The helminths and arthropods

6 The helminths and arthropods



Introduction


The term ‘helminth’ is used for all groups of parasitic worms. Three main groups are important in humans: the tapeworms (Cestoda), the flukes (Trematoda or Digenea) and the roundworms (Nematoda). The first two belong to the Platyhelminths or flatworms; the third are included in a separate phylum. Platyhelminths have flattened bodies with muscular suckers and/or hooks for attachment to the host. Nematodes (roundworms) have long cylindrical bodies and generally lack specialized attachment organs. Helminths are generally large organisms with a complex body organization. Although invading larval stages may measure only 100–200    μm, adult worms may be centimetres or even metres long. Infections are commonest in warmer countries, but intestinal species also occur in temperate regions. The arthropods are the largest and arguably most successful single group of animals. Those of most relevance to human disease are the insects, ticks and mites. Many of these have adapted to live on humans or use humans as sources of food (blood and tissue fluid). Linked with these feeding habits is the ability of many arthropods to transmit a very wide variety of microbial pathogens. Others, acting as intermediate hosts, may transmit helminth parasites when eaten, and yet other species can inflict dangerous bites and stings.



The helminths




Transmission of helminths occurs in four distinct ways


Transmission routes are summarized in Figure 6.1. Infection can occur after:



• swallowing infective eggs or larvae via the faecal–oral route


• swallowing infective larvae in the tissues of another host


• active penetration of the skin by larval stages


• the bite of an infected blood-sucking insect vector.


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Figure 6.1 How helminth parasites enter the body.


The greater frequency of helminths in tropical and subtropical regions reflects the climatic conditions that favour survival of infective stages, the socioeconomic conditions that facilitate faecal–oral contact, the practices involved in food preparation and consumption, and the availability of suitable vectors. Elsewhere, infections are commonest in children, in individuals closely associated with domestic animals and in individuals with particular food preferences.


Many helminths live in the intestine, while others live in the deeper tissues. Almost all organs of the body can be parasitized. Flukes and nematodes actively feed on host tissues or on the intestinal contents; tapeworms have no digestive system and absorb predigested nutrients.


The majority of helminths do not replicate within the host, although certain tapeworm larval stages can reproduce asexually in humans. In most, sexual reproduction results in the production of eggs, which are released from the host in faecal material. In others, reproductive stages may accumulate within the host, but do not mature. The nematode Strongyloides is exceptional in that eggs produced in the intestine can hatch there, releasing infective larvae, which re-invade the body – the process of ‘autoinfection’.



The outer surfaces of helminths provide the primary host–parasite interface


In tapeworms and flukes, the surface is a complex plasma membrane, and in both there are protective mechanisms to prevent the host damaging the outer surface. The nematode outer surface is a tough collagenous cuticle, which, although antigenic, is largely resistant to immune attack. However, smaller larval stages may be damaged by host granulocytes and macrophages. Worms release large amounts of soluble antigenic material in their excretions and secretions, and this plays an important role both in immunity and pathology.



Life cycles



Many helminths have complex life cycles


In direct life cycles, reproductive stages produced by sexually mature adults in one host are released from the body and can develop directly to adult stages after infection of another host via the faecal–oral route (Ascaris) or by direct penetration (hookworms). Indirect cycles are those where reproductive stages must undergo further development in an intermediate host (tapeworms) or vector (filarial worms) before sexual maturity can be achieved in the final host.



The larvae of flukes and tapeworms must pass through one or more intermediate hosts, but those of nematodes can develop to maturity within a single host


Most flukes are hermaphrodites, except the schistosomes, which have separate sexes. The reproductive organs of tapeworms are replicated along the body (the strobila) in a series of identical segments or ‘proglottids’. The terminal ‘gravid’ proglottids become filled with mature eggs, detach and pass out in the faeces. The eggs of both flukes and tapeworms develop into larvae that must pass through one or more intermediate hosts and develop into other larval stages before the parasite is again infective to humans. The tapeworm Hymenolepis nana, occasionally found in humans, is exceptional and can go through a complete cycle from egg to adult in the same host.


In nematodes, the sexes are separate. Most species liberate fertilized eggs, but some release early-stage larvae directly into the host’s body. Development from egg or larva to adult can be direct and occur in a single host, or may be indirect, requiring development in the body of an intermediate host. Classification of nematodes is complex, and for practical purposes only two categories of human-specific nematodes are considered here:



• those that mature within the gastrointestinal tract, some of which may migrate through the body during development (e.g. Ascaris, hookworms, Trichinella, Strongyloides, Trichuris)


• those that mature in deeper tissues (e.g. the filarial nematodes).


In addition, humans can be infected with the larvae of species that mature in other hosts (e.g. the dog parasites Toxocara canis and Ancylostoma brasiliense).



Helminths and disease



Adult tapeworms are acquired by eating undercooked or raw meat containing larval stages


Tapeworms frequently infect humans, but are relatively harmless despite their potential for reaching a large size. Humans can also act as the intermediate hosts for certain species, and the development of larval stages in the body can cause severe disease (Table 6.1).



Table 6.1 Summary of the location, transmission and other hosts used by tapeworms that infect humans

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Jul 9, 2017 | Posted by in MICROBIOLOGY | Comments Off on The helminths and arthropods

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