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  Discussion on Research Summary: Monitoring Deworming Programs on Farms
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DrO

Posted on Thursday, Nov 29, 2007 - 9:22 am:

Here is wonderfully helpful study on monitoring parasite activity on farms using fecals. This report also highlights one of the unusual problems we are beginning to recognize in adult individuals. The idea behind the study is that it is impractical to do fecals on every horse so pooling fecal samples and sampling the pool is one way to go.

Their findings were that young (less than 2 years of age) horses on contaminated pastures always developed significant numbers of parasites as measured by fecals and should be wormed regularly.

The more interesting finding is one that has been reported before. Often many adults have acceptable parasite burdens (measured by less than 100 eggs per gram on a McMasters fecal test) on pastures that are not too contaminated even when not frequently dewormed but there are the occasional individual horses that do not respond like most of the adult group. Following the eight week prepotency following a dose of ivermectin, these individuals parasite egg production blooms quickly. It appears the ivermectin is doing its job since they are egg free for 8 weeks but that the horses innate ability to resist low levels of parasites is poor so they are rapidly recolonized. But I am guessing here. This individuals need to be identified and tested and dewormed more rigorously than the rest of the herd. Using aggregated samples can help identify these individuals with less work by examining smaller and smaller aggregates of labeled samples until these "excretory" can be found. For more see the article Overview of Deworming.
DrO

Vet Parasitol. 2007 Oct 22;
The use of age-clustered pooled faecal samples for monitoring worm control in horses.
Eysker M, Bakker J, van den Berg M, van Doorn DC, Ploeger HW.
Division of Clinical Infectiology, Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80165, 3508 TD Utrecht, The Netherlands.

A study was performed on two horse farms to evaluate the use of age-clustered pooled faecal samples for monitoring worm control in horses. In total 109 horses, 57 on farm A and 52 on farm B, were monitored at weekly intervals between 6 and 14 weeks after ivermectin treatment. This was performed through pooled faecal samples of pools of up to 10 horses of the groups 'yearlings' (both farms), '2-year-old' (two pools in farm A), '3-year-old' (farm A) and adult horses (four pools on farm A and five pools on farm B), which were compared with the mean individual faecal egg counts of the same pools. A very high correlation between the faecal egg counts in pooled samples and the mean faecal egg counts was seen and also between the faecal egg counts in pooled samples and larval counts from pooled faecal larval cultures. Faecal egg counts increased more rapidly in yearlings and 2-year-old horses than in older horses. This implied that in these groups of young animals faecal egg counts of more than 200 EPG were reached at or just after the egg reappearance period (ERP) of 8 weeks that is usually indicated for ivermectin. This probably means that, certainly under intensive conditions, repeated treatment at this ERP is warranted in these young animals, with or without monitoring through faecal examination. A different situation is seen in adult animals. Based on the mean faecal egg counts on both farms and on the results of pooled samples in farm A, using 100 EPG as threshold, no justification for treatment was seen throughout the experimental period. However, on farm B values of 100 EPG were seen at 9 and 11, 13 and 14 and 14 weeks after ivermectin treatment in pools 10, 12 and 13, respectively. This coincided with the presence of one or two horses with egg counts above 200 EPG. The conclusion is that random pooled faecal samples of 10 adult horses from a larger herd, starting at the ERP and repeating it at, for instance, 4-week intervals, could be used for decisions on worm control. However, there would be a certain risk for underestimating pasture contamination through missing high-egg excretory. An alternative use of pooled samples would be as a cheap first screening to detect which adult horses really contribute to pasture contamination with worm eggs on a farm. All horses should be sampled and subsequently animals from 'positive' pools can be reexamined individually.
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