African Journal of Biotechnology
Academic Journals
ISSN: 1684-5315
Vol. 2, Num. 11, 2003, pp. 470-473
African Journal of Biotechnology Vol.
2, No. 11,
November 2003, pp. 470-473
Relative prevalence of the
human hookworm species, Necator americanus and Ancylostoma duodenale in
an urban community in Ogun State, Nigeria
AA Adenusi1* and EOA Ogunyomi2†
1Department of Biological Sciences, Olabisi Onabanjo
University, P.M.B. 2002, Ago-Iwoye, Ogun State, Nigeria. 2Primary Health Care Department, Sagamu Local
Government Secretariat, Ogun State. Nigeria.
*Corresponding author. E-mail: aaadenusi@yahoo.com. †Recent address: Primary Health Care Centre,
Ilishan, Ogun State. Nigeria.
Accepted 17 October 2003
Code Number: jb03093
ABSTRACT
In order to estimate the proportion
of hookworm infections represented by Necator americanus and Ancylostoma
duodenale, the 2 major species of human hookworms in Nigeria, stool samples
from 1253 hookworm-positive schoolchildren were cultured to the third-stage
(L3), filariform larvae, using the Harada-Mori test-tube method. N.
americanus larvae were recovered from a total of 1177 (93.9%) coprocultures
while A.duodenale larvae were recovered from a total of 274
(21.9%) stool cultures. 58.2% of the hookworm infections were due solely to N.
americanus, 6.1% solely to A. duodenale and 25.8% were
mixed infections with both species. In all mixed infections, much higher number
of N. americanus larvae were recorded compared with those of A. duodenale.
Key words: Hookworm species, Necator
americanus, Ancylostomaduodenale, filariform larvae.
INTRODUCTION
Despite considerable advances in chemotherapy and control,
hookworms rank amongst the most widespread of soil-transmitted intestinal helminth
parasites and affect a significant proportion of the world population (approximately
900 million people) mainly in the tropics and sub-tropics (Bundy et al., 1991).
Adult hookworms attach to the mucosa of the small intestine, feed on blood
and are an important cause of anaemia in school-aged and adult populations
in the tropics (Olsen et al., 1998).
Necator americanus and Ancylostomaduodenale,
the 2 major species of human hookworms are sympatric over much of their distribution
and people are often simultaneously infected with both species in endemic areas.
Despite significant differences in their life histories, the 2 species have
traditionally been considered to be identical for treatment and control strategies
(Hoagland and Schad, 1978). Failure to consider these
differences is probably responsible for reports of failed drug treatments and
rapid reinfection rates following anthelmintic treatment. Report of different
anthelmintic susceptibilities between the species (Rossignol, 1990)
indicates that administration of only 1 anthelmintic may not be sufficient
to kill both species. However, the rational design of hookworm control strategies
requires, among other things, knowledge of the species infecting a particular
human population in order for treatment to be successful in the long term.
Most studies do not attempt to
speciate hookworm infections and rely on past epidemiological data, which indicate
the predominance of one species over the other (Brooker et al., 1999),
because the eggs of the 2 species are similar and not readily distinguishable
from one another by classical parasitological methods (Hawdon, 1996).
Although, there are established morphological differences between the adult
worms (Yoshida et al., 1974a,b;
Pawloski et al., 1991), the adult stages are rarely
available for routine parasitological examination. Thus, species identification
has traditionally been done by using subtle morphological characteristics to
differentiate the infective, third-stage filariform (L3) larvae,
reared from eggs in coprocultures (WHO, 1981; Pawloski
et al., 1991).
Although, previous reports from
some parts of Nigeria have shown that N. americanus is the ubiquitous
and dominant hookworm species (Fisk, 1939; Cowper and Woodward, 1961;
Oyerinde, 1978; Adenusi, 1997)
and that infections with A. duodenale represent only a small proportion
of the local hookworm infections, over the years however, the epidemiological
situation may not be the same as previously reported. Moreover, the relative
distribution of the 2 species may vary from one endemic locality to the other.
Thus, similar studies need to be carried out in other parts of the country.
We report on a study to estimate the proportion of hookworm
infections represented by N.americanus and A. duodenale, in
schoolchildren in Sagamu, an urban community and the headquarters of Sagamu
Local Government Area of Ogun State, Nigeria.
METHODS
The study was conducted on 1253
schoolchildren (655 males and 598 females), aged between 5 and 13 years
(mean age 8 years), positive for hookworm and who were participants in a large
study (to be described in detail elsewhere) on the epidemiology of intestinal
helminth infections in Sagamu, the headquarters of Sagamu Local Government
Area of Ogun State, Nigeria. Schoolchildren were chosen because they are easily
accessible and also constitute the section of the population at greatest risk
of parasitic infection.
Each subject was given a clean,
plastic container and asked to bring freshly passed stool sample the following
day. Instructions on how to avoid contamination were also given to each subject.
A brief interview of each subject regarding the time of stool collection was
done. Only stool samples collected within 2 hours were recruited in this study.
Hookworm eggs in about 4 g freshly
passed and unfixed stool samples were cultured to the infective, third stage
(L3), filariform larvae at ambient temperature (26 to 28o C),
by the Harada-Mori test-tube method (Hsieh, 1961). The
cultured larvae were concentrated by centrifugation and stained with Lugol’s
iodine. The first 150 larvae (if more than 150) were examined under the
microscope and identified according to the criteria of WHO (1981).
RESULTS
Table 1 gives the relative prevalences
of the 2 hookworm species, N. americanus and A. duodenale in
the study population. Of the 1253 hookworm-positive coprocultures, N.americanus filariform
larvae were recovered from a total of 1177, while A. duodenale filariform
larvae were recovered from 274. 854 of the 1253 (68.2%) hookworm-positive stool
samples had only N.americanus filariform larvae while 76 (6.1%)
had only A. duodenale larvae. 323 (25.8%) of the coprocultures had double
species infection as evidenced by the recovery of filariform larvae of both
species (N.americanus and A. duodenale). However, in
all double (mixed) species infections, higher numbers of N.americanus larvae
were recovered compared to A. duodenale with the former ranging from
60 to 85% and the latter ranging from 15 to 40% in any mixed infection.
Table
1. Prevalence of N. americanus and A. duodenale in the
hookworm positive subjects.
Hookworm species
No. of cases observed
Prevalence (% of total
hookworm-positive study group (n=1253)
N. americanus
1177
93.9
A. duodenale
274
21.9
In the present study, filariform
larvae of N. americanus varied in length from 490 to 620 μm,
with majority measuring between 550 to 580 μm, while A.
duodenale larvae varied in length from 670 to 760 μm
(majority measured between 670 and 710 μm).
DISCUSSION
This
study showed that N. americanus was the dominant hookworm species
in the study population, as it solely accounted for 68.2% of all hookworm
infections. This result is in agreement with previous ones from different
parts of Nigeria (Fisk, 1939; Cowper and Woodward, 1961;
Oyerinde, 1978; Adenusi, 1997)
that N. americanus is the ubiquitous and dominant hookworm species.
In
Lagos, Nigeria, Oyerinde (1978) showed that apparently
all infections with A.duodenale occurred always in association
with N. americanus, with the latter occurring independent of the
former, and concluded that less than 1% of hookworm
infections
from the Lagos population, were due solely to A. duodenale. This report
(Oyerinde, 1978) would seem to suggest that A.
duodenale seldom occurs solely in human hosts, at least in the studied
Lagos population. A recent study in Lagos (Adenusi, 1997)
however, found A. duodenale solely accounting for 4.5% of hookworm
infections.
In
the present study, although A. duodenale was solely responsible for
6.1% of all hookworm infections, it nevertheless occurred concurrently with N.
americanus (mixed hookworm infection) in 25.8% of the subjects. These
were probably more heavily exposed, as to have been infected with A. duodenale,
whose overall prevalence was about a third that of N. americanus,
the commoner of the two in this locality.
Inspite
of the fact that a female A. duodenale lays an average of 30,000 eggs
per day compared to about 9000 by an adult female N. americanus (Piekarski, 1989),
much higher numbers of N. americanus infective larvae were recovered
in coprocultures, compared to A. duodenale in all mixed infections.
This would suggest that much higher number of adult female egg-laying N.
americanus worms were present compared to A. duodenale in mixed
infections, as all the eggs were subjected to the same culture conditions.
Moreover, none of the patients had received any form of anthelmintic therapy
in the 6 months preceding the study, which could have eliminated more A.
duodenale than N. americanus. Evans et al. (1991)
had reported that in mixed hookworm infections, a much higher percentage
cure rate is obtained for A. duodenale than N. americanus by
treatment with anthelmintics.
Epidemiological
assessment of the public health significance of hookworm infections should
not, as has been the case over the years, be focused only on estimation of
the number of hookworm infections, which occur in a given population (prevalence).
Rather, it should also include identification of the infecting hookworm species.
This is vital to the evaluation of hookworm infection as a public health
problem where therapy and control of the disease should be specific and targeted
at the infecting hookworm species. It may be worth mentioning here, that
the two hookworm species differ in susceptibility to the same anthelmintic
and dosage regimen. Thus, efficacy of anthelmintic therapy is dependent on
the infecting species of hookworm (Piekarski, 1989).
Well-known anthelmintics such as Alcopar (Bephenium) and Pyrantel are known
to be comparatively less effective against N. americanus than A.
duodenale (Rajasekariah et al., 1986).
Also
in hookworm infections, the degree of severity varies with the infecting
hookworm species (WHO, 1998). A. duodenale is
the more pathogenic of the two species as about 0.02 ml of blood is lost
per worm per day with Necator compared to about 0.1 ml with Ancylostoma (Piekarski, 1989).
Albonico et al. (1998) have recently shown that infection
with A. duodenale in Zanzibari
schoolchildren has a greater impact on anaemia than infection with N.
americanus. Therefore, where A. duodenale is more
prevalent, the effect on anaemia might be greater. This is of great public
health significance.
It
is concluded that although, N. americanus was the predominant hookworm
species in the present study, A. duodenale also occurred. The results
of the present study are reliably guaranteed by the Harada-Mori culture method,
which is commonly used for the culture of hookworm eggs and the WHO (1981)
identification scheme. Identification of infecting hookworm species based
on the above is tedious and time consuming as it requires culture of eggs
and microscopic examination of numerous larvae. A rapid and simple method
that represents a considerable savings in time over current methods used
for differentiating between the major species of human hookworms is urgently
required.
Further
studies are underway to estimate the total number of worms in each subject
(including ratio of male to female worms, as well as the ratio of adult worms
of the 2 species in mixed infections) following purgation of infected subjects
and identification of passed (dislodged) adult hookworms.
ACKNOWLEDGEMENT
We are grateful to all subjects
in this study for their excellent cooperation.
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