June 22, 2019
Roundworms: An Old Battle Continues by Carin Smith They're called the "spaghetti worm": the long, white limp strands curling around in your yearling colt's fresh manure. While older horses are plagued with bloodworms, the roundworm (ascarid, or Parascaris equuorum) creates havoc with younger animals. In spite of our ability to kill this parasite with many modern dewormers, it still remains a threat and a problem. Why can't we rid our youngsters of roundworms for good? The answer lies in the roundworm eggs' almost total indestructibility. The hard little shell surrounding an immature larva withstands freezing and heat, extreme wet and desiccating dryness. It can live for years if need be, waiting for an opportunity to infect another foal. And many disinfectants don't bother these eggs one bit. Adult roundworms are extremely prolific; an infected foal may pass millions of eggs in its manure each day. These eggs possess inherent "stickiness", enabling them to be easily transported on your feet or by shovels and other barn equipment. Even on the farm with a modern deworming program and impeccable hygiene, then, the potential for infection with roundworms is high. The typical foal, curiously nuzzling each new object it encounters, is unknowingly infecting itself all the time. SIGNS OF INFESTATION We typically think of intestinal damage from worms, but ascarids migrate through many body tissues. One of the most common causes of cough in the foal is roundworm migration. The life cycle of these nasty parasites is gruesome. Once the egg is ingested, a little larva hatches out into the foal's intestine. It doesn't waste any time there, but immediately proceeds to the liver, where it lives for a week or so. The next stop for the ascarid larva is the lungs. Here, larvae live for two to four weeks. Their migration causes inflammation, coughing, and sometimes pneumonia. When they've matured to an appropriate stage, they're coughed up and swallowed, returning to the foal's intestine. Here they grow to adulthood, eating food meant for the horse and laying eggs to ensure survival of future generations. Large numbers of roundworms will seriously deplete the foals' food supply, stunting its growth and leading to a generally unthrifty-looking animal. In severe cases, the worms may even cause impaction and rupture of the foal's gut. The time span from the foal's first infection to the time eggs are passed in its manure is 80 days or more; foals younger than three months may have negative fecal egg counts and still be heavily infested from the day they were born. DIAGNOSIS How can we diagnose these cases? The presence of roundworm eggs in other, older foals on the same farm is a good clue. Also, a rise in the foals' eosinophil count (the "allergic" white blood cell) is an indication of some kind of parasitic infection. Of course, infected foals older than three months will have large numbers of roundworm eggs in their manure samples. Since the worms don't lay the same number of eggs each day, this can't be used to estimate the actual worm burden in a particular foal. Any farm which has had a roundworm problem in the past should assume that the eggs are still present on the premises. (Any place where horses have been stabled for some time is guaranteed to have a good load of eggs.) These farms should institute an aggressive deworming program for their young animals without waiting for large numbers of eggs to appear in the foals' manure. By then, contamination of the premises with millions more eggs has already happened. CONTROL What's a good program to follow for the typical foal? The first step in an effective control program is eliminating the source of infection. While we'll never get rid of all the eggs, we can reduce their numbers considerably. Daily removal of manure is essential. Products such as Lysol or a phenol-based disinfectant should be used to scrub down stalls. Manure control in the pasture is just as important as in the barn. Composting or disposal of manure is more reliable than simply spreading it on pasture, since destruction of eggs by spreading depends on extreme heat or cold. Allowing horses to graze on pasture even months after manure has been spread may cause severe reinfestation with roundworms. Current recommendations include deworming at monthly intervals from the time the foal is 6 weeks old until he reaches 6 months. After that, the foal's program may be synchronized with older horse's bimonthly dewormings. Almost all dewormers are effective against ascarids, including piperazine, pyrantel, the benzimidazoles (except thiabendazole), ivermectin, and moxidectin. No matter what product is used, though, the foal will be reinfected almost immediately, thus the need for such frequent treatments during its first six months. IMMUNITY Older horses seldom, if ever, have a problem with ascarids. As the foal matures, it gradually builds up an immunity to these worms. The larvae are destroyed as they migrate through the liver and lungs, never even making it to the intestine. This immunity occurs over a period of time, from when the foal is 6 months to two years of age. After the first year, you'll be more concerned with the other parasites. But until then, keep on the lookout for signs of infestation. Good preventive care will ensure that you never see the devastating effects of a severe roundworm problem. Dr. Carin Smith has been a practicing veterinarian since 1984. She is a regular contributor to the Journal of the American Veterinary Medical Association and has written hundreds of articles about equine health care for national magazines. Dr. Smith is the author of EASY HEALTH CARE FOR YOUR HORSE. She lives on a Washington farm and enjoys wilderness trail riding, endurance riding, and jumping.
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