Diagnosis

Author: J BOOMKER

There are various methods the veterinarian can use to diagnose helminthosis and identify the worm species causing the problem. The method used will be based on a number of factors, including the species of animal, the type of parasite, whether the animal is alive or dead, the cost of the individual tests, and the facilities available to the veterinarian.

Faecal examination methods

Faeces provide a useful source of diagnostic material for the identification of the endoparasite infesting an individual animal, or group of animals. Larvae and the eggs of some species can be used to identify the species (see later in chapter) and the number of eggs in a faecal sample can give an indication of the severity and the relevance of an infection. It must be stressed that a faecal egg count merely gives an indication of the presence of reproductively active female worms, and there is no correlation between the egg count and the actual worm burden that the animal carries. For evaluation of worm burdens of species, refer to Table 19 in Addendum B.

Direct microscopic examination

This is a simple and practical method for the veterinarian, in the field, to use. Suspend a small amount of faeces in a few drops of water and place a cover slip over the material. It is possible to detect most eggs or larvae using this method, but due to the small sample this technique will only detect heavy infections.

Flotation methods

These are used to visualise worm eggs or parasites. By suspending the faecal material in a solution with a higher specific gravity than the worm eggs, the latter will float to the surface. The sensitivity of the method used depends on a number of factors – including the specific gravity and viscosity of the solution, the SG of eggs or parasite stages, the concentration of eggs/parasites, and the care and precision with which the sample was processed.

Various methods are used and their advantages and disadvantages are outlined below. Where applicable, details of the individual procedures are given in Addendum B.

Direct flotation method: this qualitative method is suitable for use in private practice for all species and is inexpensive. It is conducted with commercially available devices such as Fecalyzer®, Ovassey® and Ovatector®. The sample used is freshly-collected faeces from the rectum – that can be stored in a refrigerator if there is to be any delay in despatching the specimen to the laboratory. The specimens for submission to the diagnostic laboratory should be placed in a suitable specimen container with a tightly fitting screw cap, and placed in a polystyrene coolbag with an ice pack to prevent development of the eggs. Speedy delivery using an overnight courier is essential. Shredded paper towels or bubble wrap to prevent direct contact between the faecal sample and the frozen ice pack, should be used.

Centrifugal flotation methods: this is a qualitative method used in diagnostic laboratories. The sampling methods are, as for the direct flotation method. For the preparation of flotation fluid, see Addendum B.

McMaster method

This is a quantitative method for counting eggs of gastrointestinal nematodes and the results are expressed as epg (eggs per grams of faeces). This method is used as a monitoring tool for nematode infection in production animals and equids.

It can be used to monitor nematode dynamics and for the Faecal Egg Count Reduction test, which monitors the efficacy of control and can also be used as a screening test to identify the presence of anthelminthic resistance (see under control). Sampling requirements are as for direct flotation. For laboratory instructions for the McMaster method, see Addendum B.

Sedimentation methods

These methods are used for the demonstration of trematodes and lungworm larvae.

Benedek method: this method is suitable for routine diagnostics in a veterinary practice. It provides qualitative and quantitative analysis for trematode eggs, and is inexpensive. The sampling requirements are as for the direct flotation test. See Addendum B for the laboratory method of Benedek’s sedimentation method.

Pitchford-Visser and Bauermann- Wetzel methods: these are confined to use in diagnostic laboratories.

Species-specific tests

Centrifugation-flotation test

This is a specialised, qualitative test used for detecting Spirocerca lupi and Anaplocephala eggs, and must be done by an experienced diagnostician.

Adhesive swab tape test

This cheap, qualitative test can be used to detect cestode eggs in dogs and cats, as well as oxyurid eggs in equids, primates, and humans.

Blood sample examination

Stained thick and thin blood films are used to detect micofilariae in various species such as dogs and cats (Dirofilaria spp). Many filarial infections are detected incidentally in routinely prepared stained thin and thick blood films. However, species identification using this technique is highly unreliable.

Membrane filtration technique

The most sensitive technique to detect patent filarial infections is used for screening dogs and cats for specific infections to satisfy import and export requirements. A sample is taken in EDTA or heparin. The storage and dispatch requirements are as for the ‘direct faecal flotation’ method. This technique should be entrusted only to an experienced diagnostician.

Microfilarial identification by acid phosphatase staining

This is the definitive differential technique for species identification and...