Causative Agent Of Worm Infestation

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Tick (Dermacentor species)

Rickettsia rickettsii

Rocky Mountain spotted fever—fever, hemorrhagic rash, confusion

pp. 541-543

Tick (Ixodes species)

Borrelia burgdorferi

Lyme disease—fever, rash, joint pain, nervous system impairment

pp. 543-546

316 Chapter 12 The Eukaryotic Members of the Microbial World Storage area Midgut

Cysts of malaria parasite

316 Chapter 12 The Eukaryotic Members of the Microbial World Storage area Midgut

Cysts of malaria parasite

Causal Organism Worm Infestation

Body cavity

Figure 12.21 Internal Anatomy of a Mosquito Note the storage areas that allow ingestion of large amounts of blood and the salivary glands that discharge pathogens into the host.

Body cavity

Figure 12.21 Internal Anatomy of a Mosquito Note the storage areas that allow ingestion of large amounts of blood and the salivary glands that discharge pathogens into the host.

blood is sucked into a tube formed by the other mouthparts of the insect. The saliva can also cause allergic reactions (the itch of a mosquito bite). After the mosquito has taken more than one blood meal, she can transmit disease from one animal to the next. Viruses found in the blood of the first animal are then transmitted to the next, and so on.

Mosquitoes in an area of arthropod-borne disease can be trapped and identified microscopically, and the blood they have ingested can be tested to see on which kinds of animals the different species are feeding. Precise identification of species and subspecies of these genera is important because different species of mosquitoes differ greatly in their breeding areas, time of feeding, and choice of host. Identification depends largely on microscopic examination of antennae, wings, claws, mating apparatus, and other features. Correct identification is often essential in designing specific control measures. ■ epidemiology, p. 486


Fleas are wingless insects that depend on powerful hind legs to jump from place to place. Points of importance in identifying fleas include the spines (combs) about the head and thorax, the muscular pharynx, the long esophagus, and the spiny valve composed of rows of teethlike cells. Fleas are generally more of a nuisance than a health hazard, but they can transmit the bacterium Yersinia pestis, which causes plague, and a rickettsial disease, murine typhus, to humans. Larval fleas have a chewing type of mouth for feeding on organic matter. They ingest eggs of the common dog and cat tapeworm, Dipylidium caninum, serving as its intermediate host. Children acquire this tapeworm when they accidentally swallow fleas. Fleas can live in vacant buildings in a dormant stage for many months. When the building becomes inhabited, the fleas quickly mature and hungrily greet the new hosts. ■ plague, p. 723


Like fleas, lice are small, wingless insects that prey on warmblooded animals by piercing their skin and sucking blood. The

Causative Agent Pediculosis
Figure 12.22 Pediculus humanus A body louse, which is the vector for Rickettsia prowazekii, the cause of typhus.

legs and claws of lice, however, are adapted for holding onto body surfaces and clothing rather than for jumping. Human lice generally survive only a few days away from their hosts.

Pediculus humanus, the most notorious of the lice, is 1 to 4 mm long, with a characteristically small head and thorax, and a large abdomen (figure 12.22). This louse has a membranelike lip with tiny teeth that anchor it firmly to the skin of the host. Within the floor of the mouth is a piercing apparatus somewhat similar to that of fleas and mosquitoes. Pediculus humanus has only one host—humans—but easily spreads from one person to another by direct contact or by contact with personal items, especially in areas of crowding and poor sanitation.

There are two subspecies, popularly termed head lice and body lice. Body lice can transmit trench fever, which is caused by the bacterium, Bartonella quintana; epidemic typhus, which is caused by the bacterium, Rickettsia prowazekii; and relapsing fever, caused by the bacterium Borrelia recurrentis. Trench fever occurs episodically among severe alcoholics and the homeless of large American and European cities.

The crab louse, Phthirus pubis, is commonly transmitted among young adults during sexual intercourse. It is not a vector of infectious disease, but it can cause an unpleasant itch.


Ticks are arachnids. Arachnids differ from insects in their lack of wings and antennae, and their thorax and abdomen are fused together. Although like insects the immature ticks have three pairs of legs, the adults have four pairs. Dermacentor andersoni, the wood tick, is the vector for Rocky Mountain spotted fever caused by the bacterium Rickettsia rickettsii. Another tick, Ixodes scapularis, transmits with its saliva Borrelia burgdorferi, the spiro-chete that causes Lyme disease. In addition, the saliva of several genera of ticks can produce a profound paralysis, especially in children on whom the tick feeds for several days. Paralyzed humans

Scabies Mite Burrows
Figure 12.23 Sarcoptes scabiei (Scabies Mite) The female burrows into outer skin layers to lay her eggs, causing an intensely itchy rash.

and animals usually recover rapidly following removal of the tick. ■ Rocky Mountain spotted fever, p. 541 ■ Lyme disease, p. 543


Mites, like ticks, are arachnids. They are generally tiny, fast moving, and live on the outer surfaces of animals and plants. Demodex folliculorum and D. brevis are elongated microscopic mites that live in the hair follicles or oil-producing glands usually of the face, typically without producing symptoms. Other species of mites cause human disease.

The disease scabies, caused by a mite, Sarcoptes scabiei, is characterized by an itchy rash most prominent between the fingers, under the breasts, and in the genital area. Scabies is easily transmitted by personal contact, and the disease is commonly acquired during sexual intercourse. The female mites burrow into the outer layers of epidermis (figure 12.23), feeding and laying eggs over a lifetime of about 1 month. Allergy to the mites is largely responsible for the itchy rash. The diagnosis can only be made by demonstrating the mites, since scabies mimics other skin diseases. Treatment of scabies is easily accomplished with medication applied to the skin. Sarcoptes scabiei is not known to transmit infectious agents.

Mites of domestic animals and birds can cause an itchy rash in humans, as can mites sometimes present in hay, grain, cheese, or dried fruits. The dust mites that often live in large numbers in bedrooms can sometimes cause asthma when the mites and their excreta are inhaled.

The mites of rodents can transmit rickettsial diseases to humans. Rickettsial pox, caused by Rickettsia akari transmitted by mouse mites, is a mild disease characterized by fever and rash. Epidemics occur periodically in cities of the eastern United States. Serious rickettsial diseases of other parts of the world such as scrub typhus are transmitted by rodent mites.

12.5 Multicellular Parasites: Arthropods and Helminths 317


In addition to the arthropods that can lead to disease in humans, the other group of multicellular animals that causes human disease are the helminths. In humans, the helminths that cause disease generally belong to one of three classes: the nematodes, or roundworms; the cestodes, or the tapeworms; and the trematodes, or the flukes.

These multicellular parasites have been controlled in the developed nations, but they continue to kill many millions in underdeveloped parts of the world. Helminths enter the body in a number of ways. They may be eaten in contaminated food, be passed through insect bites, or directly penetrate the skin. They cause disease by invading the host tissues or robbing the host of nutrients. Some helminths have complex life cycles, involving one or more intermediate hosts where early stages of development occur, and a definitive host where the sexually mature forms occur.

Nematodes or Roundworms

The nematodes or roundworms have a cylindrical tapered body with a tubular digestive tract that extends from the mouth to the anus. There are both male and female nematodes. Nematodes include a large number of species. Many nematodes are free-living in soil and water. Others are parasites of human and other animals and plants and produce serious disease.

The nematodes that cause disease can be divided into two groups—the ones that inhabit the gastrointestinal tract of the host, and the ones that are found in the blood and other tissues of the host. Generally, diagnosis of worm infestation depends on microscopic identification of the worms or their ova (eggs), or on blood tests for antibody to the worms. Table 12.6 summarizes the major diseases caused by these parasites.

Cestodes or Tapeworms

Cestodes or tapeworms have flat, ribbon-shaped bodies that are segmented. The head (scolex) of the tapeworm has suckers for attachment and sometimes has hooks. Directly behind the head is a region that produces the reproductive segments (proglottids). Each segment has both male and female sex organs. The tapeworm does not have a digestive system but rather absorbs nutrients directly. Tapeworms are often associated with beef, lamb, pork, and fish. Transmission of these organisms to humans often occurs when the flesh of these animals is eaten either uncooked or undercooked (figure 12.24). Some tapeworms are transmitted to humans from ingesting fleas infected with dog or cat tapeworms. Table 12.6 lists specific tapeworm diseases.

Trematodes or Flukes

Trematodes or flukes are bilaterally symmetrical, flat, and leaf-shaped. They have suckers that hold the organism in place as well as suck fluids from the host. Most species are hermaphroditic (have both sex organs in the same worm). Most trematodes have a complicated life cycle, which may include one or more intermediary hosts. Usually, the worms begin with a larval form

318 Chapter 12 The Eukaryotic Members of the Microbial World

Table 12.6 Nematodes, Cestodes, and Trematodes

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  • Maddison
    Which is causative agent of worm?
    4 years ago
  • Kelsey
    Which groups of arthropods transmit the causative agents of Plague and murine typhus to humans?
    4 years ago
    What is worm infestation and what are the transmission agents?
    4 years ago
  • Selene
    Which of these groups of arthropods transmit the causative agent of malaria to humans?
    3 years ago
    What is the cosative agent of worm?
    3 years ago
  • Stuart
    What is a causative agent for tick infestation ?
    3 years ago
  • kody simpson
    What is the causative agent of lice?
    3 years ago
  • charley
    What is the causative agent of louse?
    3 years ago
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    What is the causative agent of dracontiasis?
    3 years ago
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    What is the causative agent of guinea worm?
    3 years ago
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    What is the causative organism of lice?
    3 years ago
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    What is the causative organism of tapeworm?
    2 years ago
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    What are the worms and their causative organs?
    2 years ago
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    What are the causative organism for worm infestation?
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    What are the disease cause causative agent of worm?
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    What is the causative agent of Tape worm?
    2 years ago
  • conlan paterson
    Is bladderworm the causative agent of tapeworm?
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    What is causative agent of fluke disease?
    2 years ago
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    What are the causative agents of earthworm?
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    What the causative of tapworm?
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    What is a causative agent in worms infestation?
    2 years ago
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    What is the causative organism of a round worm?
    2 years ago
  • Rosa Brandybuck
    What is causative agent of worm infestation?
    1 year ago

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