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. Author manuscript; available in PMC: 2008 Jul 24.
Published in final edited form as: J Parasitol. 2007 Jun;93(3):627–633. doi: 10.1645/GE-1052R.1

GAMETOCYTEMIA AND FEVER IN HUMAN MALARIA INFECTIONS

F Ellis McKenzie 1, Geoffrey M Jeffery 2, William E Collins 3
PMCID: PMC2483402  NIHMSID: NIHMS58074  PMID: 17626355

Abstract

We examine the charts of 408 malaria-naïve neurosyphilis patients given malaria therapy at the South Carolina USPHS facility, with daily records encompassing at least 93% of the duration of infection, and focus on the 152 patients infected with the St. Elizabeth strain of Plasmodium vivax, 82 with the McLendon strain of Plasmodium falciparum, 36 with the USPHS strain of Plasmodium malariae, and 15 with the Donaldson strain of Plasmodium ovale in whom gametocytes appeared before drug, or other, intervention. In P. vivax infections, fever and parasitemia were higher after gametocytes were first detected than before; in P. malariae infections, parasitemia was higher. In P. ovale infections, fever and parasitemia were similar before and after. In P. falciparum infections, fever, parasitemia, and fever frequency were lower after gametocytes were first detected than before. Parasitemia and temperature correlated in P. vivax infections, before and after gametocytes were first detected; parasitemia and temperature at first fever were not correlated in infections with any species. Gametocyte density correlated with parasitemia in P. malariae and sporozoite-induced P. falciparum and P. vivax infections. Fevers and detected gametocytemia coincided more often than expected by chance with P. vivax and P. ovale; fever temperature and gametocyte density were not correlated in infections with any species.

The differing dynamics of gametocytemia in Plasmodium species were noted long ago. Boyd and Kitchen (1937) wrote that “in vivax infections … gametocytes are produced at every period of multiplication … [but] falciparum gametocytes are not observed until about 10 days after the first appearance of parasites, and may not be present until after the primary attack subsides.” Hackett (1941) added that the first appearance of Plasmodium malariae gametocytes “is delayed sometimes for months.” Shute and Maryon (1951) observed that gametocytes in Plasmodium falciparum infections appeared 8–10 days after a first fever, in Plasmodium vivax 6–7 days, and in Plasmodium ovale earlier still.

The timing of the first gametocyte detection relative to a first fever or first asexual-form patency follows from the rate at which gametocytes are produced, the multiplication rate of the asexual forms from which gametocytes arise, the extended sequestration of immature P. falciparum gametocytes, the dynamics of fever induction, thresholds of parasite detection, and, surely, other factors. Virtually nothing about any part of the process is understood as yet.

In a 3-yr cross-sectional study of adults attending malaria clinics in Peru and Thailand (McKenzie, Wongsrichanalai et al., 2006), we found that P. vivax patients with gametocytemia had higher fever and higher parasitemia than those without gametocytemia, while P. falciparum patients with gametocytemia had lower fever than those without gametocytemia, but similar parasitemia. Temperature correlated with parasitemia in the gametocytemic P. vivax patients and the nongametocytemic P. falciparum patients; gametocyte density correlated with parasitemia in P. vivax, but not P. falciparum, and correlated with temperature in only 1 of the 8 site-year-species combinations.

In that paper, we also noted that relationships between gametocyte prevalence, gametocyte density, parasitemia, and clinical symptoms remain unresolved for even the best-studied populations, i.e., P. falciparum–infected children in sub-Saharan Africa, and that much of the published evidence appears contradictory. Gametocytemia may be associated with higher or lower parasitemia, or with the presence or absence of anemia, in these populations. Over somewhat wider age ranges, some reports indicate that P. falciparum gametocyte prevalence among clinic patients declines with age, though gametocyte density does not (Akim et al., 2000), and that young P. falciparum gametocyte carriers with fever have lower gametocyte densities, but higher asexual-form densities, than those without fever (Gouagna et al., 2004). The relationships are even more mysterious with other Plasmodium species, and in other age groups, across the rest of the malaria-endemic world.

In his study of gametocytemia, Schuffner (1938) posed a classic challenge: “Obviously the collective parasite picture is a composite of numerous individual parasite pictures existing in persons whose state of health, or ill health, differs widely. On the face of it, it would seem impossible to trace any fixed rules in this chaos of unlimited possibilities.” Here, we take advantage of a wealth of longitudinal data on individual patients to investigate how the dynamics of individual infections may relate to static samples, in which data are collected for each patient at a single time point, using the same statistical procedures to address, to the extent possible, the same questions as in our Peru-Thailand study.

MATERIALS AND METHODS

Current knowledge of Plasmodium spp. dynamics in infected humans derives largely from 40-yr work with malaria induced to treat neurosyphilis. Many fundamental insights are owed to these malaria therapy patients, to whom we are extremely grateful. Malaria therapy treatment and data collection procedures, including those for the determination of parasitemia, gametocytemia, and patient rectal temperature during each infection, are described in detail elsewhere (Collins and Jeffery, 1999, 2002, 2005; McKenzie et al., 2001, 2002a). The first of these citations also contains extensive information about the participation and treatment of the patient population considered here, and it is accompanied by an explicit, independent analysis of relevant ethical issues.

Here, we examine the records of adult neurosyphilis patients with no known history of previous malaria infection, treated with the St. Elizabeth strain of P. vivax, McLendon strain of P. falciparum, USPHS strain of P. malariae, or Donaldson strain of P. ovale in the U.S. Public Health Service (USPHS) facility in Columbia, South Carolina. All of the P. malariae and P. ovale infections, most of the P. falciparum infections, and roughly half of the P. vivax infections considered here were initiated by inoculation of 5 ml of whole blood from a patently infected patient. The remaining P. falciparum and P. vivax infections were initiated by bites of infectious mosquitoes that previously fed on a patient or by inoculation of glands or sporozoites extracted from infectious mosquitoes. Because our previous analyses showed no notable differences between the latter 2 routes (McKenzie et al., 2002a), we distinguish only “sporozoite-induced” from “trophozoite-induced” infections, in line with our earlier work (Collins and Jeffery, 1999). Because our previous analyses showed differences in dynamics between sporozoite- and trophozoite-induced infections, we analyze those categories separately here (Table I).

Table I.

Median values (95% confidence limits) for patient medians; for the N1, N2, N3 patients; and for the aggregate values for the N3 patients (see text). Results of Mann-Whitney U-tests, for comparisons of values before and after the first detection of gametocytes are denoted * P < 0.01, **P < 0.0002, both for the N3 patients and the N2 + N3 patients (denoted in the N2 rows). Columns are for patients infected with P. falciparum (FAL), P. vivax (VIV), P. malariae (MAL), or P. ovale (OVA). Rows show log 10 per-μl parasitemia or gametocytemia (gam), patient fever (fvr) in degrees C or daily frequency (/d), the number of days between the first fever and intervention (1st fvr–end), the first fever and first gametocytemia (1st fvr–1st gam), or the first gametocyte and intervention (1st gam–end). Thus, rows are for parasitemia at the first fever (1st fvr), temperature of the first fever, day of the first fever (with the first day of patency as day 1), day of the first gametocytemia (with the first day of patency as day 1), parasitemia before and after the first gametocytemia, fever before and after the first gametocytemia, fever frequency per day (/d) before and after the first gametocytemia, the difference in fever frequency (before–after), and the daily frequency of gametocytemia (/d).

FAL trophozoite FAL sporozoite VIV trophozoite VIV sporozoite MAL trophozoite OVA trophozoite
N1 Patients 27 20 14 27 26 9
 1st fvr parasitemia 2.76 (1.78–3.48) 2.19 (1.78–2.88) 2.15 (1.00–2.91) 1.66 (1.00–2.30) 2.88 (2.43–3.41) 2.84 (1.00–3.61)
 1st fvr temperature 39.6 (38.9–40.0) 39.8 (38.9–40.6) 39.2 (38.6–40.0) 39.1 (38.7–39.6) 38.9 (38.3–39.4) 40.1 (38.3–40.8)
 1st fvr day 2 (1–4) 2 (1–4) 2 (1–3) 2 (1–3) 12.5 (6–15) 3.5 (1–8)
 Parasitemia 3.88 (3.32–4.57) 3.58 (3.11–4.15) 3.49 (2.48–3.91) 3.01 (2.52–3.53) 3.29 (2.28–3.57) 2.06 (1.00–2.90)
 Fever 40.1 (39.6–40.6) 40.4 (40.2–40.6) 40.1 (39.3–40.7) 40.4 (39.7–40.8) 39.4 (39.0–40.0) 39.7 (38.8–40.8)
 Fever (/d) 0.85 (0.67–1.0) 0.81 (0.67–0.86) 0.82 (0.67–1.0) 0.77 (0.47–0.88) 0.37 (0.06–0.53) 0.19 (0.11–0.75)
 1st fvr–end (d) 5 (3–8) 6 (3–7) 6 (4–7) 8 (7–12) 42 (10–67) 43 (8–64)
N2 Patients 14 6 20 22 8
 1st fvr parasitemia 2.94 (1.00–3.41) 2.16 (1.30–4.35) 1.69 (1.00–2.57) 1.53 (1.00–2.08) 2.08 (1.00–3.47)
 1st fvr temperature 39.6 (38.6–40.8) 39.9 (39.2–40.6) 39.5 (38.6–40.0) 39.1 (38.3–39.8) 38.9 (38.3–39.9)
 1st fvr day 2.5 (1–4) 2 (1–4) 1 (1–3) 1 (1–3) 6 (1–12)
 1st gam day 8.5 (7–11) 12 (8–21) 9.5 (7–12) 10 (7–13.5) 29 (8–60)
 1st fvr–1st gam (d) 6 (5–9) 9 (6–19) 7.5 (3–10) 8 (5–10.5) 21 (5–48)
 Parasitemia before 3.28 (2.63–3.82) 3.71 (3.04–4.39) 3.02 (2.55–3.62) 3.26 (2.68–3.61) 3.23 (1.85–3.74)
 Parasitemia after 4.16 (3.82–4.55) 4.10 (1.60–4.69) 4.05 (3.59–4.16)** 3.88 (3.46–4.02)** 3.36 (2.92–3.99)*
 Fever before 40.3 (39.3–41.1) 40.0 (39.4–41.1) 40.3 (39.4–40.6) 40.3 (38.9–40.6) 39.8 (38.6–40.6)
 Fever after 40.8 (40.2–41.1) 40.2 (38.9–41.1) 40.6 (39.9–41.0)** 40.6 (39.9–40.8)** 39.8 (38.9–40.8)
 Fever (/d) before 0.69 (0.50–0.80) 0.71 (0.47–1.0) 0.69 (0.62–0.78) 0.72 (0.60–0.85) 0.38 (0.20–0.45)
 Fever (/d) after 1.0 (0.67–1.0)** 0.67 (0.50–1.0)* 0.93 (0.67–1.0) 1.0 (0.53–1.0) 0.48 (0.09–1.0)
 Difference (/d) −0.2 (−0.50–0.0) 0.04 (−0.33–0.50) −0.09 (−0.33–0.03) −0.22 (−0.36–0.07) −0.09 (−0.57–0.40)
 Gam (/μl) 1.40 (1.00–2.07) 1.68 (1.00–3.10) 1.61 (1.20–1.94) 1.52 (1.00–1.85) 1.07 (1.00–1.65)
 Gam (/d) 1.0 (1.0–1.0) 0.83 (0.33–1.0) 1.0 (0.75–1.0) 1.0 (0.63–1.0) 0.33 (0.08–1.0)
 1st gam–end (d) 2 (1–3) 2.5 (1–6) 3 (2–4) 3 (1–5) 5 (1–8)
N3 Patients 50 12 63 47 28 15
 1st fvr parasitemia 2.59 (2.08–3.36) 2.41 (1.00–3.02) 2.05 (1.60–2.32) 1.68 (1.48–2.00) 2.33 (1.30–2.74) 2.10 (1.48–3.04)
 1st fvr temperature 39.3 (38.9–39.8) 38.9 (38.3–40.6) 39.3 (38.9–39.7) 39.2 (38.9–39.4) 39.1 (38.6–39.7) 39.4 (38.6–40.1)
 1st fvr day 2 (2–4) 2 (2–6) 2 (1–2) 2 (2–3) 6 (3–10) 4 (2–5)
 1st gam day 12 (11–14) 12 (9–24) 8 (6–8) 8 (7–10) 16 (13–25) 10 (7–19)
 1st fvr–1st gam (d) 9 (7–11) 9 (6–18) 5 (4–7) 6 (4–9) 9 (4–21) 7 (2–15)
 Parasitemia before 3.60 (3.27–3.96) 3.82 (2.26–4.36) 3.07 (2.90–3.24) 3.09 (2.81–3.25) 2.83 (2.41–3.12) 3.04 (2.40–3.34)
 Parasitemia after 3.12 (2.54–3.28)** 2.68 (1.30–3.45) 3.80 (3.72–3.92)** 3.68 (3.46–3.82)** 3.28 (2.91–3.53)* 3.08 (2.14–3.45)
 Fever before 39.8 (39.6–40.0) 39.7 (39.0–40.3) 40.1 (39.7–40.4) 40.0 (39.6–40.4) 39.2 (38.8–40.0) 39.9 (38.6–40.6)
 Fever after 39.1 (38.9–39.4)** 38.9 (38.3–39.6)* 40.7 (40.4–40.8)** 40.6 (40.1–40.8)** 40.1 (39.4–40.5)* 40.2 (38.5–40.8)
 Fever (/d) before 0.67 (0.50–0.75) 0.56 (0.20–0.82) 0.68 (0.63–0.83) 0.63 (0.56–0.67) 0.40 (0.25–0.50) 0.50 (0.33–0.67)
 Fever (/d) after 0.27 (0.10–0.40)** 0.07 (0–0.29)** 0.60 (0.55–0.69) 0.57 (0.50–0.71) 0.34 (0.25–0.50) .33 (0–0.50)
 Difference (/d) 0.35 (0.20–0.55) 0.34 (0.13–0.82) 0 (0–0.15) 0 (0–0.19) 0 (−0.11–0.17) 0.07 (−0.07–0.50)
 Gam (/μl) 2.03 (1.18–2.24) 1.90 (1.00–2.60) 1.50 (1.28–1.77) 1.72 (1.34–1.85) 1.08 (1.00–1.30) 1.00 (1.00–1.18)
 Gam (/d) 1.0 (0.73–1.0) 1.0 (0.09–1.0) 0.90 (0.73–1.0) 0.82 (0.57–1.0) 0.30 (0.17–0.55) 0.20 (0.08–1.0)
 Aggregated (N3)
 Parasitemia before 3.63 (3.49–3.81) 3.75 (3.08–3.97) 3.08 (2.95–3.24) 3.02 (2.89–3.17) 3.00 (2.85–3.11) 3.13 (2.96–3.31)
 Parasitemia after 3.09 (2.96–3.16)** 2.92 (2.54–3.15)** 3.74 (3.66–3.79)** 3.51 (3.38–3.61)** 3.27 (3.17–3.34)** 2.51 (2.30–2.74)**
 Fever before 39.8 (39.6–40.0) 39.7 (39.2–40.0) 40.2 (40.0–40.6) 40.2 (40.0–40.4) 39.7 (39.4–40.0) 40.1 (39.4–40.4)
 Fever after 39.1 (38.9–39.4)** 39.1 (38.3–39.4)* 40.7 (40.6–40.8)* 40.6 (40.3–40.8)* 40.1 (39.8–40.3) 39.4 (38.6–40.6)
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*

P < 0.01,

**

P < 0.0002.

Malaria therapy infections generally were allowed to continue as long as possible without intervention, and were then terminated with curative doses of drug. However, at the discretion of medical staff, subcurative doses of drugs, or other interventions, might be given with the explicit aim of modulating an infection, e.g., to provide a break from fever or to reduce parasitemia. Our analyses include only the parts of charts that precede any such intervention. Further, our analyses include only charts in which extant daily records encompassed at least 93% of the preintervention duration of each infection, i.e., those in which the records were blank on <1 day in each 2 wk, on average.

Temperature was recorded only when ≥101 F (38.3 C), and we adhere to that definition of fever here. Microscopists used the Earle-Perez technique (Earle and Perez, 1932); the threshold of detection for asexual forms or gametocytes was generally 10 per μl.

In most previous analyses, we set the standard time scale for each infection to start with the day of the first detection of asexual blood forms. Here, given our interest in how static surveys reflect parasite dynamics, and the consistency of the interval from asexual-form patency to the first fever in each infection (Table I), we set our initial time point (day 1) as the day of first fever. This provides a practical reference time point for calibrating presentation at a clinic, for instance, as in our recent study in Peru and Thailand (McKenzie, Wongsrichanalai et al., 2006).

Intervention or termination sometimes occurred before, sometimes very soon after, and sometimes 6 mo, or more, after the day of first detection of gametocytes. Because biases would be introduced by considering a few days commensurate to months of gametocytemia, for the purpose of comparing densities, intensities, or even frequencies, we set a standard interval after the day of first detection of gametocytes equal to the interval between the first fever and the first detection of gametocytes, so that in each infection our analyses consider an equal number of days before, and after, the first detection of gametocytes (see the schematic in Fig. 2: interval ab = interval bc). We use the results for these patients (N3 in Table I) as a baseline, but note points at which these results differ from those incorporating charts truncated earlier (N2, ab > bc). We also provide summary statistics for the patients in whom intervention occurred before any gametocytes were detected (N1).

Figure 2.

Figure 2

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Schematics of idealized P. falciparum (top panel) and P. vivax (bottom panel) infections. The first fever occurs on day a, the first gametocytemia on day b; day c is set such that interval bc = ab (group N3; see text).

We use the Mann-Whitney U-test and Spearman’s rank correlation coefficient to compare distributions, giving P-values for 2-tailed tests, and use the G-test to investigate independence within contingency tables (Sokal and Rohlf, 1981).

RESULTS

Figure 1 shows the daily frequencies of fever and detected gametocytemia, within each species and inoculation mode, with all charts aligned from the first day of asexual-form patency, i.e., the fraction of all patient records that reported fever, or gametocytemia, on each day. The frequency of fever first exceeded 50% on day 2 with P. falciparum, day 2–3 with P. vivax, and day 7 with P. ovale; it reached a maximum 47% with P. malariae on day 27. The frequency of gametocytemia first exceeded 50% on day 12–13 with P. falciparum, and day 9–10 with P. vivax; it reached a maximum of 20% with P. malariae on day 23, and 26% with P. ovale on day 10. Thus, all else equal, one would not expect to detect gametocytemia in most patients until 10–11 days after the first fever with P. falciparum, and 7 days with P. vivax, or to detect gametocytemia in most patients on any day with P. ovale or P. malariae.

Figure 1.

Figure 1

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Day-by-day frequencies (percentage of patients; vertical axis) of fever or gametocytemia, from the first day of patency (as day 1; horizontal axis). Top panels: P. falciparum, trophozoite-induced (left, A) and sporozoite-induced (right, B). Middle panels: P. vivax, trophozoite-induced (left, C) and sporozoite-induced (right, D). Bottom panels: P. malariae (left, E) and P. ovale (right, F), both trophozoite-induced.

Figure 2 shows idealized schematics of the course of individual P. falciparum and P. vivax infections. Intervention in the infections in group N1 occurred before point b, i.e., there were no gametocytes detected before intervention; in group N2 when interval bc < ab, i.e., there were fewer days after first detection of gametocytes than before; and in group N3 when bc = ab, i.e., the number of days before and after first detection of gametocytes was identical.

We identified the median parasitemia and median fever for each patient before and after the first detection of gametocytes in the infection and used these values as the baseline data for the Mann-Whitney U-tests. Table I shows the median and 95% confidence interval for these median values for the N1, N2, and N3 patients, and the results of Mann-Whitney U-tests for the N3 patients. Note that the median interval between first fever and first gametocytemia was 9 days for P. falciparum, and 5–6 days for P. vivax. Figure 3 shows the proportion of patients in whom the median parasitemia or median fever before gametocytes were first detected was higher than the median after, i.e., 58–100% for P. falciparum infections, 10–32% for P. vivax, 25–33% for P. malariae, and 47–60% for P. ovale.

Figure 3.

Figure 3

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The proportion of patients (%; vertical axis) in whom the median parasitemia or median fever before gametocytes were first detected was higher than the median after, in P. falciparum (FAL), P. vivax (VIV), P. malariae (MAL), and P. ovale (OVA) infections, induced by trophozoites (troph) or sporozoites (sporo), in the N2 or N2 + N3 patients (see text).

The median values observed in a patient had not necessarily occurred on the same day, however. Therefore, for each species and induction mode, we also aggregated all pregametocyte-detection values, and all postgametocyte-detection values, of all patients. The last 4 rows of Table I show the median and 95% confidence interval for these values for the N3 patients. Because the interval between the first fever and the first detection of gametocytes was about 1 wk, on average, these aggregated data included roughly 7 times as many data points as the patient medians. Note that, as expected, the medians are close, but the confidence intervals narrowed relative to those in the rest of Table I. We used these values, pairing same-day same-patient observations, as the baseline data for the Spearman correlation tests.

In P. vivax and P. malariae infections, fever and parasitemia were higher after gametocytes were first detected than before. Results for P. vivax did not differ when the N2 patients were included, but the difference in fever for P. malariae was diminished (Mann-Whitney U-test, P = 0.03). In P. ovale infections, fever and parasitemia before gametocytes were first detected were indistinguishable from fever and parasitemia after (P > 0.83). In trophozoite-induced P. falciparum infections, fever and parasitemia were lower after gametocytes were first detected than before; in sporozoite-induced infections, only fever was lower. When the N2 patients were included, all of these differences for P. falciparum were diminished (P > 0.04). The results for differences in fever frequency (Table I) did not change when the N2 patients were included, however, for any species or induction mode.

Parasitemia and temperature correlated in P. vivax infections, both trophozoite- and sporozoite-induced, both before and after gametocytes were first detected (Spearman P < 0.0003). Parasitemia and temperature also correlated in P. malariae and sporozoite-induced P. falciparum infections before gametocyte detection (P < 0.0006), and in trophozoite-induced P. falciparum infections after gametocyte detection (P = 0.001). The results did not change when the N2 patients were included. Parasitemia and temperature at first fever were not correlated in infections with any species, induction mode, or patient group (N3, N2 + N3, N1 + N2 + N3; P > 0.10).

Gametocyte density correlated with parasitemia in P. malariae and sporozoite-induced P. falciparum and P. vivax infections (P < 0.00003). Gametocyte density and temperature were not correlated in any infections (P > 0.09). However, detected gametocytemia and fever (after the first gametocyte detection) coincided more often than expected by independence in P. vivax and P. ovale infections (G-test, P < 0.003; vs. P > 0.18 for P. malariae and P. falciparum). The results did not change when the N2 patients were included.

We examined the resilience of our conclusions both by comparing our baseline results with the N3 patients to results when the N2 patients were included, as noted above, and by applying the Spearman and Mann-Whitney U-tests to the patient-median and aggregated data, respectively (see Table I and Appendix). Concurrence across these 4 different sampling and statistical procedures was in most respects greater than we had expected. Overall, our conclusions for P. vivax and P. ovale appeared less sensitive to the inoculation mode and the sample (of patients and/or time points) than those for P. malariae and P. falciparum.

DISCUSSION

Our results were generally in accord with our results from the Peruvian and Thai clinics (McKenzie, Wongsrichanalai et al., 2006), i.e., the P. vivax patients with gametocytemia had higher fever and higher parasitemia than those without gametocytemia, and the P. falciparum patients with gametocytemia had lower fever than those without gametocytemia. However, in Peru and Thailand, parasitemia was generally indistinguishable between P. falciparum patients with and without gametocytemia, while here we found P. falciparum parasitemia lower after gametocytes were first detected than before. We can simulate the Peru-Thailand parasitemia result here by including the N2 patients, which serves to lower the median parasitemia before and raise the median parasitemia after gametocytes were first detected (Table I). Including the N2 patients negates the fever result, however.

Considering the idealized schematics in Figure 2, the nominal effect of including the N2 patients is to narrow the sampling interval after gametocytes were first detected by moving point c closer to point b (the first gametocyte detection). Since including these patients also alters the “typical” parasitemia curve and fever intensity for P. falciparum infections (top panel), features presumably related to the earlier intervention, and shifts point b slightly closer to point a, i.e., to slightly earlier gametocytemia, this suggests that the relationship between parasitemia and fever relative to the onset of gametocytemia differed between the populations.

Our correlation results seem to support this interpretation and were unaffected by including the N2 patients. The data for the sporozoite-induced infections were consistent with those from the Peruvian and Thai clinics in that temperature and parasitemia correlated in nongametocytemic, but not in gametocytemic, P. falciparum patients. With trophozoite-induced P. falciparum infections, the pattern was reversed, in that temperature and parasitemia correlated in gametocytemic, but not nongametocytemic, patients. Here, as in the Peruvian and Thai clinics, fever temperature and gametocyte density were not correlated in infections with either species, whether sporozoite- or trophozoite-induced. Thus, the remaining points of partial disagreement are that, here, temperature and parasitemia correlated in nongametocytemic as well as gametocytemic P. vivax infections, whether sporozoite- or trophozoite-induced, and that gametocyte density and parasitemia correlated in sporozoite-induced infections with P. falciparum as well as P. vivax (but not in trophozoite-induced infections with either species).

The adult neurosyphilis patients considered here differed from the adult patients at the Peruvian and Thai malaria clinics in many ways. For instance, whatever concurrent infections may have been present in the latter, they were less microbiologically homogeneous and less clinically severe; however, it is much more likely that they included cryptic infections with other Plasmodium species, which can affect gametocytemia, fever, and parasitemia (Price et al., 1999; McKenzie et al., 2002b; McKenzie, Smith et al., 2006). Furthermore, previous malaria exposure must have greatly affected infections in the Peruvian and Thai patients by altering immune responses. Infections in some neurosyphilis patients were affected by the transfers of trophozoites in whole blood, rather than of sporozoites passaged through mosquitoes. The differences between the induction modes noted above may have arisen from the relative timing of the blood-stage dose, the mixtures of antigenic material present in whole blood, synchronization of antigenic-variant expression by mosquito passage (Peters et al., 2002), or other unknown factors. The correlation of gametocyte density with parasitemia in the sporozoite-induced, but not the trophozoite-induced P. falciparum and P. vivax infections, seems particularly intriguing in this regard.

These and other differences between the populations make the similarities of their malaria infections still more remarkable (Winckel, 1941) and reinforce the point that such combinations of similarities and differences can be used to guide studies at different levels of resolution (Talman et al., 2004). Much the same holds for comparisons across Plasmodium species, i.e., there is no reason to assume that the factors that regulate gametocyte production, or stimulate fever, are either exactly the same or entirely different across the 4 species investigated here.

The timing of gametocytemia in relation to fever and parasitemia in any of these species may be only an epiphenomenon, marking elapsed time in an infection, or it may reflect underlying connecting mechanisms. It is conceivable, for instance, that high temperatures alter parasite or host metabolism in some way that affects gametocyte production, sequestration, decay, immune response, or some other factor that affects gametocytemia, but to the best of our knowledge there is no evidence to support or refute any such speculation. The crucial role of gametocytes in Plasmodium spp. epidemiology, evolution, transmission, and recombination would seem to argue for some link, and recent studies of gametocyte production in mixed-genotype P. falciparum infections (Abdel-Wahab et al., 2002; Sutherland et al., 2002; Nassir et al., 2005) have begun to suggest a complexity of dynamics at levels previously examined only in theoretical work (McKenzie, Ferreira et al., 2001; McKenzie, Killeen et al., 2001). The forces driving observed population-level differences in gametocyte prevalence and density with respect to age, season, and transmission intensity (Covell, 1960; Rosenberg et al., 1990; Luxemburger et al., 1996; Drakeley et al., 2000) remain mysterious at the level of individual P. falciparum infections and unexamined in other species; we look forward to their resolution.

Acknowledgments

We gratefully acknowledge the contributions of L. L. Borio, W. H. Bossert, W. P. O’Meara, R. S. Wells, and an anonymous reviewer.

APPENDIX

These day-by-day data from hundreds of patients provided an opportunity to examine the sensitivity of our conclusions, through the inclusion or exclusion of a particular subset of patients (N2), and through testing both the single median observations for each variable from each patient and roughly 1 wk of observations for each, aggregated for all patients, before and after the first detection of gametocytemia.

We performed Mann-Whitney U-tests on the aggregated data as well as the patient-median data (see Table I), and, as expected, the results were closely similar. The 3 disagreements involved the P. malariae, P. ovale, and sporozoite-induced P. falciparum infections, the categories with the smallest sample sizes. Here, in the P. ovale and sporozoite-induced P. falciparum infections, parasitemia was lower after gametocytes were first detected than before. With P. malariae, the difference in fever was greatly diminished (P = 0.33). When the N2 patients were included, the only change was that the difference in fever in the sporozoite-induced P. falciparum infections became indistinguishable (P = 0.11).

We performed Spearman tests on the patient-median data, as well as the aggregated data, pairing same-patient observations, and found an unexpected similarity in the results. With the N3 patients, parasitemia and temperature again correlated in all P. vivax infections; when the N2 patients were included, the correlations for P. malariae and P. falciparum appeared as well. Gametocyte density and temperature were again not correlated in any infections (P > 0.17). Gametocyte density correlated with parasitemia only in the trophozoite-induced P. vivax infections, however (P = 0.01); when the N2 patients were included, a correlation appeared in the P. malariae infections (P = 0.0009). Thus, the parasitemia-gametocytemia results were the most sensitive to day-day pairing.

With any of these species, the gametocytes detected on a given day did not necessarily arise from the same generation as the asexual-form parasites present on that day or any particular number of days before. Gametocytes were detected on most but not all days after gametocytes were first detected (Fig. 1; Table I). Unlike fevers, which can be taken as relatively discrete events on a day-to-day basis, gametocyte densities involve decay as well as production rates (Eichner et al., 2001), and gametocyte detection involves many further complications (McKenzie et al., 2003; O’Meara et al., 2005) on which a future paper will focus. Here, though it is hazardous to equate “after gametocytes were first detected” with “detected gametocytemia,” we do so in comparing our results to those from other studies, including our own from Peru and Thailand.

Contributor Information

F. Ellis McKenzie, Fogarty International Center, National Institutes of Health, Bethesda, Maryland 20892, e-mail: em225k@nih.gov.

Geoffrey M. Jeffery, USPHS retired. 1085 Blackshear Road, Apartment B, Decatur, Georgia 30033.

William E. Collins, Division of Parasitic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30341.

LITERATURE CITED

  1. Abdel-Wahab A, Abdel-Muhsin AM, Ali E, Suleiman S, Ahmed S, Walliker D, Babiker HA. Dynamics of gametocytes among Plasmodium falciparum clones in natural infections in an area of highly seasonal transmission. Journal of Infectious Diseases. 2002;185:1838–1842. doi: 10.1086/340638. [DOI] [PubMed] [Google Scholar]
  2. Akim NI, Drakeley C, Kingo T, Simon B, Senkoro K, Sauerwein RW. Dynamics of P. falciparum gametocytemia in symptomatic patients in an area of intense perennial transmission in Tanzania. American Journal of Tropical Medicine and Hygiene. 2000;63:199–203. doi: 10.4269/ajtmh.2000.63.199. [DOI] [PubMed] [Google Scholar]
  3. Boyd MF, Kitchen SF. On the infectiousness of patients infected with Plasmodium vivax and Plasmodium falciparum. American Journal of Tropical Medicine. 1937;17:253–262. [Google Scholar]
  4. Collins WE, Jeffery GM. A retrospective examination of sporozoite- and trophozoite-induced infections with Plasmodium falciparum. American Journal of Tropical Medicine and Hygiene. 1999;61(s1):4–48. doi: 10.4269/tropmed.1999.61-04. [DOI] [PubMed] [Google Scholar]
  5. Collins WE, Jeffery GM. A retrospective examination of sporozoite-induced and trophozoite-induced infections with Plasmodium ovale: Development of parasitologic and clinical immunity during primary infection. American Journal of Tropical Medicine and Hygiene. 2002;66:492–502. doi: 10.4269/ajtmh.2002.66.492. [DOI] [PubMed] [Google Scholar]
  6. Collins WE, Jeffery GM. Plasmodium ovale: Parasite and disease. Clinical Microbiology Reviews. 2005;18:570–581. doi: 10.1128/CMR.18.3.570-581.2005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Covell G. Relationship between malarial parasitemia and symptoms of the disease. Bulletin of the World Health Organization. 1960;22:605–619. [PMC free article] [PubMed] [Google Scholar]
  8. Drakeley CJ, Akim NI, Sauerwein R, Greenwood BM, Targett GA. Estimates of the infectious reservoir of Plasmodium falciparum malaria in the Gambia and in Tanzania. Transactions of the Royal Society of Tropical Medicine and Hygiene. 2000;94:472–476. doi: 10.1016/s0035-9203(00)90056-7. [DOI] [PubMed] [Google Scholar]
  9. Earle WC, Perez M. Enumeration of parasites in the blood of malarial patients. Journal of Laboratory and Clinical Medicine. 1932;17:1124–1130. [Google Scholar]
  10. Eichner M, Diebn0er HH, Molineaux L, Collins WE, Jeffery GM, Dietz K. Genesis, sequestration and survival of Plasmodium falciparum gametocytes: Parameter estimates from fitting a model to malariatherapy data. Transactions of the Royal Society of Tropical Medicine and Hygiene. 2001;95:497–501. doi: 10.1016/s0035-9203(01)90016-1. [DOI] [PubMed] [Google Scholar]
  11. Gouagna LC, Ferguson HM, Okech BA, Killeen GF, Kabiru EW, Beier JC, Githure JI, Yan G. Plasmodium falciparum malaria disease manifestations in humans and transmission to Anopheles gambiae: A field study in Western Kenya. Parasitology. 2004;128:235–243. doi: 10.1017/s003118200300444x. [DOI] [PubMed] [Google Scholar]
  12. Hackett LW. Malaria and the community. In: Moulton FR, editor. Human malaria. American Association for the Advancement of Science; Washington, D.C.: 1941. pp. 148–156. [Google Scholar]
  13. Luxemburger C, Thwai KL, White NJ, Webster HK, Kyle DE, Maelankirri L, Chongsuphajaisiddhi T, Nosten F. The epidemiology of malaria in a Karen population on the western border of Thailand. Transactions of the Royal Society of Tropical Medicine and Hygiene. 1996;90:105–111. doi: 10.1016/s0035-9203(96)90102-9. [DOI] [PubMed] [Google Scholar]
  14. McKenzie FE, Ferreira MU, Baird JK, Snounou G, Bossert WH. Meiotic recombination, cross-reactivity and persistence in Plasmodium falciparum. Evolution. 2001;55:1299–1307. doi: 10.1111/j.0014-3820.2001.tb00652.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. McKenzie FE, Jeffery GM, Collins WE. Plasmodium malariae blood-stage dynamics. Journal of Parasitology. 2001;87:626–637. doi: 10.1645/0022-3395(2001)087[0626:PMBSD]2.0.CO;2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. McKenzie FE, Jeffery GM, Collins WE. Plasmodium vivax blood-stage dynamics. Journal of Parasitology. 2002a;88:521–535. doi: 10.1645/0022-3395(2002)088[0521:PVBSD]2.0.CO;2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. McKenzie FE, Jeffery GM, Collins WE. Plasmodium malariae infection boosts Plasmodium falciparum gametocyte production. American Journal of Tropical Medicine and Hygiene. 2002b;67:411–414. doi: 10.4269/ajtmh.2002.67.411. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. McKenzie FE, Killeen GF, Beier JC, Bossert WH. Seasonality, parasite diversity and local extinctions in Plasmodium falciparum malaria. Ecology. 2001;82:2673–2681. doi: 10.1890/0012-9658(2001)082[2673:spdale]2.0.co;2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. McKenzie FE, Sirichaisinthop J, Miller RS, Gasser RA, Jr, Wongsrichanalai C. Dependence of malaria detection and species diagnosis by microscopy on parasite density. American Journal of Tropical Medicine and Hygiene. 2003;69:372–376. [PMC free article] [PubMed] [Google Scholar]
  20. McKenzie FE, Smith DL, O’Meara WP, Forney JR, Magill AJ, Permpanich B, Erhart LM, Sirichaisinthop J, Wongsrichanalai C, Gasser RA., Jr Fever in patients with mixed-species malaria. Clinical Infectious Diseases. 2006;42:1713–1718. doi: 10.1086/504330. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. McKenzie FE, Wongsrichanalai C, Magill AJ, Forney JR, Permpanich B, Lucas C, Erhart LM, O’Meara WP, Smith DL, Sirichaisinthop J, Gasser RA., Jr Gametocytemia in Plasmodium vivax and Plasmodium falciparum infections. Journal of Parasitology. 2006;92:1281–1285. doi: 10.1645/GE-911R.1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Nassir E, Abdel-Muhsin AM, Suliaman S, Kenyon F, Kheir A, Geha H, Ferguson HM, Walliker D, Babiker HA. Impact of genetic complexity on longevity and gametocytogenesis of Plasmodium falciparum during the dry and transmission-free season of eastern Sudan. International Journal for Parasitology. 2005;35:49–55. doi: 10.1016/j.ijpara.2004.10.014. [DOI] [PubMed] [Google Scholar]
  23. O’Meara WP, McKenzie FE, Magill AJ, Forney JR, Permpanich B, Lucas C, Gasser RA, Jr, Wongsrichanalai C. Sources of variability in determining malaria parasite density by microscopy. American Journal of Tropical Medicine and Hygiene. 2005;73:593–598. [PMC free article] [PubMed] [Google Scholar]
  24. Peters J, Fowler E, Gatton M, Chen N, Saul A, Cheng Q. High diversity and rapid changeover of expressed var genes during the acute phase of Plasmodium falciparum infections in human volunteers. Proceedings of the National Academy of Sciences USA. 2002;99:10689–10694. doi: 10.1073/pnas.162349899. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Price RN, Nosten F, Simpson JA, Luxemburger C, Phaipun L, ter Kuile F, van Vugt M, Chongsuphajaisiddhi T, White NJ. Risk factors for gametocyte carriage in uncomplicated falciparum malaria. American Journal of Tropical Medicine and Hygiene. 1999;60:1019–1023. doi: 10.4269/ajtmh.1999.60.1019. [DOI] [PubMed] [Google Scholar]
  26. Rosenberg R, Andre RG, Ngampatom S, Hatz C, Burge R. A stable, oligosymptomatic malaria focus in Thailand. Transactions of the Royal Society of Tropical Medicine and Hygiene. 1990;84:14–21. doi: 10.1016/0035-9203(90)90366-m. [DOI] [PubMed] [Google Scholar]
  27. Schuffner WAP. Two subjects relating to the epidemiology of malaria. Journal of the Malaria Institute of India. 1938;1:221–256. [Google Scholar]
  28. Shute PG, Maryon M. A study of gametocytes in a West African strain of Plasmodium falciparum. Transactions of the Royal Society of Tropical Medicine and Hygiene. 1951;44:421–438. doi: 10.1016/s0035-9203(51)80020-8. [DOI] [PubMed] [Google Scholar]
  29. Sokal RR, Rohlf FJ. Biometry. W. H. Freeman; New York, New York: 1981. p. 859. [Google Scholar]
  30. Sutherland CJ, Alloueche A, McRobert L, Ord R, Leggat J, Snounou G, Pinder M, Targett GA. Genetic complexity of Plasmodium falciparum gametocytes isolated from the peripheral blood of treated Gambian children. American Journal of Tropical Medicine and Hygiene. 2002;66:700–705. doi: 10.4269/ajtmh.2002.66.700. [DOI] [PubMed] [Google Scholar]
  31. Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V. Gametocytogenesis: the puberty of Plasmodium falciparum. Malaria Journal. 2004;3:24. doi: 10.1186/1475-2875-3-24. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Winckel CWF. Are the experimental data of therapeutic malaria applicable to conditions obtaining in nature? American Journal of Tropical Medicine. 1941;21:789–794. [Google Scholar]

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