1. Barn dogs are at a fairly high risk for acquiring parasites.

Unlike many urban and suburban pets, a barn dog gets to run around in open fields, press his nose into a wide range of smelly things and, possibly, socialize with all the other dogs who visit the stables. The downside of the barn life is that your dog may also be exposed to more intestinal parasites than the average suburban dog—especially if the “smelly things” he often encounters include dead animals or the droppings of other dogs or wild canines.

“The transmission risk is mainly from other dogs or wild canines like coyotes or wolves,” says parasitologist Lindsay Starkey, DVM, PhD, of Oklahoma State University. “Parasites do sometimes cross over from wildlife, so if you live in an area with wildlife, your dogs may be at a higher risk.”

The opportunity to scavenge also increases a dog’s risk of picking up intestinal worms and other parasites. “There will be differences in the parasites of dogs that are eating nothing but good-quality commercial dog food, versus those eating leftovers, garbage or dead animals they find in their environment,” says Thomas Craig, DVM, PhD, of Texas A&M University.

Other risk factors for your dog’s exposure to internal parasites include what part of the country you live in—the hot, humid Gulf Coast states versus the dry Western deserts, for example–as well as how much time he spends outdoors, which increases his exposure to ticks, mosquitoes and other disease-carrying creatures.

2. Your dog and horse share similar internal parasites.

The major canine helminths (worms) include hookworms, roundworms, whipworms, tapeworms and heartworms. Although these worms may belong to the same genus as those that affect horses, each species of parasite tends to thrive in only one species of host. In other words, says Starkey, “Most of these parasites are not transmitted to dogs from horses or other farm animals the dogs might be hanging out with.”

• Hookworms. Four species of hookworms may infect dogs in the United States. “The dog hookworm is very similar to the small strongyle of the horse,” says Craig. “With the hookworm, however, transmission is a little different. The larvae of the dog hookworm, instead of being picked up by grazing, penetrates the dog’s skin. If the dog lies in a grassy, shaded area where there is moisture, this is where the hookworm larvae might be.”

The hookworm larvae migrate through the dog’s body and ultimately reach the intestine, where they can cause bloody diarrhea and vomiting. “The interesting thing about the hookworm is that it may stop along the way, in the tissues,” says Craig. “In older dogs that have some level of resistance to worms, the hookworms don’t go right into the intestines. They just sit in the tissues, waiting. If something happens to remove all the adult hookworms in the intestine, such as deworming the dog, then some of these dormant young ones will wake up and go into the intestine to replace them.”

• Roundworms. “There are actually two roundworms that affect dogs, but the most common is called Toxocara,” says Craig. “In many ways it is like the ascarid that affects foals. We tend to see this infection most often in puppies. In this case, however, puppies become infected by larvae that were migrating in the mother and crossed the placenta into the pups before they were born. The eggs are very much like the eggs of the horse ascarid, in that they are very hardy and can sit around in the environment for a long time, waiting for a chance to infect a dog.” Puppies with roundworms tend to have a characteristic round “potbelly” and may have intermittent diarrhea, vomiting and constipation.

• Whipworms. These worms are picked up orally—the eggs can remain viable for long periods on hard surfaces in dirt or concrete kennels that house large numbers of dogs. The parasites infect the dogs’ cecum. Mild infections cause no symptoms, but large numbers of these worms may cause bloody diarrhea or anemia that may lead to death.

• Tapeworms. Several species of tapeworms can infect dogs. Most are consumed orally, as horses do, but with an extra step: The eggs are shed by an infected dog, then ingested by an herbivorous prey species, where the larvae encysts in the muscle, until it is ultimately consumed by a predator or scavenger. One canine tapeworm species travels by fleas, which consume the eggs; when the dog bites at the itchy spot, he swallows the fleas along with the eggs. “Tapeworms are very common, but most dog owners don’t realize their dogs have tapeworms because they are difficult to diagnose at the veterinary clinic and typically do not cause overt clinical illness,” Starkey says.

3. Heartworm preventatives also control other internal parasites.

Many of the same chemicals used to control parasites in horses are also used to treat dogs. “These drugs include pyrantel, fenbendazole, praziquantel, ivermectin and moxidectin,” says Starkey. “These same drugs have been formulated into dog products for treating similar parasites.”

In most dogs, the primary focus of parasite control is to prevent heartworms, and a number of those products also contain agents that work against other parasites as well. “The majority of heartworm products on the market today are labeled for treating hookworms and roundworms, and some of the products treat for whipworms, as well,” Starkey says. “A couple of heartworm preventive products also treat tapeworms now.”

Even if heartworm isn’t typically a concern in your area, it may still be a good idea to treat your dog. “Heartworm prevention is a great thing for any dog, anywhere in the country, not only for preventing heartworm infection but also for all the other parasites that the product treats,” says Starkey.

4. Heartworm is spreading beyond its previously recognized range.

Heartworm (Dirofilaria immitis) is one of the most serious health threats for dogs. This parasitic roundworm is spread by mosquitoes, which pick up microfilariae from the blood of infected dogs, and after a short period of maturation within the mosquito, pass them on to another susceptible host. Once established in a new host, the larvae take about six months to mature into adult worms, which can reach one foot long and live in pulmonary vessels near the heart.

Signs of infection include a persistent mild cough, exercise intolerance, decreased appetite and weight loss. Dogs with a heavy parasite burden can develop heart failure and damage to other internal organs. Heartworm preventatives are formulated to kill any larvae the dog is exposed to before they molt into adulthood. One cause for concern, though, is that in recent years heartworm has been spreading and becoming established well beyond its historic range.

“Twenty or 30 years ago the only region we considered risky for heartworms was the Southeast. It was a big problem along the Gulf Coast,” says Craig. “But today heartworm control is important for nearly all dogs, all over the United States.”

A number of factors have contributed to the spread of heartworm. For one, a warming climate has increased the range of mosquito populations. Also, says Craig, “Storms may carry infected mosquitoes hundreds of miles.”

Dogs are also moving around the country more, both with their owners and through long-range adoptions. “Mosquitoes were already present in other areas, just waiting for the chance to grow worms,” says Craig. “They got the chance when infected dogs moved into that neighborhood, and now heartworms are everywhere. After hurricanes in New Orleans, for instance, people suddenly started seeing heartworms in places like California and Washington State that had never seen it before. If one dog is infected, the other dogs will soon be infected.”

Another factor that is spreading heartworm is the increased range of wild canines, such as coyotes, who are steadily moving back into areas from which they had once been exterminated. “I read a recent scientific paper that indicated that more than half the coyotes examined were positive for heartworms, and the older they got, the more likely to be infected,” says Craig. “This is the same with dogs. Not all dogs with heartworms die of this disease, and not all infected dogs have clinical disease. But enough dogs that are infected have problems with heartworms to make it worthwhile to have them on a preventative worm control program.”

5. Collies and many other herding breeds are unusually sensitive to ivermectin.

Ivermectin is generally a safe, effective dewormer for dogs and is commonly used in low doses as a heartworm preventative and in higher doses to treat conditions such as mange. Some dogs, however, carry a genetic mutation (MDR1) that makes them more susceptible to toxicity from higher canine doses of this common drug. Signs are neural dysfunctions, including incoordination, blindness, tremors, excessive salivation and coma. A serious overdose can be fatal.

“Sensitivity has to do with ability of the drug to cross the blood-brain barrier,” says Craig. “Ivermectin and its cousins all work on the nervous system of the worms or arthropods. The reason these drugs don’t cause any damage in most mammals is that it doesn’t cross the blood-brain barrier and is kept away from the brain. If the genetics of the dog are such that the ivermectin is able to cross the blood-brain barrier, then we see problems.”

The MDR1 genetic mutation is most common in Collies (about 70 percent are affected) but also appears (at rates from 5 to 30 percent) in related herding breeds, including Australian Shepherds, German Shepherds and Shetland Sheepdogs as well as mixed breeds. Tests for the MDR1 mutation are available from the Washington State University Veterinary Clinical Pharmacology Lab.

However, finding that your dog carries MDR1 does not mean that you need to skip the heartworm preventative. “The dog heartworm prevention drug that is ivermectin-based is a very low concentration and has been proven safe for all dogs, even the susceptible breeds,” says Starkey. But do avoid letting any dog, of any breed, ingest dropped or unattended dewormers formulated for horses. “It’s safest to never use any large animal products in dogs,” she adds. “Use the dog product.”

For more information, see “A Danger to Dogs” (EQUUS 413).

This article first appeared in EQUUS issue #469, October 2016.