Evolution of Plasmodial Infection According to Age and Vector Control Method Implemented: Deltamethrin Treated Durable Lining (δDL) in Candiero or Lambdacyhalothrin Indoor Residual Spraying (λIRS) in Chisséquélé, 2 Villages around Balombo Town, Benguela Province, Angola

Several hundred millions of malaria cases were averted these last decades thanks to the large scale implementation of vector control, mainly Long Lasting Insecticide Treated Nets (“LLINs”) and Indoor Residual Spraying (“IRS”).

Due to several operational issues IRS were stopped in Angola and the National Malaria Control Program (NMCP) is based upon large scale distribution of Long Lasting Insecticide Treated Nets (LLIN) through different channels.

On the other hand the recently developped Insecticide Treated Plastic Sheeting (ITPS) could represent an interesting alternative to IRS in term of longer lasting activity, community acceptability and participation, entomological and epidemiological efficacy observed in various experimental and natural conditions. Actually the acceptability of ITPS was studied in Huambo, a town close to Balombo but not yet its epidemiological efficacy in this area.

The aim of the present study was to compare the efficacy of the 2 methods of vector control: Indoor Residual Spraying and Insecticide Treated Plastic Sheeting with 2 parasitological indicators of plasmodial infection: parasite index (“PI”) and parasite load (“PL”) in 2 villages close to Balombo: Candiero, which received 2 rounds of λIRS then δITPS; and Chisséquélé , which received δITPS only and just once; considering 4 age groups recommended in Essential Malariology: 0-2 years old; 3-5 years old; 6-9 years old and 10-15 years old.

During the 5 five consecutive years of the study, 46 cross sectional parasitological surveys were regularly done (23 in each village), 4792 thick blood films (“TBF”) were prepared and their microscopical examination revealed the presence of plasmodial infections in 925 i.e. an overall Plasmodial Index of 19.3%.

Plasmodium were noticed in 475 of the 2513 TBF (PI=18.9%) made in Candiero and 450 of the 2279 TBF done in Chisséquélé (PI=19.7%) (χ2= 0.55: P= 0.46; OR= 0.95 [0.82-1.09]).

Before vector control (VC) Parasite Index (“PI”) of babies (≤ 2 years old) were significantly lower than PI of the three other older age groups. The impact of VC on plasmodial index was different according to age groups. After VC the overall parasite index significantly dropped by #70% in Candiero (IRS village) and # 60% in Chisséquélé (ITPS village), the decrease was significant in each age group but it was less in babies than in the 3 older groups. This could lead to a “negative” conclusion of the efficacy of VC if targeting only plasmodial prevalence in babies.

On the other hand Parasite Load (“PL”) significantly dropped the first year after VC in babies in the 2 villages and not in all older age group leading to a positive conclusion of the efficacy of VC when targeting babies; the drop of PL after VC was noticed in some age groups (but not all) in Candiero but not in Chisséquélé.

These analyzes underlined the key issues of the choice of indicator and the targeted age groups because conclusion of the efficacy of one or the other method of vector control could be opposite. Another key point is that, considering the classical age groups (0-5 or 2-9 or ≤15 years old) Insecticide treated plastic sheeting appeared as efficient as Indoor Residual Spraying with the double advantage of lasting longer and involving actual community participation.

ITPS could therefore be recommended in place of IRS which have well known great operational and social issues.