Introduction to gastrointestinal parasitism

All animals are subject to infection by internal parasites. Though the internal environment provides several advantages, a large hurdle the parasite must overcome is ensuring its survival by infecting new hosts. Some endoparasites deal with this problem by taking advantage of predatory, scavenging or blood-feeding behaviors of hosts by making infective stages available in tissues or blood.

Other internal parasites can infect reproductive or milk-producing organs, or even cross placental barriers, to ultimately infect new hosts. Then again, many internal parasites produce stages that are released into the environment, most commonly via excreta (usually feces and urine), that will hopefully (from a parasite’s perspective) encounter and infect a competent host.

As a result, several endoparasitic infections can be diagnosed by finding characteristic ova, larvae, cysts, or oocysts through a routine examination of animal feces. Although a fecal examination seems straightforward, there are several issues that you will have to consider.

In the diagnostic laboratory fecal laboratories, we will help you set up and correctly run fecal diagnostic exams, as well as assist you in diagnosing internal parasites.

Considerations for using feces for the diagnosis of intestinal parasitism

By far, one of the most common samples submitted for parasite screening is feces as intestinal parasites consist of the majority of pathogenic veterinary parasites. While molecular techniques are beginning to gain popularity for characterizing intestinal parasitism, the mainstay for fecal evaluation remains fecal flotation, fecal sedimentation, dry and wet mount fecal cytology (e.g. direct fecal smear), and Baermann examination (for larvae).

Principles for fecal collection and preservation

1. Fresh feces should be used for fecal exams

   • A freshly voided sample or a rectal sample collected with a fecal loop or finger are ideal specimens for fecal examination.
   • A small composite sample of the specimen should be placed in a suitably sized and airtight container (plastic cup with lid, glass jar, or a sealed plastic bag).
   • Newspaper, tissue paper, paper towels, and cardboard boxes do not make satisfactory containers.

2. Feces should be preserved when same-day evaluation is not possible

   • If the fecal exam is to be performed immediately, preservation is not always necessary.
   • If the sample is to be kept until the next day or sent to a laboratory, place the sample in an airtight container with as much of the air removed (either squeeze the plastic bag or fill a small container).
   • Refrigeration or the addition of a 10% formalin solution (1 part feces to 5 parts formalin) is highly recommended if there will be a delay in evaluation
     • Note: Many ova and oocysts will continue to develop in formalin-preserved feces.
     • Refrigerating or preserving feces minimizes bacterial and yeast contamination.
     • Samples should not be frozen because water expands when frozen, hence oocysts and ova will deform or break.
     • Formalin-preserved samples cannot be used for fecal smear, larval culture or Baermann techniques as these techniques require living organisms.

Why is it necessary to collect fresh feces?

An important reason for obtaining fresh feces (or keeping samples refrigerated) is to facilitate nematode ova identification. Given warm temperatures and time, strongyle-type eggs will develop into larvated eggs, resulting in possible confusion with nematode eggs that are shed larvated in fresh feces (i.e.: Strongyloides sp. (Threadworms) or Physaloptera sp. (Stomach worm) Additionally, by collecting fresh or rectal samples, you prevent finding free-living nematodes (those that live in the soil) which could be confused with veterinary parasitic larvae from migrating into the feces on the ground.

Is there ever a case in which you would want to incubate the feces for a specified period of time and not refrigerate?

In some cases, it is necessary to allow the egg or oocyst to develop for identification of specific structure or larval stages for more specific identification to the genus or species level. If you need to determine what coccidial genus is infecting an animal (e.g. Eimeria v. Cystoisospora), incubating the fecal sample for a few days will allow sporulation to occur (we can determine the genus and/or species based on the number of sporocysts within the ova). This technique is especially helpful when we are trying to determine if the parasite observed is spurious or parasitic.