Introduction Human body lice

Public Health Significance of Urban Pests 291 fabric fibres Nuttall, 1917. The length of time required for eggs to hatch has never satisfactorily been determined and the only study of body lice Leeson, 1941 sho- wed that hatching was achieved between 6 and 11 days at temperatures likely to be encountered under clothing 29–37°C. After emerging from the egg, lice go through three nymphal stages, each of which takes about three days to complete development Nuttall, 1917; Lang, 1975. The final moult from the third nymphal stage gives rise to the adult louse Fig. 9.2. Male lice often develop more rapidly than their female counterparts. Lang 1975 also observed that females mature for about one day after the final moult before they are able to lay eggs. In all stages of development, lice feed at regular intervals throughout the day, with the youngest nymphs requiring more frequent blood-meals than third instar nymphs and adults Lang, 1975, presumably due to their smaller size. All lice produce dry faecal mat- ter that may accumulate in clothing or hair; this dry matter is the vehicle for transmitting the infective disease organisms for which lice may be vectors. All bionomic studies of lice are inadequate. Theoretical estimates of the growth of louse populations for example, Evans Smith, 1952 are based on limited data and appear to overestimate the rate of increase, by not addressing juvenile mortality, migration and des- truction by the host. In some cases, destruction by the host appears to account for more than 50 of juveniles, so that some people who have never knowingly had an infestation can be found with long hatched nits empty eggshells. If populations grew as fast as esti- mated, the numbers of insects would soon become apparent to the host; however, expe- rience indicates the contrary, in that infestations may go undetected for weeks or even months. When deprived of access to food, body lice survive longer than other species of human lice, presumably due to their larger body mass and to the historical selection of insects more capable of resisting the dehydration that results from starvation induced by their host removing their clothing. Buxton 1947 reported survival for up to 10 days at 15°C, with shorter survival at higher and lower temperatures, ranging from 7 days at 10°C to 2 days at 36°C. All three species of human lice are distributed throughout human populations, and it is likely that they affect all communities to some extent. However, the increase of affluence in Europe and North America over the past 50 years has relegated body louse infesta- tions to vagrants, the homeless and, in recent conflicts, refugees Gratz, 1985. Head lice Human body lice 290

9.1. Introduction

Human lice are host-specific, haematophagous, obligate ectoparasites that live their whole lives on their hosts. Few studies of their biology or physiology have been conducted Burgess, 2004, and most of them were conducted in the first half of the 20 th century, primarily and were focused on body or clothing lice Pediculus humanus. That species constituted a major health risk as a vector of transmissible disease, especially during the First World War in Europe, the depression of the 1930s, and around 19421943 in parts of North Africa and southern Europe Soper et al., 1945, 1947. Environmental health practitioners who had responsibility for lice in most countries until the 1970s considered human lice of all three species – body lice P. humanus, head lice Pediculus capitis and crab lice Phthirus pubis – as pests. Subsequently, rationalization of health and environmental services has transferred responsibility for control of lice to other services or to parent carers, as in the case head lice. Should the need arise, legisla- tion that still exists in many countries could be used to control these insects – for exam- ple, in an arthropod-borne outbreak of disease. Thus, environmental health officers, say in the United Kingdom, can address verminous conditions – that is, people infested with parasites, such as body lice – by using Section 85 of the Public H ealth Act of 1936. However, such legislation requires a foreknowledge of the existence of an infestation, which limits its effectiveness. Body lice, although relatively uncommon now in most high- and medium-income coun- tries, do still constitute a health risk. Many of the services that formerly were in place to deal with these infestations no longer exist and, for the most part, the comparatively small numbers of indivi- duals affected can obtain relief from a variety of volun- tary agencies. Recently recognized changes in disease demography, however, suggest that body lice may still constitute a small risk of disease to people Raoult Roux, 1999. T his chapter concentrates on body lice as an environ- mental health problem. Head lice and crab lice, although essentially similar in physiology and behaviour, constitute specific clinical hazards for which the remedy is found in the clinical medical context rather than the environmen- tal health context. Therefore, these two species are out- side the scope of this report and are mentioned less exten- sively.

9.2. Biological factors