Can An. Mosquitoes Co-transmit both Malaria & LF?

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This article is a review on the thesis, “Transmission Of Wuchereria bancrofti and/or Plasmodium falciparum By Anopheles (Diptera:  Culicidae) Mosquitoes In Western Region, Ghana.” The study was situated in the Ahanta West District of the Western Region, specifically in Butre and Achowa – both coastal villages. 

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Definition of some key terms

LF:  This is the abbreviated form of ‘lymphatic filariasis’, a disease characterized by swollen feet (elephantiasis). The disease is popularly known as ‘elephantiasis’, although that is not the accurate name for it. This is because the condition, elephantiasis is only a symptom of LF.

Wuchereria bancrofti:  this is the causative organism for LF.

Anopheles mosquito: This is a particular grouping of mosquitoes commonly involved in the transmission of a number of insect-borne diseases.

Endophagic: Indoor biting behaviour of mosquito.

Exophagic: Outdoor biting behaviour of mosquito.

Researcher: D.I.

Introduction

In 2006 and 2008, respectively, an estimated 8.3 million and 3.2 million malaria cases were reported for Ghana (WHO, 2009).

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Prevalence of LF is between 9.2 – 25.4% along the coast (Dunyo et al., 1996) and 20 – 40% in the northern regions (Gyapong et al., 1996).

LF prevalence in Achowa was estimated to be 30 – 50 %, whilst that in Butre to be 10 – 30 %; an estimated malaria prevalence of 10 – 30 % for Achowa and > 0 – 10 % for Butre (Kelly-Hope et al., 2006).

Work done by Dunyo et al. (1996) along the coast of Ghana revealed the presence of Anopheles gambiae s.s., which Gyapong et al. (2005) reported to be involved in the phenomenon of limitation, despite deployment of mass drug administration in the area.

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The vectors that play major roles in malaria and LF transmission in Ghana are: Anopheles gambiae Giles, and An. funestus Giles (Appawu et al., 1994 and Dzodzomenyo et al. 1999).

Work done by Muturi et al. (2006b) along the Kenyan coast revealed that Wuchereria-infected An. gambiae s.l. had significantly higher Plasmodium falciparum sporozoite rates than uninfected mosquitoes suggesting that filarial parasites may enhance malaria transmission.

There is therefore the need to study if concomitant infections of the two diseases in Anopheles vectors is transmittable to the human hosts living in those areas so that appropriate integrated vector control strategies that target both diseases simultaneously may be designed and implemented.

 Objective

To determine if concomitant infections of the two diseases, malaria and lymphatic filariasis (LF) in Anopheles vectors is transmittable to the human hosts living in those areas.

Key Findings

Morphological identification showed that anophelines were the predominant mosquito species in Butre, whereas in Achowa culicines predominated (Fig 2).

PCR revealed that in both study sites, Anopheles melas was the most dominant followed by An. gambiae s.s. This may largely be due to the abundance of saltwater in both study sites, as both sites are coastal villages. Anopheles melas is not known to transmit P. falciparum in Ghana.

Occurrence of both W. bancrofti and P. falciparum infections in Anopheles mosquitoes was found to be nil. Wuchereria bancrofti prevalence rate in Anopheles mosquitoes was 0.12 %. Only Anopheles gambiae s.s.  from Butre harboured Wuchereria bancrofti larva, indicating its ability to pick  parasites even at low densities.

There was no significant correlation between the nocturnal biting cycles of Anopheles mosquitoes in Achowa and of those in Butre (r = 0.31; p > 0.05) (Fig. 3).

In both study sites, indoor biting anophelines were more associated with the part of the community closest to the coast, whilst outdoor biting anophelines were more associated with that part of the community which was away from the coast. The part of the community closest to the coast was more windy by virtue of its proximity to the sea, accounting for this trend.

78.5% of the dissected anophelines were not older than 6 days (Fig 4). Thus the female Anopheles mosquito populations collected were not old enough to carry the individual infections, much less both infections.

Abstract

Africa accounts for about 33 and 90 % of the world’s burden of lymphatic filariasis (LF) and malaria respectively. This study set out to investigate if co-infections of Wuchereria bancrofti and Plasmodium falciparum, the causative agents of LF and malaria, in Anopheles mosquitoes was transmittable to the human populations living in areas co-endemic for the two diseases. The study was conducted in Achowa and Butre, both in the Ahanta West District of Western Region of Ghana using human landing and pyrethrum spray catches to collect adult mosquitoes.

Using morphological identifications, and Polymerase Chain Reactions (PCR), it was found that Anopheles gambiae s.l. was the most dominant mosquito species in Ahanta West District, with a frequency of 68.63 %. There was no occurrence of concomitant infections of Wuchereria bancrofti and Plasmodium falciparum in the Anopheles vectors, probably because female Anopheles mosquito populations collected were not old enough to carry the individual infections, much less both infections. 

Only Anopheles gambiae s.s. harboured Wuchereria bancrofti microfilaria, an indication of its ability to pick the parasites even at low densities. Anopheles mosquitoes at the study sites were found to be more endophagic than exophagic, and their peak biting times were observed to be towards and after midnight. Wuchereria bancrofti infection rate in the Anopheles mosquitoes was found to be 0.12 %. No clear-cut relationship could be established between malaria and filariasis transmission indices. Eighty-nine per cent (89.7 %) of the Anopheles mosquitoes collected were parous, and 78.5 % of them were not older than 6 days. 

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Some References

Appawu, M. A.;Baffoe-Wilmot, A.; Afari E. A.; Nkrumah, F. K. and Petrarca, V. (1994). Speciescomposition and inversion polymorphism of the Anopheles gambiae complexin some sites of Ghana, West Africa. Acta Tropica, 56: 15 – 23.

Appawu, M. A.;Dadzie, S. K.; Baffoe-Wilmot, A. and Wilson, M. D. (2001). Lymphatic filariasisin Ghana: entomological investigation of transmission dynamics and intensity incommunities served by irrigation systems in the Upper East Region of Ghana. TropicalMedicine and International Health, 6: 511 – 516.

Dunyo, S. K.; Appawu, M. A.; Nkrumah, F. K.;Baffoe-Wilmot, A.; Pedersen, E. M. and Simonsen, P. E. (1996). Lymphaticfilariasis along the coast of Ghana. Transactions of the Royal Society ofTropical Medicine and Hygiene, 90: 634 – 638.

Dzodzomenyo, M.;Dunyo, S. K.; Ahorlu, C. K.; Coker, W. Z.; Appawu, M. A.; Pedersen, E. M. andSimonsen, P. E. (1999). Bancroftian filariasis in an irrigation project communityin southern Ghana. Tropical Medicine and International Health, 4:13 – 18.

Gyapong,J. O.; Adjei, S. and Sackey, S. O. (1996). Descriptive epidemiology oflymphatic filariasis in Ghana. Transactions of the Royal Society of TropicalMedicine and Hygiene, 9: 26 – 30.