{"id":5849,"date":"2021-09-03T11:20:38","date_gmt":"2021-09-03T15:20:38","guid":{"rendered":"https:\/\/todaysveterinarynurse.com\/?post_type=articles&p=5849"},"modified":"2022-09-06T16:33:26","modified_gmt":"2022-09-06T16:33:26","slug":"roundworm-infections-in-dogs","status":"publish","type":"post","link":"https:\/\/navc.sitepreview.app\/todaysveterinarynurse.com\/parasitology\/roundworm-infections-in-dogs\/","title":{"rendered":"Roundworm Infections in Dogs"},"content":{"rendered":"

The nematode roundworms<\/span> are so named because of their round or cylindrical shape on cross-section.1<\/sup> In dogs, the 2 most common roundworms are Toxocara canis<\/i> and Toxascaris leonina<\/i>. By far, T canis <\/i>are the most common parasitic roundworms in dogs; an estimated 30% of dogs younger than 6 months actively shed these eggs, and virtually all puppies are born infected.2,3<\/sup> In the United States, an estimated 99% of puppies and 20% of adult dogs (>12 months of age) are infected with T canis<\/i>.4,5<\/sup> Thus, most of this article focuses on T canis<\/i>, the most common parasite of companion dogs in the United States.\u00a0<\/span><\/p>\n

\n

The Pests and Parasites series<\/a> is brought to you by Merck Animal Health, the makers of Bravecto\u00ae (fluralaner) and Sentinel\u00ae (milbemycin oxime\/lufenuron)<\/span><\/p>\n

\n

Roundworm distribution is worldwide; incidence is higher in urban areas. The definitive hosts for T canis <\/i>parasites are canids (e.g., dogs, wolves, foxes, coyotes). Adult worms can be found in the small intestine of canids, robbing the host of nutrients.1<\/sup> However, other animals can ingest the parasite and become a paratenic host or a dead-end host. A paratenic host is one that ingests the eggs and can transport them, but the parasites do not develop in the host. All small mammals can be paratenic hosts.4<\/sup> A dead-end host is an inappropriate host in which the larvae can migrate through various body organs but do not develop or become infective.3<\/sup> Humans are dead-end hosts for roundworms; the parasite cannot complete its life cycle in humans but can cause clinical illness.1<\/sup>\u00a0<\/span><\/span><\/p>\n

Roundworm Life Cycle: A Complex Story<\/h2>\n

Dogs can become infected with roundworms in multiple ways (<\/span>FIGURE 1<\/span><\/strong>). This description of life cycle <\/span>begins with adult worms in the small intestines of a dog.<\/p>\n

\"\"<\/a>

Figure 1. Life cycle of Toxocara<\/em> species roundworms. cdc.gov\/dpdx\/toxocariasis\/modules\/Toxocara_LifeCycle_lg.jpg<\/p><\/div>\n

After worm copulation, unembryonated eggs pass into the external environment in the dog\u2019s feces. A single female adult can produce >85<\/span>\u2009<\/span>000 eggs per day.2<\/sup> The period between eggs being passed out of the dog and becoming infective is termed the prepatent period. For T canis <\/i>parasites, this period is 2 to 4 weeks, depending on how the larvae are acquired. For T leonina,<\/i> it is 8 to 10 weeks.2<\/sup><\/span><\/p>\n

Many animals, including a variety of mammals as well as earthworms, can serve as paratenic, or transport, hosts. When a paratenic host ingests roundworm eggs, the host does not become infected with the roundworms, but the second-stage larvae hatch from the eggs (<\/span>FIGURE 2<\/span><\/strong>) and migrate within the host and either encyst or rest in the tissue.3<\/sup> When a dog eats an infected paratenic host, roundworm larvae are released and enter the dog\u2019s intestines directly, with no somatic migration.1,3<\/sup> Thus, the dog can shed parasite eggs into the environment as early as 2 weeks after infection.2<\/sup><\/span><\/p>\n

\"\"<\/a>

Figure 2. Second-stage Toxocara canis<\/em> larva emerging from egg. olgaru79\/shutterstock.com<\/p><\/div>\n

Roundworm eggs will embryonate in the environment, developing into first-stage larvae. The larvae will molt and become second-stage larvae within the embryonated egg. The egg can remain infective in the environment for up to 3 years.4<\/sup> If second-stage larvae are ingested, the cycle begins again. If a puppy younger than 3 months ingests infective eggs, the larvae migrate through the trachea. If the host dog is older than 3\u00a0months, migration will be somatic.1,3<\/sup><\/span><\/p>\n

Tracheal Migration<\/h3>\n

For tracheal migration, the infective egg enters the gastrointestinal (GI) tract, hatches, and releases third-stage larvae into the intestines. The larvae penetrate the GI wall, enter the bloodstream, and travel to the lungs where they enter the alveoli. The larvae move from the lungs into the trachea and are coughed up and swallowed; when they reach the small intestine, they mature into adults. Alveolar penetration of large numbers of worms can cause verminous pneumonia.1,3<\/sup>\u00a0<\/span><\/span><\/p>\n

Somatic Migration<\/h3>\n

After entering the host and traveling through the blood to the liver and lungs, larvae reenter the host\u2019s alveolar <\/span>blood vessels and travel to the muscles and organs where they become encysted and their development is arrested.\u00a0<\/span><\/p>\n

Roundworm Transmission to Puppies<\/h3>\n

Puppies can be infected while in utero or while nursing.\u00a0<\/span><\/span><\/p>\n

Transplacental\u00a0<\/span><\/strong><\/p>\n

In pregnant dogs, encysted larvae become reactivated and migrate through the umbilical vein to infect the developing puppies\u2019 liver and lungs. After the puppies are born, the larvae are coughed up, then swallowed, and move to the small intestine to complete their development. In transplacentally infected puppies, egg shedding can begin as early as 2.5 to 3 weeks of age.<\/span><\/p>\n

Transmammary<\/strong>\u00a0<\/span><\/p>\n

During lactation, encysted larvae in the dam become active and pass into the milk to directly infect the puppies with no tracheal migration.1,3<\/sup> In transmammarily infected puppies, egg shedding can begin as early as 4 weeks of age.<\/span><\/p>\n

Clinical Signs of Roundworm Infection<\/h2>\n

Puppies infected with T canis<\/i> may exhibit a variety of clinical signs, including diarrhea, vomiting (with or without adult worms in vomitus), enlarged abdomen, occasional constipation, flatulence, dull hair coat, and poor growth. Severe infections in puppies can cause verminous pneumonia, ascites, fatty liver disease, and mucoid enteritis, and can also lead to acute death at a few days of age. Death can result from pneumonitis caused by migrating worms, enteritis, and GI blockage.4,6<\/sup> Infected adult dogs exhibit diarrhea as the most common clinical sign. Signs of disease are not confined solely to the presence of adult worms in the small intestine. Immature worms migrating through the viscera or located in the small intestines can cause clinical signs with no detectable eggs.6<\/sup> Some clinical signs are related directly to larval migration and to the immature and mature adults in the GI tract.7<\/sup><\/span><\/p>\n

Diagnostics Tests for Roundworm Infection<\/h2>\n

Diagnosis can be made by direct visualization of adult worms in the vomitus or feces of an infected dog, by visualization of eggs during microscopic examination of feces, or by enzyme-linked immunosorbent assay (ELISA) testing.<\/span><\/p>\n

Direct Visualization of Worms<\/h3>\n

By far, the easiest way to detect roundworm infection is direct visualization of worms. Adult worms are 3 to 8 cm long, white, and very stout (<\/span>FIGURE 3<\/span><\/strong>). When passed, they tend to be very tightly coiled. They are most commonly seen in the vomitus or feces of puppies. It is advisable to ask clients to bring in any suspect worms for correct identification.5,8<\/sup><\/span><\/p>\n

\"\"<\/a>

Figure 3. Adult Toxocara canis<\/em> worms. MRAORAOR\/shutterstock.com<\/p><\/div>\n

Microscopic Visualization of Eggs<\/h3>\n

Before running a microscopic examination, first visually evaluate the sample for consistency; color; odor; and presence or absence of blood, mucus, or adult parasites. Microscopically examining feces for roundworm egg identification is best done with the freshest sample possible because larvated eggs are more challenging to identify (<\/span>FIGURE 4<\/span><\/strong>). Differentiating nematodes that typically reside in soil from hatched T\u00a0canis<\/i> larvae is impossible. If a sample cannot be examined soon after receipt, refrigeration can help slow down larval development, and mixing equal parts feces with 10% formalin can arrest the larval development.5<\/sup><\/span><\/p>\n

\"\"<\/a>

Figure 4A. Toxocara canis<\/em> unembryonated eggs. olgaru79\/shutterstock.com<\/p><\/div>\n

\"\"<\/a>

Figure 4B. Toxocara canis<\/em> embryonated eggs with larvae. Dew_gdragon\/shutterstock.com<\/p><\/div>\n

Direct Smear<\/strong><\/p>\n

The easiest microscopic examination is a direct fecal smear; however, the small amount of sample used may not be sufficient for detecting low parasite burdens or intermittent egg shedding. For this procedure, a small amount of feces is placed on a glass slide and mixed with a small amount (2 to 3 drops) of 0.9% saline. To enable egg identification, the resulting emulsion should be thin enough to read newsprint through; if the emulsion is too thick, it is hard to find the eggs through all the fecal debris. A coverslip is then applied and the slide is examined at 10\u00d7 and 40\u00d7 magnification, looking for T canis<\/i> or T leonina<\/i> eggs.5<\/sup><\/span><\/p>\n