The American Journal of Tropical Medicine and Hygiene - Volume 69, Issue 2, August 2003

Antibodies of all four dengue virus serotypes were detected by hemagglutination inhibition (HI) in 97% of 2,000 infants’ cord sera at the time of delivery. In comparison with 250 mother–infant’s paired sera, we found that 53% of the infants’ serum HI titers were higher than those of the mother’s. The mother/infant IgG subclasses 1, 2, 3, and 4 titers were 53.1/87.0, 8.4/11.7, 0.14/0.11, and 1.1/1.0 mg/dL, respectively. In 18 months of follow-up of 100 infants studied, we observed that antibody to dengue virus disappeared in 3% by two months of age, in 19% by four months of age, in 72% by six months of age, in 99% by nine months of age, and in 100% by 12 months of age, with a half-life of 41 days. We conclude that the antibodies to dengue virus disappeared in the first year of life. We suggest that the most appropriate age for vaccination with a live-attenuated dengue vaccine in an endemic area is one year of age.

A recombinant vaccine that expresses the envelope (E) gene of dengue virus type 4 was tested for immunogenicity and protection in Macaca fascicularis. One hundred micrograms of semipurified recombinant E protein (E4rec) expressed in Pichia pastoris was used to immunize three animals. Neutralizing antibodies to dengue 4 virus with a titer of 1:30 were detected in all immunized monkeys prior to challenge. Animals were challenged with 105 plaque-forming units of dengue 4 virus. One vaccine-immunized monkey was protected from viremia, while the other two were partially protected. Monkeys immunized with E4rec elicited the highest neutralizing antibody titers (P < 0.05) ranging from 1:85 to 1:640 at day 30. In both immunized and control animals, the longest duration of viremia correlated with earliest and highest level of IgM antibody to dengue virus. The vaccinated animals showed anamnestic antibody responses upon virus challenge, indicating successful priming by the recombinant vaccine. Our results suggest that E4rec expressed in P. pastoris can provide partial protection against viremia. However, the results were not effective enough to use it as a vaccine candidate. Further work is required to improve the quality of the immunogen.

Between January and June 1995, an outbreak of dengue fever occurred in Palau, an island nation of 32,000 inhabitants in the Western Pacific. To determine the magnitude of this outbreak and to determine modifiable risk factors to guide control strategies, we established active surveillance at the national hospital and private clinics, reviewed available clinical records, and conducted serologic and entomologic surveys. Between January 1 and July 1, 1995, 817 case-patients with acute febrile illness with body or joint aches and one of the following: headache, rash, nausea, vomiting, or hemorrhagic manifestations presented to health facilities in Palau. The epidemic peaked in the second week of April 1995. Of 338 case-patients tested, 254 (75%) had positive serologic results by an IgM capture enzyme-linked immunosorbent assay. Dengue 4 virus was isolated from 78 (51%) of 154 serum samples tested. Blood samples collected during a cross-sectional survey were tested for IgM antibody and yielded an attack ratio of 27% (95% confidence interval = 23–31%). Potential vectors included the introduced species Aedes aegypti and Ae. albopictus, and the native species Ae. hensilli. Significant risk factors (P ≤ 0.05) for infection included age < 20 years, the presence of food or water pans for animals on the property, taro farming, the presence of Ae. aegypti on the property, and presence of Ae. scutellaris group mosquitoes (Ae. Hensilli, Ae. albopictus, and a native species). This was the first outbreak of dengue 4 virus in the Western Pacific, and the first documented epidemic of dengue in Palau since 1988.

After West Nile virus (WNV) was first detected in Florida in July 2001, intensive surveillance efforts over the following five months uncovered virus activity in 65 of the state’s 67 counties with 1,106 wild birds, 492 horses, 194 sentinel chickens, and 12 people found infected with the virus. Thirteen of 28 mosquito isolations came from Culex mosquitoes. As seen in the northeastern United States, wild bird mortality was the most sensitive surveillance method. However, unlike the predominantly urban 1999 and 2000 epizootics, the Florida transmission foci were rural with most activity detected in the northern part of the state. All human cases were preceded by the detection of WNV in animals; however, only eight of the twelve cases were preceded by reports of WNV activity in the county of residence. West Nile virus-positive animals detected by multiple surveillance systems preceded seven of these cases by two weeks or more.

A 240-nucleotide sequence of the capsid/premembrane gene region of 23 Japanese encephalitis virus (JEV) strains isolated in Tokyo and Oita, Japan was determined and phylogenetic analyses were performed. All the strains clustered into two distinct genotypes (III and I). All strains isolated before 1991 belonged to genotype III, while those isolated after 1994 belonged to genotype I. In addition, the strains of the genotype I isolated in Japan showed a close genetic relationship with those from Korea and Malaysia.

Acceptance of oral baits by dogs was evaluated in Guatemala. Eight bait matrix/attractant combinations were produced using commercial materials available in the United States. Two baits were produced using local materials in Guatemala. All baits included a plastic sachet that contained a placebo vaccine (water). Bait trials were conducted February–April, 2002, at five sites using 261 dogs. Bait acceptance ranged from 50.0% to 87.1%, and the combined proportion of sachets either swallowed or punctured ranged from 23.1% to 83.9%. The four bait types with the highest acceptance by dogs were the wax-coated sachet coated with poultry oil and poultry meal (87.1%), the dog meal polymer coated with poultry oil and poultry meal (82.8%), the fish meal polymer coated with poultry oil and poultry meal (77.4%), and the chicken head bait (77.8%). These four bait types were accepted most often as determined both by consumption and combined proportion of sachets swallowed or punctured (P = 0.0001). Future trials should demonstrate efficacy of oral rabies vaccination in Guatemala based on the use of selected bait matrices and the poultry oil/poultry meal attractant.

Vivax malaria reemerged in the Republic of Korea in 1993. Most of the cases occurred among soldiers in the region adjacent to the Demilitarized Zone (DMZ) until 1995. To determine the rate of dispersion of vivax malaria, we evaluated its epidemiologic characteristics. Of 13,903 cases of vivax malaria reported in 2000, 40.1% (5,577) were reported among Republic of Korea military personnel, 26.2% (3,641) among veterans discharged less than two years from the military, and 33.7% (4,685) among civilians. Cases of vivax malaria have rapidly increased annually among counties bordering the DMZ, and have spread to approximately 40 km south of the DMZ. Chemoprophylaxis administered to military personnel may have been responsible for the decreasing number of cases among the Republic of Korea military population. The first mosquito-transmitted cases appeared in early June. Therefore, chemoprophylaxis should be instituted in early April to reduce the number of infected mosquitoes. Extensive intervention is warranted to reduce the spread of vivax malaria in the Republic of Korea.

In vitro drug sensitivity assay is an important tool for various on-going studies aiming to establish the correlation between candidate molecular markers for drug resistance and drug response in laboratory-adapted strains and field isolates of Plasmodium falciparum. A widespread use of this technique in the field would require a suitable substitute that can replace human serum. In this study, several alternative sources of serum substitutes and supplements were evaluated for their capacity to sustain parasite growth for a single life cycle and their compatibility with in vitro assays for clinical isolates that have not been adapted to in vitro culture. Albumax, a commercial preparation of lipid-enriched bovine albumin, did not support parasite growth as much as human serum and fetal calf serum in several isolates. Other serum supplements (AmnioMax and Ultroser) supported parasite growth relatively well. The 50% inhibitory concentrations (IC50s) of chloroquine and antifolates determined with 0.05% Albumax were generally two or three times higher than with human serum. With 10% fetal calf serum, IC50s for chloroquine and antifolates were approximately two times higher and three times lower than with human serum, respectively. The use of AmnioMax and OptiMAb resulted in a greater than two-fold increase in IC50s and several uninterpretable assays. Despite possible batch-to-batch differences, fetal calf serum may be a suitable substitute for in vitro drug assays while awaiting the results of further studies on other serum substitutes and supplements.

Clinical observations have shown that pyrimethamine resistance develops rapidly in endemic countries where antifolate drugs are used massively for the treatment of Plasmodium falciparum infections. To analyze this phenomenon, the in vitro response of clinical isolates to pyrimethamine and the dihydrofolate reductase (dhfr) gene sequence were analyzed in 2000–2001 and compared with the results obtained since 1994 in Yaounde, Cameroon. Of 139 samples obtained in 2000–2001, 10 (7.2%) isolates were of the wild-type, 116 had pure mutant alleles (2 [1.4%] with a single mutation, 11 [7.9%] with double mutations, and 103 [74.1%] with triple mutations), and 13 (9.4%) had mixed alleles. With the exception of a single isolate with triple mutations (50% inhibitory concentration [IC50] = 84.3 nM), all isolates with pure wild-type dhfr alleles (IC50 < 100 nM) and those with pure mutant dhfr alleles (between 1 and 3 point mutations; IC50 ≥ 100 nM) were clearly distinguished by in vitro drug sensitivity assays. The results of the two methods are highly correlated, and both methodologic approaches indicate an increasing proportion of pyrimethamine-resistant isolates in Yaounde over the past eight years (42–45% in 1994–1995, 63–67% in 1997–1998, and 88–92% in 2000–2001). At present, clinical isolates carrying triple dhfr mutations predominate in Yaounde. This situation calls for a regular surveillance of the efficacy of antifolate drugs by all available means, including clinical evaluation, in vitro drug sensitivity assays, molecular markers, and pharmacologic studies.

Atovaquone is a new broad-spectrum antiprotozoal drug with high in vitro activity against multidrug-resistant Plasmodium falciparum. Its specific action against protozoans is based on the inhibition of the parasite cytochrome bc1 complex of the mitochondrial electron transport system. Protozoans may develop atovaquone resistance by the selection of a mutant cytochrome b gene. With the increasing availability of atovaquone-proguanil combination for prophylaxis and treatment of malarial infections, it is necessary to establish baseline data on atovaquone sensitivity before the drug is introduced massively in an endemic region. For this purpose, the activity of atovaquone was assessed indirectly by in vitro drug sensitivity assays with several serum substitutes and DNA sequencing of the cytochrome b gene. Using the standard in vitro assay procedures with 10% human serum, the geometric mean 50% inhibitory concentration (IC50) for atovaquone was calculated to be 1.15 nM (range = 0.460–4.17 nM), while the use of 10% fetal calf serum resulted in lower IC50s (geometric mean = 0.575, range = 0.266–2.20 nM). The use of Albumax, a lipid-enriched bovine albumin, over the same concentration range (0.25–16 nM) showed poor results. None of the 37 isolates with an atovaquone IC50 < 4.17 nM displayed any mutation. Further monitoring of atovaquone-resistant P. falciparum is warranted for the rational use of this new antimalarial drug.

Amodiaquine, a 4-aminoquinoline compound, is being considered as an alternative to chloroquine and pyrimethamine/sulfadoxine where resistance in Plasmodium falciparum to both drugs has been selected. Although amodiaquine is more potent than chloroquine, its effectiveness is reduced in areas where chloroquine resistance is high. We report an association of the P. falciparum chloroquine resistance transporter (pfcrt) gene and the P. falciparum multiple drug resistance 1 (pfmdr1) gene, two chloroquine resistance markers, with chloroquine and amodiaquine efficacy in vivo in southern Sudan. The data show that the allele of the pfcrt gene with a lysine to threonine change at codon 76 is strongly associated with both chloroquine and amodiaquine resistance. No such association was observed with the pfmdr1 gene.

Experimental systems that model some of the complex interactions between parasite and host can be extremely valuable in identifying and developing new prophylactics and therapeutics against human diseases. Because primates have similar immune systems to humans, we have characterized a baboon model for understanding host response to Plasmodium knowlesi. Ten intact olive baboons (Papio anubis) of either sex were experimentally infected with P. knowlesi H strain erythrocytic parasites. The infection in these baboons was either acute or chronic. Animals with acute infection developed multiple system organ dysfunction and cerebral involvement. In chronically infected animals, only the spleen was moderately enlarged. The P. knowlesi parasitemia profile in baboons and rhesus monkeys was comparable. However, some clinical symptoms of the baboons and P. falciparum-infected humans were similar. These studies demonstrate for the first time that P. anubis is a suitable host for P. knowlesi for studying clinical symptoms and pathology.

A polymerase chain reaction (PCR)–based diagnostic assay was developed that rapidly and reliably differentiates the sibling species of the Anopheles claviger complex, An. claviger s.s. and An. petragnani. The assay makes use of nucleotide differences in the internal transcribed spacer 2 ribosomal DNA sequences to generate PCR products of specific length for each of the two species. In evaluating the test, 580 of 592 field-collected An. claviger s.l. specimens were unambiguously identified as one of the two sibling species. Due to poor DNA quality, the remaining 12 specimens yielded no PCR product. Of the 592 mosquitoes, 407 larval specimens had been identified morphologically prior to species-specific DNA amplification, and in all instances PCR identification corroborated with morphologic identification. Mosquitoes identified as An. claviger s.s. came from various localities all over Europe and from Israel. Those identified as An. petragnani were collected in southern France and Spain. The species-diagnostic PCR assay would facilitate data collection on the temporal and spatial distribution of the two An. claviger sibling species because they represent possible vectors of disease in Europe, the Near and Middle East, and north Africa.

Anopheles funestus is a major vector of malaria in Africa. It belongs to a group of sibling species that can be identified morphologically only at certain stages of their development. A diagnostic polymerase chain reaction (PCR)-based tool made it possible to differentiate five species of the group. The assay seems to be applicable over all their distribution area for four of these species: An. funestus, An. leesoni, An. parensis, and An. vaneedenip. The fifth species, An. rivulorum, is the second most abundant species of the group and can be mistaken at its adult stage for the major vector of malaria An. funestus. Molecular and morphologic observations of specimens identified as An. rivulorum from Cameroon and South Africa showed that they belong to two different taxa. The species identified in Cameroon, and named here An. rivulorum-like, might extend to western Africa and central Africa. The species-specific PCR assay is supplemented by a primer specific to An. rivulorum-like and thus makes it possible to differentiate the five species of the An. funestus group and the newly defined taxon.

Prolonged efficacy of a combination of bacteria (Bacillus thuringiensis var. israelensis [Bti] and copepods (Mesocyclops aspericornis) in controlling immature forms of Aedes aegypti in peridomestic water containers was achieved by adding various products from local villages as supplementary food for copepods. In all experiments, 100 first-instar larvae were added into the breeding containers every day for eight weeks. Combinations of biological control agents and each local supplementary food were applied once at the beginning of the experiment. At the end of the experiment, the average number of mosquito larvae in containers with a combination of copepods and Bti with one gram of rice grain had decreased to only 0.5% of that with no control agent. In comparison, the average numbers of mosquito larvae in containers with Bti only, or copepods only, were approximately 10% and 33% of those in containers with no control agents, respectively. In addition, the number of copepods in containers with mosquito larvae and supplementary food was at least three times higher than those with mosquito larvae alone.

Ninety-nine individuals with stools positive for Blastocystis hominis but negative for other parasites were identified from medical records of healthy adults who had received a physical examination at Taipei Veterans General Hospital from November 2000 to October 2002. The medical records of these 99 positive cases and 193 randomly selected controls, matched for age, sex, and date of examination, were retrospectively reviewed. The pathogenicity of B. hominis could not be demonstrated due to a lack of association with the development of gastrointestinal symptoms or pathologic findings on endoscopic examination. Multivariate analyses revealed that chronic hepatitis B infection was a predisposing condition to the acquisition of B. hominis (odd ratio = 2.848, 95% confidence interval = 1.299–6.242, P = 0.009), and concentration of urate was significantly lower in B. hominis-positive individuals (mean ± SD = 361.64 ± 87.44 versus 392.57 ± 93.38 μmol/L; P = 0.009). Among the 64 individuals who underwent gastric biopsy, Helicobacter pylori was found more frequently in the individuals harboring B. hominis (19 of 26 versus 15 of 38; P = 0.017).

A cross-sectional study was undertaken to determine the prevalence of isosporiasis and its clinical and laboratory pattern in Venezuelan patients infected with human immunodeficiency virus (HIV) (n = 397). At enrollment, they underwent a thorough clinical history and physical examination, and provided stool specimens for the identification of Isospora belli and other parasites. Isospora belli was identified in 56 subjects (14%) and diarrhea, either acute or chronic, was present in 98% of these cases (P < 0.001). Eosinophilia was strongly associated with isosporiasis (P = 0.01). It was also found that the presence of eosinophilia was more common in I. belli-infected patients without weight loss (P < 0.001). Twenty-six (81.25%) subjects with I. belli infection had CD4+ cell counts < 200 cells/mm3 (P = 0.03). In addition, the data and its description shows the association to be < 100 cells/mm3. This infection seems to be seasonal because the recovery of oocysts occurred mainly in months with significant rainfall. In fact, isosporiasis should be suspected in HIV-infected patients from tropical countries with diarrhea, weight loss, eosinophilia, and low CD4+ cell counts.

Epidemiologic data suggest that 30–40% of Taenia solium-seropositive people become spontaneously negative without acquiring cysticercosis. To compare the responses of these individuals with those of patients with neurocysticercosis, we screened seropositive persons among family members of 16 patients. We searched for specific antibodies in patients and their 118 households by an enzyme-linked immunoelectrotransfer blot assay using specific glycoproteins of T. solium metacestodes. We found six seropositive individuals without neurocysticercosis among members of four families. The matching patients were young, harbored viable cysts, and had short evolution of disease. The baseline response of healthy seropositive individuals was scarce and showed a low frequency of antibodies against glycoproteins GP39-42 and GP24, which are immunodominant in patients with neurocysticercosis. Moreover, they became spontaneously negative in few months. The response of patients was heterogeneous as shown in other studies. The results of this work support a highly dynamic host-parasite immunologic interaction and suggest individual susceptibility or level of exposure among family members.

It has proven very difficult to determine the causes of early infant mortality and morbidity in Africa. We undertook a two-year, prospective birth cohort study in a rural Kenyan District Hospital to estimate cause-specific mortality and severe morbidity in infants too young to gain benefit from routine immunization approaches. A total of 2,359 infants eligible for the cohort were delivered. Of these, 136 (6%) were stillborn and 77 (3.5%) subsequently died. Prematurity (34%), birth asphyxia (27%), and infection (18.5%) were the predominant causes of death in the first 98 days of life, although infection accounted for 36% of all life-threatening illness episodes in the same period. The data suggest that health system constraints are likely to impede programmatic efforts to reduce early infant mortality and morbidity, and that infection prevention measures offer some promise for mortality reduction. Assessing the cost effectiveness of the latter, particularly for very specific interventions such as further maternal vaccination, will require very large trials.