PREVALENCE STUDIES
Prevalence studies in epidemiology are studies conducted in order to examine the prevalence of a particular disease/condition at a certain point in time. It denotes the number of cases of the disease under study existing in the source population at a particular time. It is arrived at by comparing the number of people found to have the condition with the total number of people studied and is usually expressed as:
Prevalence rate = (number of current cases of an illness in a specified time period) / (population at midpoint of the time period) × k .
where K is usually a constant that can take value nearest to whole number.
Prevalence studies are also referred to as cross sectional studies, they frequently take the form of surveys. Cross sectional studies can be used to investigate associations between risk factors and disease. The population at risk/source population is usually the population living in the study area or it is defined by geographical, administrative, demographical, occupational or other parameters such as health services clients. Prevalence rate is reported on a population base e. g 5 cases of a disease per 100 inhabitants (5%).
In prevalence study, disease prevalence is measured at a specific point in time, rather than over a specific risk period. However, the point in time may not necessary be the same date for each study participant. E.g studies of congenital malformations usually involve measuring the prevalence of congenital malformations at birth. Thus the source population may be all babies born in this city during 2017 and the time at which prevalence is measured may be at birth, which will be a different date and time for each member of the source population.
MEASUREMENTS OF PREVALENCE
Prevalence is considered a simple measure of burden disease. In infectious disease, its interpretation would require a more extensive knowledge of the mechanisms by which the infection is transmitted, pre clinical and clinical evolution of the disease, as well as the duration of the infection.
The most commonly used types of prevalence rate are: point and period or lifetime prevalence.
Point Prevalence – Is the total number of individuals with a disease at a specific point in time divided by the population at risk of having the disease at this point in time.
Period Prevalence – Is the total number of individuals with a disease in a given period of time divide by the population at risk of having the disease during the same period of time.
Lifetime Prevalence – Is the total number of person known to have had the disease at least part of their life.
Sampling methods commonly used in prevalence studies includes :
Random sampling –Prevalence studies generally require random sampling of a population. A probability sample substantially increases the chance that the participants will be representative of the target population and so assures the internal validity of the study. This also allows extrapolaration of study results to other communities (external validity).
Systematic sampling – A systematic sampling will use some type of pre-established sequence to select participants, for examples, from files of medical histories, houses on a street, or patients presenting spontaneously in ambulatory clinics.
Stratified sampling –This involves dividing the population into distinct subgroups according to some important characteristics and selecting a random sample of each subgroup. If the proportion of sample drawn from each strata is the same as the proportion of the total population, then all strata will be fairly represented in with regard to the number of person in the sample.
PRIMARY AND SECONDARY SOURCE OF DATA IN PREVALENCE STUDIES
Collecting primary data – Self administered questionnaires and interview are common ways of obtaining information on morbidity, frequency of symptoms and variables of interest in prevalence surveys. In addition, laboratory tests for the detection of biological markers can be used to measure the prevalence of infection/disease and risk factors.
Collecting secondary data – Secondary data are data obtain from information systems of control programmes. Secondary data are useful in prevalence studies because it helps to build up time series pertaining to a particular disease.
MEASURING EXPOSURE IN PREVALENCE STUDIES
In prevalence studies, exposure is measured in all members of the source population. Although a prevalence study involves measuring disease status at one point in time, information can be collected on historical exposures.
For example, a prevalence survey of coronary heart disease (CHD) among people who are physically active( exposed) and the prevalence among people who are not physically active ( unexposed).
Present CHD Absent CHD Total
Active (exposed) 50 a b 700 750
Not active (unexposed) 50 c d 200 250
Total 100 900 1000
Pi =a/(a+b) ×100 = 50/750 ×100 = 6.7% prevalence of CHD among people who are active
Po =c/(c+d ) × 100 = 50/250 × 100 =20% prevalence of CHD among people who are not active.
PREVALENCE ODDS RATIO
The prevalence odd ratio (POR) is calculated in the same manner as the odds ratio.
POR =ad/bc
PREVALENCE RATIO
The prevalence ratio (PR) is the ratio of the proportion of the persons with the disease over the proportion with the exposure. It can be expressed as:
The prevalence ratio can be calculated from the information on CHD and physical activity.
PR = a/(a+b) / c/(c+d) = 50/750 / 50/250 = 0.06/0.2 = 0.3
USES OF PREVALENCE STUDIES IN EPIDEMIOLOGY
Prevalence studies are used to evaluate the proportion of a population with disease or with risk factors for disease such as the prevalence of asthma in children.
It is used to estimate the occurrence of risk factors in segments of the population characterized by age, sex, race or socioeconomic status.
It can be used to investigate disease trends over time.
Data gotten from prevalence studies can be used for health service planning.
PREVALENCE OF MALARIA AMONG CHILDREN 1-10 YEARS OLD IN COMMUNITIES IN EGOR LOCAL GOVERNMENT AREA, EDO STATE NIGERIA
Introduction
Malaria is a major cause of illness and death especially among children under 5 year’s old and pregnant women. It is estimated that more than one million children living in Africa especially in remote areas with poor access to health services die annually from direct and indirect effects of malaria.(Fawale and Onadeko, 2001).Fatally affected children often die within less than 72hours after developing the symptoms. In those children who survive, malaria drains vital nutrients from them impairing their physical and intellectual development. (WHO, 1998).Malaria infections represent substantial social costs due to school absenteeism and reduced economic productivity.
In Nigeria, malaria consistently ranks among the five most common causes of death in children. As a result of increased mortality and morbidity there is need for proper understanding of the epidemiology of the disease among the most at the risk group, hence this study was carried out. The plasmodium species responsible for malaria infections in Nigeria are Plasmodium falciparum, Plasmodium malaria and Plasmodium ovale.
The objective of this study is to find out the following:
To ascertain the prevalence of malaria parasites infections among children 1-10 years in fours randomly selected communities in Egor Local Government Area.
To determine the predominant plasmodium species causing the infection in the study area.
The study was guided by the following research questions:
What is the prevalence of malaria among children of ages 1 – 10 years?
What is the prevalence of malaria among children by sex?
What is type of plasmodium species are identified among various groups?
What is the parasite density in the children (1-10 years) studied?
Methodology
The study is a descriptive survey aimed at finding out and describing the prevalence of malaria among children 1-10 years old in the study area. The study area is Egor local government area in Edo state. The study communities lie within the tropical rainforest occurring in flat-floored valleys and adjoining low lying area. Vegetative cover comprises of tall trees, elephant grass, climber trees etc. This vegetation provides enough breeding of mosquitoes’ both during and after the rainy seasons thereby predisposing them to mosquito bites.
The study was carried out between the months of March and August. In the study, a sample of 1000 children aged 1- 10 years were randomly selected from 20 primary and 31 nursery schools in the four randomly selected communities in Egor local government area. The selected communities are Use, Uwelu, Egor and Iguikpe. Based on the sample size, 60% of the total was allocated to primary school pupil and 40% to nursery school pupil.
Data collection
Two milliters (2ml) venous blood sample was collected from each of the 1000 pupils (600 primary pupils and 400 nursery pupils) and stored in an anticoagulant specimen bottle with the consent from the school authorities and parents. Both thick and thin smears were prepared, stained and examined microscopically under oil immersion objective. During the examination of stained films, if only one or two rings were found, it would be practically impossible to decide which species it was. However, small rings were found in large numbers and no other forms were present, it was almost certainly it is the excluded malignant tertian except in moribund cases, and the identification rested between tertian and quartan malaria. Also 12 hours human bait collection method was used for identification of types of mosquitoes found in the study communities.
Table 1: Prevalence of malaria by age among children 1-10 years old.
Age Number examined Number positive Percentage of positive
1 87 62 71.30
2 108 67 62.04
3 123 94 76.40
4 59 42 71.20
5 36 20 55.60
6 90 42 46.70
7 46 25 54.40
8 77 42 54.50
9 93 47 50.50
10 281 141 50.20
Total 1000 582 58.2%
Table 2: Prevalence of malaria by sex
Source of variation Number examined Number of positive % of positive Number of negative % of negative
Male 500 296 59.2 204 40.80
Female 500 286 57.2 214 42.80
Total 1000 582 58.2 418 41.80
Table 3: Types of plasmodium species involved in malaria parasitaemia among children 1-10 years
Source of variation 1-5yrs N = 400 6-10 yrs N = 600
Species of Plasmodium Number of positive % of positive Number of positive % of positive
P.falciparium 268 67 311 51.8
P.malaria 0 0 3 0.5
P.vivax 0 0 0 0
P.ovale 0 0 0 0
Examined No. 268
400 67.0 314
600 52.3
Data interpretation
After the study, the total prevalence of malaria among the 1000 children examined was 582(58.2%), malaria infection is most prevalent in children 3 yrs old (76.40%), followed by children of one year (71.30%),having the highest level of infection 4 yrs (71.20%),2 yrs old (64.42%),5 yrs old (55.60%),8 yrs old (54.50%),7 yrs old (54.40%),9 yrs (50.50%) and 10 yrs old (50.20%) while children 6 yrs old are less infected (46.70%).This shows that 0-1 year olds had low prevalence for plasmodium infection. This is attributed to the maternal antibodies they derived. The markedly increased level of parasitaemia in children 2-3 years could be attributed to the gradual loss of theses maternally derived antibodies.
Although it has been established that the residual immunity derived from mothers could be very effective in younger children but environmental condition and inability of children of this age in the study area to ward off environmentally induced mosquito attack predisposed them to malaria attack.
The prevalence of malaria is lower among children above 5 years; this could be attributed to the fact that children of this age have developed immunity against plasmodium parasite.
Infection prevalence among the males and females shows that out of five hundred male children examined 296 (59.20%) of males were positive for malaria while out of five hundred female children examined 286 (57.20%) were positive for malaria. It shows that malaria is more prevalent among male children.
On the involvement of different species of plasmodium in malaria parasitaemia of children 1-10 years, it reveals that plasmodium falciparium is the predominant specie found in the blood of children with malaria from 1-10 years old. Out of the 400 blood sample from children 1-5 years examined,268 (67%) were positive for plasmodium faciparium infections. Among 6-10 years old out of 600 children examined,311 or 51.8% were positive for plasmodium faciparium infection while 3 (0.5%) had plasmodium malariae infection. None of the children were positive for plasmodium vivax and plasmodium ovale infections.
Conclusion and recommendations
The study clearly shows that malaria is still posing problems in Egor Local Government Area of Edo State. The prevalence rate is still high among the younger age group (1-5 years).
For these reasons, the following recommendations are made:
Public health education campaign for mother and health care givers to create awareness that may lead to reduction of vectors of malaria infection and control of the disease especially in young children.
Free or subsidized insecticide treated bed net should be made available to mothers so that the infection of malaria could be controlled in children.
Mothers and other caregivers need to be empowered to treat malaria infection at home.
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