IN the second in our series of articles related to parasites in horses, we will look at diagnostic tests, when and why they should be performed and some common misconceptions. We will also look at how samples should be taken and stored and how these tests can or cannot be applied to the parasite associated disease we discussed in the first article.

The true goal of parasite control in horses is to limit parasite infections so animals remain healthy and clinical illness does not develop. The goal is not to eradicate all parasites from a particular individual. Not only is eradication impossible to achieve, but the inevitable result is also accelerated development of parasite drug resistance.

Further contamination

As discussed in the previous article, both small and large strongyles (and ascarids) cause the greatest disease during their larval stages, which are refractory to most anthelmintics treatments (wormers). Consequently, most treatments that kill only adult strongyle worms, yield limited direct benefit to the horse. However, treatments effective against adult stages have an indirect benefit in that they prevent further contamination of the environment with infective stages. This prevention of environmental contamination is the ultimate aim of any treatment plan. Monitoring the egg shedding gives us some information as to the level of environmental contamination.

Remember!

There is a common misconception that a faecal egg count (FEC) tells us how many worms a horse has. This is not true!

What are the purposes of the FEC if not to tell us how many worms are present?

  • To determine the shedding status of strongyle spp, of the adult horse at the time of sampling.
  • To determine whether parasite burdens in foals and weanlings are primarily Parascaris spp. or Strongyle (see Fig 1).
  • To evaluate the anthelmintic efficacy using the faecal egg count reduction test (FECRT).
  • To evaluate and monitor the strongyle egg reappearance period (ERP) of the most recently administered dewormer.

    • HORSES develop immunity to parasites over time. This immunity is dependent on exposure to the parasites (hence some worms are a good thing) and the time required varies between parasites.

    This immunity means that parascaris spp are rarely seen in horses greater than 18 months of age and that disease associated with cyathostomins is rarely seen in horses older than five years of age.

    This immunity results in the host (horse) being able to limit the development of larvae into adults and also to decrease the egg shedding of adults that do develop. This immunity varies from animal to animal but is consistent within the individual for their adult life.

    This fact has resulted in adult horses being defined as low, medium or high strongyle shedders, based on their FEC results. It is important to note that this only applies to the shedding of strongyles as other parasites shed eggs intermittently.

    Number of eggs

    85% of strongyle eggs are shed by only 15% of horses. Therefore, identifying these high-shedders allows us to target treat them to reduce the eggs shed into the environment. Limiting the eggs in the environment limits exposure for all animals sharing that environment.

    The FEC therefore tells us how many eggs are being shed, but the number of eggs in a FEC is not a reflection of the actual numbers of parasites in that given horse, this is thought to be the result of a number of factors:

  • The effect of the host immune response reducing worm fecundity.
  • The mixed species nature of cyathostomin infections and a substantial fecundity variation between species.
  • Density-dependent egg-shedding, where the egg production per worm is decreased, when worm burdens increase.

    • When deciding to treat individuals based on FEC, remember:

  • You are treating to reduce shedding of eggs of that species.
  • Some parasites are good for the development of an immune response; therefore the aim is not 0 in a FEC and treatment of horses with less than 500epg is no longer recommended as it not only inhibits the development of an immune response but also selects for, and speeds up, the development of resistance.

    • Why young horses can have high parasite numbers

    AS young horses are still in the process of development of an immune response, they frequently harbour higher number of parasites than adult horses and shed higher numbers of eggs. However, FECs in young animals can be useful to not only determine the shedding status at that time but are also indicative of how effective other control mechanisms are. If the environmental contamination level is high, then this is likely to be reflected in the FEC of the young animal.

    Limitations of faecal egg counts

  • Faecal egg counts (FECs) tell us nothing about larval stages of parasites as only the adults can shed eggs. Therefore, we can have a severe parasitic disease related to larval stages and a negative FEC.
  • Tapeworm infections are often missed or underestimated by faecal float techniques, and modified techniques are required. Tapeworm eggs are also shed intermittently.
  • Parascaris spp. eggs are also shed intermittently and therefore a negative FEC for ascarid eggs does not mean that there are no parasites of this species, just that there were none shedding on that day.
  • Oxyuris eggs are usually missed since they are adhered as egg packets around the anus rather than being shed in the faeces.

    • Recommendations for faecal sampling and storage

  • Samples should be stored in airtight and leak-proof containers or plastic bags.
  • Collected manure should be as fresh as possible. Samples less than 12 hours old are acceptable but should be refrigerated immediately after collection. At least 10g should be collected (see fig 2).
  • Refrigeration is always recommended for storage of faecal samples, but anaerobic storage at room temperature will also prevent eggs from hatching. Anaerobic storage can be achieved by squeezing all the air out of the bag, or by using a vacuum-sealing device. Note that anaerobic storage works best on wet faeces; if faeces are dry, it is difficult to achieve an anaerobic state (see fig 3).
  • Samples should preferably be tested within seven days of collection, although there are indications that eggs can remain intact for longer if adequately refrigerated.
  • Faecal samples that are or have been frozen are not acceptable as this will damage the eggs and decrease the recovery rate.
  • Diarrhoea samples are not acceptable for FEC.

    • Small strongyles - diagnosis

    ONLY patent or adult infections can be diagnosed with the use of faecal egg counts. This is especially important to remember if larval cyathostomiasis is suspected. The emerging larvae will not yet be capable of producing eggs and therefore no eggs will be detected in faeces despite what is a severe parasite-related disease.

    A serum ELISA test is now available as an aid in the diagnosis of Larval cyathostomiasis. However, it must be remembered that this test is qualitative and not quantitative. This means you get the same result (a positive or negative) regardless of the number of parasites present. It can be useful in young horses but it is of questionable benefit in older horses and as yet there are few controlled studies evaluating its use. This is likely to be an area of research over the next few years and recommendations may change with regard to this test

    Large strongyles - diagnosis

    Based on the detection of eggs in faeces but like small strongyles this means that only adult or patent infections can be diagnosed, while it is the larval or migrating parasite that is most responsible for disease.

    Ascarids – diagnosis

    Adult Parascaris spp only shed eggs intermittently, therefore not identifying them in faeces does not necessarily mean that the animal does not harbour any ascarids. It could mean that; or may mean that they were simply not shedding eggs at that time.

    So what use is it?

    If ascarid eggs are identified in any animal within a group, then all animals in the group should be treated. If one animal is shedding eggs that means it has ascarids and since all animals sharing the same environment are exposed to the same infection levels of the same parasites, then all animals in the group are likely to be harbouring ascarids. Remember, this only applies to youngstock as they have not yet developed their immune response. This would not be in the case in adult horses as a single adult individual may simply have a different immune response than others.

    Clinical signs

    Larval stages of parascaris spp cannot be diagnosed definitively and therefore disease based on larval migration is largely diagnosed based on clinical signs.

    Tapeworms - Diagnosis

    Diagnosis is difficult as egg shedding is intermittent and there is no correlation between faecal egg counts and the number of parasites present. There has been limited sensitivity of FEC using standard methods and therefore some modified techniques have been used which have improved sensitivity. This is likely to improve significantly with wider use of automated equipment.

    Commercial diagnostic tests (ELISA) which detect antibodies in serum or saliva are available but can generate false positives due to persistence of antibodies for up to four months.

    Studies have shown that these may be beneficial particularly in older horses as part of an overall parasite control programme and if used judiciously could reduce the use of anthelmintics aimed at this group of parasites.

    The next article in the series (January 28th edition) will look at the anthelmintic (deworming) products that are on the market. It will encompass data of product efficacy and documented resistance to the products. It will also cover non-anthelmintic means of parasite control.