Animal parasites commonly occur in a great variety of hosts. They can be collected from almost any vertebrate and many invertebrates. Being small in size and often living in internal organs, they are not usually conspicuous but can be found with a little search. Once collected and preserved they are not difficult to handle and their study is of interest not only in connection with the parasites themselves but also because it involves problems of host relationship, geographical distribution, and life cycles. Also, many parasites cause disease in man and domestic animals. The following notes pertain to the collection, killing, and preservation of the more common metazoan parasites.

External Parasites

External parasites such as lice, fleas, or ticks can usually be collected by forceps and placed directly in 70% alcohol. Fleas or lice may be combed from fur with a fine-toothed comb. However, most ectoparasites, particularly fleas, leave the host upon its death. Recently killed birds or small mammals should be sealed as soon as possible in some container such as a can or a cellophane bag which prevents escape of the fleas or lice.

Scabby or mangy areas of the skin may contain mites which can be recovered by scraping the tissue with a scalpel and teasing apart in water. Boardman (1944) recommends killing ticks in a solution of 98 parts 20% alcohol and 2 parts ether, and storing in 70% alcohol. This method is also effective for insect parasites.

Larvae of flies and mosquitoes, and small arthropods such as ‘chiggers’ (larval mites) may be killed by dropping them into boiling water, then storing in 70% alcohol.

Monogenetic trematodes and Crustacea are common parasites on the gills of fishes. Some of these can be secured by washing the gills in a strong stream of tap water into a dish and decanting. Many however remain attached. These can be collected by scraping the gills under water, decanting, and examining the sediment. Refrigeration of the host for several hours will relax and kill these parasites. Crustacea should be stored page 57 in 70% alcohol or formalin. Monogenea should be treated in the same manner as Digenea, as described below.

Internal Parasites

Internal organs of the host should be slit open and each shaken in tap water or physiological salt solution. The digestive tract is the most common location of parasitic helminths. Larger and detached helminths can be removed with forceps, needle or brush. Small and attached helminths can be freed by deeply scraping with a scalpel the mucosa under water. Allow the turbid mixture to settle, preferably in a tall container such as a beaker or battery jar, and then decant. Parasites are heavier than most of the intestinal content. Examine the settlings over a black background using a hand lens or dissecting microscope. Good light is essential. Moist chambers or petri dishes are good for examination of this sediment. A medicine dropper is useful in swirling the mixture and removing specimens.

Killing Trematodes. Trematodes tend to contract badly when placed in killing solution. Since usually only a few specimens are present, it is best to kill them individually beneath a coverglass on a slide. See that the worm lies flat on the slide; remove excess water; add a few drops of killing solution (preferably Formal-Acetic-Alcohol); then quickly cover with a coverglass and apply gentle pressure with a needle. Trematodes are often soft bodied and care must be taken to apply only enough pressure to flatten the worm to more or less natural shape. Some species are non-muscular and require no pressure at all. After a few minutes remove the specimen to a vial of killing solution. If specimens are numerous, try placing some in distilled water in which some species relax, or dropping into hot killing solution.

Killing Cestodes. Here again the chief problem is to prevent contraction and curling. Cestodes usually relax in lukewarm tap water and upon death can be fixed (in Formal-Acetic-Alcohol or 10% formalin). Refrigeration also tends to prevent contraction. Specimens may be stretched out on a piece of glass and the killing solution applied with a camel hair brush. Or the tapeworm can be stretched gently by hand as the killing solution is applied. Overstretching causes distortion and breaking.

Killing Acanthocephala. Acanthocephala are intestinal parasites of vertebrates. Place specimens in distilled water until dead. This treatment usually causes the proboscis to be extended. Fix in Formal-Acetic-Alcohol and preserve in 70% alcohol.

Killing Nematoda. Nematodes are best killed in steaming hot 70% alcohol, or in hot 5 to 10% formalin, and stored in the same solution. Later, a few drops of glycerine may be added.

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Although specimens will keep well in this killing solution for some weeks, it is best to remove them, after some minutes or hours, to 70% alcohol for storage.

Staining. Trematodes and cestodes stain well with Delafield's haematoxylin or with carmine stains.

Van Cleave and Ross (1947) found that if Acanthocephala and nematodes are soaked for a time, usually a few hours, in a solution of 0.25% trisodium phosphate in distilled water, they are more permeable and can be stained, dehydrated, and mounted with less difficulty. Dried specimens of helminths and other invertebrates can be reclaimed by soaking in this same solution.

Nematodes should not be stained but cleared in lactophenol solution or in glycerine.