Characteristics of Fungi

Fungi are eukaryotes but are quite different from plants and animals. Fungal colonies are vegetative structures because they are made up of cells involved in catabolism and growth. Whether microscopic or macroscopic, they generally grow as filamentous, multinucleated organisms that form threadlike filaments called hyphae, or as unicellular fungal organisms (Figure 8.2, A). Morphologically, fungi are often divided into three groups.

Most fungi that are identified as human pathogens are microscopic. Hyphae are long, branching filamentous cells of a fungus and they represent its main vegetative growth. A collective mass of the threadlike hyphae is called the mycelium (Figure 8.2, B). A mycelium can be small, forming colonies too small to see with the naked eye, or they can be extensive, such as Armillaria gallica, which was described as extending through 37 acres of forest soil and was estimated to weigh at least 9700 kg.

Depending on environmental conditions such as temperature, fungi can exhibit different morphological characteristics, a condition referred to as dimorphism. They may appear as yeasts, molds, or fleshy fungi. For instance, some fungi can grow as a mold at room temperature (about 25° C) and as yeast at 37° C. Fungi often adapt to an environment that is hostile to bacteria. They are free-living and heterotrophic organisms (i.e., they use organic compounds as their source of carbon). Although fungi, like bacteria, absorb nutrients rather than ingest them, they differ in several nutritional characteristics as well:

Fungi have a cell wall, typically composed of a strong, flexible, nitrogenous polysaccharide called chitin, and a plasma membrane that contains ergosterol. Fungi differ from plants because they lack chlorophyll and therefore do not use photosynthesis in their metabolic processes. Moreover, their ribosomal RNA is an 80S rRNA, which is the same as in humans and animals. For medical/treatment purposes, important aspects of fungal metabolism include the following:

Fungal pathogens can be classified on the basis of their growth forms—filamentous, yeast, or dimorphic—and on the type of infection they can cause. For example, in superficial mycoses, the fungus grows on the surface of the skin or hair. In cutaneous and subcutaneous mycoses, nails and deeper layers of the skin are involved, and in systemic (deep) mycoses, the fungus spreads to internal organs (Box 8.1). Patients with a compromised immune system are especially susceptible to fungal infections. Whereas superficial and cutaneous mycoses are generally mild, systemic mycoses are serious and difficult to treat.

Yeasts

Yeasts exist as single cells that reproduce by budding. However, some species may become multicellular through the formation of connected budding cells referred to as pseudohyphae, or true hyphae (Figure 8.3). They can be classified according to the presence or absence of capsules, the size and shape of the cells, the mechanism of daughter cell formation, formation of hyphae, presence of sexual spores, and other physiological/genetic data. Yeasts may require oxygen for their metabolic activities as obligate aerobes or may be able to continue their metabolic activities in the absence of oxygen as facultative anaerobes. Yeasts do not require light for their growth.

MEDICAL HIGHLIGHTS

Yeast Infections—Candidiasis: The Fungus Among Us

Candidiasis is an infection that can be caused by more than 20 species of Candida, most commonly by Candida albicans. These fungi are often part of the normal human flora, but under certain conditions can become opportunistic organisms and cause yeast infections. The most common infections caused by Candida albicans are vaginal yeast infections, thrush (oral infection), and diaper rash.

Molds

Molds are rapidly growing, asexually reproducing fungi that grow on a variety of substances. They are characterized by the development of hyphae (long filaments of cells joined together), which results in colony characteristics that can be used for identification purposes. Hyphae can grow to immense proportions (see Armillaria gallica, earlier this chapter). In general, the hyphae in molds contain cross-walls called septa, which divide the hyphae into distinct uninuclear, cell-like units called septate hyphae. Some classes of fungi have coenocytic hyphae, which do not contain septa and appear as long, continuous cells with many nuclei (Figure 8.4). In an appropriate environment, the hyphae grow to form a mycelium that is visible to the naked eye.

Life Cycle of Fungi

Although all fungi have some means of asexual reproduction (mitosis), most fungi can also reproduce sexually. Asexual reproduction does not involve genetic recombination between two sexual types, whereas sexual reproduction does involve genetic recombination. Sexual reproduction, as expected, introduces variation in a population.

Asexual Reproduction

Yeast typically bud in a manner similar to the binary fission of prokaryotic organisms (see Chapter 3, Cell Structure and Function; and Chapter 6, Bacteria and Archaea). Filamentous fungi can reproduce asexually by fragmentation of their hyphae, and by spore formation either sexually or asexually. Asexual reproduction results in the formation of sporangia, which ultimately release spores into the environment. Spores, after entering the respiratory tract, are common causes of infection (see Chapter 11, Infections of the Respiratory System). These asexual spores are often categorized on the basis of the nature and manner of spore development:

• Sporangiospores form inside a sac, the sporangium, which is attached to a stalk, the sporangiophore, located on the tips or sides of hyphae. The spores are released when the sporangium ruptures.

• Conidia (conidiospores) are produced at the tips or sides of hyphae but are not enclosed by a saclike structure. Their development involves the pinching off of the tip of a fertile hypha or the segmentation of a preexisting vegetative hypha. They are the most common of the asexual spores and exist in different forms (Figure 8.5):

• Arthrospore: A rectangular spore, formed when a septate hypha breaks at the cross wall

• Chlamydospore: A spherical conidium produced by the thickening of a hyphal cell that is released when the surrounding hyphae crack

• Blastospore: A spore that buds from a parent cell

• Phialospore: A spore that buds from the mouth of a vase-shaped, spore-bearing cell

• Microconidium and macroconidium: Spores formed by the same fungus under different conditions, either as one cell (microconidium) or as two or more cells (macroconidium)

• Porospore: A spore that grows out through small pores in a spore-bearing cell

Classification of Fungi

Traditionally, fungi are divided into four major subgroups: Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota. Some fungi form partnerships with either green algae or cyanobacteria and are considered a separate group: the lichens.

• Zygomycota are coenocytic (multinucleate) molds, most of which are saprobes (fungi that derive their nutrition from nonliving organic material); the remaining are obligate parasites of insects or other fungi.

• Ascomycota are the primary fungi causing food spoilage. This group also includes plant pathogens, both producing a negative impact on the economy. However, many of the Ascomycota are beneficial and probably the best known fungus in this category is Penicillium (Figure 8.6, A), the mold responsible for the production of the antibiotic penicillin (see Chapter 22, Antimicrobial Drugs). Another fungus in this group, Saccharomyces (Figure 8.6, B), ferments sugar to produce alcohol and carbon dioxide, the basic processes necessary in the baking and brewing industries. Furthermore, Neurospora, the pink bread mold, is an important contributor in genetic and biochemical research.

• Basidiomycota include mushrooms (fleshy fungi) and other fruiting bodies of fungi (Figure 8.7). Many of the mushrooms are edible and some produce toxins and/or hallucinatory chemicals. In addition, the fungus Cryptococcus neoformans grows in the form of yeast in humans and is the leading cause of fungal meningitis. Besides the edible and poisonous forms, Basidiomycota are important decomposers, digesting cellulose and lignin of dead plants, which returns essential nutrients back to the soil.

• Deuteromycota are somewhat different than the other subgroups, and sometimes are referred to as “imperfect fungi.” Whereas stages of sexual reproduction in the previously described groups have been well established, the sexual stages of Deuteromycota are unknown. Either this group of fungi does not produce sexual spores, or their sexual spores have not been identified at this time (recall that sexual spores may not form diploid nuclei for centuries).

• Lichens (Figure 8.8) consist of hyphae (mold) of a fungus and cyanobacteria or green algae. The mold, often an ascomycete, surrounds the photosynthetic cell and provides nutrients, water, and protection from drying out and excessive light. Conversely, the algae or cyanobacterium supplies carbohydrates and oxygen, the by-products of photosynthesis, to the fungus.

Characteristics of Algae

All algae have two things in common: all are photosynthetic organisms and all are aquatic. Other than that, they differ in distribution, morphology, reproduction, and biochemical preferences; they are not a unified group. The classification of algae is problematic and has not been completely determined. Historically, the differences in photosynthetic pigments, storage products, and cell wall composition have been used to classify algae into several groups. The groups according to their photosynthetic pigments are as follows: green algae, red algae, brown algae, golden algae, and yellow-green algae.

Life Cycle of Algae

All algae are capable of asexual reproduction. In unicellular algae the nucleus divides by mitosis and when the newly formed nuclei move to the opposite poles of the cell, the cell divides into two new cells by cytokinesis (see Chapter 3, Cell Structure and Function). Multicellular algae also may reproduce asexually by fragmentation, and each fragment is capable of forming a new thallus (the entire vegetative structure) or filaments.

Unicellular algae can also reproduce sexually and each algal cell then serves as a gamete, which can fuse with another gamete to form a zygote. This zygote then undergoes meiosis to return to the haploid state. When multicellular algae reproduce sexually, every cell in the reproductive structures of the algae becomes a gamete. The sexual reproduction of many algae can occur by alternation of haploid and diploid generations (Figure 8.9). In these life cycles, the diploid organism undergoes meiosis to produce male and female haploid spores, which then develop into haploid male and female thalli. In some algae, each of these will produce gametes that fuse to form a zygote, which then creates a new diploid thallus.

In some of the algal species, sexual and asexual reproduction can alternate, depending on the available growth conditions. Other species may alternate generations, whereby the offspring resulting from sexual reproduction reproduce asexually, and the next generation then reproduces sexually.

Classification of Algae

As mentioned earlier, the classification of algae is problematic and a summary of the algal groups described in this section and their characteristics is shown in Table 8.2.

• Chlorophyta are green algae whose pigments are chlorophylls a and b. They use sugar and starch as food reserves. Because of these and other similarities with plants, green algae are often considered to be progenitors of plants. Most green algae are unicellular or filamentous, living in freshwater ponds, lakes, and pools (Figure 8.10). They form a characteristic green to yellow scum.

• Rhodophyta (red algae) contain a red accessory pigment (thus their name) and use glycogen as their energy-storing molecule. Red algae have cell walls containing agar, a gelatinous substance used as a solidifying substance in microbiological culture media. This polysaccharide has many more applications, including its use as a thickener for soups, jellies, and ice cream, to name a few.

• Phaeophyta are commonly referred to as brown algae because of the presence of brown pigments (xanthophylls) in addition to chlorophylls and carotene. Depending on the amount of brown pigments, these algae may appear brown, tan, yellow-brown, greenish brown, or green. Most brown algae are marine organisms.

• Chrysophyta is a diverse group of algae and some taxonomists lately have grouped them with brown algae because of similarities in the nucleotide sequences and flagellar structure. Diatoms, comprising one taxon in this group (Figure 8.11), have a unique cell wall composed of silica, and are a major component of marine phytoplankton. Phytoplankton include photosynthetic microorganisms that form the basis of food chains in the oceans. Significantly, because of their massive numbers, diatoms are the major source of the world’s oxygen.

Characteristics of Protozoans

Protozoans are unicellular organisms that vary in size and are defined by the three characteristics described in the previous section. Protozoa consist of a diverse group of microbes and with the exception of one subgroup they are motile due to cilia, flagella, and/or pseudopodia (see Chapter 3, Cell Structure and Function). Protozoans may vary in size, but all require a moist environment to survive. Most species live in ponds, streams, lakes, and oceans, whereas others live in moist soil, beach sand, and decaying organic matter. Aquatic protozoans are important components of plankton, the basis of the food chain of aquatic animals.

Most protozoa are chemoheterotrophs and obtain their nutrients from various sources, such as by phagocytizing:

However, some protozoans are photoautotrophic, such as the dinoflagellates and euglenoids. These organisms are sometimes classified as algal plants rather than as protozoans, which adds to the problematic nature of their classification.

Protozoans exist in a motile, active, feeding state called the trophozoite during times of plentiful food and moisture. If the environment becomes unfavorable for feeding and therefore growth, many species of protozoans are capable of entering a dormant stage in which the organism exists as a cyst. In this stage the organisms can be distributed by air, which may play a role in transmitting diseases such as amoebic dysentery. Given appropriate moisture and nutrients the cyst breaks open and becomes an active trophozoite.

A few protozoans are pathogens that infect the human body as intracellular or extracellular parasites. Intracellular parasites infect a wide variety of cells including erythrocytes, macrophages, epithelial cells, muscle cells, and cells of the nervous system. Extracellular parasites reside in the blood, intestine, or urogenital system (Table 8.3).

Archaezoa

Archaezoa are protists that do not have mitochondria but instead possess mitosomes—unique cell organelles that appear to be a remnant of mitochondria, but the function of which has not yet been identified. Mitosomes have been seen only in organisms that do not have mitochondria, including anaerobic or microaerophilic organisms. Because these protozoans do not contain mitochondria they cannot use oxidation for energy transformation. Mitosomes were first described in Entamoeba histolytica, an intestinal parasite of humans, and more recently have been identified in some species of Microsporidia as well as in Giardia intestinalis (also called Giardia lamblia). Although there are some similarities between mitochondria and mitosomes, mitosomes do not have genes within them. The genes for the components of these organelles are within the nuclear genome.

Many Archaezoa are spindle shaped and have two or more flagella, located mostly on the anterior (front) end, that are used for whiplike motility and allow the cells to move through their environment (Figure 8.13). Human parasites include Trichomonas vaginalis (see Chapter 17, Sexually Transmitted Infections/Diseases) and Giardia spp. (see Chapter 12, Infections of the Gastrointestinal System). Trichomonas vaginalis lacks a cyst stage and must be transferred from host to host rather quickly before it dries out. Giardia lamblia, on the other hand, is a cyst-forming organism that is excreted in the feces and can survive in the environment before ingested by its next host.

Microspora

Like Archaezoa, Microspora do not have mitochondria and also lack microtubules. They are obligate intercellular parasites in a variety of animals including humans. The organisms have been reported to be the cause of several human diseases, mostly in patients with AIDS.

Ciliophora

Ciliophora or ciliates are probably the best known and most frequently observed unicellular organisms. Ciliates are found almost everywhere there is water; in lakes, ponds, oceans, soil, and they can be obligate and opportunistic parasites. As their name indicates, they have cilia by which they either move themselves or move the water around their surface. Some of the ciliates have only one cilium, whereas others are covered with cilia, or have isolated tufts. All ciliates have a macronucleus and a micronucleus and therefore are capable of sexual as well as asexual reproduction. They are predators of bacteria and other single-celled organisms. A commonly studied organism in college teaching laboratories, and a well-known member of this group, is Paramecium, found in freshwater environments, especially ponds.

In general, ciliates are harmless, and Balantidium coli is the only ciliate pathogenic to humans, causing a disease called balantidiasis. It is the largest protozoan parasite of humans and is known to be able to exist as a trophozoite as well as a cyst. It produces proteolytic enzymes that break down and digest the intestinal epithelium (see Chapter 12, Infections of the Gastrointestinal System).

The classification of ciliates has always been difficult and has undergone several changes, especially lately because of the advances in genetic research. Genetic analysis has shown that many ciliates now grouped together on the basis of their morphological similarity are not necessarily genetically closely related. In the future it can be expected that the taxonomy of ciliates, just as for other eukaryotic microbes, will undergo many revisions as genetic technology continues to advance.

Euglenozoa

Euglenozoa are a large group of flagellated protozoa, including a variety of common free-living species and some important parasites, some of which can infect humans. Euglenoids are well-known flagellates, found in freshwater rich in organic materials. Many are photoautotrophs, but others feed by phagocytosis, preying on bacteria and smaller flagellates. The euglenoids have a semirigid plasma membrane and move with the aid of a flagellum located at the anterior end of the cell. The photoautotrophic euglenoids have a red “eyespot,” an organelle containing carotene, which senses light and directs the cell in the appropriate direction for photosynthesis. Other euglenoids are facultative chemoheterotrophs and ingest organic matter.

Hemoflagellates are blood parasites, transmitted by the bite of blood-feeding insects. They have long, slender bodies and an undulating membrane, giving them mobility in the circulatory system of the host. The genus Trypanosoma (Figure 8.14) includes T. brucei, which is transmitted by the tsetse fly and is the causative agent of sleeping sickness (see Chapter 14, Infections of the Circulatory System).

Amoebozoa

Amoebozoa, or amoebas, are a major group of amoeboid protozoans. Most of them are unicellular and common habitants in soil and water. They move by means of cytoplasmic flow, by extending blunt, lobelike projections of the cytoplasm, called pseudopods (Figure 8.15). The primary way in which these protozoans obtain nutrition is by phagocytosis, whereby the cell surrounds potential food particles and seals them into vacuoles, where digestion followed by absorption occurs. The only organism pathogenic to humans is Entamoeba histolytica, a parasite in the human intestine causing amoebic dysentery (see Chapter 12, Infections of the Gastrointestinal System). Entamoeba is transmitted between humans through ingestion of cysts that are present in the feces of infected persons. Another amoeba that can infect humans, typically immunocompromised people, is Balamuthia, which has been reported to cause brain abscesses referred to as primary amoebic meningoencephalitis (see Chapter 13, Infections of the Nervous System and Senses). Balamuthia is a free-living organism found in water and can enter the body through the lower respiratory tract or through open wounds. The organism cannot be transmitted by person-to-person contact.

Apicomplexa

Apicomplexa are a large group of protozoans characterized by the presence of a unique, complex organelle called the apical complex (Figure 8.16). This complex can be visualized only by electron microscopy and it has been shown to consist of an apical conoid into which the ducts of saclike organelles called rhoptries lead. Furthermore, microtubules extend backward from this complex, apparently to support the surface of the organism. These rhoptries are believed to secrete an adhesive substance that facilitates attachment to the host cell, followed by entry of the parasite through the membrane.

Apicomplexans have a complex life cycle requiring transmission between several hosts. An example is Plasmodium, the causative agent of malaria. Malaria is a devastating parasitic disease, endemic to tropical and subtropical areas of Asia, North and South America, the Middle East, North Africa, and the South Pacific. Plasmodium vivax is the most common among the four human malaria species: P. falciparum, malariae, ovale, and vivax. The organism is transmitted through the bite of infected Anopheles mosquitoes. The infective stage of Plasmodium carried by the mosquito is called the sporozoite. Once injected into the human the sporozoites are carried in the bloodstream to the liver, where they undergo schizogony to produce thousands of progeny called merozoites. Leaving the liver, the merozoites then invade red blood cells and reproduce. The young trophocytes (cells that provide nourishment to other cells) form ringlike structures referred to as a ring stage, which enlarges and divides repeatedly. Eventually the red blood cells rupture and release more merozoites, which then can infect more red blood cells to perpetuate their asexual reproduction. The waste products of the merozoites are responsible for the resulting fever and chills, characteristics of malaria. Some merozoites develop into male and female sexual forms (gametocytes), which can be picked up by a bite of another Anopheles mosquito. The gametocytes then enter the mosquito’s intestine and begin their sexual cycle by uniting to form a zygote. The zygote produces an oocyst in which cell division occurs and asexual sporozoites are formed. After rupturing of the oocytes the sporozoites migrate to the salivary glands of the mosquito and the cycle can start again (Figure 8.17).

Other apicomplexans are Babesia microti, Toxoplasma gondii, and Cryptosporidium.

Babesia microti is a parasite that infects erythrocytes and causes fever and anemia in humans. Before entering the human host, the life cycle of Babesia involves a rodent, primarily the white-footed mouse Peromyscus leucopus, and a Babesia-infected tick that introduces sporozoites into the mouse host. Here the sporozoites undergo asexual reproduction and some parasites differentiate into male and female gametes. Once ingested by an appropriate tick, usually the deer tick Ixodes dammini, they undergo a sporogonic cycle resulting in the production of sporozoites (Figure 8.18). Humans enter the cycle when bitten by infected ticks. The sporozoites enter erythrocytes and undergo asexual reproduction, which is responsible for the clinical symptoms of the disease—babesiosis. Humans are dead-end hosts and there is little chance that subsequent transmission occurs from ticks feeding on an infected person. However, human-to-human transmission can occur through blood transfusions.

Toxoplasma gondii is another intracellular parasite in humans and its life cycle includes domestic cats. The organism is the causative agent of toxoplasmosis, which is discussed in Chapter 16 (Infections of the Reproductive System).

Cellular Slime Molds

Cellular slime molds are typical eukaryotic cells that resemble amoebas and spend most of their lives as single amoeboid cells feeding on fungi and bacteria by phagocytosis. Under unfavorable conditions large numbers of these cells aggregate to form a single structure. This aggregation occurs in response to the release of chemicals by some individual amoebas and others respond by migrating toward the released chemical (cyclic adenosine monophosphate, or cAMP). The aggregated amoebas become enclosed in a slimy sheath called a slug, which migrates toward light. After migration, the slug begins to form differentiated structures; some form a stalk whereas others go up the stalk and form a spore cap. Most of these differentiate into spores, are released under appropriate conditions, and germinate to become single amoebas (Figure 8.19). Cellular slime molds are of great interest to biologists because they provide a relatively simple and easily manipulated system for understanding cellular migration and aggregation.

Plasmodial Slime Molds

Plasmodial slime molds are masses of protoplasm containing thousands of nuclei. This life form is called a plasmodium; it moves as a giant amoeba engulfing organic material and bacteria. These organisms distribute oxygen and nutrients throughout the plasmodium by a process called cytoplasmic streaming, in which the plasmodium moves and changes its speed and direction. The plasmodium grows as long as food and moisture are available. When conditions become unfavorable, the plasmodium separates into many protoplasmic groups, each of which forms a stalked sporangium in which spores develop. The spore nuclei undergo meiosis to form haploid cells. On release, in improved conditions, the spores will germinate, fuse, and form diploid cells, which then develop into a multinucleated plasmodium.

Characteristics of Helminths

Many parasitic helminths spend much of their life cycle in a mammalian host. Most of the helminths affecting humans are either flatworms (platyhelminths; Platyhelminthes) or roundworms (nematodes; Nematoda). Adult animals are usually large enough to be seen with the naked eye, but the microscope is necessary to identify their eggs and larvae.

All helminths are multicellular eukaryotes that generally contain digestive, circulatory, nervous, excretory, and reproductive systems. Parasitic helminths differ from their free-living counterparts, because parasites must be highly specialized to live inside their hosts. For example, their reproductive system is often dominant over other systems, and the worms may be reduced to a series of flattened sacs filled with ovaries, testes, and eggs to optimize infection. They might lack a digestive system because they can absorb the necessary nutrients from the food of the host and body fluids, or tissues of the host. Motility may be reduced, because the parasites are transferred from host to host. Furthermore, parasitic helminths do not have an extensive nervous system, because they do not have to search for food or adapt to environmental changes.

Life Cycle of Helminths

The life cycle of parasitic helminths includes the fertilized egg (embryo), larva, and adult. It can be extremely complex, involving a succession of intermediate hosts, and a definitive host for the adult parasite.

Adult helminths may have male reproductive organs in one individual and female reproductive organs in another, in which case the worms are called dioecious. Reproduction in these species occurs only when the host contains worms of both sexes.

If a helminth has both male and female reproductive organs it is called monoecious (hermaphroditic), and two worms can fertilize each other. Very few types of hermaphrodites fertilize themselves. For continued survival of the species, the parasite must complete the life cycle by transmitting an infective form to the body of another host of the same or different species. The infective stage of a worm is usually in the form of an egg or larva. In general, larval development is supported by the intermediate host, whereas adulthood and mating occur in the definitive (final) host.

Humans are definitive hosts for many parasitic worms, and the sources of infection include contaminated food, soil, water, or infected animals. The route of infection (see Chapter 9, Infection and Disease) can be by oral intake or skin penetration.

Classification of Helminths

The helminths are classified according to shape, size, and degree of organ development, and by the presence of hooks, suckers, or other special structures. In addition, the mode of reproduction, the type of host, and the morphology of eggs and larvae are also considered in their classification.

Platyhelminths

Members of the phylum Platyhelminthes are dorsoventrally flattened, thus the name flatworms. Parasitic flatworms include trematodes and cestodes.

• Trematodes (flukes) are generally flat and leaf shaped, with oral and ventral suckers holding the organism in place (Figure 8.20). They have complex life cycles and their common names are given according to the tissue of the definitive host in which the adults live (e.g., lung fluke, liver fluke, blood fluke).

• Cestodes or tapeworms are intestinal parasites (see Chapter 12, Infections of the Gastrointestinal System). Adult tapeworms share a basic body structure: a scolex (head), a neck, and one or more proglottids (segments; see Figure 8.21). The scolex of the worm attaches to the intestine of the definite host, and the neck is a relatively undifferentiated mass of cells forming new segments—this is where all growth in the adult tapeworm occurs. The body of the worm is composed of successive units of proglottids collectively called strobila. It makes the worm look like a strip of tape and this is the source of its common name. Mature proglottids are released from the mature tapeworm and leave the host in its feces. The most mature proglottids are at the tail of the tapeworm and the most immature are closest to the neck.

Nematodes

Nematodes, or roundworms, are cylindrical and tapered at each end and are the most numerous multicellular animals on earth. Nematodes possess digestive, nervous, excretory, and reproductive systems, but lack defined circulatory and respiratory systems. They are found everywhere in freshwater, marine, and terrestrial environments. Many are free-living and harmless; others are parasitic and can be pathogenic to plants and animals, including humans. Nematode infections in humans are divided into two categories, depending on whether the eggs or the larvae are the infecting agent.

• Enterobius vermicularis, the pinworm or seatworm, spends its life in a human host. The adult worms reside in the large intestine, from where it migrates to the anal area to deposit its eggs (hence the name, seatworm). The eggs can then be transmitted to another host by ingestion or via a fomite (contaminated clothing or bedding). Ascaris lumbricoides (a large roundworm that infects humans) is a parasite without an intermediate host. It spends its adult life in the human small intestine and feeds primarily on predigested food. Eggs can be excreted with feces and are able to survive in soil for long periods of time until accidentally ingested by another host. The eggs hatch in the small intestine and the larvae dig out of the intestine into the bloodstream. They are transported to the lungs, where they grow and eventually are coughed up, swallowed, and returned to the intestine for maturation into adults.

• Necator americanus and Ancylostoma duodenale are also called hookworms (Figure 8.22), and live in the small intestine of humans. Their eggs are excreted in feces, and their larvae hatch in the soil. The larvae feed on bacteria and can enter their human host by penetrating the host’s skin, after which they enter a blood or lymphatic vessel to be carried to the lungs. The larvae will be coughed up in sputum, swallowed, and carried to the small intestine, where the worms mature.

MEDICAL HIGHLIGHTS

Ascariasis: Nematode Freeloaders

The most prevalent intestinal worm infections are caused by nematodes and are generally transmitted by contact with soil contaminated with human feces containing nematode eggs. The most common human worm infection is due to Ascaris lumbricoides, a large roundworm that ranges in length from 6 to 13 inches. The infections are common throughout the world in both temperate and tropical areas, but are more prevalent in areas of poor sanitation, and are usually a result of human carelessness. Often, symptoms may not occur in infected individuals; however, they are active carriers of the infection.

Eggs are swallowed and pass to the intestine, where they develop into larvae. The larvae can penetrate the intestinal wall, enter the circulatory system, and reach the lungs. A large number of larvae within the lungs at any point in time may cause pneumonia. Once in the lungs the larvae will travel via the bronchial tree to the pharynx, where they are swallowed. On entering the small intestine again the larvae will grow, mature, and mate, and their progeny will be released into the environment to find a new host.