Parasitic helminths-1
Parasitic Helminths
Objectives intermediate hosts. Humans can serve as the in- After completing this exercise, you should be termediate host for the dog tapeworm. The larva able to: encysts as an hydatid cyst in a variety of tissues 1. Differentiate nematode, cestode, and trema- including the lungs or liver. tode. Definitive host The adult (reproductively ma- 2. Explain how helminth infections are diag- ture) stage of a parasite lives in a definitive host. nosed. Humans can serve as the definitive host for beef, pork, and fish tapeworms. Eggs infective The eggs of some parasitic Background roundworms are infective for humans. Adult Helminths are multicellular eukaryotic animals pinworms are found in the large intestine. From that generally possess digestive, circulatory, there, the female pinworm migrates to the anus nervous, excretory, and reproductive systems. to deposit her eggs on the perianal skin. The Some are free-living in soil and water. eggs can be ingested by the host or by another Helminths are studied in microbiology because person exposed through contaminated clothing they cause infectious diseases and most are di- or bedding. agnosed by microscopic examination of eggs or Larvae infective Some parasites are infective larvae. Eggs may have striations (lines), a spine, for mammals in the larval stage. Larva of filarial or an operculum (hatch by which the larva worms such as Wucheria that infect humans and leaves). dog heartworm are transmitted to their mammal- Helminths infect more than one-third of the ian host by mosquitoes. They mature into adults world population. Helminth infections differ in the mammal. from bacterial or protozoan infections because Flatworms the worms do not usually increase in number in Platyhelminthes, or flatworms, are flattened the host. Symptoms are usually due to mechani- from the dorsal to ventral surfaces. The classes cal damage, eating host tissues, or completing of this phylum include trematodes and cestodes. for vitamins. In this exercise, we will examine Trematodes, or flukes, often have flat, leaf- prepared slides of parasitic helminthes. shaped bodies with ventral and oral suckers (Mi- Life cycle crobiology Figure 12.25a). The suckers hold the Parasitic helminths are highly modified com- animal in place. Flukes obtain food by absorbing pared to free-living helminths. They often lack it through their outer covering, called the cuti- sense organs such as eyes, and may even lack a cle. Flukes are given common names according digestive system. Their reproductive system, to the tissue of the definitive host in which the however, is often complex, which ensures infec- adults live (for example, lung fluke, liver fluke, tion of new hosts. Some flukes can produce blood fluke). 25,000 eggs per day. Cestodes, or tapeworms, are intestinal para- Adult helminths may be dioecious; male re- sites. Their structure is shown in Microbiology productive organs are in one individual, and fe- Figure 12.27. The head, or scolex (plural: scole- male reproductive organs are in another. In those ces), has suckers for attaching to the intestinal species, reproduction occurs only when two mucosa of the definitive host; some species also adults of the opposite sex are in the same host. have small hooks for attachment. Tapeworms do Adult helminths may also be monoecious, or not ingest the tissues of their hosts; in fact, they hermaphroditic—one animal has both male and completely lack a digestive system. To obtain female reproductive organs. Two hermaphro- nutrients from the small intestine, they absorb dites may copulate and simultaneously fertilize food through their cuticle. The body consists of each other. segments called proglottids. Proglottids are Intermediate host Some parasites have a dif- continually produced by the neck region of the ferent host for each larval stage, these are called Parasitic helminths-2 scolex, as long as the scolex is attached and Procedure alive. Each mature proglottid contains both male 1. Using a compound light microscope, examine and female reproductive organs. Each proglottid the male and female pinworms. Sketch the matures as it is pushed away from the neck by parts of the worms that show anatomic dif- new proglottids. Each proglottid contains both ferences. Using a dissecting microscope, male and female reproductive systems and eggs measure the length of the male and female are fertilized as the proglottid reaches the middle pinworms. of the worm. The proglottids farthest away from 2. Observe the blood smear containing Wuche- the scolex are basically bags of fertilized eggs ria. Estimate the length and width of the that will be shed in feces. worm by comparing it to blood cells. (White Roundworms blood cells =15 µm Red blood cells =7 µm.) Nematodes, or roundworms, are cylindrical Note that these filarial worm larvae are filled and tapered at each end. Roundworms have a with a column of nuclei. Do the nuclei extend complete digestive system, consisting of a all the way to the tail? mouth, an intestine, and an anus (Microbiology 3. Examine and sketch a Trichinella cyst with a Figure 12.29). Most species are dioecious. The compound light microscope. Using a dissect- reproductive system consists of long tubules that ing microscope, measure the size of Trichi- serve as ovaries or testes. In females, the repro- nella cysts. In what tissue are the nematodes ductive tubule (ovary) is usually double. Males encysted? are smaller than females and have one or two 4. Using a dissecting microscope, examine the hardened spicules on their posterior ends that tapeworm slide. Measure the length and guide sperm to the female’s genital pore. Spe- width of immature and mature proglottids. cies identification is often based on spicule a. Examine the neck region. Measure the structure. width of a proglottid. Count the number of proglottids that fit into one millimeter. Es- Materials timate the length of one proglottid Number of proglottids Prepared slides (= ). Enterobius vermicularis (pinworm) 1 mm b. Using a compound light microscope, ex- Wucheria bancrofti (filarial worm) amine and sketch an hydatid cyst, What Trichinella spiralis cyst structures are visible in the cyst? Taenia sp. (tapeworm) 5. Examine the liver fluke. Measure the length Echinococcus (hydatid cyst) of the ovaries and the total body. Clonorchis sinensis (liver fluke) 6. Examine the fecal smears containing worm Fecal smears of eggs eggs. Sketch one egg of each species ob- served. Be aware that the slides may have Techniques Required debris so you will need to search for the regu- Compound light microscope, Exercise 1 larly-shaped eggs. Dissecting microscope, Appendix E Parasitic helminths-3
Laboratory Report Parasitic Helminths Name ______Date ______
Purpose ______
______
______
Results Roundworms Pinworm. Sketch to show differences between the sexes.
Length: Female______mm Male ______mm
Wucheria Length ______µm Width ______µm
Sketch, label the nuclei.
Parasitic helminths-4
Trichinella cyst
______mm
Tissue: ______
Flatworms Tapeworm Mature proglottid Immature proglottid Neck proglottids/mm: Width: ______Width: ______Width: ______Length: ______Length: ______Length: ______
Hydatid cyst
Sketch and label the visible structures.
_____ ×
Fluke Body length______mm Ovary length ______mm
Eggs
Species ______
______× ______× ______× Parasitic helminths-5
Conclusions 1. Are pinworms visible without a microscope? ______
2. How many times longer is a mature proglottid than the neck-region proglottid? ______
What occupies most of the space in the mature proglottid?______
3. What percentage of a fluke’s body occupied by reproductive structures? ______
4. Which of the eggs has an operculum (escape hatch for the larva)? ______
5. Which has a rough surface? ______
6. Which egg is flattened on one side?______
Questions 1. Approximately 10% of the body of a free-living flatworm is occupied by reproductive tissue. Why is the value you obtained for the fluke so much different? ______
______
______
2. The nematode, Wucheria, lives in lymph capillaries in the human body. The adult worm can be a few millimeters long. How does it fit through capillaries?______
______
3. Are humans the intermediate or definitive host for Trichinella? ______
How can you tell?______
______
4. To what phylum and class does this animal belong? ______List two characteristics that put it in this phylum. Name the body parts. What is the name of the encysted larva of this animal? ______
Parasitic helminths-6
5. To what phylum and class does this animal belong? ______List two characteristics that put it in this phylum. Identify the intestine, ovary, and genital pore. What is the name of the encysted larva of this animal? ______
Critical Thinking 1. A woman found a worm in her laundry basket. The worm measured approximately 6 centi- meters in length (figure a). Eggs were removed from the worm and examined at 400× magni- fication (figure b). What is your diagnosis? Based on what criteria?
a.
b.
2. A 25-year-old previously healthy woman had a physical examination, including a chest x-ray, as one of her pre-employment requirements. The x-ray showed a cyst-like lesion in her left lung. The cyst measured 3-4 cm in diameter (see the figure). What is your diagnosis? Based on what criteria?