AUSTRALIAN MILITARY FORCES
H.Q. 2 Aust Corps
18 August, 1943
DDMS 2 Aust Corps
D of H LHQ
ADMS 9 Aust Div
ADMS 6 Aust Div
DADMS 17 (Cairns) L of C Sub Area
SCRUB TYPHUS – 24 AUST. BRIGADE
Throughout its jungle training period on Atherton Tableland from mid April to mid June, 1943, no case of Scrub Typhus was reported from 24 Brigade.
On June 16, 24 Brigade transferred to M.R. 6185 (Cairns 1:63360) for amphibious training. A case of Scrub Typhus was reported from Brigade Headquarters, 28 June, 1943. Subsequently during the period 2 July, 1943 to 20 July, 1943, cases were reported almost daily. In all 37 cases were evacuated in 3 weeks – 28 from 2/32 Aust. Infantry Battalion.
Distribution of cases through the Brigade was:
The relatively very heavy incident of infection upon 2/32 Battalion naturally attracted epidemiological inquiry to that Unit and suggested that most profitable field of study would lie in the record of its exercises. Additional support for this view derived from the circumstance that whereas in 20 Brigade the incidence of Typhus had been closely associated with bivouacs, 2/32 Battalion alone of the Units of 24 Brigade had made a practice of night bivouacs on combined operations exercises. This circumstance too, offered itself as a ready explanation of the anomaly that of all A.I.F. and Allied troops using the same training areas, embarkation and landing points, a heavy incident of Typhus was experienced by only one Unit – 2/32 Battalion.
As a preliminary datum upon which to base inquiry, the notification of cases strongly indicated a definite incubation period. The complete freedom of 24 Brigade from Scrub Typhus throughout its training on the Tableland, the heavy incidence of infection some time after its transfer to the amphibious training area and the abrupt cessation of cases soon after its return to KAIRI area, warranted the primary assumption that infection was localised in the combined operations area. On this ground, the first case indicated an incubation period of not more than 12 days and the last case one of not less than 15 days.
Syllabus of 2/32 Battalion training shows that whilst day training was undertaken in areas used just as intensively by other A.I.F. and Allied troops, the following exercises involved bivouacs out of camp.
(a) Night of 21/22 June: Bivouac south end of Trinity Beach. Embarkation. Landing Yorkeys Beach, advance inland and hold Bridgehead. This exercise involved a night bivouac in forest heavily grassed country behind a beach, followed by a first light landing and subsequent advance through heavily grassed forest fringed by jungle. On the basis of a 12 – 15 day incubation period this bivouac could account for 6 cases reported 3 – 7 July in 2/32 Battalion but would not explain 3 cases in other units during the same period nor 22 others occurring earlier and later in the same Battalion.
(b) Night of 23/24 June: Bivouac Yorkeys Beach. Embarkation. Landing White Cliff Beach. This exercise involved traversing similar but not identical terrain to that preceding it. An incubation period of 12 – 15 days from the date of this bivouac would embrace 3 new cases in 2/32 Battalion but would not explain 3 cases in other Units occurring the same period.
(c) Night of 28/29 June: Bivouac Yorkeys Beach. Embarkation. Landing White Cliff Beach. Although embarkation point in this exercise was the same as in that of 23/24 June, bivouac area at Yorkeys and landing point at White Cliff were different. An incubation period of 12 – 15 days from this bivouac would account for 10 cases from 2/32 Battalion but 2 cases from other Units not taking part would remain unexplained.
(d) Night 2/3 July: Bivouac north end of Trinity Beach. Embarkation. Landing on Palm Beach. Incubation period of 12 -15 days from this bivouac would embrace 11 cases in 2/32 Battalion and attached troops.
It is apparent that within the limits of the arbitrary incubation period adopted it is not possible to incriminate one area more specifically than another. To incriminate the Yorkeys area which features most consistently in the exercises it would be necessary to accept an incubation period ranging from 7 – 22 days. Since four separate sections of Yorkeys area are involved it would in addition be necessary to assume an intensity and extent of infection throughout the area impossible to reconcile with the comparative immunity of other A.I.F. troops using it and the complete immunity of the many allied troops quartered and operating there.
The latter objection applies with equal force to the alternative explanation that infection was widely scattered through all the exercise areas. All had been safely used earlier, later and simultaneously by other troops, A.I.F, and Allied, and none could account for all cases in 24 Brigade.
On the other hand the spread of the cases in 2/32 Battalion over the period 2-20 July without discernible relation to any one exercise or bivouac locality is susceptible of another explanation than either a wide incubation range or an extensive dispersion of infection, viz. continuing and repeated infections in the camp area.
Inspection of this camp M.R. 614863 (Cairns 1:63360) showed that 2/32 Battalion was tented on the northern bank of a dry jungle fringed creek. The soil was of light bull dust character with a heavy grown of ‘kunai’. The situation closely resembled the bivouac area of Severin Creek to which cases in 20 Brigade were traced. Tracing of cases by platoons revealed that infections for the most part were drawn from the tents pitched nearest the fringe of jungle along the creek bank. Personnel in tents along the road remote from the creek and on the rising ground beyond the road were free.
This point was checked in other affected Battalions. It was found that 2/28 Battalion all cases came from Headquarters Coy., which was tented nearest the jungle fringe.
Mite survey of the 2/32 area was difficult as heavy traffic had, at the time of inquiry, largely denuded the ground of vegetation and converted it to dust. Mites however were found in the vicinity of A Coy Lines. One of these despatched to Capt. McCulloch in New Guinea and identified by him as LEEUWENHOEKIA AUSTRALIENSIS.
Two features require explanation if infection in areas outside the camp is to be excluded-
(a) Fluctuation in case incidence in a Battalion more or less continually occupying the infected area.
(b) The late appearances of the majority of cases – nearly four weeks after first exposure in the area. It may reasonably be argued that most cases, if infection were contracted in camp area, should have occurred early unless a long incubation period be postulated.
From both of these aspects Fig. 1 is of considerable interest. The upper graph represents daily record of 9 a.m. Dry Bulb temperature recorded at Cairns from 19 June to 3 July. The lower records number of cases reported each day from 1 July to 15 July, 1943. The figure reveals a remarkable correlation between temperature and cases occurring 12 days later. Infections are increased by warmth, decreased by cold, indicating that feeding of mites is influenced by variations of temperature around 65∞F. The band above the temperature graph represent nights spent out of the camp area and these broadly suggest that even where temperature was favourable, absence on bivouac was associated with reduced incidence 12 days later. Viewed broadly the Figure explains the time distribution of cases, confirms their origin in the camp area and rationalise an incubation period of 12 days.
The very restricted localisation of the infecting area and its limitation to the vicinity of part of 2/32 Battalion lines are in close conformity with the findings in 20 Brigade outbreak. The reason for this remarkable feature of Typhus epidemiology is not clear but possible explanations which suggest themselves and which merit further investigation are –
(a) Confinement of the vector mite to extremely few very restricted and widely separated localities, in which it exists in large numbers. This focal intensity of mite population and sparse dispersion of foci may be determined by the distribution of a grass or plant favoured by the adult. The identification of this plant, if it exists, would very appreciably facilitate recognition of dangerous areas and contribute towards rational and successful measures of control.
(b) Limitation of infection to very few and widely separated colonies of a generally prevalent species of mite. This limitation might be due to infrequency of Typhus amongst rodents or to restricted migration and limited distribution of susceptible species. Identification of such species and study of their feeding and breeding habits would facilitate recognition of potentially infected areas.
(c) Infection of the adult mite by rickettsial plant or insect disease, infection of rodents like that of man incidental and not definitive. However, improbably and unprecedented this suggestion may appear, the very narrow limits of dispersion which characterise typhus infection of the mite appear to be incompatible with the migratory habit of rodents and in the absence of other satisfactory explanations, demand an inquiry in a wider field.
SUMMARY:
- Outbreak of scrub typhus in 24 Brigade was due to occupancy of infected camp site M.R. (Cairns 1:63360) 614863.
- The infected area was very restricted – affecting chiefly 2/32 Battalion and in that Battalion platoons camped nearest the creek bank.
- Incubation period approximately 12 days.
- The biting of mites and resulting infection is directly influenced by warmth. Dry bulb temperature of 65∞F seems favourable for the transmission.
- Mites had largely disappeared by the time search could be attempted. The only mite identified was LEEUWENHOEKI AUSTRLIENSIS, the same species as was found in the infected 20 Brigade area of Severin Creek.
- Pending more precise localisation of infected areas, troops unprotected by repellent should not rest on flat kunai clad banks of jungle fringed creeks. High ground over 50 yards from the bank appears safe even if jungle clad.
Signed. C.E. Cook. Major,
D.A.D.H. 2 Aust. Corps