Imported malaria*

Bull. Org. mond. Santeu 1974, 50, 329-336 Bull. Wld Hlth Org. Imported malaria* MYRON G. SCHULTZ ' There have been 4 waves of imported malaria in the USA. They occurred during the colonization of the country and during the Second World War, the UN Police Action in Korea, and the Viet-Nam conflict. The first 3 episodes are briefly described and the data on imported malaria from Viet-Nam are discussed in detail. Endemic malaria is resurgent in many tropical countries and international travel is also on the rise. This increases the likelihood ofmalaria being importedfrom an endemic area and introduced into a receptive area. The best defence for countries threatened by imported malaria is a vigorous surveillance programme. The principles of surveillance are discussed and an example of their application is provided by a description of the methods used to conduct surveillance of malaria in the USA. In his excellent review of imported malaria four years ago, Bruce-Chwatt (1) rightly characterized it as a growing world problem. The present paper is not an attempt to update the Bruce-Chwatt review on a worldwide scale-for one thing, data on im- ported malaria from many countries are either un- obtainable or non-existent-but rather an attempt to describe the problem of imported malaria in the USA. This approach has been taken for several reasons. First, imported malaria in the USA is a subject that is well known to the author, a problem dealt with on a day-to-day basis. Secondly, this country has just experienced a major resurgence of imported malaria. Thirdly, the experience of the USA, a country that is supposedly malaria-free, is probably relevant to the countries of Europe and northern Africa that are either non-malarious or sparsely malarious.2 The concepts of surveillance and our methodology of dealing with imported malaria will thus be emphasized in the hope that these techniques will be found useful in countries facing similar problems. * Presented at the Symposium on Malaria Research, Rabat, Morocco, 1-5 April 1974. 1 Chief, Parasitic Diseases Branch, Bureau of Epidemi- ology, Center for Disease Control, Public Health Service, Atlanta, Ga. 30333, USA. 'These were the countries represented at the above- mentioned Symposium on Malaria Research at which this paper was presented. To place malaria in its proper context, it should be recalled that infectious diseases often move in great waves. In the past, smallpox, plague, typhus, and the treponematoses swept through nations and conti- nents, often closely following the paths of pilgrims and armies. Even today the world is in the midst of the seventh pandemic spread of cholera, and waves of influenza sweep across the globe every other year. The same is true of malaria. The USA has just experienced its fourth wave of imported malaria. The epidemic curve (Fig. 1) with its sharp single peak is shaped like a tidal wave; fortunately, the epidemic caused few fatalities. Before the details of malaria imported from Viet-Nam are discussed, however, the previous three waves of imported malaria will be described briefly. The first wave occurred during the colonization of the country 300 years ago. Malaria, according to available evidence, did not exist in North America prior to its settlement by European colonists (2). Plasmodium vivax and P. malariae probably were introduced mainly by Spanish, English, Dutch, and French settlers and P. falciparum mostly by African slaves. Once introduced, malaria spread rapidly. Noah Webster (3) noted that about 40 or 50 years after the settlement of New England the infectious fevers, some of which were intermittent agues, were common. Duffy (4) stated: " The significance of malaria in colonial history can scarcely be overrated, for it was a major hurdle in the development of the 3196 -329 M. G. SCHULTZ 4 500 4 000 3 500 3 000 2 500 n 2 000 1 500 1 000 500 MELITARY /CIVILIAN ~~r = O _ _ _ _ _ _ /~~~~~ ~~~____ __ _. , 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 Fig. 1. Number of military and civilian cases of malaria in the USA, 1959-1972 American colonies... Directly and indirectly, ma- laria was one of the most fatal of colonial diseases and shares with dysentery first place among the colonial infections ". At the time of the American Revolutionary War malaria was widely prevalent from Georgia to Pennsylvania; in the Mississippi, Ohio, and Tennessee valleys; in the Western Re- serve; in the Indian Territory; in river bottoms of what are now Iowa, Nebraska, and Kansas; and in the Sacramento and San Juaquin valleys of Califor- nia. It was moderately endemic in New England. This early history is cited to demonstrate how significant imported malaria can be, particularly in the life of a new nation. In the USA it became firmly established as a major endemic disease. The next 200 years of the history of this disease in the USA concern the advance, the decline, and the ultimate conquest of indigenous malaria and are therefore not relevant here. This subject has been reviewed recently in considerable detail by Russell (2). The second wave of imported malaria occurred during the Second World War. At that time malaria was declining in the USA owing to the cumulative effects of an improved standard of living, urban migration, extensive drainage of anopheline breeding areas, larviciding, and the widespread availability of quinine. Malaria was, however, still endemic in rural areas of the 13 traditionally malarious states of the Atlantic and Gulf coasts and the Mississippi delta area. With the return of many thousands of Second World War veterans from Africa and from the Mediterranean and Pacific areas to endemic regions of the USA, public health officials became concerned that there would be a resurgence of malaria. A major control programme was undertaken, particularly around military bases in the south-eastern states. The reported incidence of malaria in the USA at the beginning of the 1940s was approximately 70 000 cases a year; at the end of the decade it was approximately 4 000 cases a year. It is impossible to say exactly how many of these cases were imported; however, an indirect measure is provided by the incidence rates from the 13 malarious states and the 35 other non-malarious states (6). The incidence declined each year in the 13 malarious states, whereas in the non-malarious states it crested from 656 cases in 1942 to 13 618 cases in 1945, indicating at least that many cases were imported. Langmuir (7) has commented on an interesting phenomenon that occurred during this period-a phenomenon involving a simple concept but one that deserves repeated emphasis. During the war period, US$31 million were expended in the USA for the malaria control programme, which included tradi- tional control techniques of ditching and larvicid- ing as well as extensive DDT spraying, at the time a new technique. The rationale for this accelerated programme was largely the fear of imported malaria. Yet epidemiological services to determine the true incidence of the disease were not organized until 1947, and the results of their studies were in fact quite surprising. Simple appraisal of morbidity and mortality reports indicated that malaria was being grossly over-reported. States with the highest inci- dence rates were submitting weekly reports of the number of cases seen rather than the names of individual patients. Such a system encourages exag- geration. When the reporting system was changed and diagnoses had to be confirmed by examination of blood smears, the reported incidence of malaria in one southern state dropped from 17 764 cases to 914 cases in the very first year that epidemiological appraisals were made. This change alone showed that the country was much closer to the goal of eradi- cation than had been thought, and undoubtedly saved millions of dollars from being spent needlessly. The "moral " of this story is simple: epidemiological anal- ysis is an essential part of any malaria control or eradication scheme and is an absolute requirement in differentiating imported malaria from indigenous malaria. This concept will be expanded upon later. 330 IMPORTEX) MALARIA The third wave of imported malaria came during the Korean police action. This episode was brief. In 1950 the reported incidence of malaria in the USA was 2 184 cases. Two years later at the height of the conflict it was 7 023 cases, and 2 years after that, in 1954, it had dropped to 715 cases. In the area of fighting, the annual hospitalization rate among US Army troops did not exceed 1.2 %, reflecting in part the effective replacement of atabrine by chloroquine for prophylaxis. However, after their return to the USA, troops experienced considerably higher vivax attack rates, which reached 6.6% in one study (8) and 17.5% in another (9). This was partly due to delayed primary attacks, which are characteristic of Korean vivax malaria. It was during this police action that terminal chemoprophylaxis with primaquine was first used. In a closely supervised study 294 asymptomatic military personnel returning to the USA were given either primaquine or a placebo. The vivax attack rate in the placebo group was 17.5% whereas in the primaquine group it was zero (9). It was also during this period that the USA experienced its most notorious out- break of introduced malaria. Thirty-five cases of P. vivax malaria occurred among a group of Camp Fire girls attending camp at Lake Vera, California, during the summer of 1952 (10). In 9 of the cases primary attacks occurred within a few weeks after the girls' exposure at an outing; however, in 26 cases there were latent periods of 217-316 days. The index case was a member of the US Marines who had returned home and spent a few nights camping near the Camp Fire girls' outing. After the Korean police action the incidence of malaria in the USA declined; in the following decade the annual incidence ranged from 50 to 171 cases. These were isolated imported cases occurring in businessmen, seamen, missionaries, and aliens. The last case of indigenous malaria in the USA, Puerto Rico, or the Virgin Islands (USA) was reported in 1957, and in 1969 the USA was certified by the World Health Organization as malaria-free (6). It is ironic that this certification occurred at a time when the country was in the midst of its fourth (and, hopefully, last) wave of imported malaria. Before the epidemiological characteristics of im- ported malaria from Viet-Nam are discussed, it might be well to describe how the US malaria surveillance system functions. The goal of this system is to detect every case of malaria diagnosed in the USA. Since indigenous malaria no longer exists, virtually all cases reported to the Center for Disease Control (CDC) are imported from other countries. These patients have already been diagnosed and presumably treated; hence, their welfare is generally not at stake. Any programme dealing with imported malaria must have as its focal concern the prob- lem of introduced malaria, i.e., malaria acquired by mosquito transmission from imported cases in an area where malaria transmission is not a regular occurrence. This indeed is the focus of our concern at CDC. The US population of over 200 million people has no immunity to malaria; suitable ano- pheline vectors, Anopheles freeborni in the west and A. quadrimaculatus in the east, exist in most areas of the country; and the presence of human carriers who have imported their parasites from the tropics creates the risk of reestablishing the disease. Needless to say, all unusual cases of malaria that are detected by our surveillance system are immediately investigated. Cases of malaria are reported to CDC by means of the Malaria Case Report form. This form was developed at the Center itself and is distributed to all state health departments and to the hospitals of the Armed Forces, the Public Health Service, and the Veterans Administration. The form is simple to fill out and provides a brief description of the epidemio- logical and clinical features of each case. In addition to these forms, the military services furnish CDC with duplicates of the telegrams they are required to send to their Surgeons General for reporting cases of infectious diseases. These reports are reviewed at CDC, where we are in frequent telephone contact with our colleagues in state health departments and military installations so as to share information about cases. All cases must be confirmed by the presence of parasites on peripheral blood smear. A clinical impression of malaria without a slide diag- nosis is not counted as a case except in special situations; for example, when the donor is being traced in an episode of transfusion-induced malaria, serological tests (immunofluorescent and haemag- glutination) are used as epidemiological tools. Because patient histories of an attack of malaria occurring abroad are often imprecise, the first onset of symptoms in the USA is considered as the initial attack even though in a minority of cases it may be a relapse. Cases are classified according to World Health Organization terminology, i.e., as indigenous, introduced, imported, induced, relapsing, and cryptic. Case records are computerized so that any previously reported case is easily accessible and all the data can be readily analysed on a periodic basis. It should be emphasized, however, that there is 13 331 M. G. SCHULTZ nothing magical about the computer. For malaria services that wish to adopt this type of surveillance system, particularly in countries where the incidence of malaria is not overwhelming, simple line listings or marginal punch cards will serve the same purpose. What is important is that the surveillance system should be looked upon not as a static collection of facts but rather as a dynamic interchange of infor- mation between field and central units. Any data collection system that facilitates this interchange is worth while. The system just described has served the USA well during the past few years, which saw the fourth wave of imported malaria. The increase began in 1966 when there were 764 cases reported; the incidence then rose rapidly in 1967 to 2 857 cases, fell slightly in 1968 to 2 698 cases, rose again in 1969 to 4 059 cases, peaked in 1970 at 4 245 cases, and then fell to 3 172 cases in 1971 and 588 cases in 1972. Thus, over this 7-year period there was a total of 18 383 reported cases of malaria imported into the USA. Of these, 94.2% occurred in military personnel. Virtually all of these cases were imported from Viet-Nam. At the peak of the conflict the hospitalization rate for malaria among the troops stationed in Viet-Nam was 3.6% a year (no more than 0.7% for P. vivax infections, at least 2.9% for P. falciparum) (11). The attack rate for malaria in military personnel who had returned to the USA was approximately 1 %. The attack rate in combat troops was almost three times higher than the rate in supporting troops (11). From an historical viewpoint these attack rates are relatively low for a military force in a tropical area, particularly when one considers the fact that most P. falciparum infections acquired in Viet-Nam are resistant to chloroquine suppression and treatment. Besides the problem of drug resistance by the para- sites, there was also a problem of behavioural resis- tance on the part of military personnel. One study showed that 70% of troops, irrespective of rank, failed to take the prescribed prophylactic regimen when returning to the USA (12). The magnitude of the problem of imported malaria, particularly those cases due to P. vivax, would have been significantly diminished had compliance been better. During the 7-year period, 81.6% of the infections imported from Viet-Nam were due to P. vivax; the percentages for the other species were P. falciparum, 12.9%; P. malariae, 0.6%; P. ovale, 0.03%; mixed infection, 2.2%; and unknown, 2.6%. These figures are interesting in light of the fact that the majority of infections occurring in military personnel while in Viet-Nam were due to P. falciparum whereas after the return of troops to the USA the majority of infections seen were due to P. vivax. Several factors explain this difference. P. falciparum infections ac- quired in Viet-Nam commonly escape suppression and become manifest shortly after the infection is acquired, and the duration of falciparum malaria is usually less than 1 year; in contrast, P. vivax infec- tions that are inadequately treated can relapse 2 or 3 years after infection. Indeed, at the beginning of the Viet-Nam conflict the relapse rates for P. vivax infections were 29.4% for 1 relapse, 8.6% for 2 re- lapses, and 1.4% for 3 relapses. The situation re- mained the same until the US Army changed its regimen for radical cure from unsupervised treat- ment with 1 chloroquine-primaquine tablet weekly for 8 weeks to supervised treatment with 15 mg of primaquine base daily for 14 days (13). At the end of the Viet-Nam conflict the vivax malaria relapse rates were 5.0% for 1 relapse, 0.5% for 2 relapses, and zero for 3 relapses (14). The distribution of cases among the different branches of the US Armed Forces was as follows: Army, 83.9%; Marines, 10.6%; Navy, 0.4%; Air Force, 0.2 %. The branch of service was unknown in 4.7% of cases. Military hospitals in the USA treated 71.6% of cases, Veterans Administration hospitals 18.1 %, civilian hospitals 7.6%, and Public Health Service hospitals 0.9 %. In 99% of patients, the onset of illness occurred within 12 months after their return to the USA. Only 18.6% of P. vivax infections became manifest within the first month of the patients' return, whereas 63 % of P. falciparum infections became manifest during this same time period. This point is particularly significant because troops were either discharged or given one month of home leave immediately upon their return to the USA. It is during this period that they were most likely to develop falciparum malaria and also most likely to be seen by civilian physicians. From the CDC analysis of mortality rates in falci- parum malaria it is known that civilian physicians were not as capable as military physicians in diagnos- ing and treating this dangerous infection (15). During the past 10 years 42 deaths due to malaria have been reported to CDC; 37 were due to P. falciparum. The case/fatality ratio for patients who first received care from civilian physicians was 5.7%; for those cared for in military or Veterans Administration hospitals the ratio was 0.2%. Similar findings were recently reported in England (16). This striking 24-fold difference in case/fatality ratio is accounted for 332 IMPORTED MALARIA in part by differences in the interval that elapsed between the time when patients were admitted to the hospital and the time when the correct diagnosis was recognized. In fatal civilian cases a median of 4 days elapsed between admission to hospital and diagnosis of malaria, whereas the median interval for military cases was less than 1 day. One group of civilians that are at particularly high risk of acquiring malaria and dying from it are merchant seamen. During the past decade 194 cases of malaria in seamen were diagnosed in the USA and reported to CDC (17). Seamen had higher mortality rates for malaria-43.5 per 1 000 cases-than any other occupational group. To touch briefly on one consequence of imported malaria, the USA has recently seen an increase in induced malaria directly related to the fourth wave of imported malaria. During the past 7 years, 36 cases of transfusion-induced malaria have been reported to CDC. There has been a significant increase in the percentage of cases due to P. falciparum as compared with the percentage of such cases reported prior to the Viet-Nam conflict, and the majority of the infective donors identified were military personnel who had recently returned from Viet-Nam (18, 19). Troops returned from Viet-Nam were also respon- sible for several episodes of induced malaria due to the illicit use of heroin. Two outbreaks in California involved a total of 56 individuals who were infected by contaminated syringes (20, 21). The other consequence of imported malaria, and, as indicated earlier, the one that is the main concern of CDC, is introduced malaria. There have in fact been 10 documented outbreaks of malaria in the USA since the end of the Second World War in which the parasite was transmitted by mosquitos. The most recent outbreak occurred in 1970. The 10 outbreaks involved a total of 59 patients, all were due to P. vivax, and none resulted in a second generation of cases. Probably the most important factor pre- venting the reestablishment of malaria in this country is the high standard of living, which limits the opportunity for man-mosquito contact; however, it is also believed that our surveillance activities are essential for prompt detection and control. While this review has been devoted mainly to the subject of imported malaria in the USA, it would be incomplete if it did not touch on the wider implica- tions of imported malaria. It is well known that endemic malaria is currently resurgent in many parts of the world. It is also known that international travel is increasing at a rapid rate. The International Civil Aviation Organization, which represents 128 countries, has just reported that the world's airlines carried 480 million passengers over 380 thousand million passenger-miles in 1973. This represents an 11 % increase over 1972. Moreover, in many nations where malaria has been eradicated or controlled it is a well-known fact that anophelism still exists and climatic conditions favour the transmission of ma- laria. In Europe, for example, where indigenous malaria no longer occurs, the incidence of imported malaria rose steadily from 839 cases in 1967 to 1 240 cases in 1972, and an outbreak of introduced malaria occurred in Corsica in 1971-1972 (22). Therefore, even if wars can be successfully prevented or con- tained, there remains an increasing risk of civilian travellers' carrying the seed of malaria from endemic areas into receptive areas where it can sprout in epidemic form. How then can imported malaria be controlled? It is wishful thinking to believe that effective controls can be imposed on the movement of individuals or populations. An aeroplane can carry a person incubat- ing malaria halfway around the world, unbeknown to anyone, in less than a day. Moreover, travellers leaving malarious areas cannot be compelled to take prophylactic drugs. This was amply demonstrated by the recent experience with US military personnel returning home from Viet-Nam. The answer to the problem of controlling imported malaria, in my opinion, lies in surveillance. Just as a vigorous surveillance programme has been the best defence against imported malaria in the USA, there is good reason to believe that it has the same potential for other nations faced with this threat. Surveillance in the traditional public health sense used to mean the observation of an individual who had been exposed to a serious communicable disease so as to detect the early signs of infection. In the modern context, the term applies to diseases in populations rather than in individuals. Surveillance embraces the continuing watchfulness over the inci- dence and trends of disease in order to identify any problems and to advise on control measures. Surveil- lance has a unique role to play in malaria and other infectious diseases that require national and interna- tional control. Indeed, the shift away from the old " fortress mentality " reflected by draconian quarantine measures towards the concept of surveil- lance as the best means to prevent the international spread of communicable disease has been endorsed by the World Health Organization and incorporated 333 M. G. SCHULTZ into the new International Sanitary Regulations that came into force in January 1971. There are three main features of surveillance (23). They are: (1) the systematic collection of pertinent epidemio- logical data; (2) the orderly consolidation and evaluation of these data; and (3) the prompt dissemination of the results to those who need to know them. To recapitulate and to illustrate the application of these principles to the surveillance of malaria in the USA, the first characteristic of surveillance-the systematic collection of pertinent data-is embodied in morbidity and mortality reporting by state, local, and military health departments through the use of the Malaria Case Report form discussed earlier, and epidemic field investigations. Secondly, the orderly consolidation and evaluation of the data collected is carried out by epidemiologists on the staff of the Parasitic Diseases Branch, Bureau of Epi- demiology, CDC, who interpret the reports that are received. As for the dissemination of epidemio- logical evaluations to those who submitted the basic data and to all who are in a position to act and need to know the facts, this is achieved by the sending of periodic reports to the Malaria Commission, Armed Forces Epidemiologic Board, to advise on current trends and necessary control measures; the publica- tion of case histories and periodic summaries in CDC's Morbidity and Mortality Weekly Report, which now has a circulation of over 30 000; and the publication of CDC's Malaria Surveillance Annual Summary. In conclusion, imported malaria is indeed a grow- ing world problem, both currently and potentially, as Bruce-Chwatt has pointed out (1), and I believe that the best means for its control lies in the intelligent application of the principles of disease surveillance. ACKNOWLEDGEMENTS The author thanks the following Epidemic Intelligence Service Officers and staff of the Center for Diseas Control who participated in the malaria surveillance programme during the past decade: Dr Robert Scholtens Dr Hans Lobel, Dr George Fisher, Dr Karl Western, Dr Arthur Dover, Dr Peter Walzer, Dr James Gibson Mrs Stella Howell, Miss Mel Thornton, and Mrs Bee Hayes. RItSUMIt PALUDISME IMPORTt L'auteur expose certains aspects de l'importation du paludisme aux Etats-Unis d'Amdrique et les mesures prises pour faire face a cette situation. Les Etats-Unis ont connu quatre vagues d'importation du paludisme. La premiere a eu lieu pendant la periode de colonisation, il y a 300 ans. Le paludisme, amene par les colons europeens et les esclaves d'Afrique, s'est solidement etabli dans le pays. La deuxieme s'est pro- duite pendant la deuxieme guerre mondiale. A cette epoque, le paludisme etait en regression aux Etats-Unis, bien que toujours endemique dans certains Etats. Au debut des annees 40, le nombre approximatif des cas a atteint 70 000 par an; A la fin de la decennie, on en comptait 4000 environ par an. La troisi6me vague a colncid& avec la guerre de Coree. En 1950, l'incidence du paludisme etait de 2184 cas; deux ans plus tard, elle atteignait 7023 cas, mais en 1954 elle n'etait plus que de 715 cas. La guerre du Viet-Nam a ete a l'origine de la qua- trieme et derniere vague d'importation du paludisme, dont l'auteur s'attache a decrire les caracteristiques epidemiologiques. Il expose A cette occasion le fonction- nement du systeme de surveillance mis en place aux Etats-Unis afin de depister chaque cas de paludisme diagnostique. L'augmentation de l'incidence du palu- disme s'est manifestee en 1966 avec 764 cas. On a enregis- tre en 1967: 2857 cas; en 1968: 2698; en 1969: 4059; en 1970: 4245; en 1971: 3172; et en 1972: 588, soit au total, en 7 ans, 18 383 cas. Les infections etaient dues A Plasmodium vivax: 81,6%; P. falciparum: 12,9%; P. malariae: 0,6 %, P. ovale: 0,03 %. L'auteur insiste sur la menace permanente que reprd- sente le paludisme importe. Dans beaucoup de regions tropicales, le paludisme endemique a fait sa reapparition et l'augmentation des 6changes internationaux accroit considerablement le risque d'introduction de la maladie dans des pays ou persistent des populations de vecteurs potentiels. 334 IMPORTED MALARIA 335 REFERENCES 1. BRUCE-CHWATT, L. J. Imported malaria-a growing world problem. Trans. Roy. Soc. Trop. Med. Hyg., 64 (2): 201-209 (1970). 2. RUSSELL, P. The United States and malaria: debits and credits. Bull. N.Y. Acad. Med., 44 (6): 623-653 (1968). 3. WEBSTER, N. A brief history of epidemic and pesti- lential diseases. London, Woodfall, 1800, p. 285. 4. DuFFY, J. Epidemics in Colonial America. Baton Rouge, Louisiana State University Press, 1953, p. 214. 5. FAUST, E. C. Malaria incidence in North America. In: Boyd's Malariology, Philadelphia, Saunders, 1949, vol. 1, pp. 749-756. 6. PAN AMERICAN HEALTH ORGANIZATION. Report for Certification and Registration of Malaria Eradication from United States of America. Washington, D.C., 1969. 7. LANGMUIR, A. The surveillance of communicable diseases of national importance. New Eng. J. Med., 268: 189-192 (1963). 8. YOUNG, M. D. & BURGESS, R. W. Susceptibility of Anopheles quadrimaculatus to Korean vivax malaria. Publ. Hlth Rep., 67: 14-16 (1952). 9. COATNEY, G. R. ET AL. Korean vivax malaria. V. Cure of the infection by primaquine administered during long-term latency. Amer. J. Trop. Med. Hyg., 2: 985-988 (1953). 10. BRUNETIT, R. ET AL. An outbreak of malaria in California 1952-1953. Amer. J. Trop. Med. Hyg., 3: 779-788 (1954). 11. FISHER, G. V. ET AL. Malaria in soldiers returning from Vietnam-epidemiologic, therapeutic and clini- cal studies. Amer. J. Trop. Med. Hyg., 19 (1): 27-39 (1970). 12. BARRETT, 0. ET AL. Malaria imported to the United States from Vietnam-chemoprophylaxis evaluated in returning soldiers. Amer. J. Trop. Med. Hyg., 18 (4): 495-499 (1969). 13. FISHER, G. U. Recent trends in malaria in the United States. Bull. N.Y. Acad. Med., 45 (10): 1016-1026 (1969). 14. UNrrED STATES PUBLIC HEALTH SERVICE, CENTER FOR DISEASE CONTROL. Malaria surveillance, annual summary 1971, Atlanta (issued June 1972). 15. WALZER, P. D. ET AL. Malaria fatalities in the United States. Amer. J. Trop. Med. Hyg., in press. 16. BRUCE-CHWATr, L. J. Malaria epidemiology. Brit. Med. J., 2: 91-93 (1971). 17. UNrrED STATES PUBLIC HEALTH SERVICE, CENTER FOR DISEASE CONTROL. Malaria surveillance, annual summary 1972, Atlanta (issued September 1973). 18. DOVER, A. & SCHULTZ, M. G. Transfusion-induced malaria. Transfusion, 11 (6): 353-357 (1971). 19. FISHER, G. U. & SCHULTZ, M. G. Unusual host- parasite relationship in blood-donors responsible for transfusion-induced falciparum malaria. Lancet, 2: 716-718 (1969). 20. LYMAN, D. 0. ET AL. Malaria among heroin users. Health Services Reports, 87 (6): 545-549 (1972). 21. FRIEDMANN, C. T. H. ET AL. A malaria epidemic among heroin users. Amer. J. Trop. Med. Hyg., 22 (3): 302-307 (1973). 22. ZULUETA, J. DE. Malaria eradication in Europe: the achievements and the difficulties ahead. J. Trop. Med. Hyg., 76 (11): 279-282 (1973). 23. LANGMUIR, A. Changing concepts of surveillance of communicable diseases. In: Proceedings of the VIlIth International Congress of Tropical Medicine and Malaria, Teheran, 7-15 Sep. 1968. DISCUSSION BRUCE-CHWATT: In 1973 the number of cases imported into the United Kingdom reached 537, which reflects not only the increase in travel but also a better diagnosis of malaria. There were 25 fatal cases among 466 imported cases of falciparum malaria during the period 1970-73. This high mor- tality from falciparum malaria (5.2 %Y per annum) is due mostly to delayed diagnosis and treatment. Of importance in this connexion is the delay-for as long as 9 or 10 months, and occasionally longer- in the appearance of symptoms of vivax malaria in those under incomplete suppressive treatment. Units for the surveillance of exotic diseases, and in particular of malaria, should be established in countries where the number of imported cases is increasing. DE ZULUETA: There was a high fatality rate from falciparum malaria in Europe (2.7% among 1 709 imported cases reported) during the period 1967-72. The malaria introduced into European countries as well as into Lebanon has always been due to P. vivax. It seems to be difficult for Mediterranean anophelines to become infected with P. falciparum of tropical origin, but more experimental work is needed in this respect. M. G. SCHULTZ LYSENKO: In the USSR, during the period 1963-72, 1 787 imported cases were recorded, 54% of which were in Soviet citizens and 46% in visiting foreigners. Acute malaria predominated in the former, who were incompletely protected by chemoprophylaxis; clini- cal symptoms were often delayed for up to 3 years in these patients. Among the foreign visitors-chiefly Africans-asymptomatic malaria predominated. Most (64%) of the Soviet citizens were infected with P. vivax, but as many as 9% were infected with P. ovale; the corresponding figures for the foreign visitors were 60% and 3 %. It is noteworthy that P. ovale was found in Soviet citizens returning from Guinea and Mali, where this parasite had not been reported before. Small foci originating from imported cases were discovered in the Caucasus, in Bielorussia, in Kiev, and in the Moscow region. All these out- breaks were due to P. vivax imported from Asia. There were no introduced cases from imported cases from Africa. Only one fatal case of falciparum malaria (com- ing from Africa) has been found in the USSR in recent years. Soviet citizens travelling abroad are advised of the risks of malaria in tropical areas and their health card contains a note concerning their travel. This is of immediate value to the physician in case of sickness after return to the home country. CAMBOURNAC: Imported malaria is still a problem in Portugal and the Cape Verde Islands, from which the disease has been eradicated (in the case of Portugal, this was certified by WHO in 1973). Despite the great increase in imported cases (due mostly to P. falciparum) which rose to 584 in 1972, there has been no transmission of malaria in Portugal for a number of years. This may be the result of an effective surveillance system, but the decreased popu- lation of the vector, A. atroparvus-always a very zoophilic species-may have contributed to the absence of transmission. There has been no evidence of transmission in the Cape Verde Islands. After a drought of 7 years, rain fell in August 1973, and after this some cases were found, as well as infected A. gambiae. This may have been due to the lack of animals and to important ecological changes affect- ing this vector. Adequate surveillance measures resulted in the elimination of the focus by December 1973. SCHULTZ: Through malaria surveillance, 3 cases of human babesiosis have been detected in the past in the USA. Although chloroquine has been used for treatment of this disease, diamidines are probably more effective. LUPASCU: Romanian citizens travelling to endemic areas are supplied with enough chloroquine/pyri- methamine tablets for 2 months' chemoprophylaxis and are requested to contact a doctor in case of fever on their return. FERNEX: It is a significant finding that 3 introduced cases of P. falciparum were found in the vicinity of the international airport of Kloten (Zurich) in 1971-72. DUPUCH: About 30 cases were reported in a small outbreak of malaria in Corsica in 1970-71. Although I had advised that tourists coming to the island be warned of the outbreak, it was not possible to do this, with the result that several tourists were unneces- sarily infected with malaria. SCHULTZ: I agree with Professor Bruce-Chwatt that travel agencies contribute greatly to the problem of imported malaria owing to ignorance or by deliberately withholding information from travellers. 336