SPRING... AN EFFECTIVE DEWORMING PROGRAMME
Thursday, September 16, 2010 at 5:44PM 
Spring... the problem of more parasites.
(Photo : Leigh Willson)
"Spring Cleaning"
Spring is a time of new growth, greening of pastures, foals being born, and warmer, wetter weather. Along with this comes the problem of more parasites.
Worms would not be a problem if they lived in perfect harmony with their hosts. At low levels of infection, such as those seen in wild horses and Zebra, they cause little harm and are well tolerated. However, with domestication and intensification, we have turned the balance in favour of the worms.
Worm damage results in scar tissue which can accumulate over a number of years, reducing the area available for the absorption of nutrients and making it difficult for the horse to maintain its weight and condition. Regular worming assists in promoting good condition throughout the horse's life.
Our aim, as horse owners, is to ensure that we control the worm burdens in our horses, and with prudent parasite control, to optimize the health and performance of horses. For a deworming program to work, it is important that an effective dewormer be used at the most effective time, the appropriate interval between treatment be adhered to, and the correct dose given.
There will be variations in advice given relating to varied geographical locations, management practises and weather but the objectives remain the same - to bring the level of parasitism down to a level where there is no clinical disease whilst allowing a degree of acquired immunity to develop over the years.
As a starting point the current parasitic burden can be measured using Faecal Egg Counts (FEC). In cases where any individual FEC or the average in a group is over 200 eggs per gram, those horses should receive a dose of a broad - spectrum anthelmintic. Repeating the FEC two weeks post treatment will then give an indication of the efficacy of that treatment.
The goal in deworming is to treat strategically when the greatest risk of exposure is present. The risk of exposure and subsequent infection depends on the susceptibility of the horse (age, health status), the type of parasite, climatic and seasonal variation, and the horse's environment. E.G. Horses concentrated on a small pasture are more likely to be heavily infected with parasites than horses kept in clean stalls. As a general rule of thumb, in wet hot seasons more doses of anthelmintic will be required than in dry times. This is due to the parasitic life cycles favouring wetter years or climates.
No one wormer will kill all types of worms so an effective program, taking into account seasonal variations, worm activity and chemical efficacy is needed to control all worms.
The ingredients in dewormers influence the interval between worming times, because the time it takes for worms to re-infect horses and produce eggs that are detectable in the faeces (ERP - Egg reappearance period) varies by anthelmintic class of drug.
Typical ERP's for common dewormers are as follows :
Ivermectin - 8 weeks
Moxidectin - 12 weeks
Pyrantel 4 - 6 weeks
Benzimadoles (fenbendazole) - 4 weeks
What do the active ingredients do?
Praziquantel : Controls tapeworms.
Types of worms
Cyathostomes (small strongyles or small redworms) are the most important internal parasite of horses. They often cause sub-clinical impairment of intestinal function, which may lead to poor performance, weight loss or failure to thrive. They may cause overt disease with diarrhoea, severe weight loss and oedema. They are also the worms that are most likely to develop resistance to dewormers.
Pinworms : Pinworm eggs are picked up by horses from contaminated feed, water, or bedding. These worms are irritating, causing the horse to rub its tail, resulting in hair loss and sometimes injury to the tail and rump.
Ascarids (large Roundworms) : The adult stages of the large roundworm are found in the small intestine, where the female passes large numbers of eggs into the manure. In about two weeks, these eggs become infective and the horse picks them up while grazing. The larvae migrate into the blood vessels and are carried to the liver and lungs. The immature worms are coughed up and swallowed, maturing in the small intestine to complete the life cycle.
Bots : Bots are the immature maggot stages in the life cycle of the Bot fly, the adult of which resembles the honeybee in general appearance. The females lay their eggs, during early spring and late autumn, by attaching them to the hairs of the front legs, throat, and under line. As the horse licks itself, the larvae attach themselves to the lips and tongue and burrow into these tissues. After about three weeks they attach themselves to the lining of the stomach, where they may remain for several months, causing additional damage, before passing out in the manure and maturing into adult Bot Flies. Deworming with an Ivermectin based dewormer in Spring and late autumn will target the Bot larvae in the gut.
Large Strongyles (Blood Worms) : Adult strongyles are found firmly attached to the walls of the large intestine, where the females pass large numbers of eggs into the manure. These eggs hatch and the larvae climb blades of grass and are swallowed. The larvae then migrate to the large arteries which supply the intestines. As the artery walls are damaged, blood clots form and break away, causing colic.
Tapeworms : Have a life cycle that takes roughly six months to complete, so a twice yearly dose of Praziquantal is sufficient to control them. They are to be found in the large gut at the junction of the small intestine, caecum and colon.
Once infected, tapeworms contribute to digestive problems, colic and malnourishment and can cause obstruction. Tapeworms are often undetected using normal faecal flotation methods.
Management Control Measures include resting pastures where possible, cleaning pastures of droppings regularly, deworming all horses at the same time to minimise pasture contamination, deworming new arrivals before putting them into a general pasture and ensuring feed bins and stables are kept clean and regularly disinfected.
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