provisionally characterized by enzymatic restriction of its amplified DNA.

Following a report of three cases of scrub typhus in suburban Bangkok, we performed a serosurvey in the patients' communities. Both IgG and IgM antibodies were measured in an indirect immunoperoxidase assay, using separate spots of antigen from Rickettsia tsutsugamushi (scrub typhus), R. typhi (murine typhus), and TT-118 spotted fever group rickettsiae. Of 215 people donating blood, antibody levels indicative of most recent exposure to R. tsutsugamushi were most prevalent (21%), followed by R. typhi (8%), and TT-118 (4%). Seroprevalence suggesting most recent exposure to R. tsutsugamushi varied by location (range 13–31%), gender (26% of females and 13% of males), age (61–80-year-old individuals were the highest, with a prevalence of 38%), and contact with orchards and orchid farms (29% of those with extensive contact, 38% of those with occasional contact, and 10% of those with no contact). These patterns indicated that exposure to R. tsutsugamushi was related to occupation and behavior, as has been observed in areas of rural transmission. Expansion of metropolitan Bangkok has created a situation in which people employed in agriculture live with people employed in the city. As a result, a serosurvey in suburban Bangkok reveals evidence of murine typhus, which is usually transmitted in urban areas, as well as scrub and tick typhus, which are usually transmitted in rural areas.

Attempts to isolate viruses from 178,181 unengorged female mosquitoes collected from different ecologic areas of Sri Lanka yielded 31 isolates: 17 of Japanese encephalitis (JE) virus, nine of Getah virus, three of a Batai-related bunyavirus, and two of Arkonam virus. Culex tritaeniorhynchus and Mansonia uniformis mosquitoes were found to carry JE virus in a dry zone nonepidemic area, and Cx. pseudovishnui was found to carry it in a wet zone nonepidemic area. Japanese encephalitis virus was isolated from Cx. tritaeniorhynchus, Cx. gelidus, Cx. fuscocephala, and Cx. whitmorei during a human epidemic in the dry zone. Getah virus was isolated from Cx. tritaeniorhynchus, Cx. gelidus, and Cx. fuscocephala collected in the vicinity of swine. Isolations of Getah, Arkonam, and Batai-related viruses from Sri Lanka are reported for the first time.

The impact of annual screening and treatment with praziquantel on schistosomiasis japonica was examined on Jishan Island in PoYang Lake, Jiangxi Province, China. Prevalence of infection in the community decreased from 39% in year 1 to 33% in year 3 with a corresponding decrease in the geometric mean egg count from 51 eggs per gram of stool (epg) to 31 epg. The most dramatic changes in infection status and intensity of infection were observed in younger individuals (0–19 years of age). The prevalence of hepatosplenomegaly also significantly decreased, again primarily in younger individuals. No change in the community prevalence of schistosome-induced hepatic fibrosis was observed as determined by ultrasonography. Longitudinal cohort analysis, however, demonstrated significant improvement in treated individuals with advanced hepatic fibrosis. These data indicate that annual screening and treatment had a significant impact on infection status and morbidity and suggest that community therapy may be an effective approach to control schistosomiasis japonica in lake regions and marshlands in China. Further studies are necessary to determine the optimal and most cost-effective approach for drug delivery.

Anemia is an increasingly recognized health problem in African children. To determine the prevalence of and risk factors for anemia in young children, we enrolled 252 pregnant women and studied their newborn infants in Mangochi District in southern Malawi. At the first follow-up visit after birth at approximately two months of age, the mean hematocrit value of the 252 infants was 29.5%, and 64 infants (25%) were anemic (hematocrit value < 25%). Placental malaria infection was the strongest risk factor for an infant having anemia at the first follow-up (relative risk = 2.0, P = 0.003). Infants who had Plasmodium falciparum parasitemia at the first follow-up had lower hematocrit values than infants without parasitemia (median 28% versus 31%; P = 0.02). Neither the mother's hematocrit at enrollment, her hematocrit at delivery, sex of the infant, nor fever illness in the infant was associated with having a hematocrit less than 25% at the first follow-up. Although infants with hematocrit values less than 25% were more likely than infants with higher hematocrit values to die during the first year of life, this difference was not statistically significant (relative risk = 1.7, P = 0.15). In rural Malawi, anemia commonly affects young infants, is acquired early in life, and is probably a risk factor for death in infancy. Strategies to reduce anemia in infants must address P. falciparum infection, both during pregnancy and in the first few months of life.

Recently described enzyme-linked immunosorbent assay (ELISA) and immunoblot methods for the detection of serum IgG against Strongyloides stercoralis larval antigens were prospectively evaluated for the diagnosis of endemic strongyloidiasis. A modification of the ELISA involved preincubation of sera with Onchocerca gutturosa phosphate-buffered saline-soluble extract to remove cross-reactivity with other helminths. The sensitivity of the ELISA increased from 80% to 85% following preincubation. Similarly, there was an increase in specificity from 94% to 97%. The IgG recognition of 41-, 31-, and 28-kD filariform larval components showed sensitivities of 100%, 85%, and 65%, respectively. The specificities of recognition of these proteins were 94%, 89%, and 75%, respectively. Both the ELISA following incubation of sera with O. gutturosa extract and serum IgG reactivity to a 41-kD larval component using immunoblotting are sensitive and specific techniques for diagnosing endemic strongyloidiasis.

We successfully treated a serious case of Entamoeba histolytica infection showing brain abscess, meningitis, and liver abscess by oral administration of metronidazole, intramuscular injection of dehydroernetine, and drainage of the brain abscess. The diagnosis of amebic brain abscess was based on a combination of clinical symptoms and signs, computed tomography and magnetic resonance imaging findings, a positive serologic test result for E. histolytica, a dramatic response to anti-amebic drugs after an ineffective therapeutic history with antibacterial drugs, and application of the polymerase chain reaction method.

The pathogenesis of renal diseases associated with Plasmodium malariae infections is still not fully understood. The present work is concerned with the infection caused by P. inui in the rhesus monkey Macaca mulatta as a potential model for human quartan malaria, which the monkey parasite resembles in morphology and schizogonic behavior. Various aspects of the disease were studied. Changes in the levels of serum complement components C3 and C4 indicate activation of complement through the classical pathway. A few days after infection, IgG antibody titers increased, coinciding with low levels of parasitemia, which suggests that some of these antibodies are protective. Immunofluorescence testing of kidney tissue showed a predominance of IgM antibodies over IgG, C3, C4, albumin, and fibrinogen, which was detected in a number of the infected monkeys. These findings were consistent with those seen in humans with P. malariae infection and indicate that the P. inui/rhesus monkey model is likely to be appropriate for the study of different aspects of quartan malaria.

The inhabitants of four villages endemic for Schistosoma mansoni in central Mali (n = 1,106 of both sexes, age range 2–80 years) and of two nonendemic villages in another part of the country were examined parasitologically and ultrasonographically to establish the prevalence of periportal liver fibrosis (PF) and other features of hepatosplenic schistosomiasis. The prevalence of S. mansoni infection ranged from 36% to 93% in the endemic villages. A severe infection (> 400 eggs/g of stool) was found in 16% of the infected individuals. No case of grade III PF (echogenic bands usually > 10 mm in diameter around the central part and major branches of the portal vein and streak-like fibrous bands that extended into the periphery of the liver) and only eight cases of grade II PF (echogenic bands usually > 10 mm in diameter around the central part and major branches of the portal vein) were found; no other signs of severe hepatosplenic disease were encountered. However, grade I PF (echogenic bands usually > 4 mm in diameter that were best visible in the area of the portal vein bifurcation and gall bladder neck) was detected in 21% of all individuals, mainly in adults. In the nonendemic villages, the prevalence of grade I PF in adults was 9%. Generally, there was no significant correlation between the grade of PF and S. mansoni egg output. In one village with a high endemicity level, however, the prevalence of grade PF I increased with the intensity of infection. Morphometric data revealed no significant influence of S. mansoni infection on portal vein stem diameter and spleen size. It is concluded that schistosomiasis mansoni did not progress to severe hepatosplenic disease in the examined villages; grade I PF was prevalent, especially in adults, but seemed to be related to factors other than schistosomiasis.

Some strains of Plasmodium falciparum form erythrocyte rosettes that are believed to result from a lectin interaction between malaria-infected and uninfected erythrocytes. The sulfated glycoconjugate heparin and certain heparin derivatives have been observed to disrupt rosettes. To investigate this interaction further, we have studied the effects of four sulfated glycoconjugates on 15 fresh isolates of P. falciparum from Papua New Guinea. A broader range of sulfated glycoconjugates has been tested against a laboratory strain. A concentration of 1,000 µg/ml of dextran sulfate (molecular weight [MW] 500,000) was the most potent disrupter of rosettes. Fucoidan, heparin, and dextran sulfate (MW 5,000) were of decreasing effectiveness in 14 of 15 fresh isolates. The same relationship was true for the laboratory strain. Pentosan polysulfate and sulfatide also disrupted rosettes; chondroitin sulfates A, B, and C and keratan sulfate gave either minimal or no rosette disruption. Thus, some sulfated glycoconjugates are potent disrupters of P. falciparum erythrocyte rosettes. Sulfated glycoconjugates that are potent disrupters of P. falciparum rosettes may prove useful in identifying ligands involved in rosette formation.

Twenty-eight nonimmune patients with acute uncomplicated falciparum malaria returning from subSaharan Africa were treated with a micronized formulation of halofantrine hydrochloride (three doses of 250 mg at 6-hr intervals) to investigate the drug's efficacy, tolerance, and pharmacokinetics. In vitro drug susceptibility patterns were determined by the isotopic semimicrotest. Twenty-four of 28 patients were cured. Two of the four patients experiencing recrudescence were associated with low absorption of the drug and parasites susceptible in vitro to halofantrine. The other two patients had adequate plasma concentrations of halofantrine and its main human metabolite, N-desbutylhalofantrine, but the isolates were also resistant in vitro to the drugs, suggesting drug resistance as the cause of treatment failure. Only mild, transitory side effects were noted. A wide interindividual variation in plasma concentrations of halofantrine and its metabolite was observed. Pharmacokinetic studies suggested that the micronized formulation of halofantrine hydrochloride may not increase drug absorption considerably. Further studies using higher doses or longer treatment periods are needed to ensure that adequate plasma concentrations of the drug are used.

Plasmodium falciparum chemosensitivity to the various antimalarial drugs is presently determined in the laboratory by setting up multiple microcultures of the parasite and estimating the amount of growth inhibition caused by known concentrations of drug. Parasite growth inhibition is assessed either by microscopy, radiolabeled substrate uptake, or calorimetrically. The obligate requirement for serum in this assay presents difficulties in the direct comparison of results among laboratories. We now have evidence that antimalarial drug sensitivity assays can be reliably performed in a serum-free medium. The overall comparison of 50% inhibitory concentration (IC50) values obtained with serum-free media (bovine albumin, Cohn fraction V [BAM] and BAM combined with glucose and lipids-cholesterol-rich mixture) and those obtained in serum-supplemented medium was r = 0.56; n = 60; P < 0.01.

In a double-blind clinical study, 109 adult Egyptian patients infected with Shigella spp. and 45 infected with Salmonella spp. were randomly assigned to three treatment groups: 1) norfloxacin in a single 800-mg dose, 2) norfloxacin, 400 mg twice a day for three days, and 3) trimethoprim (160 mg)-sulfamethoxazole (800 mg) (TMP-SMX), twice a day for three days. Among Shigella-infected patients, diarrheal symptoms had resolved in 86–97% and bacteriologic failure (repeat positive stool culture) occurred in only two patients five days after the start of the three treatment regimens. Among Salmonella-infected patients, diarrheal symptoms had resolved in 76–82% of patients and bacteriologic failure was common (18–36%) five days after the start of therapy. These data indicate that short-course therapy with either norfloxacin or TMP-SMX can be effectively used to treat shigellosis in adults in developing countries. However, for uncomplicated Salmonella spp. infection, short-course therapy with norfloxacin and TMP-SMX may not lead to a rapid resolution of symptoms or consistently eliminate this enteropathogen.

Three strains of Plasmodium falciparum, Vietnam Oak Knoll (FVO), Uganda Palo Alto (Hawaii) (FUP-H) and Uganda Palo Alto (Cayenne) (FUP-C), were examined in 154 Aotus nancymai monkeys as suitable models for testing blood-stage vaccines. The Vietnam Oak Knoll strain had the greatest number of animals with maximum parasite counts > 200,000/µl. Uniformity of the parasitemia curve increased from passage 4 to passage 6 with an accompanying decrease in the number of days required to reach maximum parasitemia or required treatment. The Uganda Palo Alto (Hawaii) strain was highly infectious, but many animals had extended prepatent periods and extended days to maximum parasitemia. The FUP-H strain would require a greater number of animals per test group to detect partial protection because of the greater number of low-density maximum parasite counts in control animals. The Uganda Palo Alto (Cayenne) strain was poorly adapted to intact A. nancymai. However, five of six splenectomized monkeys inoculated during passage 6 with 105 parasites had maximum parasite counts > 200,000/µl. For the testing of vaccines against primary parasitemia in the A. nancymai model system, the FVO at passage 4 level would appear preferable to passage 6 parasites following a challenge with 105 parasites. A similar pattern could be obtained using FUP-H if the challenge was 106 parasites. To measure immune memory against recrudescence or rechallenge infection, FUP-C at an early passage in splenectomized A. nancymai would appear to be the appropriate model.

Sporogonic development of cultured Plasmodium falciparum was compared in six species of Anopheles mosquitoes. A reference species, A. gambiae, was selected as the standard for comparison. Estimates of absolute densities were determined for each lifestage. From these data, four aspects of parasite population dynamics were analyzed quantitatively: 1) successive losses in abundance as parasites developed from gametocyte to ookinete to oocyst stages, 2) oocyst production of sporozoites, 3) correlation between various lifestage parameters, and 4) parasite distribution. Parasite populations in A. gambiae incurred a 316-fold loss in abundance during the transition from macrogametocyte to ookinete stage, a 100-fold loss from ookinete to oocyst stage, yielding a total loss of approximately 31,600-fold (i.e., losses are multiplicative). Comparative susceptibilities in order were A. freeborni ≫ A. gambiae, A. arabiensis, A. dirus > A. stephensi, A. albimanus. The key transition(s) determining overall susceptibility differed among species. Despite species differences in oocyst densities and infection rates, salivary gland sporozoite production per oocyst (approximately 640) was the same among species. The most consistent association among lifestage parameters was a positive correlation between densities and infection rates of homologous lifestages. A curvilinear relationship between ookinete and oocyst densities in A. gambiae indicated a threshold density was required for ookinete conversion to oocysts (approximately 30 ookinetes per mosquito). The same relationship in A. freeborni was linear, with no distinct threshold. Ookinete and oocyst populations were negative binomially distributed in all species. Indices of heterogeneity in mosquito susceptibility to infection indicated that gene frequencies determining susceptibility fluctuated with time in all species, except A. freeborni where susceptibility remained homogenous throughout the study. This approach provides a framework for identifying mechanisms of susceptibility and evaluating Plasmodium sporogonic development in naturally occurring vector species in nature.

Rates of acquisition and loss of parasitic infection in the vector host of Wuchereria bancrofti were estimated using a new method. The age of parasitic infections was estimated from the abdominal condition, determined on the basis of Sella's stages of blood digestion and ovary development, and parity of the vector host and larval stages of the parasite. A negative exponential relationship between age and number of W. bancrofti infections provided estimated daily losses of 25.3% and 33.2% of the initial infection from the resting and biting populations, respectively, of the mosquito Culex quinquefasciatus. The rates per day of acquisition of infection estimated from resting and biting female Cx. quinquefasciatus were found to be 9.6% and 8.6%, respectively. The mean minimum developmental time for W. bancrofti infective larvae estimated from the natural resting and biting populations were 8.9 ± 1.41 (± SEM) and 8.33 ± 0.85 days, respectively, which was somewhat less than the duration observed under laboratory conditions. Comparisons of the estimates (mean age, rates of acquisition, and loss of infections) based on the resting and biting data suggest that the rates are independent of the method of collecting mosquitoes.