Multicellular Parasites Arthropods and Helminths

A number of disease-causing multicellular organisms are also studied using the same microscopic and immunological techniques that are used to study microorganisms and viruses. As a result, they are included here. Most of the medically important multicellular parasites fall into one of two groups: arthropods and helminths. The arthropods are more highly advanced on the evolutionary scale and include the insects, ticks, lice, and mites. Their main medical importance is that they serve as vectors that may transmit microorganisms and viruses to humans. The helminths, which include the nematodes (roundworms), cestodes (tapeworms), and the trematodes (flukes), are more primitive animals. In only a few instances do they transmit microbial infections to their host animal. Instead, they cause disease by invading the host's tissues or robbing it of nutrients.

Most multicellular parasites have been well controlled in the industrialized nations, but they still cause death and misery to many millions in the economically underdeveloped areas of the world. Our need to know about these problems has come about because more people are traveling farther, more people are moving from one place to another, and more goods are being exchanged worldwide. A clear example of this occurred in New York City in the summer of 1999 when West Nile fever was contracted by a number of people. At least 61 persons suffered serious disease and seven people died. A significant number of crows died at the same time and were found to be carrying the disease. In addition to birds and people, horses, cats, and dogs were also found to carry the virus. It is not clear how the virus arrived in New York City, but it perhaps could have been carried by a traveler from Africa, West Asia, or the Middle East, where it is commonly found. It could possibly have been brought by an imported bird from the same areas. Worldwide travel makes us more vulnerable to diseases from other parts of the world.

12.5 Multicellular Parasites: Arthropods and Helminths 315

In addition, worldwide climatic conditions are changing and bringing increases in certain insect populations to areas that were previously free of them. As a result, more cases of multicellular parasitic infections are being seen by physicians in the United States than previously.


The arthropods include insects, ticks, fleas, and mites. Arthropods act as vectors for transmitting diseases. In some instances, an arthropod such as a fly simply picks up a pathogen on its feet from some contaminated material such as feces and then lands on food that is then eaten by humans, thus transmitting the pathogen. In this case, the fly acts as a mechanical vector. In other cases, such as with Plasmodium sp., the cause of malaria, the vector, a mosquito, is a host for the organism before it transfers that organism to a human through a bite on the skin. In this case, the vector acts as an essential part of the life cycle of the organism and is known as a biological vector. The pathogen actually multiplies in number within the vector. ■ mechanical vector, p. 491 ■ biological vector, p. 491

Examples of some important arthropods, the agents they transmit, and the resulting diseases are shown in table 12.5.


The female mosquito needs the blood of a warm-blooded animal for the proper development of her eggs. To get this, she needs to bite such an animal. The mosquito can take in as much as twice its body weight in blood, thus giving it a relatively good chance of picking up infectious agents such as malarial parasites circulating within the host's capillaries. The anatomy of a mosquito is particularly adapted to transmit disease (figure 12.21). The mouthparts of the female mosquito consist of sharp stylets that are forced through the host's skin to the subcutaneous capillaries. One of these needlelike stylets is hollow, and the mosquito's saliva is pumped through it. The saliva increases blood flow and prevents clotting as the victim's

Table 12.5 Some Arthropods That Transmit Infectious Agents


Infectious Agent

Disease and Characteristic Features


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