MTOC Meeting 3/73
Agenda Item

"During the first quarter, as well as the quarter under review, relatively high densities of Anopheles farauti were observed at various localities along the coastal strip east of Kukum, Guadalcanal (Koli, Ilu Farm, Lunga). These observations coincided with the finding of falciparum cases."

A very topical comment but in fact one taken from the Eleventh Quarterly report, of the Malaria Eradication Pilot Programme for April-June 1964. No definite conclusions were reached from studies carried out at that time, but the findings were of high densities of An. farauti combined with low parous rates suggesting the actual density of potentially dangerous females would be comparatively low.

2. Some five years later the report by Kranendonk and Ray (Independent Assessment team, 7 December 1968 to 12 January 1969; Section 4.6.5.2; para (a)(v) - dealing with Guadalcanal and Savo) states: "The attached table also indicates that there was some increase in the number of indigenous cases (1968 cf. 1967). This alone should be sufficient indication of the urgent need to review the entire spray programme in this area".

3. The current epidemics in many areas of north Guadalcanal give cause for much concern. The problems in certain of the bush areas can be partially attributed to the failure of the Field Operations Officer (Central) to follow verbal instructions and thus neglecting spraying of low-lying villages until after my written memo. to him of 5th January. The resultant gap between the CSO cycles was thus around 8 months. A second factor is the surprising resurgence of An. punctulatus which I have commented on elsewhere.

The problems in the coastal and peri-coastal areas are more complex. For instance, the last spraying cycle in the current Zones 3 and 4 epidemic area, Kukum to Ngalimbiu, was in November last. How efficient the CSO cycle was is difficult to assess. The recent report, 6/3/1-249, by Dr. Mackay also comments on related aspects such as new houses and smoke blackening.

In the small group of houses, which I will refer to as Burn's Creek, there have been three P. falciparum cases during January and February. The cases, a father and two of his children, live in the same house. This house is principally of timber and leaf construction with the centre of the roof being corrugated iron. The following studies have so far been made:

The failure of the egress traps to collect farauti is not over-surprising since Sweeney (1968) commented that, during the night of 18th May 1964, he collected 1967 farauti man-biting but his window-trap caught only 12 specimens. The failure to trap the farauti females in window-traps is strongly indicative that the vectors seek egress from the house immediately after feeding. If the vectors were still in the house at dawn then one would expect them to be attracted by the light coming in through the windows. Numerous houseflies and several Culex annulirostris mosquitoes were trapped in this way.

4. The studies at Arohane on San Cristobal showed, however, that the DDT has a deterrent effect which is quite marked, at least for some four months after application. This should lead to a reduction in entry and, provided the humans are indoors, thus to a reduced opportunity to feed, in turn a high mortality would ensue. Sweeney also found this to be true, except the reduced feeding in his studies was due mainly to use of mosquito nets. His mortality rate for unfed farauti was 41.5% in 24 hours and only 1.6% for bloodfed farauti females.

There is also the favourable factor that resting for as little as 15 minutes on a DDT-sprayed wall causes death within a short time. We found this to be true at Boromole on Big Ngella last August as well as in the Lunga tests.

5. In my agenda item 10/73 of MTOC meeting 1/73, I raised the question of an alteration in the timing of CSO's in Zone 94 (San Cristobal). This was refuted on the grounds that transmission had been interrupted in this Zone and that the S.P.R was steadily falling. However, the "Monthly Malaria Bulletin" for February 1973 notes that "Main foci of positives (on San Cristobal) are E.Bauro 24 (13 Pf); Star Harbour Hanunu 5 (3 Pf), indicating almost certain resumption of transmission in Bauro and Hanunu."

At the last meeting of the MTOC, Item 9/73, I referred to my deterrency study results and suggested there was a case for an extra spray round on the north coast of Makira and, if necessary, this should be sprayed on a 4-monthly cyclical basis. Dr. Mackay in his report similarly suggested a possible 4-monthly CSO in Zone 4. Admittedly in Zones 1 to 5 we have had a system of "Interval Spraying" since mid-1971 but this only covers surfaces below six feet. Further the original intention was for IS, to replace follow-up spraying but, in fact, FUS has proved necessary to cover the new houses which spring up so rapidly.

6. Mass Drug Administration, in one form or another, has been and is being utilised in an attempt to enhance the effect of insecticide spraying but this is very demanding in man-power and depends on extremely good public cooperation.

The results, moreover, so far are not over-encouraging particularly among the age-group 1 to 4 years. The recent Mass Blood survey on Ngella by the WHO Laboratory Specialist revealed a fair number of vivax infections. The apparently excellent results on Wagina, from the October 5-day MDA followed with fortnightly pyrimethamine doses, were somewhat muted by a resurgence of vivax from mid-December onwards. A second 5-day MDA has now been carried out and a third is planned.

The detection of a fairly large number of falciparum cases with gametocytes, 16 out of 99 in Central District, including 5 out of 30 in Zones 1 to 6, where ACD is on a weekly basis and supposedly closely supervised, suggests fever cases are not coming forward during the ACD Agent's visits or the falciparum is to some extent asymptomatic. Additionally some 22 vivax cases out of a Central District total of 210 were detected from Follow-up and House Contact slides.

7. It could be suggested that we should took for an alternative insecticide. Unfortunately, it seems that no other insecticide has yet been discovered that combines the qualities so desirable in DDT, i.e. low cost, good residual action, low mammalian toxicity, acceptability to the public, etc. Malathion is perhaps a contender but the cost is higher (more than $1.50 per 6 litres diluted in the spray-can, using the retail price of Agricultural grade malathion ex-Greenacres, obviously a higher figure than if ordered direct and in bulk, compared to 25¢ for DDT at our current purchase price). Moreover, the Arohane results so far indicate a deterrency as great if not greater than that of DDT; this is not necessarily an undesirable factor.

8. Despite the somewhat gloomy picture painted in section 3 above there is no doubt that in areas of low pre-spraying vector density DDT applied at 2.0 g/m² on a twice yearly basis has been successful in interrupting transmission. I propose, therefore, that the policy for spraying in the following areas should be three cycles per year, i.e, at 4-monthly intervals.

On a final note of persuasion it is perhaps worth pointing out that, if one compares the monthly slide positivity rates for each Zone with the dates of cyclical spraying, in a number of Zones there has been a tendency for the SPR to be highest in the fifth and sixth months following each CSO.

Dr. B. Taylor
Government Entomologist
7.3.73

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