It is rare for one parasitologist to have the opportunity to continue investigations over half a century. Don Thomas (University of Sussex, UK) has had the insight to realize how important this kind of observation can be and has contributed a fascinating chapter concerning the ecology of helminth parasites and their fish hosts. He first carried out ecological studies involving salmonid fish in Welsh rivers in the early nineteen fifties, his interest being their feeding behaviour, helminth parasites and intermediate hosts. An opportunity to follow up these studies in 1998 made it possible to ascertain how changing environmental conditions might have impacted on the salmonid hosts and their parasites. Don uses this study as a backdrop to delve into many important questions concerning interactions between populations of helminth parasites, their hosts and their environment. There is much to stimulate the reader here together with ideas for future research.

Almost twenty years have passed since the last detailed review on avian schistosomes within the genus Trichobilharzia by Blair and Islam (1983). So what has been happening in the interim? In a comprehensive chapter Petr Horak, Libuse Kolarova (Charles University, Czech Republic) and Coen Adema (University of New Mexico, USA) bring together a wealth of new information concerning recognized Trichobilharzia species, their biology, interactions with their hosts and the pathology associated with infection. Particular attention is given to problems concerning the systematics of the genus and to morphological and molecular methods available for species characterization. Due to the non-specific penetration of the cercariae through the skin, contact with water bodies containing infected snails and cercariae can lead to cercarial dermatitis in man. Cercarial dermatitis is a common condition, which with few exceptions occurs globally. Of perhaps greater concern is that developing schistosomula have now been reported from various organs and nervous tissue of mammals, which may lead to further complications. The chapter ends by considering the epidemiology of cercarial dermatitis including the identification of potential transmission sites and possible control measures.

In the third chapter, Bob Snow and Kevin Marsh, from the Kenya Medical Research Institute, discuss the complicated relationships between intensity of malarial transmission, which can range from fewer than one infective mosquito bite per year to several thousand such bites, and the burden of mortality and morbidity. At the lower end of the range, all age groups are at risk of developing severe malaria with, mainly, cerebral involvement, whereas at the higher end it is predominantly infants who are at risk of severe disease, and malarial anaemia rather than cerebral malaria dominates the clinical picture. This of course reflects the increasingly rapid development of immunity as transmission intensity increases. The authors conclude that it is in areas of moderate to high transmission that interventions such as the use of insecticide-treated bed nets (ITNs) will have the greatest effect in reducing disease and mortality. In areas of high transmission intensity, the data suggest that malarial and all-cause mortality appear to saturate, although the authors agree that this is contentious. If this is so, initial reductions in mortality may be difficult to sustain as even the reduced mortality rate may lie within the saturated region of the curve. However, the overall benefits of using ITNs in reducing the disease burden regardless of transmission intensity and in producing even limited reduction of infant mortality rates emphasize that their use should be encouraged in all areas of Africa where malaria is endemic.

Most helminth infections induce immune responses characterized by production of Th-associated cytokines and antibodies. These have usually taken to be host-protective, with a highly regulated and controlled host inflammatory response directed against parasite antigens and leading to a chronic infection with mild symptoms. The final review by Karl Hoffmann and David Dunne of the Department of Pathology of the University of Cambridge UK, and Thomas Wynne of the Immunology Section, The National Institutes of Health USA, using schistosomiasis as a model demonstrates that, while most chronic induced type-2 associated mediated responses are held in check by regulated control mechanisms, prolonged cytokine biases can be not only undesirable but, in fact, lethal. Uncontrolled polarized type-1 immune responses can also lead to a serious increase in host immuno-pathology.

John Baker Ralph Muller David Rollinson

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