INTRODUCTION
Analyses are being made of archival data from patients infected with malaria between 1940 and 1963 for the treatment of neurosyphilis to better understand the development of immunity to infection and the relationships between the parasite and its human host and its vectors.1–16 Previously, blood-stage dynamics of P. vivax12,14 was reported, and mosquito relationships for P. ovale8 and P. falciparum15 were presented. Sporozoite-induced infections were often used because of the potential problems associated with blood passage. Whenever a new patient needed to be treated, mosquitoes would be fed on an infected patient, if available.
Reported here are the results of a retrospective examination of archival data from mosquito feedings on 166 induced infections with P. vivax for the treatment of paresis and other mental disorders associated with tertiary syphilis. The goal was to further document 1) the relationship between microgametocyte count and mosquito infection, and 2) the effect, if any, of asexual parasite count or fever on percentage of mosquitoes infected.
MATERIALS AND METHODS
Patient management.
Consent for whatever treatments the hospital staff determined necessary for the patients was granted by the families of the patients or the courts when patients were admitted to the hospital. The decision to infect a neurosyphilitic patient with a specific malaria was made as part of standard patient care by the medical staff of the South Carolina State Hospital. Patient care and evaluation of clinical endpoints (e.g., fever) were the responsibility of the medical staff. As previously reported,2 during infection, the temperature, pulse, and respiration were checked every four hours and hourly during paroxysms (fevers) by hospital personnel. During paroxysms, patients were treated symptomatically. Infections were terminated at the direction of the attending physician. The U.S. Public Health Service personnel provided the parasites for inoculation, monitored the daily parasite counts to determine the course of infection, provided mosquitoes to be fed on the patients, and performed mosquito dissections and examinations. All patients undergoing malariatherapy lived in screened wards of the hospital to prevent possible infection of local anophelines.
Treatment.
Infections were terminated by treatment with various antimalarial drugs. In addition, various drugs, such as primaquine, pyrimethamine, quinine, and chlorguanide are all capable of at least temporarily preventing mosquito infection, without permanently eliminating the infection in the human host. Patients receiving treatments that may have had an effect on mosquito infection were excluded from the current analysis.
Strain of Plasmodium vivax.
The 166 patients were infected with the St. Elizabeth strain that was obtained from St. Elizabeth Hospital in Washington, D.C. in the 1930s. The exact parasite origin is unknown.
Parasitemia.
Patients were infected by the intravenous inoculation of parasitized erythrocytes or via sporozoite inoculation. Thick and thin peripheral blood films were made daily by the method of Earle and Perez,17 stained with Giemsa, and examined microscopically for the presence of parasites. The threshold of detection was approximately 10 parasites/μL. Asexual and sexual parasites were recorded per microliter of blood. Infections often persisted for many weeks. However, most of the patients were fed upon during the initial 40 days of patent parasitemia.
Mosquitoes.
Anopheles quadrimaculatus mosquitoes were laboratory reared, caged, and allowed to feed to repletion directly on the patients. Mosquitoes that fed were subsequently dissected and the number of oocysts present on the midguts counted and recorded.
Data analysis.
Poisson regression was used to investigate the effect of fever and parasitemia on malaria infection rates. Orthogonal contrasts were used to compare infection rates by fever status at varying levels of parasitemia. Pearson correlation was used to quantify the association between infectivity and microgametocyte count. SAS version 8.0, (SAS Institute, Cary, NC) was used to analyze data. Statistical significance was set at P < 0.05.
RESULTS
During primary infection with the St. Elizabeth strain of P. vivax, 845 lots of An. quadrimaculatus mosquitoes were fed and examined (Figure 1). Of these, the average percentage infection of mosquitoes per lot was 56.6%, with greater than 50% infection rates between days 17 and 38. Thus, there was a high probability of infection when mosquitoes were fed anytime during this period. An examination was made of the possible effect of microgametocyte count on percentage infection. Counts were recorded for 986 of the days on which mosquitoes were fed. Counts ranged from > 1 to > 500/μL (Figure 2). In spite of the high percentage infection, there were still many occasions where lots were lightly infected or not at all; yet, there was a low moderate positive trend for increased percentage infection with increased microgametocyte count (r = 0.22, P < 0.001).
An examination was made of the possible relationship between fever, the asexual parasite count, and percentage infection (Figure 3). Parasitemia was categorized at four levels, namely, < 2,500, 2,501–5,000, 5,001–10,000, and > 10,000/μL. Fever status was defined as having no fever (<101°F), 101–104.8°F, and ≥ 105°F. Infection rates for persons with very high (≥ 105°F) fevers remained low and relatively constant across the four levels of parasitemia with 42.1%, 41.9%, 43.7%, and 43.9% infection, respectively. Persons with no fever had significantly (P < 0.001) higher rates of infection than did persons with fevers ≥ 105°F at each level of parasitemia (except at < 2,500 parasites/μL) with 44.6%, 64.1%, 68.3%, and 67.6% infection, respectively. Persons with more moderate fevers (101–104.8°F) had significantly (P < 0.001) higher rates of infection at all levels of parasitemia with 51.3%, 53.6%, 61.0%, and 62.0% infection, respectively, than did persons with very high fever. Between days 9 and 16 (Figure 1), the lots of mosquitoes fed had lower average percentage infection; this coincided with the days when fever was most prevalent.
A total of 76 lots of An. quadrimaculatus mosquitoes were fed on patients during their secondary reinfections with P. vivax following previous P. vivax infection (Figure 4). Of these, the average percentage infection was 49.8% versus the 56.6% obtained during the primary infections.
DISCUSSION
An examination of the archived data showed that mosquitoes were not fed in a systematic pattern, but rather on an ad hoc basis. When infected mosquitoes were needed to transmit the infection to another patient for malariatherapy, mosquitoes were fed for one or two days with little or no rationale for expected infection. Apparently, experience had indicated that P. vivax readily infected mosquitoes and that infection could apparently be expected by feeding on any day that asexual parasites were present. An examination of the results of these feedings indicated that the success obtained was remarkable high.
Anopheles quadrimaculatus were indeed readily infected by feeding on patients infected with the St. Elizabeth strain of P. vivax. Infection did not appear to be related to the density of microgametocytes nor to the course of the parasitemia. Highest percentage mosquito infection (≥ 50%) occurred following the first two weeks of patent parasitemia and continued for the following four weeks. This long window of relatively high-density mosquito infection allowed for the continued cyclical transmission from patient to patient.
A more detailed examination of the data showed that days when mosquitoes were not infected or were of low density were more associated with days of fever, regardless of asexual parasite count or microgametocyte count. Mosquito lots fed during days of fever were more likely to have lower percentages of mosquito infection. The reasons for this are unclear, but suggest that high temperature may somehow interfere with the sexual process. The highest percentages of mosquito infection were obtained by feeding on high microgametocytemic, non-fever days during the third to sixth week of patent parasitemia.
Percentage infection of 845 lots of Anopheles quadrimaculatus mosquitoes fed on patients during primary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 845 lots of Anopheles quadrimaculatus mosquitoes fed on patients during primary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 845 lots of Anopheles quadrimaculatus mosquitoes fed on patients during primary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 986 lots of Anopheles quadrimaculatus mosquitoes fed on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to density of microgametocytes.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 986 lots of Anopheles quadrimaculatus mosquitoes fed on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to density of microgametocytes.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 986 lots of Anopheles quadrimaculatus mosquitoes fed on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to density of microgametocytes.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Distribution of percentage of Anopheles quadrimaculatus mosquitoes infected by feeding on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to presence of fever (101–104.8°F and ≥ 105°F) and asexual parasite count/μL.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Distribution of percentage of Anopheles quadrimaculatus mosquitoes infected by feeding on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to presence of fever (101–104.8°F and ≥ 105°F) and asexual parasite count/μL.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Distribution of percentage of Anopheles quadrimaculatus mosquitoes infected by feeding on patients infected with the St. Elizabeth strain of Plasmodium vivax as related to presence of fever (101–104.8°F and ≥ 105°F) and asexual parasite count/μL.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 76 lots of Anopheles quadrimaculatus mosquitoes fed on patients during secondary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 76 lots of Anopheles quadrimaculatus mosquitoes fed on patients during secondary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Percentage infection of 76 lots of Anopheles quadrimaculatus mosquitoes fed on patients during secondary infections with the St. Elizabeth strain of Plasmodium vivax.
Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 70, 6; 10.4269/ajtmh.2004.70.638
Authors’ addresses: William E. Collins, Division of Parasitic Diseases, Centers for Disease Control and Prevention, Mailstop F-36, 4770 Buford Highway, Atlanta, GA 30341, E-mail: wec1@cdc.gov. Geoffrey M. Jeffery (Public Health Service, retired), 1093 Blackshear Drive, Decatur, GA 30033. Jacquelin M. Roberts, Division of Parasitic Diseases, Centers for Disease Control and Prevention, Mailstop F-22, 4770 Buford Highway, Atlanta, GA 30341, E-mail: jmr1@cdc.gov.
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