Where have all the parasites gone? Unusual Plasmodium falciparum monoparasitaemia in a cross-sectional malariometric survey in northern Nigeria

Introduction

Malaria is caused by one of five currently known Plasmodium species causing diseases in humans. These are P. falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi. Scientists have modelled the anopheles mosquito vector extensively based on its characteristics, but not so much the parasite¹. An absence of the Duffy antigen on red blood cells of West Africans has long been postulated to be responsible for the absence of P. vivax in these areas, but cases of P. falciparum, P. malariae and P. ovale, in order of decreasing occurrence, have been found. The World Malaria Reports of 2014² and 2015³ reported 100% of confirmed cases in Nigeria as being due to P. falciparum. It is generally thought to account for about 98% of all malaria cases, with P. malariae accounting for the rest, often as a co-infection with P. falciparum⁴. This figure likely over-estimates the proportion of cases as P. falciparum is responsible for most severe cases of malaria; these are the cases most commonly reported alongside confirmed cases of malaria, which are tracked by passive surveillance in Nigeria. It may also be because of limited expertise in identifying other species of Plasmodium.

Available data from across Nigeria shows a mixed picture; large areas across northern Nigeria in 1967/68 found an average proportion of 22% of malaria parasitaemia due to P. malariae in a study at a time when most of Nigeria was considered holoendemic for malaria. P. malariae was often seen in coinfection with P. falciparum, particularly among younger age groups. P. ovale was responsible for 5% of malaria infections, being more common in children under five years of age, and P.falciparum ranged between 84.4% and 90.5% across age groups⁵. The proportion of P. malariae was high, probably because the data was obtained from a survey of both asymptomatic and symptomatic participants.

More recently, in 2010, prior to the commencement of nationwide LLIN distribution, a study including 4209 individuals in Jos, northern Nigeria, found a P. malariae rate as low as 1.6%, with P. falciparum responsible for 98.7% of infections, sometimes in co-infection with P. malariae⁴. Children aged less than 10 years and all individuals in every third household were selected for this abridged malaria indicator survey (MIS). Crucially, however, no P. vivax or P. ovale species were seen either in this location or another in south-eastern Nigeria, which was shown to have a higher rate of P. malariae infections (around 30%) and lower rate of P. falciparum infections (68.1%) in a study also conducted in 2010⁶. This study, however, included 2,936 individuals from 1400 clusters in Abia state, spread out across the state. Quality control measures in the identification of parasite species were implemented in the study, including a WHO-certified malaria microscopist, giving credibility to the results obtained⁶. In south-western Nigeria, a study in Ikorodu in 2012 recruited 1,496 participants of all ages, which included 237 children under the age of five years and 509 children aged 15 years and below. Microscopy and DNA evaluation was used to determine parasite species, which found that 93.6% of participants had P. falciparum infection, with the remainder being P. malariae⁷. This is in contrast to previous studies in 1976 around the same location in south-western Nigeria, which found that 13% to 16% of parasitaemia was due to P. malariae, with 62 to 76% being due to P. falciparum⁸. These proportions were the age-group specific parasite prevalence in the study, which included mostly children aged five to 10 years (1,500 participants) in an attempt to compare spleen and parasite rates among individuals with sickle cell trait and those with normal adult haemoglobin. It also found P. ovale in 2% to 3% of participants in 5–10 and 2–4 year age groups, respectively. These findings, although limited in scope, perhaps suggest a changing trend, with the disappearance of P. ovale from Nigeria over time. P. falciparum is an undisputed leader in all the studies performed. Its high proportion perhaps accounts for high rates of malaria-related anaemia in most studies, explained by the ability of the parasite to invade and destroy both young and senescent red blood cells^7,9.

We conducted this study to determine relative proportions of parasites causing clinical malaria in Sokoto, north-western Nigeria.

Methods

Results

Participants included

We screened a total of 1136 participants for inclusion in the study after they consented to participation in the study. We excluded 109 because they had been treated with antimalarials in the two weeks prior to enrolment and excluded 10 from the analysis due to incomplete data. We included 1017 participants in the analysis (Figure 1)¹⁸.

Figure 1. Flow chart for inclusion in analysis for clinical malaria.

The age-sex distribution showed that all ages were equally represented in the study, as shown in Table 1.

Table 1. Age and gender distribution of the subjects included in the study.

Age (completed years)	n	Gender
		Male, n (%)	Female, n (%)
2	123	60 (5.9)	63 (6.2)
3	117	63 (6.2)	54 (5.3)
4	110	54 (5.3)	56 (5.5)
5	107	52 (5.1)	55 (5.4)
6	111	62 (6.1)	49 (4.8)
7	111	73 (7.2)	38 (3.7)
8	110	50 (4.9)	60 (5.9)
9	113	54 (5.3)	59 (5.8)
10	115	57 (5.6)	58 (5.7)
Total	1017	525 (51.6)	492 (48.4)

Parasite species causing malaria

279 participants were found across the seasons to have clinical malaria and all of them had P. falciparum malaria, irrespective of the season and nature of their clinical presentation. The relative proportions of parasites are depicted in Figure 2. The most common presenting feature among these was fever (92%), followed by vomiting (38%), refusal to feed/poor appetite (32%) and body weakness (25%).

Figure 2. Parasite species causing clinical malaria.

Of the clinical cases of malaria, 9.6% had complicated malaria, as indicated by the WHO criteria for severity¹⁹. The number of severe malaria cases was significantly lower in the dry season than the rainy season, as shown in Table 4. Different participants had various combinations of the criteria for severity, although the common criteria were hyperpyrexia, prostration and persistent vomiting.

Table 4. Prevalence of severe malaria across seasons.

Season	N	Severe malaria Freq (%)	Other cases Freq (%)
Rainy season	511	24 (4.7)	487 (95.3)
Dry season	506	2 (0.4)	504 (99.6)
Total	1017	26 (2.6)	991 (97.4)

χ² = 18.883; df = 1; p = 0.000.

Discussion

The prevalence of malaria in this study, when compared with serial MIS’s performed in 2010¹³ and 2015²⁰ shows a progressive reduction; from 48.1% to 37.1% for north-western Nigeria, and a prevalence of 46.6% for Sokoto in 2015 compared with 34.8% for our study. Prevalence in the MIS’s was measured among children aged six to 59 months and is probably higher than for the children included in this study because including the children from 6–10 years is likely to reduce the overall prevalence, as is excluding those aged six months to two years, who generally have a higher prevalence rate²¹.

Although there were studies performed in the past in Sokoto, they are limited in comparison to the present study by virtue of having been conducted in a different age group, or hospital in lieu of community setting and the seasons in which these studies were conducted. The prevalence of 34.8% found here was higher than the 27.9% found by Abdullahi et al.¹² in Sokoto; however, samples in the previous study were collected from patients visiting two hospitals within the metropolis and was thus not community-based. Furthermore, all ages from 0 to 65 years were included in the study, which is likely to further dilute the findings and give a falsely low prevalence because the incidence of malaria is generally lower among adolescents and adults, as indicated in the study. The overall picture supports the suggestion of a reduction in the prevalence of malaria, likely owing to better access to malaria prevention and increasing urbanisation; both of which cause a decline in malaria parasite rates generally²².

The prevalence found in this study is also lower in comparison to the 45.4% prevalence rate found in a study by Jiya et al.²³, conducted in Sokoto between 2007 and 2009. Additionally, it was lower than the prevalence of 49.6% found among children under the age of five years in the same study. Considering both age-specific prevalence rates, there is a reduction in prevalence, although being a hospital-based study, the prevalence for the former study is likely to be higher than the current. It is, however slightly, higher than the projected national average of 29% for 2015, with wide inter-regional differences²⁴. The Nigerian MIS of 2015 found a higher prevalence of 46.6% than this study, although the age of included participants ranged from six to 59 months, which will limit the comparability of results from this study due to the different age ranges of participants²⁰.

The prevalence by age in this study roughly indicated a progressive decline with age. The highest age-specific prevalence was among two-year-olds (50.4%), with a statistically significant difference among the age groups. This finding is in conformity with the steady-state assumption and is similar to findings in previous studies that showed higher prevalence among younger age-groups.

With respect to the parasite species causing uncomplicated malaria, all parasitaemia in this study was found to be due to P. falciparum. This is in keeping with recent studies in Adamawa²⁵ and Cross River states⁹ in 2011 and 2013, respectively. Another study from Ihiala, in Anambra state of south-eastern Nigeria, found P. falciparum mono-parasitaemia even though this study considered all types of malaria, both severe and uncomplicated²⁶. This is, however, unlikely to affect the findings, as most cases of severe malaria in this area are due to P. falciparum²⁶. An earlier report from Sokoto between 2005 and 2006, carried out at Usmanu Danfodiyo University Teaching Hospital by Jiya and Sani^23,27 likewise did not find any Plasmodium species apart from P. falciparum, although they only considered cases of severe malaria, which are unlikely to be due to a different species of Plasmodium within Nigeria. Meanwhile, only three cases out of 582 (0.01%) were positive for P.malariae in another study by Nwaorgu and Orajaka²⁸ in Awka, south-eastern Nigeria. The finding of P. falciparum mono-parasitaemia supports the fact that P. falciparum is the dominant species of Plasmodium in Sub-Saharan Africa, with the tendency to exclude other forms of parasitaemia, as expounded by Lucas and Gilles²⁹ in 1998 with time and sustained malaria control activities.

Other studies have shown the presence of other forms of parasitaemia, notably with P. malariae either as mono-infection or coinfection with P. falciparum. In Abia and Plateau states (2010), P. malariae accounted for 32.0% and 1.4% of malaria infections, respectively⁶. In a study in north-central Nigeria, 6.1% of examined participants had P. malariae infection³⁰ and as high as 41% and 4%, respectively, had P. malariae and P. ovale infections in a historical study in Garki, Abuja (1968)³¹. Outside Nigeria, there has been a shift towards mono-parasitaemia with P. falciparum as well, and this has been documented in the Horn of Africa³² and Benin in West Africa³³. Some authors have suggested it be an evidence of failing control measures but this is at variance with data from this study, which shows a reduction in prevalence from previous data, including a reduction in the number of severe cases of malaria, which were very few in this study.

Severe malaria was seen in 26 of the 1017 participants analysed in this study, with an overall prevalence of 2.6%. It was higher during the rainy than the dry season, probably due to higher prevalence of the disease and higher parasitaemia, as earlier discussed. This is lower than expected from other hospital-based studies for which children presenting to the hospital are more likely to be ill than those found in a community-based survey such as this. In one such study in Ilorin by Olanrewaju and Johnson²⁹ found that a third of all children admitted with malaria had a severe form of malaria.

Data availability