The American Journal of Tropical Medicine and Hygiene - Volume 27, Issue 2, March 1978

It is always rewarding and pleasurable to attend the Annual Meeting of the Society. But the pleasure takes on an added dimension when one has the very satisfying task and privilege to introduce a colleague who has been an outstanding intellectual force in the development of our discipline. The Craig Lectureship was initiated by our Society in 1936 to honor Col. Charles Franklin Craig who so strongly stimulated and influenced studies on tropical medicine and parasitology in the United States. This year's selection committee, consisting of Drs. Irwin Sherman, Jordi Casals-Ariet and myself had no difficulty in rapidly reaching agreement that Dr. William Trager's nomination to be the forty-first Craig Lecturer would pay high honor to Col. Craig's memory.

Dr. Trager received his doctorate in 1933 from Harvard University and for the next 40-odd years made his professional home at Rockefeller University.

Charles Franklin Craig was still very much alive in the 1940s when I was already well started on a career in parasitology. His research and his classic books on malaria and other parasitic protozoa and on mosquitoes and other vectors had a pervasive influence on me, as on all parasitologists. And so I am very grateful to the Society for this honor of giving the lecture bearing Dr. Craig's name. Furthermore, as it turns out, this has provided me with an unusual opportunity.

It is not often that one has the good fortune to be able to speak about one's lifelong interest at the same time that the subject is attracting widespread attention. I have always enjoyed growing things. Even as a schoolboy with an interest in insects, collecting them seemed to me a relatively dull sport compared with raising them.

Many disciplines are included in the term “Tropical Medicine”—in fields of human and animal diseases, parasitology, medical entomology, preventive medicine, curative medicine, pharmacology and therapeutics, immunology, and the practical topics of vector control and sanitary engineering. The modern era embraces a century, and workers from the New World and the Old World, the Southern and the Northern Hemispheres have been involved. Americans have been involved with the nationals of many other countries. The world's most terrifying maladies have been brought under effective, although certainly not absolute control. A not entirely unexpected result of these efforts has been the appearance of new team members—the demographer, the population biology student and prophet, and increasingly more prominently, the ecologist, the David with coat of many colors, advising us of the importance of the balanced ecosystem for mankinds' future well-being. As pestilence has, at least temporarily, receded in importance as a major contributor to the negative side of the equations of population balance, we are warned that famine is threatening to take lead place.

The disposition of sulfalene was studied in eight individuals before and during an infection with a chloroquine-resistant strain of Plasmodium falciparum. Isoniazid acetylator phenotype was determined in each individual prior to the administration of sulfalene. Following the administration of sulfalene before infection with malaria, a significant difference in half-life of non-acetylated sulfalene and percent acetylation of sulfalene in plasma was observed between rapid and slow acetylators. When sulfalene was administered during malaria, this difference was no longer apparent. Individuals who did not respond to the therapeutic administration of sulfalene alone were treated with a combination of sulfalene and pyrimethamine. Three individuals were cured by sulfalene without pyrimethamine and one was cured by the drug combination. Three of the four individuals who were not cured by any dose of sulfalene or the drug combination were slow acetylators. There was no distinct correlation between clinical response and maximum levels or half-life of nonacetylated sulfalene. These findings suggest that acetylator phenotype does not influence the therapeutic response of individuals infected with falciparum malaria to sulfalene or to the combination of sulfalene and pyrimethamine. Further information is presented, however, to confirm the importance of an as yet unidentified host factor(s) in determining therapeutic response to these agents.

Systematic surveys of the wild macaques of South Asia by blood culture resulted in the discovery that trypanosomiasis is enzootic in the simians of Indonesia, Malaysia, India, and Thailand. The isolates obtained differ in morphology, metabolism, and ability to multiply in arthropods. Following this discovery, interest focused on possible transmissions of these trypanosomiases. Laboratory-reared and wild-caught insects were studied to determine which are satisfactory intermediate hosts and potential natural vectors. Successful results were obtained with insectary-reared reduviids and Indonesian isolates. In Rhodnius prolixus and Triatoma rubrofasciata the Indonesian trypanosomes multiply for periods which can exceed 40 days. The flagellate infections are in the digestive tract, whereas trypanosomes have never been seen in the salivary glands or in the hemolymph. The feces of trypanosome-carrying reduviids are infective, suggesting a stercoreal method of infection of mammals, and infection was produced in experiments in which feeding by the insects was not possible. The relevance of these findings to natural transmission in Indonesia is not known. Experiments with insects and all other trypanosomal isolates have been negative. The natural transmission mechanism(s) of the simian trypanosomiases in South Asia remains an unsolved problem.

Naegleria fowleri, a free-living ameboflagellate, is the causative organism of primary amebic meningoencephalitis. Intransal inoculation of N. fowleri in mice produces an infection similar to human disease. Mice immunized with live N. fowleri by intraperitoneal injection were found to be more resistant to subsequent intranasal challenge. The survival rate was 27% in immunized animals, compared to 0% in the control group. These results may provide a lead to the development of immunotherapy for this virulent disease for which satisfactory chemotherapy is presently unavailable.

Strain IP-106 of Entamoeba histolytica was isolated 12 yr ago from a case of amebic dysentery and has been maintained in axenic culture for the past 11 yr. Hamsters were inoculated intrahepatically, intraperitoneally, and intracecally with 6.5, 5.0, and 8.0 × 104 axenic trophozoites, respectively. With the first two routes of inoculation all the animals developed liver abscesses, and most developed amebic metastases to other sites. After intracecal inoculation only 3 out of 6 animals developed abscesses at the primary site and metastases to other sites. Most numerous metastases were obtained following intrahepatic inoculation. Intrahepatic inoculation with 1 and 2 × 104 axenic amebae resulted in an increase in the size of the liver abscess for a period of 2 wk and its apparent total resorption by day 17 post-inoculation, accompanied by a corresponding gain of body weight. With 4 and 6 × 104 amebae there was a steady increase of the size of the liver abscess and corresponding loss of body weight. High antiamebic indirect hemagglutination titers were observed only on day 17 post infection in animals inoculated with 2, 4, and 6 × 104 amebae. The extent of amebic metastases was roughly proportional to the size of the inoculum.

Virulence of axenically cultured Entamoeba histolytica as a function of lesion size in adult hamster liver was examined in a statistically analyzed model. Several potential sources of variation inherent in animal hepatic inoculation experiments were considered. The effects of growth stage in culture was examined. Log or exponential growth phase cells produce significantly larger lesions than cells from either lag or decline phase of axenic culture. A comparison of the size of lesions produced by unmanipulated HM-1 or substrains of HM-1 previously passed through the liver of hamsters (control strains) was made with the size of lesions produced by λ substrains (obtained by the passage of control strains through hamster liver one additional time). Control strains were paired with corresponding λ substrains and inoculated into the right or left hepatic lobe of the same animal. Statistical analysis of the data demonstrated an overall significant increase in mean lesion size due to a single additional liver passage. A similar comparison was made of the size of lesions produced by unmanipulated HM-1 and by substrains which had been passed through the liver two or four times. This revealed that a significantly larger increase in lesion size resulted from multiple hamster liver passages alternating with axenic culture. Hamsters killed 1 mo after inoculation with a substrain passed four times through hamster liver had large lesions characteristic of human amebic liver abscess. Hamsters inoculated with unmanipulated HM-1 had no grossly demonstrable lesions at 1 mo. Increased virulence conferred by hamster liver passage could still be demonstrated 45 wk after final isolation from liver of a substrain passed four times. These observations support the concept that multiple hepatic passages significantly enhance virulence of trophozoites of E. histolytica which have become minimally pathogenic during long-term axenic cultivation. Periodic hamster liver passage offers a method of restoring and maintaining virulence in E. histolytica kept in axenic culture. Comparative studies of attenuated axenic strains and those with enhanced virulence could reveal modifications necessary for pathogenicity of this protozoan.

Mebendazole (methyl-5-benzoylbenzimidazole-2-carbamate: Vermox), a broad spectrum anthelmintic, cured 22 (88%) children with symptomatic trichuriasis when given as a single 6-day course in a dosage of 100 mg twice daily. A further 3 (12%) were cured after a repeat 6-day course of therapy. Thus complete parasite eradication was achieved in all. The administration of an antidiarrheal agent, loperamide hydrochloride (Imodium), appeared to enhance the efficacy of mebendazole. Both drugs were well tolerated and completely free of any toxic effects.

Observations were made of details of tegument development of schistosomes grown in mouse, hamster, and rat hosts. In permissive hosts (mouse and hamster) the surface of the worm alters rapidly during early maturity and is characterized by fusing of a highly undulate surface network into smooth folds and spine-covered tubercles. In non-permissive hosts maturation of the tegument is both delayed and incomplete, and the tubercles are aspinous. Scanning views of the oral cavity and the gynecophoral canal, both sites of transitional tegumental organization, are also shown. The gynecophoral canal tegument seems to be a site of active lipid secretion.

Extramedullary eosinophilopoiesis is described as a regular pathological feature of murine and cricetine schistosome infections. Using the Dominici staining technique, colonies of developing eosinophils were commonly found in certain tissue sites of dense oviposition, particularly in the liver, and in associated lymphoreticular tissues. The relationship of these observations to the role of eosinophilic leukocytes in schistosomiasis is discussed.

Studies of granulomatous hypersensitivity to Schistosoma japonicum eggs were performed at various time periods up to 20 wk after the induction of light infections in mice. Cell populations which were determined in granulomas isolated from the livers revealed a maximum in the total number of cells at 6 wk with a decline of 36% by 20 wk. Large mononuclear cells were predominant at all time periods, with eosinophils being the second most common cell. Measurements of granuloma diameters around single viable eggs in the livers also revealed peak size at 6 wk with a decline of 51% between 16 and 24 wk. Immunodiffusion analysis demonstrated the presence of precipitating antibodies as early as 7 wk after infection. Investigations of lymphocyte blastogenesis revealed a profound depression in response to T-cell mitogens by 8 wk of infection. Studies of footpad swelling to soluble S. japonicum egg antigens revealed massive immediate reactions starting at 6 wk, but no delayed reactivity over a period from 3 to 20 wk. All of these results are related to differences in the biology of S. japonicum in comparison with S. mansoni with respect to the earlier onset of egg production, the much larger numbers of eggs produced, and the possibility of differences in the antigens emitted by the eggs.

Counterimmunoelectrophoresis (CIE) was used to detect antibodies to Schistosoma japonicum soluble egg antigen in 118 sera from people living in a schistosomiasis endemic area in the Philippines. The sera were also tested for antibodies by the circumoval precipitin test (COPT); 53% were found positive by CIE and 48% positive by COPT. No significant differences were found between the tests (P = 0.18 by McNemar's test). Crossreactions with sera from patients with intestinal capillariasis and monkeys with experimental angiostrongyliasis were not found and no false reactions were detected with sera from “normal” controls. The CIE test as used in the present studies was found to be comparable to the COPT in sensitivity and specificity as related to cross-reactivity with sera from nematode infections such as intestinal capillariasis and angiostrongyliasis. Results are usually available quickly with nearly 70% of any reactions occurring within 1 h. The test should be of value in seroepidemiologic surveys in schistosomiasis-endemic areas.

Forty-seven men on the Isthmus of Panama were exposed to histoplasmosis in an old bunker inhabited by bats. The resulting epidemic was studied with serial clinical, serological, and radiological examinations. Thirty-seven (78.7%) of the men showed serological evidence of infection and 26 (70.3%) had symptoms. Incubation periods ranged from 4 to 30 days. A general relationship between severity of illness and degree of exposure was noted. The agar gel diffusion test for precipitin antibodies was more sensitive than the complement-fixation test or slide test in detecting infection with Histoplasma capsulatum. Decontamination procedures and environmental studies are described.

The correlation between transmission of e-antigen (e-Ag) and hepatitis B antigen (HBsAg) was investigated in primary and secondary cases of type B hepatitis occurring in 15 families. The e-Ag was not consistently transmitted in the familial environment, as were the d and y subtypes of HBsAg, but occurred in erratic fashion among genetically and epidemiologically related cases. It is concluded that e-Ag is probably the non-transmissible product of a specific individual response to hepatitis B infection.

A serological survey of 939 Neotropical bats of 22 species from an enzootic focus of Venezuelan encephalitis (VE) virus on the Pacific lowlands of Guatemala during 1971–1975 revealed VE virus specific antibodies in seven species, three belonging to the genus Artibeus. VE virus was isolated from the blood of one Uroderma bilobatum. Antibody frequency was considerably lower in bats than in terrestrial mammals, and tended to vary within any given species from locality to locality and from year to year. At the village of La Avellana where VE virus was most active, antibody rates in the two best-sampled Artibeus species were 0.14 in 1971, 0.11 in 1972, 0.03 in 1973, and 0.11 in 1975. The positive rate of 0.10 over all years at La Avellana for the genus Artibeus suggests that these and possibly other bats regularly are infected by VE virus, and may possibly serve as alternate hosts to maintain virus circulation if most terrestrial animals become immune.

Eighty-nine Neotropical bats of five species were inoculated subcutaneously with epizootic or enzootic strains of Venezuelan encephalitis (VE) virus. Viremia was detected in 92.5% of all bats, but no illness attributable to virus infection was observed. Detectable viremias averaged slightly over 4 days in Artibeus jamaicensis and A. lituratus, and 2.8 days in Phyllostomus discolor, and maximal viremia titers in these three species averaged 6.9, 6.6, and 4.6 log10 SMicLD50 per ml of blood, respectively. In general Artibeus developed and maintained detectable levels of both hemagglutination-inhibition (HI) and neutralizing antibody for as long as tested (up to 506 days), although HI antibody to enzootic VE virus strains disappeared in some A. lituratus. The detectable antibody response of P. discolor was slower and of lower magnitude and shorter duration than that of Artibeus, although individual P. discolor which had lost detectable HI and N antibody resisted challenge. Vertical passage of antibody was observed in three A. lituratus offspring. Artibeus jamaicensis were found to be only slightly less susceptible to VE virus infection than a U.S. subspecies of the cotton rat (Sigmodon hispidus). No virus was recovered by mouse inoculation of organ pools of bats killed as early as 2 days and as late as 299 days after the last day of detectable viremia.

A total of 80 Neotropical bats of five species was inoculated with one of four strains of Venezuelan encephalitis (VE) virus. Virus was detected in the oropharynges of 56% of bats, and most regularly in Artibeus jamaicensis (75%). Titers of virus in oropharyngeal secretions were occasionally very high (8.5 log10 SMicLD50/ml in one A. jamaicensis). Only 2 of 123 urine samples from 50 bats and 2 of 86 fecal samples from 46 bats yielded VE virus. No contact or aerosol virus transmission from bat to bat was detected. VE virus passed transplacentally from two infected mothers to their fetuses, which were aborted. Virus did not pass from one infected mother to her nursing young.

After prolonged selection, two hybrid strains of Culex tarsalis were evolved that were highly resistant to infection following ingestion of western equine encephalomyelitis virus. These strains were greater than 25,000-fold more resistant than the most susceptible parental strain when fed on viremic chicks. Resistance was associated with a mesenteronal barrier since both refractory and parental strains were equally susceptible to infection by intrathoracic inoculation. Susceptibility was dominant, possibly incompletely dominant, over resistance. Inheritance was probably polyfactorial but this could not be determined with certainty since a small proportion of individuals appeared to become infected by nongenetic mechanisms.

The implications of the Keystone virus-Aedes atlanticus transmission cycle are explored in the context of a quantitative model. Among the variables considered are the vertical transmission rate, the effect of the virus upon vector fertility and survival, vector densities and distributions, the proportion susceptible in the vertebrate population, the attractiveness of different vertebrates to the vector and vector survival rates. The logical relationships between these several variables are explored. It is concluded that the current view of Keystone virus maintenance is quantitatively feasible, and that certain predictions may be made as to the magnitude of several parameters which have not yet been measured. Such predictions allow direct testing of the model. The general structure of the model is such that it may prove useful in describing the epidemiology of other vector-borne infections in which vertical transmission is essential for infection maintenance at certain periods of the year.