West African Immigrants and New Patterns of Malaria Imported to North Eastern Italy

B-cell memory in malaria: Myths and realities

B-cell and antibody responses to Plasmodium spp., the parasite that causes malaria, are critical for control of parasitemia and associated immunopathology. Antibodies also provide protection to reinfection. Long-lasting B-cell memory has been shown to occur in response to Plasmodium spp. in experimental model infections, and in human malaria. However, there are reports that antibody responses to several malaria antigens in young children living with malaria are not similarly long-lived, suggesting a dysfunction in the maintenance of circulating antibodies. Some studies attribute this to the expansion of atypical memory B cells (AMB), which express multiple inhibitory receptors and activation markers, and are hyporesponsive to B-cell receptor (BCR) restimulation in vitro. AMB are also expanded in other chronic infections such as tuberculosis, hepatitis B and C, and HIV, as well as in autoimmunity and old age, highlighting the importance of understanding their role in immunity. Whether AMB are dysfunctional remains controversial, as there are also studies in other infections showing that AMB can produce isotype-switched antibodies and in mouse can contribute to protection against infection. In light of these controversies, we review the most recent literature on either side of the debate and challenge some of the currently held views regarding B-cell responses to Plasmodium infections.

The effect of declining exposure on T cell-mediated immunity to Plasmodium falciparum - an epidemiological "natural experiment"

BACKGROUND

Naturally acquired immunity to malaria may be lost with lack of exposure. Recent heterogeneous reductions in transmission in parts of Africa mean that large populations of previously protected people may lose their immunity while remaining at risk of infection.

METHODS

Using two ethnically similar long-term cohorts of children with historically similar levels of exposure to Plasmodium falciparum who now experience very different levels of exposure, we assessed the effect of decreased parasite exposure on antimalarial immunity. Peripheral blood mononuclear cells (PBMCs) from children in each cohort were stimulated with P. falciparum and their P. falciparum-specific proliferative and cytokine responses were compared.

RESULTS

We demonstrate that, while P. falciparum-specific CD4⁺ T cells are maintained in the absence of exposure, the proliferative capacity of these cells is altered considerably. P. falciparum-specific CD4⁺ T cells isolated from children previously exposed, but now living in an area of minimal exposure ("historically exposed") proliferate significantly more upon stimulation than cells isolated from children continually exposed to the parasite. Similarly, PBMCs from historically exposed children expressed higher levels of pro-inflammatory cytokines and lower levels of anti-inflammatory cytokines after stimulation with P. falciparum. Notably, we found a significant positive association between duration since last febrile episode and P. falciparum-specific CD4⁺ T cell proliferation, with more recent febrile episodes associated with lower proliferation.

CONCLUSION

Considered in the context of existing knowledge, these data suggest a model explaining how immunity is lost in absence of continuing exposure to P. falciparum.

Cross-border malaria: a major obstacle for malaria elimination

Movement of malaria across international borders poses a major obstacle to achieving malaria elimination in the 34 countries that have committed to this goal. In border areas, malaria prevalence is often higher than in other areas due to lower access to health services, treatment-seeking behaviour of marginalized populations that typically inhabit border areas, difficulties in deploying prevention programmes to hard-to-reach communities, often in difficult terrain, and constant movement of people across porous national boundaries. Malaria elimination in border areas will be challenging and key to addressing the challenges is strengthening of surveillance activities for rapid identification of any importation or reintroduction of malaria. This could involve taking advantage of technological advances, such as spatial decision support systems, which can be deployed to assist programme managers to carry out preventive and reactive measures, and mobile phone technology, which can be used to capture the movement of people in the border areas and likely sources of malaria importation. Additionally, joint collaboration in the prevention and control of cross-border malaria by neighbouring countries, and reinforcement of early diagnosis and prompt treatment are ways forward in addressing the problem of cross-border malaria.

IFNγ/IL-10 co-producing cells dominate the CD4 response to malaria in highly exposed children

Although evidence suggests that T cells are critical for immunity to malaria, reliable T cell correlates of exposure to and protection from malaria among children living in endemic areas are lacking. We used multiparameter flow cytometry to perform a detailed functional characterization of malaria-specific T cells in 78 four-year-old children enrolled in a longitudinal cohort study in Tororo, Uganda, a highly malaria-endemic region. More than 1800 episodes of malaria were observed in this cohort, with no cases of severe malaria. We quantified production of IFNγ, TNFα, and IL-10 (alone or in combination) by malaria-specific T cells, and analyzed the relationship of this response to past and future malaria incidence. CD4⁺ T cell responses were measurable in nearly all children, with the majority of children having CD4⁺ T cells producing both IFNγ and IL-10 in response to malaria-infected red blood cells. Frequencies of IFNγ/IL10 co-producing CD4⁺ T cells, which express the Th1 transcription factor T-bet, were significantly higher in children with ≥2 prior episodes/year compared to children with <2 episodes/year (P<0.001) and inversely correlated with duration since malaria (Rho = −0.39, P<0.001). Notably, frequencies of IFNγ/IL10 co-producing cells were not associated with protection from future malaria after controlling for prior malaria incidence. In contrast, children with <2 prior episodes/year were significantly more likely to exhibit antigen-specific production of TNFα without IL-10 (P = 0.003). While TNFα-producing CD4⁺ T cells were not independently associated with future protection, the absence of cells producing this inflammatory cytokine was associated with the phenotype of asymptomatic infection. Together these data indicate that the functional phenotype of the malaria-specific T cell response is heavily influenced by malaria exposure intensity, with IFNγ/IL10 co-producing CD4⁺ T cells dominating this response among highly exposed children. These CD4⁺ T cells may play important modulatory roles in the development of antimalarial immunity.

Despite reports of decreasing malaria morbidity across many parts of Africa, the incidence of malaria among children continues to be very high in Uganda, even in the setting of insecticide-treated bednets and artemisinin-based combination therapy. Additional control measures, including a vaccine, are sorely needed in these settings, but progress has been limited by our lack of understanding of immunologic correlates of exposure and protection. T cell responses to malaria are thought to be important for protection in experimental models, but their role in protecting against naturally acquired infection is not clear. In this study, we performed detailed assessments of the malaria-specific T cell response among 4-year-old children living in Tororo, Uganda, an area of high malaria transmission. We found that recent malaria infection induces a malaria-specific immune response dominated by Th1 T cells co-producing IFNγ and IL-10, and that these cells are not associated with protection from future infection. IFNγ/IL-10 co-producing cells have been described in several parasitic infections and are hypothesized to be important in limiting CD4-mediated pathology, but they may also prevent the development of sterilizing immunity. These observations have important implications for understanding the pathophysiology of malaria in humans and for malaria vaccine development.

Malaria and travellers visiting friends and relatives

Among all travel-acquired illnesses, malaria carries the greatest burden not only considering the number of imported cases but also the potential of a fatal outcome. The increased number of imported malaria cases in developed countries in the last decades has been attributed to the increasing number of travel to tropical destinations in combination with the enormous influx of immigrants. At present, immigrants visiting friends and relatives (VFRs) constitute the most significant group of travellers for malaria importation in developed countries, with sub-Saharan Africa destinations carrying the highest risk. VFRs typically demonstrate travel and behavioural patterns which render them at high risk for acquisition of this largely preventable infection. Pre-travel services are rarely sought by VFRs, whereas misconceptions that they possess life-long immunity against malaria make them less likely to receive or adhere to antimalarial chemoprophylaxis recommendations. There is an urgent need to increase awareness about malaria of this group of travellers.

Imported malaria in immigrants to Italy: a changing pattern observed in north eastern Italy

BACKGROUND

Seventy percent of imported malaria cases in Italy occur in immigrants, generally with milder clinical presentation due to premunition acquired through repeated infections. Nevertheless, premunition could be progressively lost after a long period of nonexposure. We investigated the changing pattern of malaria in immigrants in two definite 5-year periods one decade apart.

METHODS

We retrospectively examined the main laboratory findings of all malaria cases observed in immigrants from 1990 to 1994 and from 2000 to 2004. We stratified patients by reason for traveling: subjects in Italy who traveled to visit friends and relatives (VFR) or new immigrants (NI).

RESULTS

Forty-eight cases of malaria in immigrants occurred from 1990 to 1994, while 161 were observed from 2000 to 2004. Patients admitted in the latter period had a significantly higher parasitemia (median 6,298 vs 3,360 trophozoites/microL, p= 0.028) and lower platelet count (median 96.5 vs 132 x 10(9)/L, p= 0.012) and hemoglobin (median 12.6 vs 13.4 g/dL, p= 0.049). While NI did not show any significant difference in the two study periods, in the VFR subgroup a higher parasitemia (median 8,845 vs 2,690 trophozoites/microL, p= 0.003) and lower platelet count (median 96 vs 131 x 10(9)/L, p= 0.034) were observed during the second period, during which three cases of severe malaria occurred in VFR. A longer stay in Italy was reported in VFR admitted during the second study period (median 8.3 vs 5.7 years).

CONCLUSIONS

We found a changing pattern of malaria presentation in immigrants over a decade. The most likely explanation is the longer average stay outside endemic countries and subsequent loss of premunition observed in the second cohort. Immigrants living in Italy for some time and traveling to VFR should no more be considered a low-risk group for severe malaria. Pretravel advice should be particularly targeted to this group.

Imported malaria in adults and children: epidemiological and clinical characteristics of 380 consecutive cases observed in Verona, Italy

BACKGROUND

Since the year 2000, in Italy, there has been a constant decrease in the number of cases of imported malaria in immigrants. Nevertheless, immigrants still account for about 70% of reported cases. To our knowledge, no data are yet available on imported malaria in children. This paper describes the main characteristics of malaria cases observed in recent years in the three main hospitals in Verona (roughly representing 10% of all cases reported in Italy in the period), with a special focus on the poorly known problem of imported malaria in children.

METHOD

All malaria cases occurring from 2000 to 2004 were retrospectively examined. Semi-immune and nonimmune patients were analyzed for clinical, laboratory, and parasitological findings. A separate analysis was carried out for children who traveled to endemic areas to visit relatives and friends (VRF) and children born in endemic countries who came to Italy for immigration purposes.

RESULTS

A total of 380 cases of imported malaria occurred in Verona in the 5-year period, 43 being children. Semi-immune patients had a significantly lower parasitemia (p = 0.0032) and parasite clearance time and significantly shorter fever duration than nonimmune (p = 0.025 and p = 0.0026). VRF children presented significantly higher parasitemia and significantly lower platelet count (p = 0.016 and p = 0.042) than recent immigrants. Parasitemia clearance time and fever duration were longer in VRF children than in recent immigrants (p = 0.014 and p = 0.0085). We observed 23 cases of severe malaria, including 4 cases in immigrants.

CONCLUSIONS

Our data confirm a significant difference both in clinical and in parasitological findings between semi-immune and nonimmune patients. We identified two populations of immigrant children: semi-immune (recent immigrants) and nonimmune (VRF). The latter is a high-risk group for severe malaria. Educational actions should be specially designed for immigrants traveling to VRF, focusing on the risk of severe malaria for both adults and children.

Imported malaria in children: a review of clinical studies

Imported malaria is a preventable disease, yet it is responsible for several thousand cases and a substantial number of deaths every year. There has been a pronounced rise in the incidence of imported malaria in most developed countries over the past three decades and, more concerning, Plasmodium falciparum, which is responsible for almost all cases of severe malaria, is now the most prevalent species. Children account for around 15-20% of all imported malaria cases and must be considered separately from adults because they have different risk factors for developing malaria and a higher risk of developing severe disease since they are more likely to be non-immune to malaria. We did a thorough review of the literature since 1980 to identify and critically assess clinical case series on children with imported malaria with respect to travel destination, reason for travel, the use of antimalarial prophylaxis, clinical presentation, delay in diagnosis, laboratory features, complications, management, and outcome. Children living in non-endemic countries and travelling during school holidays to visit family and relatives in their parents' country of origin currently account for the largest proportion of cases in many European countries. This group of travellers deserves special attention because they often do not take antimalarial prophylaxis or other preventive measures. There is a need for standardised recommendations on management and prevention of imported malaria in children, which should be supported by large multicentre clinical trials. A prospective national surveillance study on imported malaria in children was launched in the UK and Ireland through the British Paediatric Surveillance Unit in 2006, which may provide answers to some of the questions raised in this Review.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

Clinicoepidemiological study of imported malaria in travelers and immigrants to Madrid

BACKGROUND

The number of Spanish travelers visiting malaria endemic areas, and the number of immigrants from malarial countries arriving in Spain are continuously increasing. However, little information about imported cases in Spain is available.

METHODS

A prospective clinicoepidemiological study of imported cases of malaria diagnosed at a referral teaching hospital in Madrid, Spain.

RESULTS

Of the 160 patients, sixty (37.5%) were immigrants and 100 (62.5%) Spanish nationals. Malaria was acquired in Africa by 98.3% of immigrants and in 83.0% of travelers. Falciparum malaria accounted for 71.8% of the cases, P. vivax for 11.9%, P. ovale for 10.6% and P. malarie for 5. 0%. Eleven (6.9%) patients, all immigrants, were asymptomatic. Severe complications were recorded in 17 (10.6%): 7, severe anemia; 3, cerebral malaria; 2, renal failure; 1, spontaneous splenic rupture; 1, acute pulmonary edema; 1, sepsis; 1, acute cerebrovascular accident; and 1, disseminated intravascular coagulation. There were no fatal cases. Among the 100 Spanish nationals, 44% did not follow any prophylaxis, 29% followed a correct prophylaxis, 27% were considered defaulters, and 39% took self-treatment without cure.

CONCLUSIONS

There is a changing pattern of imported malaria in Madrid, with one third occurring in immigrants and two thirds in nationals. This data provides information about the reemergence of imported malaria to Europe.

The Changing Pattern of Imported Malaria in the Academic Medical Centre, Amsterdam

Background: Since 1988, the incidence of malaria imported into the Netherlands has been stable, but the population groups have remarkably changed. Methods: The records of all patients with malaria in the Academic Medical Centre, Amsterdam, between October 1991 and December 1994 were analyzed. Results: Of the 286 patients, 149 (52%) were Dutch citizens and 114 (40%) were originally from malaria endemic areas (92 immigrants, 22 asylum-seekers), whereas between 1979 and 1988 these figures were 85 and 15%. The remaining 23 (8%) patients were 11 children born in the Netherlands to immigrants, 10 foreigners from nonmalarious areas, and 2 for whom the origin is unknown. Plasmodium falciparum was found in 197 (69%) patients, mostly acquired in subSaharan Africa; P. vivax (61 patients, 21%) was mainly acquired in Asia. Two vivax infections proved to be chloroquine-resistant. The compliance with the malaria chemoprophylaxis was poor: only 38% (30/80) of the Dutch citizens and 8% (4/52) of the settled immigrants and children were fully compliant. Severe complicated falciparum malaria developed in 18 (10%) patients, two of whom died. The majority of the falciparum cases were treated with halofantrine or sulfadoxine-pyrimethamine. Artemisinin was used in two. Conclusions: Among the patients with imported malaria, settled immigrants and their (nonimmune) children constitute a growing number. Compliance with chemoprophylaxis is decreasing in Dutch travelers and remains poor in the immigrants. Quinine was increasingly used both as initial treatment for severe falciparum malaria as well as in patients with nonsevere malaria who were nauseated or vomiting.