a Centre for Paediatric Epidemiology and Biostatistics, Institute of Child Health, London, UK.
b Laboratoire de Parasitologie et de Pathologie Exotique, Hôpital de la Croix Rousse, Lyon, France.
c Medical Research Council Clinical Trials Unit, London, UK.
Dr Ruth Gilbert, Centre for Paediatric Epidemiology and Biostatistics, Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK.
Abstract
Background The aim of prenatal serological screening for toxoplasmosis is to identify and treat maternal infection as soon as possible in order to prevent transmission of the parasite to the fetus. However, despite widespread provision of prenatal toxoplasma screening across Europe, the effectiveness of prenatal treatment is uncertain. The study aimed to determine the effect of the timing and type of prenatal treatment on mother to child transmission of Toxoplasma gondii.
Method A cohort of 554 infected pregnant women were identified in Lyon, France between 1987 and 1995 and their children were followed to determine congenital infection status. We determined the effect of prenatal treatment on transmission by examining the effect of the delay between maternal seroconversion and start of treatment. We also compared the effect of the type of treatment and no treatment on the risk of mother to child transmission. Analyses were adjusted for gestation at maternal seroconversion.
Results Compared to treatment within 4 weeks from seroconversion, the adjusted odds ratios (OR) for mother to child transmission after a treatment delay of 47 weeks was 1.29 (95% CI : 0.61, 2.73) and after more than 8 weeks, 1.44 (95% CI : 0.60, 3.31). The adjusted OR associated with spiramycin alone compared with pyrimethamine-sulfadiazine treatment was 0.91 (95% CI : 0.45, 1.84) and the OR for no treatment compared with pyrimethamine-sulfadiazine treatment was 1.06 (95% CI : 0.37, 3.03).
Conclusions The authors hypothesize that the absence of an effect of prenatal treatment is due to transmission before the start of treatment.
Keywords Congenital toxoplasmosis, treatment, vertical transmission
Accepted 24 May 2001
Routine testing for toxoplasma infection during pregnancy is offered in many European countries.1 The aim is twofold. To treat infected pregnant women with antibiotics in order to reduce the risk of fetal infection and, if fetal infection has occurred, to reduce the risk of impairment in the child.2 In this report, we present results relating to the first aim of treatment, the reduction in mother to child transmission of infection.
Two possible mechanisms for the transmission of infection from mother to child have been suggested.3,4,5,p.156 First, infection is transmitted to the fetus via the placenta shortly after maternal infection and is most likely to occur in immunocompetent women when three factors occur more or less concurrently': maternal parasitaemia in the presence of well-developed placental blood flow before the development of competent maternal cellular and humoral immunity to Toxoplasma gondii.5,p.156 This hypothesis underpins the rationale for the frequent re-testing of susceptible women in France in order to start treatment as soon as possible after detection of maternal infection.6
An alternative mechanism is that the parasite forms inflammatory foci in the placenta and tachyzoites are released into the fetal circulation at a later point in pregnancy.3,7 Wide acceptance of this hypothesis has resulted in the standard policy of continued treatment throughout pregnancy in infected women even following a negative diagnosis of fetal infection.8
Almost all women diagnosed with infection during pregnancy receive treatment. However, the delay between maternal seroconversion and start of treatment varies due to differences in the frequency of serological testing. We therefore examined the relationship between treatment delay and the risk of mother to child transmission in order to provide information on the effect of treatment. Further information on the timing of transmission was sought by investigating the association between the interval between maternal seroconversion and fetal diagnosis and the results of fetal diagnosis in children subsequently found to have congenital toxoplasmosis. Finally, we determined whether the risk of transmission differed according to whether women were prescribed spiramycin or the more potent regimen of pyrimethamine-sulfadiazine915 compared with those not treated.
Methods
Patients
This retrospective study was based on women in whom seroconversion to toxoplasma infection was confirmed by the Hôpital de la Croix Rousse, Lyon, France, between August 1987 and October 1995. The methods for ascertainment, diagnosis, treatment and follow-up of the cohort have been described elsewhere.16 In brief, women were included if they seroconverted from negative to positive specific IgG antibodies. We excluded women identified on the basis of detectable IgG antibodies as preconceptional infection cannot be excluded unless mother to child transmission occurs. In order to avoid selection bias, women referred for suspected fetal abnormalities were excluded. Dates of serology samples, prescription of anti toxoplasma drugs, detection of fetal abnormalities by ultrasound, and the results of fetal diagnosis were obtained from clinical records.
The standard treatment policy spiramycin (9 x 106 units/day) was prescribed immediately after confirmation of maternal infection. Exceptions to this regimen were women with a positive diagnosis of fetal infection and those infected after 32 weeks of gestation. Women in the latter two groups were prescribed pyrimethamine (50 mg/day) and sulfadiazine (3 g/day) alternating three weekly with spiramycin (subsequently referred to as pyrimethamine-sulfadiazine). Treatment was continued until delivery.
In children born to infected women, congenital toxoplasmosis was diagnosed if IgG persisted beyond 12 months, although in non live births positive findings by polymerase chain reaction (PCR) or mouse inoculation of amniotic fluid, fetal blood, or fetal tissue was accepted. The sole criterion for excluding congenital toxoplasmosis was a decline in specific IgG antibody below detectable levels before one year of age in the absence of treatment. Clinicians responsible for the clinical management of the mother routinely recorded the woman's compliance with treatment based on questioning after delivery.
Rationale for analyses
As toxoplasma infection is usually asymptomatic, seroconversion during pregnancy is used to identify maternal infection. However, as Figure 1 illustrates, the date when seroconversion occurred is not known for certain, only the interval between the last negative and first positive test. We used a statistical model to take account of the uncertainty of the date of seroconversion and the strong effect of gestation at seroconversion on transmission by determining the probability of IgM seroconversion (change from negative to positive specific IgM antibodies) on all possible dates.16 Bayes' Theorem was used to calculate the probability of seroconversion based on information on the presence or absence of specific IgG antibodies at the first IgM positive test (using a previously derived function16), and whether the child was infected or not (Figure 2
).
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Effect of type of treatment compared with no treatment on transmission risk
If there is a significant delay between maternal infection and transmission to the fetus, the risk of transmission in women prescribed the more potent regimen of pyrimethamine-sulfadiazine would be expected to be lower than in women prescribed spiramycin and those not treated.915 These three treatment groups were compared but, as pyrimethamine-sulfadiazine is routinely prescribed to women after a positive fetal diagnosis, we limited this analysis to women who did not undergo prenatal diagnosis.
Association between time to fetal diagnosis and result of fetal diagnosis
This analysis was limited to cases where fetal diagnosis was performed and where the infant had a definitive diagnosis of congenital toxoplasmosis. We examined whether the time interval between seroconversion and fetal diagnosis was associated with detection of fetal infection (defined as positive mouse inoculation of amniotic fluid or fetal blood, or PCR analysis of amniotic fluid). If delayed transmission was common, samples taken soon after seroconversion would be less likely to be positive than those taken later.
Analysis
In all the analyses, we took account of the confounding effects of gestational age at maternal seroconversion. Parameters were estimated by the method of maximum likelihood using a model that has been described previously.16 We assumed that the effects of treatment and gestation at seroconversion were additive on a logistic scale. Significance tests were based on the likelihood ratio test. Confidence intervals were obtained using the profile likelihood method.17
Results
Patients
Between 1987 and 1995, 788 mother-child pairs were identified. A total of 84 pairs were excluded from analysis due to: suspicion of referral for fetal infection (8); inadequate confirmation of maternal infection (9); incomplete data on prenatal treatment (7) and unknown congenital infection status (60) (Figure 3). Some 150 women identified through tests of recent infection were also excluded from analysis because pre conceptional infection could not be ruled out. In all, 554 women infected with T. gondii during pregnancy (identified through proven IgG serconversion) and their children were therefore analysed. Of these, 141/554 (25%) were IgM positive at their first prenatal test.
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Table 1 shows the prenatal treatment prescribed by whether fetal diagnosis was performed. Treatment consisted of: spiramycin alone (416 women, 75%); pyrimethamine-sulfadiazine alternating with spiramycin (11 women, 2%); and spiramycin followed by pyrimethamine-sulfadiazine alternating with spiramycin (96 women, 17%). In 4 (0.8%) of the 523 treated women, compliance was recorded as possibly inadequate. Of the 31 women (6%) who were not treated, 25 had infection confirmed after delivery and no information is available for the remainder.
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Association between time to fetal diagnosis and positive fetal diagnosis
Amniocentesis and/or cordocentesis were performed in 281 women (51%). In 274 (98%) of them mouse inoculation of amniotic fluid was performed and in 32 (11%) amniotic fluid was analysed with PCR. Of the 52 babies who had congenital toxoplasmosis confirmed postnatally, 39 (75%) had a positive fetal diagnosis and 13 were negative. The interval between seroconversion and fetal diagnosis was not associated with a positive diagnosis of fetal infection: adjusted OR for detection of fetal infection associated with a one-week increase in the time between seroconversion and fetal diagnosis was 0.86 (95% CI : 0.61, 1.19).
Discussion
We found no evidence that the risk of mother to child transmission was related to the timing of prenatal treatment despite reportedly good compliance. However, we cannot exclude a clinically important effect of treatment given within 4 weeks compared with 4 weeks after maternal seroconversion. Although women were generally treated promptly (median 6 days) after the first positive IgM test, only an estimated 50% of women were prescribed treatment within one month of seroconversion. This interval is strongly determined by the frequency of antibody testing in susceptible women. In France, where monthly re-testing was made a legal requirement in 1992,18 the interval between seroconversion and start of treatment is probably the shortest achieved in any screening programme.
Possible explanations for findings
One possible explanation for our failure to detect an effect of the timing of treatment is that transmission occurs after a prolonged delay.3,7 However, other findings suggest that delayed transmission is unlikely. First, although pyrimethamine-sulfadiazine is more potent than spiramycin,915 we found no evidence that the effect of these two treatment regimens differed. Second, there was no association between the timing of fetal diagnosis and detection of fetal infection. Third, if delayed transmission was predominant it would be expected that fetuses exposed to maternal infection for longest would be most likely to be infected. Instead, the reverse pattern is observed as shown by the steep increase in the risk of transmission with gestation at seroconversion.16
Fourth, detection of infected placentas in uninfected children would provide evidence of delayed transmission, but such cases appear to be extremely rare.1921 Finally, delayed transmission would be expected to result in an association between increasing interval between seroconversion and prenatal diagnosis and a positive diagnosis. However, we found no evidence for this.
An alternative explanation for our failure to detect an effect of the timing of treatment or an effect of more potent treatment, is that, even with monthly testing and very prompt treatment after a positive test result, transmission occurred before treatment started. Experimental studies in animals show that mother to fetus transmission of infection occurs during the phase of maternal parasitaemia.5,9,10,22,23,24,p.146 Less is known about the timing of transmission in humans, as it is rarely possible to accurately date the onset of maternal or fetal infection or the duration of parasitaemia. However, studies report a failure to detect parasitaemia in women identified by serological testing.5,p.147 This suggests that, in most women, parasitaemia ceases by the time seroconversion occurs. Information on the timing of seroconversion after infection is limited to an outbreak investigation of 14 infected individuals, which found that all had detectable antibodies to T. gondii within 2 weeks of infection.25
Findings of other studies
Our results on prenatal treatment support the findings of a study of 144 pregnant women referred to five European centres. The study found no evidence for an effect of prenatal treatment compared with no treatment, or the timing of treatment, on the risk of mother to child transmission after taking account of gestation at maternal infection.26 Two systematic reviews27,28 of the effect of prenatal treatment on mother to child transmission of infection found no controlled trials and none of the observational studies adequately accounted for the steep rise in the risk of mother to child transmission of infection with gestation at maternal infection.16 Without adjusting for this effect, apparently big treatment effects on transmission might be due to differences in timing of seroconversion between treatment groups.
Conclusions
We did not detect a significant effect of prenatal treatment on the risk of mother to child transmission of infection. Although this was the largest study reported to date, the sample size was not large enough to exclude a clinically important effect of treatment. We hypothesize that mother to child transmission of infection predominately occurs during maternal parasitaemia, which ceases when a serological response to toxoplasma infection is detectable. If our hypothesis is true, prenatal treatment of women identified by serological screening will not materially reduce the risk of mother to child transmission of infection.
Acknowledgments
The study was funded by The Wellcome Trust and by the European Commission BIOMED programme (BMH4-CT98-3927). We thank the staff of Laboratoire de Parasitologie et de Pathologie Exotique, Hôpital de la Croix Rousse, Lyon, for help with data collection, Catherine Peckham, for helpful comments on the paper and Eskild Petersen for commenting on drafts of the report and facilitating the collaboration.
References
1 Raeber PA, Biedermann K, Just M, Zuber P. [Prevention of congenital toxoplasmosis in Europe]. Schweiz Med Wochensch 1995;65(Suppl.): 96S102S.
2 Jeannel D, Costagliola D, Niel G, Hubert B, Danis M. What is known about the prevention of congenital toxoplasmosis? Lancet 1990;336: 35961.[ISI][Medline]
3 Thalhammer O. Perinatal Medicine. Sixth European Congress, Vienna 1978. Prevention of congenital infections. Introductory remarks. Some problems surrounding prevention of prenatal toxoplasma infection. Thalhammer O, Baumgarten K, Pollak A. Stuttgart: Georg Thieme Publishers, 1979.
4 Desmonts G. Toxoplasmosis in France. Asean J Clin Sci 1987;7:89109.
5 Remington JS, McLeod R, Desmonts G. Toxoplasmosis. In: Remington JS, Klein J (eds). Infectious Diseases of the Fetus and Newborn. 4th Edn. Pennsylvania: WB Saunders, 1995, pp.140267.
6 Chevallier M. Etude cout-avantage d'un système de prévention de la Toxoplasmose Congénitale. Bull Statist Sante-Securite Sociale 1974;3: 7184.
7 Desmonts G, Daffos F, Forestier F, Capella Pavlovsky M, Thulliez P, Chartier M. Prenatal diagnosis of congenital toxoplasmosis. Lancet 1985;i:50004.
8 Stray-Pedersen B, Foulon W. Effect of treatment of the infected pregnant woman and her foetus. In: Amboise-Thomas P, Petersen E (eds). Congenital Toxoplasmosis. Scientific Background, Clinical Management and Control. Paris: Springer Verlag, 2000, pp.14152.
9 Schoondermark van de Ven E, Galama J, Vree T et al. Study of treatment of congenital Toxoplasma gondii infection in rhesus monkeys with pyrimethamine and sulfadiazine. Antimicrob Agents Chemother 1995;39:13744.[Abstract]
10 Schoondermark-van de Ven E, Melchers W, Camps W, Eskes T, Meuwissen J, Galama J. Effectiveness of spiramycin for treatment of congenital Toxoplasma gondii infection in rhesus monkeys. Antimicrob Agents Chemother 1994;38:193036.[Abstract]
11 Leport C, Vilde JL, Katlama C, Regnier B, Matheron S, Saimot AG. Failure of spiramycin to prevent neurotoxoplasmosis in immunosuppressed patients. JAMA 1986;255:2290.
12 Dannemann B, McCutchan JA, Israelski D et al. Treatment of toxoplasmic encephalitis in patients with AIDS. A randomized trial comparing pyrimethamine plus clindamycin to pyrimethamine plus sulfadiazine. Ann Intern Med 1992;116:3343.[ISI][Medline]
13 Luft BJ, Remington JS. Toxoplasmic encephalitis in AIDS. Clin Infect Dis 1992;15:21122.[ISI][Medline]
14 Chamberland S, Kirst HA, Current WL. Comparative activity of macrolides against Toxoplasma gondii demonstrating utility of an in vitro microassay. Antimicrob Agents Chemother 1991;35:90309.[ISI][Medline]
15 Derouin F, Nalpas J, Chastang C. [In vitro measurement of the inhibitory effect of macrolides, lincosamides and synergestines on the growth of Toxoplasma gondii]. Pathol Biol Paris 1988;36:120410.[ISI][Medline]
16 Dunn D, Wallon M, Peyron F, Petersen E, Peckham CS, Gilbert RE. Mother to child transmission of toxoplasmosis: risk estimates for clinical counselling. Lancet 1999;353:182933.[ISI][Medline]
17 Cox DR, Oakes D (eds). Analysis of Survival Data. London: Chapman and Hall, 1984, pp.5152.
18 Thulliez P. Screening programme for congenital toxoplasmosis in France. Scand J Infect Dis 1992;84(Suppl.):4345.
19 Desmonts G, Couvreur J. Congenital toxoplasmosis. A prospective study of 378 pregnancies. N Engl J Med 1974;290:111016.[ISI][Medline]
20 Fricker Hidalgo H, Pelloux H, Racinet C et al. Detection of Toxoplasma gondii in 94 placentae from infected women by polymerase chain reaction, in vivo, and in vitro cultures. Placenta 1998;19:54549.[ISI][Medline]
21 Naessens A, Jenum PA, Pollak A et al. Diagnosis of congenital toxoplasmosis in the neonatal period: a multicenter evaluation. J Pediatr 1999;135:71419.[ISI][Medline]
22 Dubey JP. Toxoplasma-induced abortion in dairy goats. J Am Vet Med Assoc 1981;178:67174.[ISI][Medline]
23 Dubey JP, Sharma SP, Lopes CW, Williams JF, Williams CS, Weisbrode SE. Caprine toxoplasmosis: abortion, clinical signs, and distribution of Toxoplasma in tissues of goats fed Toxoplasma gondii oocysts. Am J Vet Res 1980;41:107276.[ISI][Medline]
24 Schoondermark van de Ven E, Melchers W, Galama J, Camps W, Eskes T, Meuwissen J. Congenital toxoplasmosis: an experimental study in rhesus monkeys for transmission and prenatal diagnosis. Exp Parasitol 1993;77:20011.[ISI][Medline]
25 Sulzer AJ, Franco EL, Takafuji E, Benenson M, Walls KW, Greenup RL. An oocyst-transmitted outbreak of toxoplasmosis: patterns of immunoglobulin G and M over one year. Am J Trop Med Hyg 1986;35:29096.[ISI][Medline]
26 Foulon W, Villena I, Stray-Pedersen B et al. Treatment of toxoplasmosis during pregnancy: a multicentre study of impact on fetal transmission and children's sequelae at age 1 year. Am J Obstet Gynecol 1999;180:41015.[ISI][Medline]
27 Eskild A, Oxman A, Magnus P, Bjorndal A, Bakketeig LS. Screening for toxoplasmosis in pregnancy: what is the evidence of reducing a health problem? J Med Screen 1996;3:18894.[Medline]
28
Wallon M, Liou C, Garner P, Peyron F. Congenital toxoplasmosis: what is the evidence that treatment in pregnancy prevents congenital disease? Br Med J 1999;318:151114.