IN-DEPTH: IMMUNOLOGY/VACCINATION/PARASITE CONTROL


value for treatment. By treating adult horses ex- ceeding egg counts of 200 EPG with an anthelmintic with 99% efficacy, it has been shown that about 50% of horses will be left untreated, while maintaining an overall egg reduction above 95%.22 Thus, by performing systematic surveillance over time, fecal egg counts can reliably identify consistent low, me- dium, or high strongyle egg shedders within a herd of adult horses. Typical cut-off values used for cat- egorizing horses into these three groups are as fol- lows: Low shedders 200 EPG, medium shedders  200 to 500 EPG, and high shedders  500 EPG. However, the choice of cut-off may vary depending on conditions on the given farm. As op- posed to with the FECRT, the McMaster technique is generally very suitable for identifying high, me- dium, and low shedders. Also, it should be empha- sized that the consistent egg shedding patterns are much less pronounced with horses younger than 4 years-of-age, as egg count levels are generally higher and more variable. Therefore, selective therapy is not recommended in foals and young horses.


Correlation of Fecal Egg Counts to Clinical Parasitism


The previous sections have illustrated how useful information can be generated with fecal egg counts. Despite this, many veterinarians and horse owners remain skeptical about their reliability. The main reason for this is probably that fecal egg counts are not useful tools for diagnosing clinical parasitic dis- ease. There are several reasons for this:


1. The mere presence of parasite eggs and/or larvae in the feces does not equate parasitic disease. The majority of equine parasites are more or less omnipresent, and parasitic dis- ease represents the exception rather than the rule. In addition, false negative egg counts are not uncommon.23


2. Migrating or encysted strongyle larvae do not produce eggs but remain the most pathogenic stages. Egg or larval counts strictly repre- sent progeny produced by less pathogenic adult parasites present in the intestinal lu- men. Currently, there are no commercially available diagnostic assays capable of detect- ing migrating or encysted parasite larvae in horses, but several are under development.24


3. There is no direct linear relationship between the actual egg counts and the size of the worm burden,23 so a higher egg count simply does not mean more worms. However, cutoff val- ues in the range of 100 to 500 EPG all corre- sponded to lower worm burdens in horses below the cutoff compared to those above.23 Thus, there is data supporting a cutoff in this EPG range.


Taken together, a fecal egg count does not provide useful information when dealing with cases of sus-


404 2014  Vol. 60  AAEP PROCEEDINGS


pect parasitic disease, and there is no reason to consider it part of a clinical work-up.


3. Baseline Treatments


There is general consensus that a foundation of baseline treatments, which should be considered for all horses, can be identified. Parasite surveillance performed on the farm will help identify needs for additional treatments. In adult horses, the general foundation is two


yearly treatments, preferably with a macrocyclic lac- tone, typically applied in the spring and early fall. These are generally applied to break the life cycle of the more pathogenic large strongyles, but a tape- worm drug can be included if there is evidence of high tapeworm transmission. Egg counts will iden- tify consistent high strongyle shedders that could receive additional treatment during the grazing sea- son. A large majority of adult horses are likely to not need more than the two baseline treatments. It should be emphasized that these baseline treat- ments could be integrated with other preventive health measure programs such as yearly vaccina- tions and dental work. In foals and young horses, parasite control should


be approached in a different manner. Egg counts can provide useful information in foals, but they should be used differently than in adult horses. Given the ubiquity of Parascaris equorum, it is jus- tified to focus the program on this parasite during the first few months of the foal’s life. The main concern is the risk of small intestinal impaction, which typically occurs at the age of 4 to 8 months.25 Ironically, treatment with an efficacious anthel- mintic has been identified as a significant risk factor for this condition,25 so treatments should be planned with great consideration. It should also be noted that in case of macrocyclic lactone resistance, no drug formulation will provide efficacy against mi- grating larvae when given in a single dose. Larvae migrate in the liver and lungs for about four weeks before returning to the intestinal lumen, so there is little or no value to treating foals at less than 30 days-of-age. The first treatment for foals should be considered


around 2 to 3 months-of-age. This allows the first ascarid parasites to complete their migration, reach the intestine, and become available for treatment. Benzimidazole-type drugs appear to be the best choice for treatment at this time because of the apparent lack of anthelmintic resistance in P. equo- rum, but other drug classes can be considered if they have been found efficacious. Furthermore, other drug classes with a paralytic mode of action may be more associated with the risk of verminous small intestinal impactions than the benzimidazoles, which interact with parasite metabolism.25 The second treatment should be considered about three months later, which could coincide with weaning in many foaling operations. It is recommended to con- sider anthelmintic treatment prior to weaning.


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