Parasitology diagnostics in your practice (Proceedings)

At a minimum, veterinary practices should be able to perform in-house diagnostic tests for heartworm infection and detection of parasites in fecal samples.   Despite the increasing availability of commercial laboratories that can perform these tests, they cannot replace the advantage of having test results rapidly available, particularly in cases of clinical disease where results guide therapeutic choices. 

Additionally, parasitology diagnostic tests like the routine fecal flotation are inexpensive to perform and are a profitable part of veterinary practice-fecal flotations shoud be perfomed on dogs and cats in the U.S. at least annually  .  Certified veterinary technicians receive extensive training in parasitology and can provide excellent diagnostic skills. 

Heartworm

In-house diagnostic tests for heartworm use ELISA and immunochromatographic techniques to detect female adult worm antigen.  These tests have been refined over the years to be highly specific and sensitive so that the decision about which test to use can be guided by other considerations, such as cost and convenience.  The sensitivity and specificity of the tests are very high, but it is always important to confirm a positive antigen test in an asymptomatic dog.  Repeating the in-house test can eliminate any errors in test performance (too long an incubation period, switched blood samples, etc.). 

The presence of Dirofilaria microfilariae on a Knott's test would confirm antigen test results, but microfilariae may be absent in an infected dog.  Lesions observed with imaging techniques are also helpful, but their absence also would not rule out the presence of heartworm.  In the absence of any other confirmatory evidence, it is helpful to send a sample from the dog to a laboratory using an antigen test from a different manufacturer.  If the second test is also positive, the likelihood of a true heartworm infection becomes much higher.

Giardia

Another immunologic test that is often used in veterinary practices is the IDEXX Snap Giardia test.  This is a very useful test for animals presenting with the clinical signs of giardiasis.  Studies indicate that the test is comparable to flotations performed with zinc sulfate solution and read by an experience technician and the test is much superior to flotations read by staff who are uncertain about the identification of Giardia cysts.   The Companion Animal Parasite Council recommends testing “symptomatic dogs and cats…with a combination of direct smear, fecal flotation with centrifugation, and a sensitive, specific fecal ELISA optimized for use in companion animals.” Repeat testing performed over several (usually alternating) days may be necessary to identify infection.

Fecal flotation

The fecal flotation exam has been the standard procedure for identification of internal parasite infections in small animals for many years and will probably continue to be used for many years to come.  It is inexpensive and easy to perform, and common parasites are easily recognized without extensive training.   Many practices have begun sending their fecal samples out to the large commercial labs.  This may be satisfactory for clinically normal animals on a monthly anthelmintic treatment program, which often have negative fecal exams.   However, it is important to maintain the ability to do fecal flotation tests so that you can rapidly assess the parasite status of clinically affected animals when determining a treatment protocol.  Fecal flotations are also a profitable part of practice and earn far more than the cost of supplies and technician time to perform them.

Selecting the best fecal flotation solution should be based on what you are most likely to use the flotations for.  No fecal solution is perfect for every situation.  For example, the highest specific gravity solutions are more likely to detect tapeworm eggs and some of the more unusual nematodes like Physaloptera.  However, if you are interested in being able to detect Giardia cysts, then 33% zinc sulfate solution seems to be more effective, but its lower specific gravity makes it less likely to float tapeworm eggs.  See Table 1 for a comparison of fecal flotation solutions.  Whatever flotation solution you use, fecal flotation tests should be centrifuged.   Centrifugation significantly increases parasite recovery.  This is very important in cases of low egg numbers, exemplified by some clinical cases of Trichuris infection.

Another parasite diagnostic test that can easily be performed in practice is the Baermann test.  This is the best test for detecting larvae in fecal samples.  Larvae detection is important in infections of cats with the lungworm Aelurostrongylus, and canine infections of Strongyloides and some unusual/exotic parasites like Crenosoma and Angiostrongylus.  All that is required for a Baermann test is a plastic disposable wine glass with a hollow stem.  A fecal sample of 10 or 20 g can be wrapped in a gauze pad and secured with an elastic band.  A pencil is passed through the band and the packet of feces is suspended over the bowl of the glass, which is then filled with lukewarm tap water. 

The test is allowed to sit at least eight hours or preferably overnight.  Larvae from the sample move out of the feces, fall through the water and are concentrated at the base of the hollow stem. The next morning the feces is discarded and a transfer pipette is used to suck up a few drops of fluid at the base of the hollow stem of the wine glass.  A drop or two is placed on a slide and examined for larvae.  If they are present, a drop of Lugol's iodine (easily available and inexpensive) can be added to kill larvae so they can be identified.  Ten percent formalin can also be used but tends to make larvae curl up when they die and they are harder to see.

Table 1. Common fecal flotation solutions

Flotation solution

Specific gravity

Availability

Advantages

Disadvantages

Sodium nitrate (NaNO3)

Fecasol®

Saturated NaNO3

1.2

1.33

Commerically available

Prepare as for saturated NaCl solution

Fecasol floats common helminth and protozoa eggs and cysts.

Distorts Giardia cysts rapidly.

Does not float most fluke, some unusual tapeworm and nematode eggs.

Saturated solution not routinely used

Sheather's Sugar solution

1.25

Commercially available

or combine 1 lb sugar (454 g) and

12 oz  (355 ml) tap water over low heat.  Add 22 ml of 10% formalin to prevent mold growth (if using formalin reduce water by same amount).

Floats common helminth and protozoa eggs and cysts, preferred for Cryptosporidium oocysts.  Best for tapeworm eggs

Does not float most fluke, some unusual tapeworm and nematode eggs.  Less sensitive than ZnSO4 for Giardia.  Creates sticky surfaces. 

33 % Zinc sulfate

(ZnSO4)

1.18

Commerically

Available

or combine 330g

zinc sulfate with water to a volume of 1000 ml

Floats common helminth and protozoa eggs and cysts. Preferred for Giarida.

and some lungworm larvae.

Does not float most fluke, tapeworm and some uncommon nematode eggs. 

Saturated Sodium chloride (NaCl)

1.2

Add pickling salt (no additives) to warm tap water until saturated (should have some undissolved salt on the bottom of the container)

Floats common helminth and protozoa eggs and cysts

Distorts Giardia cysts rapidly.

Does not float most fluke, some tapeworm and nematode eggs. 

Saturated Magesium sulfate (Epsom Salts)

1.32

Add Epsom Salts to water until saturated as for sodium chloride sollution

Floats common helminth and protozoa eggs and cysts

Distorts Giardia cysts rapidly.

Does not float most fluke, some tapeworm and nematode eggs. 

References

American Heartworm Society.  Current Canine Guidelines. August, 2011. http://www.heartwormsociety.org/veterinary-resources/canine-guidelines.html

Bowman D, Little SE, Lorentzen L, Shields J, Sulliva MP, Carlin EP.  2009.  Prevalence and geographic distribution of Dirofilaria immitis, Borrelia burdorferi, Ehrlichia canis, and Anaplasma phagocytophilum in dogs in the United States:  Results of a national clinic-based serologic survey.  Vet. Parasitology 160:138-148.

Companion  Animal Parasite Council.  Recommendations on Giardiasis, August , 2011. http://www.capcvet.org/recommendations/giardia.html

Dryden MW, Payne PA, Ridley R, Smith V.  2005.  Comparison of common fecal flotation techniques for the recovery of parasite eggs and oocysts.  Vet Therapeutics 6:14-28.

Zajac AM, Conboy GA.  2006.  Veterinary Clinical Parasitology, 7th ed.  Ames IA.  Blackwell Publishing