by Leslie Slusher, Ph.D.

Reprinted from AKC Gazette Magazine, June 2020

Hookworm (Ancylostoma caninum) and Roundworm (Toxocara canis) are two of the parasites that can affect a puppy in the first weeks of its life. Understanding the lifecycle of these parasites and how they are transmitted to puppies is important for recognizing the symptoms of infection and determining the best mode of prevention and treatment. While these parasites are transmitted to adult dogs from the environment, the mode of transmission to neonates is usually vertical, meaning that the parasite is transmitted directly from the mother to her puppy.


Eggs of hookworms are deposited in the soil from the feces of infected dogs. The eggs will hatch in a day or two depending on the temperature with warmer summer temperatures decreasing hatching time. The larvae hatch into the soil and mature there to an infective stage in four to five days.  The usual route of infection for the adult dog is by ingesting the larvae from sniffing or eating infected feces or soil.

The newborn puppy, however, is infected through a different route.  Infective larvae present in the soil can penetrate through the skin of a dog that comes into contact with them. Once in the body, the larvae use the blood vessels to move through the body. They migrate from the blood vessel into muscle tissue and enclose themselves in a cyst.  They remain in the muscle tissue where they completely arrest metabolic activity. Since they are not metabolically active, the dog’s immune system does not detect them nor can they be effectively treated with anthelminthic drugs.

Under the influence of pregnancy hormones, the encysted larvae activate around Day 42 of gestation. The larvae then migrate to the mammary glands. As the pup nurses it ingests these larvae. Once in the pup, the larvae migrate to the small intestine where they mature. In about two weeks, the larvae will have matured into blood sucking adults. The nursing pup can accumulate a large number of larvae. As the worms mature, they secrete enzymes which dissolve the gut tissue and prevent coagulation of the blood. The worms will move around in the gut leaving bleeding sores in the intestine. Heavily infected pups will have significant anemia which causes pale mucus membranes. The adult worms suck up to 0.2 mls/day of blood which makes anemia a concern in the infected puppy. If, for example, the puppy is infected with 10 worms, it can lose up to 2 mls of blood/day. An adult dog can lose this volume of blood and not be severely affected. This amount of blood loss, however, can be fatal to a puppy.

The level of infection will depend on how many arrested larvae are in the mother’s tissue. Not all of the larvae come out of the arrested state at the same time. They begin to “wake up” around Day 42 of pregnancy and will continue to activate throughout the remaining pregnancy and lactation. In addition, not all of the arrested larvae will activate with the first pregnancy. The bitch can deliver larvae to her first three litters.


Roundworms are also vertically transmitted from mother to puppy.  Unlike hookworms, the initial infection with roundworms is through the placenta. Adult roundworms living in the intestine of an infected animal lay eggs. These eggs are deposited in the feces and excreted.  As with hookworms, the eggs are orally ingested by other dogs as they sniff or ingest infected feces or soil.  Once ingested, the eggs will hatch into the small intestine in about 2 weeks. The larva can then migrate through the intestinal wall and are carried in the blood to other tissues. Similar to hookworms, the larvae encyst waiting for the bitch to become pregnant. Around Day 42 of gestation, the worms in the mother will “wake up” and move in the blood through the umbilical cord where they migrate across the placenta to the liver of the puppy.

Once the puppy is born the worm larvae will move from the puppy’s liver through the blood into the lungs. In the lungs they will break through the capillaries into the lung space. From there they climb up through the trachea where they will be swallowed by the puppy infecting his gut. The larvae then migrate from the stomach to the small intestine. Adult roundworms can be found in the small intestine about 4 weeks after the puppy ingests the larvae. Roundworms live in the lumen of the small intestine. Unlike hookworms, roundworms do not suck blood but instead eat the contents of the small intestine.

The problem with a heavy infection of roundworms in a puppy is the large number of worms breaking through the lung capillaries. There is a lot of bleeding into the lungs causing difficulty breathing. The puppy can literally drown in his own blood. If the infection is not heavy, the puppy may frequently cough but not have additional respiratory symptoms.

Roundworm larvae can also get into the mammary gland and be suckled and swallowed by the puppies in a manner similar to hookworms. This happens when the puppies are about 6 weeks of age. Since puppies are usually weaned around this age, there is only a narrow window of time during which the roundworms can infect puppies through this route.

Puppies can accumulate large numbers of roundworms living in the lumen of intestine. They are about 4 to 5 inches long. Because the worms are in the lumen, the peristaltic action of the intestine keeps moving the worms down the intestine. To counteract this action, the worms coil themselves up into a spring-like shape and keep moving forward. With a large number of worms doing this, it causes friction against the intestinal wall resulting in edema and inflammation. This reduces the number of intestinal microvilli that absorb food creating a nutritional problem in the puppy. The edema in the gut also causes a pot belly in the puppy. The belly is very tender to the touch and the puppies are very uncomfortable.

It is important to note that a female roundworm can lay as many as 100,000 to 200,000 eggs per day. If, for example, there are 50 worms in a pup, it will be passing millions of eggs each day in its feces which have the ability to persist in the soil for 5 to 7 years. For at least the first 2 years these eggs will persist in the top levels of soil where they are the most infectious. The point here is that once you have a roundworm infection in your kennel, it is very difficult to eliminate.


Treatment for both hookworm and roundworm infections are similar. Both worms are vertically passed to their offspring so the same drugs can be used to treat both types of parasite. Although there are several drugs that are approved by the FDA for treatment of hookworms and roundworms in dogs, only Febendazole (Panacur and Safe-guard) and Pyrantel pamoate (Nemex and Heartgard Plus) are labeled for pups as young as 2 weeks. Moxidectin (Advantage Multi) and Milbemycin oxime (Interceptor and Sentinel) should not be given until 7 weeks of age. All of the drugs target the adult worms so you have to allow the worms to become adults before treatment. The pups and their mother should be treated at 2, 4, 6 weeks of age and every two weeks after this until weaned. Four weeks after weaning the pups should be placed on a monthly parasitic preventative (e.g. Heartgard) which should be continued monthly for the entire life of the dog.

Picking up dog feces on a daily basis is the best way to limit infection by both hookworm and roundworm. Keeping your pens clean removes the parasite before it becomes infective. Hookworm larvae are living organisms that do not have a protective shell so they are easier to control than roundworm eggs. Hookworm larvae will die if they dry out or freeze. They are also susceptible to some disinfectants. Roundworm eggs, however, have a strong protective coat that insulates them from the environment. The eggs have survived storage in formalin and hatched larvae. Very few disinfectants are effective against roundworms. Steam cleaning the kennel, however, will kill the eggs of both organisms. Again, picking up feces on a daily basis is the best way to limit infection by these parasites.

Several studies have been conducted in which topical selamectin (Revolution) or moxidectin and imidacloprid (Advantage Multi) was applied to the pregnant bitch at Days 10 and 40 of pregnancy. Results from these studies demonstrated a large reduction in the number of roundworm larvae passing through the placenta. Even a single dose given on day 56 of pregnancy will significantly reduce the number of larvae. If the bitch is on these preventatives year-round there will be a constant blood level that should kill most larvae and you don’t have to worry about the timing. The pups should still be treated but you will not risk losing the pups during their first two weeks of life. Fenbendazole (Panacur) can also be given daily beginning at day 40 of pregnancy. Fenbendazole does not leave the gut very well so it needs to be started early and given daily to be effective. It will, however, greatly reduce the number of roundworm larvae passing through the placenta and hookworm larvae entering the milk.


Much of the information for this article was provided in a Breeders Excellence Seminar given by Dr. Thomas Nolan, Ph.D. Director of the Clinical Parasitology Laboratory at Ryan Hospital University of Pennsylvania School of Veterinary Medicine

About the Author:  Leslie Slusher, Ph.D. received her doctoral degree from Pennsylvania State University College of Medicine where she remained following graduation in order to complete a five-year National Institutes of Health sponsored postdoctoral fellowship in Molecular Genetics. She joined the Department of Biology Faculty at West Chester University of Pennsylvania in 1991. She spent the next 26 years teaching Genetics, Human Genetics, Bioethics, Medical Genetics and Molecular Genetics at both the graduate and undergraduate levels. She was the recipient of many teaching awards perhaps the most significant being named Professor of the Year by the University’s Honor Students the year proceeding her retirement.