Human landing catch (HLC) practices have been used for a long time in estimating the human malaria vectors abundance. It has been a most reliable method in estimating abundance because it targets the host-seeking behaviour of mosquitoes 12. The entomological inoculation rate and vectorial capacity are important indices of malaria transmission. A main component of both entomological inoculation rate and vectorial capacity is the human biting rate (i.e. the number of female anopheline mosquitoes biting a person per night). The main negative impact underlying this traditional method of sampling mosquitoes is the variation of attractiveness between individuals 34, as well as ethical and practical reasons for minimizing the use of this method 5. Due to these negative impacts of the HLC, an alternative sampling method has to be found and used in its place. The method should be considered to be similarly or more sensitive than HLC, preferably it should be useable in different conditions than HLC, and it should be acceptable in protecting human participation in disease vector sampling. Apart from HLC, other sampling methods were evaluated in this study, including odour-baited entry trap (OBET), indoor resting collection (IRC) and pit shelter (PS). The odour-baited trap was formerly used to evaluate the mosquito responsiveness and preference to host odours 67. Indoor resting collection method has been used before for the estimation of the indoor resting density, as described in WHO manual 1. Pit shelters have been used for the estimation of the outdoor resting mosquito density 1. All these sampling methods where done individually in different sites, they have shown to be efficient in sampling the malaria vectors.

Therefore, it was the objective of this study to compare the performance of human landing catch against odour-baited entry trap, Indoor resting collection and pit shelters in sampling (trapping) malaria vectors, within the same environmental setting in lower Moshi rice irrigation schemes in northern Tanzania.

A total of 3,074 mosquitoes was collected. Among these 1,780 (57.9%) were Anopheles arabiensis and 1294 (42.1%) were Culex quinquefasciatus. Each method sampled different number of mosquitoes, indoor resting collection collected 335 (10.9%), OBET-cow 1,404 (45.7%), OBET-man 378 (12.3%), Pit shelter 562 (18.3%) and HLC 395 (12.8%). The density distribution results are shown in Figure 1. The abdominal status of mosquitoes collected is shown in Figure 2.

In the evaluation of these sampling methods, a total of 150 (30 days) experiments were done. In these experiments, OBET trap with human or cow were treated as two different traps due to different hosts occupied traps. Therefore the total number of sampling methods was five. General linear model univariate analysis method was used, position of the sampling method had no effect on mosquito density catch (DF = 4, F = 35.596, P = 0.78). Days variation had no effect on the collected density too (DF = 29, F = 4.789, P = 0.09). The sampling methods had significant impact on the caught mosquito densities (DF = 4, F = 34.636, P < 0.0001). Further analysis was done to compare between the individual sampling method against HLC. The Wilcoxon pair wise comparison between mosquitoes collected in HLC and Pit shelters was significant (Z = -3.849, P < 0.0001), HLC versus indoor resting collection was not significant (Z = -0.502, P = 0.615), HLC versus OBET-human was significant (Z = -2.687, P = 0.007), HLC versus OBET-cow was significant (Z = -3.127, P = 0.002).

Species identification was not done as the previous studies in the area showed that more than 95% of the Anopheles gambiae sensu lato were An. arabiensis 12.

The results of this study have demonstrated the usefulness of using different methods for sampling disease vectors in surveillance and control programmes in disease endemic areas for the purpose of developing better option than human landing catch. The field evaluation of HLC, PS, OBET-human, OBET-cow and IRC in the same ecological setting enabled the efficient comparison of these sampling methods in mosquito collection. In this study, only IRC had no statistical significant results in sampling mosquitoes against HLC (Z = -0.502, P = 0.615), The main advantage of PS is that, it samples the outdoor-resting mosquitoes, while IRC samples the indoor resting mosquitoes but in both mosquitoes have different abdominal status 1. OBET-human, OBET-cow and HLC had advantages much as they were sampling host-seeking mosquitoes 71012.

OBET-cow caught the highest number of anopheline followed by the PS, the least was IRC preceded by OBET-human and HLC (Figure 1 and 2). These results are supported by the previous OBET trap studies 713, and reports contrary to what found in Madagascar by Laganier et al 14 using the OBET trap. The high proportion of mosquitoes caught in PS method were gravid, semi-gravid and fed, which reflects the number of mosquitoes resting while seeking for oviposition sites and blood meal digestion 1. In this context, HLC seems to be weaker and more risk to the collector than other sampling methods where volunteer is protected from host-seeking mosquitoes. This is supported by the other study compared HLC and odour-baited resting boxes in the same study areas 9. In this study, the higher number of mosquitoes collected in OBET-cow, PS and with IRC in cowshed might have been a result of the exophilic and zoophilic behaviour of the vectors in this area 78911.

In this study factors, such as position of sampling methods, days and time, did not appear to influence mosquito collection. Only the sampling method used seemed to have influenced the number of vectors collected, but different sampling methods have shown varying ability in collecting mosquitoes of different abdominal condition, which can be more informative in malaria epidemiology than only have unfed host-seeking vectors in HLC.

In vector population currently, there is a high level of insecticide resistance against malaria vectors 15. This might influence the high transmission rate despite of the coverage of the ITNs. These findings are merely supported by other findings reported by Rubio-Palis and Curtis 5.

There is a need to advocate other methods for sampling mosquitoes other than HLC in areas with An. arabiensis for surveillance and control purposes, as other potential sampling methods are available 1913. In OBET-human and OBET-cow, both visual and physical stimuli are present. Host-seeking mosquitoes are first attracted by the olfactory stimulus and then will move toward the host via additional host stimuli such as visual cues, temperature, and humidity 31617. Thus, the mosquito in response to such stimuli can target the appropriate site for taking its blood meal before coming in contact with human or cow.

Despite the differences observed in the mean numbers of vectors collected in this study, the age population structure of the malaria vectors were overall similar in HLC and OBET with cow or human, while it was similar for PS and IRC. These odour-baited based methods have provided qualitatively comparable abilities in collecting host-seeking malaria vectors populations as found previously for HLC and light traps 1217181920.

Due to the increase of malaria drug resistance distribution 21 and more needs of measuring the disease exposure OBET with human or cow, and PS offers a reasonable alternative to HLC, because they do not expose the collector to malaria infected mosquitoes and because collection can be done overnight without interruptions. Based on the findings of this study, OBET with human or cow could be used to explain the malaria transmission estimation and exposure better than HLC 1213.

The use of host-seeking sampling techniques should be considered for further evaluation in different ecological settings. This will be more appropriate in planning for the campaigning for ceasing human landing catch practices. The ecological characteristics should be used to deploy the appropriate sampling technique for existing vectors.

The authors declare that they have no competing interests.

EJK and AMM contributed equally to this study, in conceiving, its design, collecting and analysing data, and in writing this manuscript. Both agreed with the final version before submission.