Transmission of malaria has continued on the north coast of Guadalcanal despite the "regular" cyclical spraying of all structures since early 1963.

The most frequently voiced reason for this continuing transmission is the apparent change in the biting habits of Anopheles farauti when under the pressure of DDT residual intra-domiciliary spraying.

In order to elucidate the validity of this reason I have continued the fixed station studies initiated by my predecessors, both W.H.O. and Government employed. However, because of the need to extend the entomological coverage of the Protectorate without a vast increase in staff, I have reduced the fixed stations on Guadalcanal to two, Koli and Namuvalu, one to the east of Honiara and the latter to the west.

Unfortunately, there is a lack of detailed long term pre-spraying data available from the B.S.I.P. The most involved study so far was on Malaita but there I concentrated efforts on a fairly comprehensive vector distribution study during the nine months between my arrival and the commencement of cyclical spraying in July 1970. Some fixed station work was carried out and I hope to make a fairly detailed report in the near future.

The island of San Cristobal, or Makira, represents the last opportunity to make any comprehensive study of an unsprayed area in the Solomons. Starting in October 1970, an almost complete vector survey has been made of the island. With the posting of a senior technician to the island in March 1971 a series of all-night catches has been underway at five fixed stations. Limited climatic records are also being kept.

Results

These are of a preliminary nature only.

Graph 1 shows the man-biting activity of Anopheles koliensis and the indication from these results is that there is a difference between the peaks of biting indoors and outdoors. However, this difference is perhaps not significant.
Graph 2 shows the pre-spraying man-biting activity of' An. farauti and here there is a clearly marked difference between the peak of outdoor biting (in the hours after sunset) and the peak of indoor biting (around midnight).
Graph 3 shows the post-spraying biting activity of An. farauti in late 1967, i.e. some four and a half years after the commencement of cyclical spraying operations. These collections unfortunately stopped at 2230 hours and it is not possible to state whether or not later biting activity was taking place. Very marked is the high numbers taken outdoors as opposed to indoors.
Graph 4 shows the post-spraying biting activity of An. farauti in the last few months. Although relatively few females were collected the peak of activity for both indoor and outdoor biting was in the hour or so after sunset.

The evidence so far presented shows what is a potentially alarming situation. Here is a vector previously mainly found biting late at night and indoors (i.e. feeding on sleeping humans) and thus likely to be affected by DDT residual intradomiciliary spraying but after spraying the pattern changes, and I have other evidence to show this occurs within about 18 months after the commencement of spraying. Now we have a vector which feeds primarily out of doors and early in the evening. This correlates with the human behaviour which is to sit outside and eat and talk early in the evening, often with sleeping children lying in the parental group.

How serious is this situation?

The transmission of malaria is affected by what has been called the vectorial capacity. This is a largely theoretical approach to assessing the factors involved in transmission but it is clear that if one can measure these factors one has a useful tool.

In the Solomons the probability of transmission taking place is possibly somewhat easier to assess than in many other countries. Firstly there are relatively few alternative hosts as domestic animals are in a small minority and, secondly, there is only one vector to worry about (An. koliensis and An. punctulatus become very rare after spraying starts).

Graph 5 shows the variations in level of the total numbers of An. farauti collected between 1830 and 2100 by one collector at Koli fixed station during the years 1967 to the present time. Catches have been made at about twice monthly intervals throughout this time. I have also shown the proportion parous as a measure of daily survival, and the date of each cyclical spraying operation. The wettest months on north Guadalcanal are February and March with the driest period being August to October or November. The peak months for malaria cases are April and May.

Two features emerge from this graph.

(1) The highest levels of vectors are collected,
(a) during the dry season,
(b) shortly before a spraying cycle is due, i.e. sorne 5-6 months after the previous spraying operation.
(2) The peak levels of malaria transmission are related to the highest levels of the parous rate and not to the actual vector density. These peaks are towards the end of the wet season.

We are fortunate that the best conditions for breeding do not appear to coincide with the best conditions for survival.

Conclusions

The trend over the last 18 months has been for a dramatic drop in malaria cases on north Guadalcanal. This has been due to a drop in the number of imported cases previously coming from Ngella and Malaita Islands.

The recent introduction of interval (inter-cyclical) spraying should help to keep the vector numbers down. We have not had anything like the numbers in this month (September) that we have had in previous years but whether this will reduce the annual upsurge of malaria cases in April and May only time will tell.

What gives me the most concern is that the level of vector population required to maintain transmission seems to be very low given the right climatic conditions, and the need for 100% efficient spray coverage is a paramount need until we are satisfied our case detection mechanism can really cope particularly with regard to case treatment.

Brian Taylor
Government Entomologist
Malaria HQ, P.O. Box 349, Honiara
British Solomon Islands Protectorate
30th September, 1971

href="\malaria\malaria1.htm"