Martin K. Nielsen
Internal Parasite Screening and Control
For several decades, diagnosing parasitic infections in horses received little attention. Horses were widely exposed to ubiquitous parasite species capable of causing serious clinical disease and ill-thrift, but the availability of affordable, safe, and widely efficacious anthelmintic paste and gel formulations often precluded the need for accurate diagnostics. The philosophy was to prevent parasitic infection through regular anthelmintic treatments applied year-round. For some time, this appeared to be a simple and safe approach to parasite control. Calendar-based treatment programs became widely used, and most horses still receive four to eight anthelmintic treatments at regular intervals year-round.
Equine parasite control is now undergoing dramatic changes because ever-increasing levels of anthelmintic resistance have forced the industry to change strategy and take a more sustainable approach. In cyathostomins (small strongyles), resistance to both benzimidazoles and pyrantel salts is common worldwide, and there is great concern about emerging resistance to ivermectin and moxidectin (Table 77-1). Even more concerning is the worldwide occurrence of ivermectin and moxidectin resistance in Parascaris equorum. The large strongyles (Strongylus spp) appear to remain fully susceptible to anthelmintic treatment, which explains their rare occurrence in managed horse populations. The development of resistance has in turn led to departure from the calendar-based treatment approach and a move toward deworming regimens based on systematic parasite surveillance. The rationale is to reduce the anthelmintic treatment intensity in order to maintain anthelmintic efficacy for as long as possible. In the European Union, several countries have implemented prescription-only restrictions on all anthelmintic drugs. This legislation prevents prophylactic treatment and requires that parasitic infection be diagnosed before the anthelmintics can be prescribed. Consequently, many veterinarians are now performing fecal egg counts as a routine measure, and treatment frequency in those countries has been considerably reduced. However, this is not just a European trend: in the United States, an increasing number of equine veterinarians are now offering fecal egg counts as a means of monitoring parasite levels, and there appears to be general acceptance of the need to avoid further development of anthelmintic resistance. This has created an increased focus on understanding the methods available for diagnosis of equine parasitism.
Diagnostic Testing
Fecal Egg Counts
Quantitative fecal egg counts are and will remain the cornerstone of equine parasitologic diagnostics. Therefore it is important to understand the nature of egg counts in order to appropriately interpret results from the numerous egg counting techniques and their various modifications. The widely used classic egg counting techniques include the Stoll, Wisconsin, and McMaster methods. These methods are all based on flotation of eggs to separate them from the fecal matter and make them available for microscopy and counting. The two most important features characterizing any given egg counting technique are the detection limit and the variability between repeated counts. The detection limit (synonymous with the multiplication factor for the technique) is the smallest egg count detectable with the method and is particularly important for the fecal egg count reduction test (FECRT), in which it is crucial to detect low egg numbers after treatment. The variability is considerable with most egg counting methods, and this should be taken into account when results are interpreted. As a rule of thumb, any egg count should be interpreted with a ±50% margin. That is, an egg count of 200 eggs per gram (EPG) really represents the interval of 100 to 300 EPG. A recently developed modification of the McMaster principle, the FLOTAC, has been thoroughly validated and has much less variability than the classic techniques. Because the detection limit with the FLOTAC technique is as low as 1 EPG, this method appears to be highly suitable for the FECRT.
Egg counts can be performed for several purposes, but it is important to realize that individual tests are not equally useful for all these. The three most important purposes are for (1) screening of anthelmintic efficacy (FECRT), (2) identification of horses as high, medium, and low egg shedders for selective deworming with a specific anthelmintic, and (3) clinical diagnosis of parasitism in a single horse.
Fecal Egg Count Reduction Test
The FECRT (Box 77-1) remains the gold standard for detection of anthelmintic resistance, which is always diagnosed in the parasite population that is shared among the herd. Each horse represents only one sample of the parasite population, and because of variability, FECRT must be generated as the mean reduction observed on the herd level. The limit of detection is very important for the FECRT because a small reduction in anthelmintic efficacy will be expressed as a small number of eggs in the post-treatment samples. In essence, a high detection limit can falsely overestimate the efficacy of the drug. For example, if a pretreatment egg count is 300 EPG, and the horse is treated with a drug with 90% efficacy, the post-treatment egg count should be 30 EPG. If the egg counting method had a detection limit of 50 EPG, the post-treatment result would be falsely reported as 0 EPG, leading to a calculated FECRT of 100%. There are two possible ways to avoid this bias. One is to ensure that horses selected for the FECRT have very high egg counts before treatment (e.g., 1000 EPG) so that a detection limit of 50 EPG would be able to detect eggs in the post-treatment samples, even when treatment efficacy is 90%. However, such high egg counts are rare in adult horses, and it is unlikely to find more than a few on any given farm. The other option is to use an egg counting method with a lower detection limit. The McMaster techniques are widely used in veterinary practice because of their simplicity and typically have detection limits in the range of 25 to 50 EPG, making them unsuitable for the FECRT. The FLOTAC technique, with its detection limit of 1 EPG, is highly suitable but comes with the added costs of specially manufactured counting chambers, more technician time, and appropriate centrifuge equipment. Thus the choice of egg counting technique always represents a tradeoff between the costs and ease of use on one hand and the detection limit and variability on the other.
Box 77-1
Fecal Egg Count Reduction Test
The FECRT evaluates the efficacy of an anthelmintic drug on the basis of its ability to reduce fecal egg output after treatment. Fecal egg counts (FEC) are performed just before (or at the time of) and 14 days after treatment. The fecal egg count reduction (FECR) is calculated for each individual horse according to the formula:
It is recommended to use an egg counting technique with a detection limit of 25 EPG or less. Always use the same technique consistently. Include horses with the highest possible pretreatment egg counts, and never use horses with counts below 200 EPG.
The FECRT should be established at the farm level by calculating the FECR for a number of individual horses and then subsequently calculating the average FECR for the treated group. It is recommended to include at least 5 to 10 horses on each farm if possible.
Suggested cutoff values for resistance depend on the drug tested and the number of horses investigated, but for the range of 5 to 10 horses, the following cutoff values are recommended as general guidelines for strongyle nematodes:
If the farm average FECR falls below these values, anthelmintic resistance should be suspected. However, it is important to rule out other causes of decreased efficacy, such as misdosing and inappropriate storage. One must also consider how many horses were tested and how high the starting FECs were. Because of inherent variability in the measurement of FEC when performing FECRT, interpretation of the data can sometimes be difficult when results fall into the borderline zones. In such cases, it is recommended to repeat the FECRT.
Egg Reappearance Period
The egg reappearance period (ERP) is the number of weeks from anthelmintic treatment until eggs are found in the feces of treated horses. The simplest approach for measuring ERP is by performing FECRTs more than 2 weeks after treatment. Initially, ERPs were defined for each drug type to identify useful treatment intervals for a calendar-based approach. Today, ERP is a useful tool to monitor emergence of anthelmintic resistance because ERP will be shortened before the FECRT shows reduced efficacy of the anthelmintics 2 weeks after treatment. This is well illustrated in the case of cyathostomins, in which treatment with ivermectin and moxidectin initially had ERPs of 8 weeks and 12 to 16 weeks, respectively. Now, several studies have revealed that ERPs are down to 4 to 5 weeks for both drugs on many farms. Thus it will be useful to monitor for moxidectin and ivermectin resistance by performing ERP for cyathostomins. This will require performing a modified FECRT in which the post-treatment egg counts are performed about 5 weeks after treatment rather than the usual 2 weeks.
Identification of Consistent Egg Shedders
Individual adult horses, particularly low egg shedders (<200 EPG), have a strong tendency to return to the same strongyle egg count levels regardless of anthelmintic treatment. For this reason, horses have a certain strongyle contaminative potential (SCP), which they are highly likely to maintain over time. This is the foundation for the selective therapy principle, in which egg counts exceeding a threshold value identify horses for treatment. If adult horses with egg counts greater than 200 EPG are treated with an anthelmintic of 99% efficacy, about 50% of horses can be left untreated while maintaining an overall egg reduction of more than 95%.
Systematic fecal egg counts over time can reliably identify consistent low, medium, or high strongyle egg shedders within the herd. Typical cutoff values are less than 200, 200 to 500, and more than 500 EPG as designations for low, medium, and high shedders, but choice of cutoff may vary depending on conditions on a given farm. Also, it should be emphasized that egg shedding patterns are much less consistent in horses younger than 4 years of age. Because egg counts are generally higher and more variable in these animals, selective therapy is not recommended.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
