Malaria in infants is classified according to the time of infection. Congenital malaria, defined as asexual parasites detected in the cord blood or in the peripheral blood during the first week of life , is due to transmission from the mother through the placenta just before or during delivery , while neonatal malaria, which can occur within the first 28 days of life, is due to an infective mosquito bite after birth . Differentiating between congenital and acquired neonatal malaria can be difficult, especially in areas of intense malaria transmission . Various studies across Africa have demonstrated that 7–10% of newborns may have malaria parasites in their cord blood , in some cases either without evidence of an active maternal infection or with parasite genotypes different to those found in the mother. This suggests transplacental passage of parasites followed by clearance from maternal and placental blood, with persistence in the foetus [8, 23, 24].
Recent reports suggest that malaria in infants under six months of age may not be uncommon, although data on prevalence and outcome are still contradictory . Prevalence of infection can vary between 0% and 27% [9, 25–33], while the percentage of deaths attributed to malaria (as determined by verbal autopsy) may be between 20.1% and 46.2% . The occurrence of infection in utero is also reflected by the prevalence of splenomegaly at one month of age, which can be as high as 80%, indicating an early development of a splenic response to the infection [5, 35].
In Mozambique, clinical malaria incidence in infants aged 1– < 6 months was substantial (320/1,000 child-years at risk in 2003–2004 and 146/1,000 child-years at risk in 2004–2005) . Reports of congenital malaria have predominantly come from Nigeria; the prevalence of parasitaemia in infants in these studies as well as in those from other malaria-endemic regions varied widely – between 0.7% and 46.7% [20, 33, 37–51]. Parasite densities in neonatal blood were also varied, although most infections were of low density [20, 40, 42, 48–50, 52, 53]. However, differences in transmission dynamics, small sample sizes and lack of details on quality control and sample selection procedures make the interpretation of these findings difficult [20, 25].
Malaria infections (mean geometric parasite density = 533 parasites/μl of blood, as detected by PCR) were common in Ghanaian infants less than 6 months of age (13.6% in newborns; 1.5–9.7% in those aged from 2–26 weeks), although clinical malaria symptoms were rare or uncommon (fever, vomiting, diarrhoea and coughing occurred in 1.8%, 1.8%, 3.0% and 4.8% of infants aged 0–3 months, respectively, and in 4.5%, 3.0%, 8.0% and 10.6% of infants aged 3–6 months) . In Mozambique, although malaria in infants aged under six months represented less than 20% of the total outpatient visits, infants with malaria were admitted in a significantly higher proportion than children aged 1–4 years . The impact of malaria in this age group is also illustrated by a greater difference in mean haematocrit between malaria and non-malaria cases in infants aged from 2–12 months (4.9%) than in children aged 5 years or above (1.8%). Among infants aged under six months attending an urban hospital in Nigeria, 27.1% had a positive blood slide and a significantly lower haematocrit (33.0%) compared with those who were uninfected (35.1%)(p = 0.003). Indeed, a haematocrit of less than 33% was the most common clinical finding among infected infants, while the occurrence of fever was 4% .
A systematic review of age-patterns of malaria revealed that as transmission increased, there was a shift of clinical malaria towards younger age groups, regardless of seasonality . As transmission intensity increased and seasonality decreased, severe cases became more frequent in the younger ages. Nevertheless, wherever a comparison between age groups was possible, malaria mortality was higher in younger children, with the peak age shifting towards infants as transmission became more intense .