Insecticide susceptibility of some vector fleas and mosquitoes in Burma.

Rat fleas and mosquitoes are insect vectors of public health importance in Burma. Plague is endemic in Central Burma and DDT has been the principal insecticide used for its control to date. Dengue haemorrhagic fever, recently introduced and transmitted by Aedes aegypti, has been spreading to major towns since 1971. The rodents, Rattus rattus, R. exulans, Bandicota bengalensis, Mus musculus, as well as shrews were commonly caught during routine trapping in the country. Rattus norvegicus, prevalent in Rangoon City, is not found in Central Burma. The rat fleas, Xenopsylla cheopis and X. astia, were found to be infesting these rodents, the former being the principal vector of plague. Insecticide susceptibility tests have been carried out periodically in plague endemic areas and Rangoon since 1966 and it is now observed that rat fleas in most of these endemic towns and Rangoon Port are no longer susceptible to DDT. Subsequent rechecks in towns where fleas became resistance to DDT show that fleas are persistently resistant to the insecticide. Aedes aegypti is found to be highly prevalent both in rural and urban areas of almost every major town and townships below 900 meters. Insecticide susceptible tests on this mosquito in limited areas show that the mosquito is generally resistant to DDT but susceptible to other insecticides. With the development of DDT resistance in fleas, it is now necessary to change to an effective insecticide in the control of plague in Burma.

Dengue haemorrhagic fever in Burma.

Although sporadic from 1965 to 1969, a major outbreak of dengue haemorrhagic fever (DHF) occurred for the first time in Rangoon in 1970. Since then the disease has occurred every year in Rangoon and is now observed to be expanding to other urban areas in the country. The clinical diagnosis of DHF was confused by concurrent outbreaks of influenza A in 1971 and influenza A and B in 1972. A laboratory study of 3,447 clinically diagnosed haemorrhagic fever cases showed that 1643 cases (47.8%) were due to dengue and chikungunya, 296 (8.6%) to influenza A, 85(2.5%) to influenza B, 12(0.3%) to measles and 1411(40.8%) were of unknown aetiology during the 5 year period 1970-1974. Ae. aegypti mosquitoes are widely distributed in the country up to and including 900 meters above sea level but breeding is not found above that altitude. The absolute larval population which is highest in July as well as landing rate correlated with the peak incidence of DHF cases.

Epidemiological features of dengue and chikungunya infections in Burma.

A serological survey for antibody to dengue and chikungunya was carried out in all 14 divisions and states and 2 border towns in Burma during 1973-74. Dengue HI antibody prevalence rate of less than 10% was observed in Arakan and Shan States, 10 to 30% in the Irrawaddy, Pegu, Mandalay Divisions and Kachin, Mon and Karen States, 31 to 60% in Sagaing Division, and over 60% in Rangoon, Magwe and Tenasserim Divisions. Similarly, chikungunya HI antibody prevalence rate of less than 10% was observed in Arakan State, 10 to 30% in the Irrawaddy, Pegu, Mandalay and Sagaing Divisions and Kachin State, 31 to 60% in Rangoon Division and Mon State. Both dengue and chikungunya antibodies were detected where Aedes aegypti mosquitoes were prevalent but the antibody prevalent rates were not directly proportional to the premises index. No HI antibody to dengue nor chikungunya was detected in Aedes aegypti free hilly areas, Chin and Kayah States, but was detected in the Shan State, Dengue and chikungunya infections were observed both in rural and urban populations. Dengue and chikungunya infections affected all socioeconomic classes in Rangoon equally but in Mandalay high socioeconomic class was nearly 3 times less affected than lower socioeconomic class. The infrequencies of dengue and chikungunya infections were observed to be 2 to 3 times higher in residents of Rangoon City than those of other towns. In Rangoon the antibody prevalence rates to dengue increased progressively with age while in other towns no appreciable increase in rates with age was observed. Both sexes were equally affected. This study provides strong circumstantial evidence that dengue and chikungunya viruses are highly and widely distributed throughout Burma, and that new outbreaks of haemorrhagic fever could occur in previously free areas following introduction of dengue viruses into populations previously exposed to one type of dengue.