1077 C. Pison, Nuclear Medicine Service, Hotel-Dieu
Hospital
for technical
help. Hotel-Dieu Hospital, Section of Endocrinology, Metabolism, and Nutrition
and INRS-Santé, L H. Lafontaine Hospital, Montreal, Canada
A. GATTEREAU P. BIELMANN J. DURIVAGE P. LAROCHELLE
University Eye Hospital,
ECONOMIC ASPECTS OF REDUCED SMOKING
90220 Oulu 22, Finland
SIR,-Professor Atkinson and Mrs Townsend refer to my review of the McGuiness and Cowling study in the section of their paper headed Restrictions on Advertising (Sept. 3, p. 492). They say "Moreover a different interpretation of their [McGuiness and Cowling] results by Johnston gives a longterm elasticity with respect to advertising of less than half the reported value. Taking Johnston’s figure, total abolition of television advertising and the reduction of expenditure on advertising in the press by three-quarters could reduce consumption
by 10%." It would be a serious mistake to regard my amendment of the McGuiness and Cowling results as a reliable base for such a forecast. My review contained several criticisms of the study, one of which was that a conceptual and mathematical error had been made in the definition of the long-run elasticity. To demonstrate the effect of this error alone, I showed that their own statistical work implied an elasticity of approximately half the reported figure. The amended figure is still vitiated by the other criticisms. Even with a well-founded statistical study, prediction of the effects of very large changes, such as those considered by Atkinson and Townsend, is subject to ever-
increasing error. Department of Econometrics, University of Manchester * Present address:
J. JOHNSTON
Department of Economics, Emory University, Atlanta, Geor-
gta30322, U.S.A.
TRANSMISSION OF TOXOPLASMOSIS BY TROPHOZOITES
SIR,-It is generally accepted that toxoplasmosis is transby transplacental infection, cysts, and oöcystS.1.2 The
mitted
trophozoites of Toxoplasma gondii are assumed to be unable to resist changes in osmotic pressure, and they are thought not to survive outside the body long enough to transmit the infectionHowever, this assumption has not been proved experimentally. We have studied the role of trophozoites in the transmission of toxoplasmosis by testing their survival and pathogenicity in liquid media. In acute Toxoplasma infections the trophozoites are widely distributed throughout the body and they are liberserous exudates, fseces, urine, saliva, sputum, nasal and conjunctival secretions, vaginal discharges, semen, and milk.4. The trophozoites remain infectious in secretions for several days, in milk for 6 days, in saliva for 5 days, in urine for 7 days, and in tears for 4 days.6 In these media the trophozoites are relatively resistant to changes in osmotic pressure but they will not withstand drying or freezing.3·g The trophozoites
ated in
easily
penetrate intact
mucous
membranes from infected
se-
cretions,9 the smallest infective dose in man being approximately ten parasites.10 These observations suggest that tropho1. Frenkel, J. K., Dubey, J. P. J. infect. Dis. 1972, 126, 664. 2. Hartley, W. J., Munday, B. L. Aust. vet. J. 1974, 50, 224. 3. Jacobs, L. J. Wildlife Dis. 1970, 6, 305. 4 French, J. G., Messinger, H. B., MacCarthy, J. Am. J. Epidem. 185. 5. Janitschke, K. Germ. Med. 1971, 1, 23. 6. Saari, M., Räisänen, S. Acta ophthal., Copenh. 1974, 52, 847. 7 Raisänen, S., Saari, M. Med. Biol. 1976, 54, 152. 8 Saari, M., Räisänen, S. Nordic Coun. arct. med. Res. Rep. 1976, 16, 9 Räisänen, S. Acta univ. tamp. A, 1977, 84, 1. 10 Brown, J., Jacobs, L. Ann. intern.Med. 1956, 44, 565.
1970, 91,
31.
zoites may transmit the disease during acute stages of toxoplasmosis. Transmission of toxoplasmosis by trophozoites may explain why persons in contact with animals possess Toxoplasma antibodies significantly in excess of the average level, transmission of the disease in cattle, and occurrence of the parasite in areas where felines are not present.
Institute of Biomedical Sciences,
University of Tampere
K. M. SAARI S. A. RÄISÄNEN
IS CHILOMASTIX HARMLESS?
SIR,-Chilomastix mesnili is
a cosmopolitan .1agellate proin warm than cool climates. It is generally regarded as a harmless commensal in man transmitted by the faecal-oral route in the cyst form, with trophozoites living in the caecum and ascending colon. I have seen three patients from abroad who have had diarrhoea with C. mesnili and no other significant microbial pathogen detected in the fseces. In at least one patient treatment with metronidazole was followed by rapid and lasting improvement. The first patient was a 5-year-old Libyan boy who underwent partial colectomy for Hirschsprung’s disease of the sigmoid colon. 12 days after closure of colostomy he had severe, offensive diarrhoea. The faeces contained many trophozoites of C. mesnili and a few cysts of Entamaeba coli and ova of Trichuris trichiura. No enteropathogenic bacteria or yeasts were cultured. In view of these findings and the severity of the symptoms he was treated with a 5-day course of metronidazole, and he rapidly improved. Subsequent foccal examination showed no parasites. The second patient was a 24-year-old nurse who had had diarrhoea whilst travelling overland from Australia through India, Afghanistan, Turkey, and Europe. She presented in England with recurrent diarrhoea, and faecal examination showed many cysts of C. mesnili and a few of Entamaeba coli. Routine bacterial cultures yielded no pathogens. A 2-week course of metronidazole was prescribed, but I do not know the outcome. The third patient was a 33-year-old Iranian woman who has lived in England for 17 years without travelling abroad except for a 7-month visit to Pakistan in 1976-77. A few months before travelling she complained of intermittent diarrhoea; faecal examination revealed no causative organisms. After her return she presented with further diarrhoea, and faecal examination showed numerous trophozoites of C. mesnili but no other parasites or bacterial pathogens. Her symptoms were moderately improved after a course of metronidazole; diarrhoea reduced from ten to four times a day, and parasites were not subsequently seen. She is being investigated for malabsorption. Estimates of infection with C. mesnili of less than 1 to more than 10% of populations have been made, varying with the communities and the age-groups studied.I.2 I know of no figures for the prevalence in Britain or in travellers; perhaps this is because the organism is considered non-pathogenic. Laboratories may mistake the trophozoite for that of Giardia lamblia, which is motile and of a similar size. The cyst is a little smaller than that of G. lamblia, more rounded, and it bears a swelling at one end; it has been described as pear or lemon shaped. Since G. lamblia is now the commonest reported intestinal parasite in Britain3 it is important that it be distinguished from C. mesnili. Most parasitology textbooks record that there is no evidence that C. mesnili is a pathogen. The 1966 edition of Manson’s Tropical Diseases discussed the difficulty in assessing its patho-
tozoon more
prevalent
1. Craig and Faust’s Clinical Parasitology; p 63. New York, 1970. 2. Smyth, J. D. Introduction to Animal Parasitology, p. 48. London, 1976. 3. Commun. Dis. Rep Quarterly edition, 77/1-13; p. 18. Communicable Disease Surveillance Centre, Public Health Laboratory Service, 1977.
1078
genicity and suggested that in some cases it might be a cause of "flagellate dysentery". However, the 1972 edition says "... there is no evidence of pathogenicity."4 In the first patient there was no other microbiological explanation of diarrhoeaEntamaeba coli and small numbers of T. trichiura are very unlikely to cause this-and there was a prompt response to metronidazole. These features suggest that C. mesnili was a pathogen in this case. In the other patients it was considered that diarrhoea with profuse excretion of the flagellates in the absence of any other microbiological explanation deserved treatment with metronidazole. The results of treatment are not known (case 2) or difficult to interpret (case 3, probably an infection superimposed on long-standing diarrhoea of different
aetiology). G. lamblia has for long been known as a cause of diarrhoea in children but it is only lately that it has been recognised as an important parasite in adults, notably travellers. Endemic giardiasis is usually asymptomatic but travellers to endemic areas run a high risk of disease; the reasons for this are not clear.s A comparable situation might exist with C. mesnili which in tropical areas is often found in patients without symptoms: it may be more harmful to the traveller.
PROTECTION AGAINST POLIOMYELITIS
SIR,-During the first half of 1977, 12 cases of poliomyelitis reported in England and Wales. This prompted us to look for evidence of immunity against polio by serological studies on specimens received in this laboratory. 1016 sera sent in during the period May, 1976, to March, 1977, were tested for neutralising antibodies to polio viruses 1, 2, and 3 by a microplate technique with the serum at a dilution of 1/4. Sera from children under 1 year of age were excluded because immunisation is not normally completed until a child is a year old. The only other criteria for inclusion of a specimen were that the age and sex of the patient and the locality from which the specimen was submitted were known and that were
for our purposes remained after the test had been done. 457 (45%) of the 1016 specimens requested examined were submitted by hospitals and general practitioners in Newcastle; most of the others came from Northumberland, Tyne & Wear, Durham, and Cleveland. 539 (53%) of the specimens were from males. sufficient
serum
TABLE I-PRESENCE OF ANTIBODY TO INDIVIDUAL POLIO VIRUSES
I thank Mr H. H. Nixon and Dr 1. Grimble for permission to publish details of patients under their care and Mr P. Sargeaunt of the London School of Hygiene and Tropical Medicine for confirming the identity of the parasites m the first patient.
Department of Bacteriology, St Mary’s Hospital (Harrow Road), London W9 3RL
M. BARNHAM
FREE-LIVING AMŒBÆ IN HOME DIALYSIS UNIT
SIR,-During April and May of this year a 46-year-old man with chronic renal failure had several episodes of pyrexia with rigors. He was on haemodialysis which, since December, 1974, had been done at home on a Dylade III B unit with a multipoint Kiil dialyser. His symptoms seemed to be caused by pyrogenic toxins. Microbiological examination of his dialysis unit revealed high bacterial colony-counts, predominantly of Aeromonas hydrophila at several points, and large numbers of amoebae resembling Acanthamaeba in the Hartmannellida;.6Small numbers of both A. hydrophila and amocbae were found in the mains water supply. Three more filters (25, 2, and 0-2 pm pore size) were placed between the water softener and the dialyser unit. Other measures included allowing several litres of dialysate to run to waste before starting dialysis and draining as much of the tubing as possible afterwards. Despite the use of 4% formalin and the additional filters counts remain high. It may be that the dialysate sterilising bath increases endotoxin levels by heat disruption of cells. However, the patient is now well and has returned to work. The most beneficial measures were, therefore, probably flushing and running to waste of dialysate before dialysis begins. Aeromonads are known to be endotoxic and pathogenic and hartmannellid amoebae have also been shown to be toxigenic’7 and they have been associated with primary amoebic encephalitis,’ chronic brain abscess,9 and destructive eye lesions’o Public Health Laboratory, General Hospital, Middlesbrough, Cleveland
4.
5. 6. 7. 8. 9. 10.
D. P. CASEMORE
Wilcocks, C., Manson-Bahr, P. E. C. Manson’s Tropical Diseases; p. 982. London, 1972. Morbid. Mortal. wkly Rep. 1976, 25, no. 38, p. 307. Center for Disease Control, Atlanta, 1976 Page, F. C. J. Protozool. 1967, 14, 709. McIntosh, A. H., Chang, R. S. ibid. 1971, 18, 632. Carter, R. F. Trans. R. Soc. trop. Med. Hyg. 1972, 66, no. 2. Jager, B. V., Stamm, W. P. Lancet, 1972, ii, 1343. Nagington, J. ibid. 1974, ii, 1537.
TABLE II-DISTRIBUTION BY AGE OF IMMUNE STATE TO POLIO
The age distribution is shown in table I, with the percentage of each age-group with detectable antibodies to the individual polio viruses. Table ii shows the percentage in each age-group who have detectable antibodies to three, two, one, or no polio viruses. 70% of sera had antibodies to polio 1, 75% to polio 2, and 57.5% to polio 3. For all three viruses the percentage of sera with antibodies was higher in the 30-40 year age-group than in any other age-group. This difference was statistically significant for polio 3 (r=0-0005). 49% had antibody to all three viruses, and it is disturbing to see that only 43% of children aged 1-5 and 40% of those aged 5-19 had full immunity. A further 21c’, of the population had antibodies to two of the three viruses.
These results suggest that the immune state against poliovirus of the population examined is unsatisfactory; this is particularly so for those under 20 years of age. There is no indication that a reduction in the numbers receiving primary vaccination has been compensated for by transmission of vaccine virus to unvaccinated contacts of those recently vaccinated. If this level of immunity persists or falls even further, the introduction of wild polio strains, given suitable seasonal and environmental conditions, is likely to lead to outbreaks and an increase in the incidence of paralytic disease. Public Health Laboratory, General Hospital, Newcastle upon Tyne NE4 6BE
A. A. CODD E. WHITE
Hospital
for technical
help. Hotel-Dieu Hospital, Section of Endocrinology, Metabolism, and Nutrition
and INRS-Santé, L H. Lafontaine Hospital, Montreal, Canada
A. GATTEREAU P. BIELMANN J. DURIVAGE P. LAROCHELLE
University Eye Hospital,
ECONOMIC ASPECTS OF REDUCED SMOKING
90220 Oulu 22, Finland
SIR,-Professor Atkinson and Mrs Townsend refer to my review of the McGuiness and Cowling study in the section of their paper headed Restrictions on Advertising (Sept. 3, p. 492). They say "Moreover a different interpretation of their [McGuiness and Cowling] results by Johnston gives a longterm elasticity with respect to advertising of less than half the reported value. Taking Johnston’s figure, total abolition of television advertising and the reduction of expenditure on advertising in the press by three-quarters could reduce consumption
by 10%." It would be a serious mistake to regard my amendment of the McGuiness and Cowling results as a reliable base for such a forecast. My review contained several criticisms of the study, one of which was that a conceptual and mathematical error had been made in the definition of the long-run elasticity. To demonstrate the effect of this error alone, I showed that their own statistical work implied an elasticity of approximately half the reported figure. The amended figure is still vitiated by the other criticisms. Even with a well-founded statistical study, prediction of the effects of very large changes, such as those considered by Atkinson and Townsend, is subject to ever-
increasing error. Department of Econometrics, University of Manchester * Present address:
J. JOHNSTON
Department of Economics, Emory University, Atlanta, Geor-
gta30322, U.S.A.
TRANSMISSION OF TOXOPLASMOSIS BY TROPHOZOITES
SIR,-It is generally accepted that toxoplasmosis is transby transplacental infection, cysts, and oöcystS.1.2 The
mitted
trophozoites of Toxoplasma gondii are assumed to be unable to resist changes in osmotic pressure, and they are thought not to survive outside the body long enough to transmit the infectionHowever, this assumption has not been proved experimentally. We have studied the role of trophozoites in the transmission of toxoplasmosis by testing their survival and pathogenicity in liquid media. In acute Toxoplasma infections the trophozoites are widely distributed throughout the body and they are liberserous exudates, fseces, urine, saliva, sputum, nasal and conjunctival secretions, vaginal discharges, semen, and milk.4. The trophozoites remain infectious in secretions for several days, in milk for 6 days, in saliva for 5 days, in urine for 7 days, and in tears for 4 days.6 In these media the trophozoites are relatively resistant to changes in osmotic pressure but they will not withstand drying or freezing.3·g The trophozoites
ated in
easily
penetrate intact
mucous
membranes from infected
se-
cretions,9 the smallest infective dose in man being approximately ten parasites.10 These observations suggest that tropho1. Frenkel, J. K., Dubey, J. P. J. infect. Dis. 1972, 126, 664. 2. Hartley, W. J., Munday, B. L. Aust. vet. J. 1974, 50, 224. 3. Jacobs, L. J. Wildlife Dis. 1970, 6, 305. 4 French, J. G., Messinger, H. B., MacCarthy, J. Am. J. Epidem. 185. 5. Janitschke, K. Germ. Med. 1971, 1, 23. 6. Saari, M., Räisänen, S. Acta ophthal., Copenh. 1974, 52, 847. 7 Raisänen, S., Saari, M. Med. Biol. 1976, 54, 152. 8 Saari, M., Räisänen, S. Nordic Coun. arct. med. Res. Rep. 1976, 16, 9 Räisänen, S. Acta univ. tamp. A, 1977, 84, 1. 10 Brown, J., Jacobs, L. Ann. intern.Med. 1956, 44, 565.
1970, 91,
31.
zoites may transmit the disease during acute stages of toxoplasmosis. Transmission of toxoplasmosis by trophozoites may explain why persons in contact with animals possess Toxoplasma antibodies significantly in excess of the average level, transmission of the disease in cattle, and occurrence of the parasite in areas where felines are not present.
Institute of Biomedical Sciences,
University of Tampere
K. M. SAARI S. A. RÄISÄNEN
IS CHILOMASTIX HARMLESS?
SIR,-Chilomastix mesnili is
a cosmopolitan .1agellate proin warm than cool climates. It is generally regarded as a harmless commensal in man transmitted by the faecal-oral route in the cyst form, with trophozoites living in the caecum and ascending colon. I have seen three patients from abroad who have had diarrhoea with C. mesnili and no other significant microbial pathogen detected in the fseces. In at least one patient treatment with metronidazole was followed by rapid and lasting improvement. The first patient was a 5-year-old Libyan boy who underwent partial colectomy for Hirschsprung’s disease of the sigmoid colon. 12 days after closure of colostomy he had severe, offensive diarrhoea. The faeces contained many trophozoites of C. mesnili and a few cysts of Entamaeba coli and ova of Trichuris trichiura. No enteropathogenic bacteria or yeasts were cultured. In view of these findings and the severity of the symptoms he was treated with a 5-day course of metronidazole, and he rapidly improved. Subsequent foccal examination showed no parasites. The second patient was a 24-year-old nurse who had had diarrhoea whilst travelling overland from Australia through India, Afghanistan, Turkey, and Europe. She presented in England with recurrent diarrhoea, and faecal examination showed many cysts of C. mesnili and a few of Entamaeba coli. Routine bacterial cultures yielded no pathogens. A 2-week course of metronidazole was prescribed, but I do not know the outcome. The third patient was a 33-year-old Iranian woman who has lived in England for 17 years without travelling abroad except for a 7-month visit to Pakistan in 1976-77. A few months before travelling she complained of intermittent diarrhoea; faecal examination revealed no causative organisms. After her return she presented with further diarrhoea, and faecal examination showed numerous trophozoites of C. mesnili but no other parasites or bacterial pathogens. Her symptoms were moderately improved after a course of metronidazole; diarrhoea reduced from ten to four times a day, and parasites were not subsequently seen. She is being investigated for malabsorption. Estimates of infection with C. mesnili of less than 1 to more than 10% of populations have been made, varying with the communities and the age-groups studied.I.2 I know of no figures for the prevalence in Britain or in travellers; perhaps this is because the organism is considered non-pathogenic. Laboratories may mistake the trophozoite for that of Giardia lamblia, which is motile and of a similar size. The cyst is a little smaller than that of G. lamblia, more rounded, and it bears a swelling at one end; it has been described as pear or lemon shaped. Since G. lamblia is now the commonest reported intestinal parasite in Britain3 it is important that it be distinguished from C. mesnili. Most parasitology textbooks record that there is no evidence that C. mesnili is a pathogen. The 1966 edition of Manson’s Tropical Diseases discussed the difficulty in assessing its patho-
tozoon more
prevalent
1. Craig and Faust’s Clinical Parasitology; p 63. New York, 1970. 2. Smyth, J. D. Introduction to Animal Parasitology, p. 48. London, 1976. 3. Commun. Dis. Rep Quarterly edition, 77/1-13; p. 18. Communicable Disease Surveillance Centre, Public Health Laboratory Service, 1977.
1078
genicity and suggested that in some cases it might be a cause of "flagellate dysentery". However, the 1972 edition says "... there is no evidence of pathogenicity."4 In the first patient there was no other microbiological explanation of diarrhoeaEntamaeba coli and small numbers of T. trichiura are very unlikely to cause this-and there was a prompt response to metronidazole. These features suggest that C. mesnili was a pathogen in this case. In the other patients it was considered that diarrhoea with profuse excretion of the flagellates in the absence of any other microbiological explanation deserved treatment with metronidazole. The results of treatment are not known (case 2) or difficult to interpret (case 3, probably an infection superimposed on long-standing diarrhoea of different
aetiology). G. lamblia has for long been known as a cause of diarrhoea in children but it is only lately that it has been recognised as an important parasite in adults, notably travellers. Endemic giardiasis is usually asymptomatic but travellers to endemic areas run a high risk of disease; the reasons for this are not clear.s A comparable situation might exist with C. mesnili which in tropical areas is often found in patients without symptoms: it may be more harmful to the traveller.
PROTECTION AGAINST POLIOMYELITIS
SIR,-During the first half of 1977, 12 cases of poliomyelitis reported in England and Wales. This prompted us to look for evidence of immunity against polio by serological studies on specimens received in this laboratory. 1016 sera sent in during the period May, 1976, to March, 1977, were tested for neutralising antibodies to polio viruses 1, 2, and 3 by a microplate technique with the serum at a dilution of 1/4. Sera from children under 1 year of age were excluded because immunisation is not normally completed until a child is a year old. The only other criteria for inclusion of a specimen were that the age and sex of the patient and the locality from which the specimen was submitted were known and that were
for our purposes remained after the test had been done. 457 (45%) of the 1016 specimens requested examined were submitted by hospitals and general practitioners in Newcastle; most of the others came from Northumberland, Tyne & Wear, Durham, and Cleveland. 539 (53%) of the specimens were from males. sufficient
serum
TABLE I-PRESENCE OF ANTIBODY TO INDIVIDUAL POLIO VIRUSES
I thank Mr H. H. Nixon and Dr 1. Grimble for permission to publish details of patients under their care and Mr P. Sargeaunt of the London School of Hygiene and Tropical Medicine for confirming the identity of the parasites m the first patient.
Department of Bacteriology, St Mary’s Hospital (Harrow Road), London W9 3RL
M. BARNHAM
FREE-LIVING AMŒBÆ IN HOME DIALYSIS UNIT
SIR,-During April and May of this year a 46-year-old man with chronic renal failure had several episodes of pyrexia with rigors. He was on haemodialysis which, since December, 1974, had been done at home on a Dylade III B unit with a multipoint Kiil dialyser. His symptoms seemed to be caused by pyrogenic toxins. Microbiological examination of his dialysis unit revealed high bacterial colony-counts, predominantly of Aeromonas hydrophila at several points, and large numbers of amoebae resembling Acanthamaeba in the Hartmannellida;.6Small numbers of both A. hydrophila and amocbae were found in the mains water supply. Three more filters (25, 2, and 0-2 pm pore size) were placed between the water softener and the dialyser unit. Other measures included allowing several litres of dialysate to run to waste before starting dialysis and draining as much of the tubing as possible afterwards. Despite the use of 4% formalin and the additional filters counts remain high. It may be that the dialysate sterilising bath increases endotoxin levels by heat disruption of cells. However, the patient is now well and has returned to work. The most beneficial measures were, therefore, probably flushing and running to waste of dialysate before dialysis begins. Aeromonads are known to be endotoxic and pathogenic and hartmannellid amoebae have also been shown to be toxigenic’7 and they have been associated with primary amoebic encephalitis,’ chronic brain abscess,9 and destructive eye lesions’o Public Health Laboratory, General Hospital, Middlesbrough, Cleveland
4.
5. 6. 7. 8. 9. 10.
D. P. CASEMORE
Wilcocks, C., Manson-Bahr, P. E. C. Manson’s Tropical Diseases; p. 982. London, 1972. Morbid. Mortal. wkly Rep. 1976, 25, no. 38, p. 307. Center for Disease Control, Atlanta, 1976 Page, F. C. J. Protozool. 1967, 14, 709. McIntosh, A. H., Chang, R. S. ibid. 1971, 18, 632. Carter, R. F. Trans. R. Soc. trop. Med. Hyg. 1972, 66, no. 2. Jager, B. V., Stamm, W. P. Lancet, 1972, ii, 1343. Nagington, J. ibid. 1974, ii, 1537.
TABLE II-DISTRIBUTION BY AGE OF IMMUNE STATE TO POLIO
The age distribution is shown in table I, with the percentage of each age-group with detectable antibodies to the individual polio viruses. Table ii shows the percentage in each age-group who have detectable antibodies to three, two, one, or no polio viruses. 70% of sera had antibodies to polio 1, 75% to polio 2, and 57.5% to polio 3. For all three viruses the percentage of sera with antibodies was higher in the 30-40 year age-group than in any other age-group. This difference was statistically significant for polio 3 (r=0-0005). 49% had antibody to all three viruses, and it is disturbing to see that only 43% of children aged 1-5 and 40% of those aged 5-19 had full immunity. A further 21c’, of the population had antibodies to two of the three viruses.
These results suggest that the immune state against poliovirus of the population examined is unsatisfactory; this is particularly so for those under 20 years of age. There is no indication that a reduction in the numbers receiving primary vaccination has been compensated for by transmission of vaccine virus to unvaccinated contacts of those recently vaccinated. If this level of immunity persists or falls even further, the introduction of wild polio strains, given suitable seasonal and environmental conditions, is likely to lead to outbreaks and an increase in the incidence of paralytic disease. Public Health Laboratory, General Hospital, Newcastle upon Tyne NE4 6BE
A. A. CODD E. WHITE
