Current Advice on Parasite Control:
Intestinal Parasites - Ascarid (also Roundworm, also Toxocara)
*Baylisascaris procyonis is an ascarid of raccoons that is occasionally found in dogs.
Overview of Life Cycle
- Dogs and cats become infected with ascarids via ingestion of larvated eggs from a contaminated environment (all ascarid species), ingestion of other vertebrate hosts that have consumed larvated eggs and thus have larvae in their tissues (all species), and ingestion of larvae in the milk of an infected dam (Toxocara spp.).
- Transplacental transmission of larvae from the bitch to the fetal pups in utero is an important route of infection for T. canis and circumstantial evidence suggests it may occur with Baylisascaris spp. However, transplacental transmission has not been shown to occur in T. cati or Toxascaris leonina. Recent work suggests that transmammary transmission of larvae does not occur with T. cati as was once thought to be the case.
- Migration of larval ascarids within the host is complex. Following acquisition, larvae of Toxocara spp. migrate through the liver and lungs, are carried up the mucociliary apparatus, and then are swallowed to develop in the small intestine. When this migration occurs in fetal pups, the migrating larvae wait in the liver and lungs until the pups are born, at which time they resume their migration across the lungs to the airways.
- Larvae acquired from ingestion of vertebrate tissues do not migrate in the dog or cat definitive host, but instead travel to the small intestine to become adult worms. Toxascaris leonina is different from the other dog and cat ascarids in that migration outside the intestinal tract does not occur in their usual definitive hosts.
- Nonembryonated ascarid eggs are passed in the feces of an infected dog or cat; the most commonly seen species are
- T. canis in the dog and T. cati in the cat.
- Infective, larvated eggs of Toxocara spp. are found in fecal-contaminated soil.
- Adult ascarids in the small intestine of the infected dog or cat can be identified by their size, stout appearance, and the presence of three lips on the anterior end.
- Adult T. cati has distinct cervical alae on the anterior end which allow definitive identification; the alae of T. canis and Toxascaris leonina are less distinct. Baylisascaris spp. do not have salient alae.
Toxocara canis viewed upon retraction
- Disease in dogs caused by infection with T. canis is most severe in young pups. This parasite is observed less commonly in dogs more than one year of age.
- Adult dogs—even dogs that are infected in the uterus of an infected dam—can be repeatedly infected with adult T. canis if they are orally infected with a few (25 to 100) infective eggs.
- Pups infected in utero may present with ill thrift, failure to gain weight, and a poor hair coat; a pot-bellied appearance is also commonly observed. Severe infections in neonatal pups can result in acute death at a few days of age as large numbers of larvae that were acquired in utero cross the alveoli en route to the small intestine. Pups with heavy infections may expel a large mass of worms in vomitus at 4 to 6 months of age; this phenomenon can cause distress for the client as the worms are large and usually alive when expelled.
- Toxocara cati causes ill thrift and a pot-bellied appearance in kittens. Cats are susceptible to infection with this parasite throughout life. In adult cats, irritation of the gastric mucosa by adult T. cati ascarids that have migrated from the small intestine may cause vomiting. Adult ascarids are often found in the vomitus of infected cats.
- Infections with adult Toxascaris leonina and B. procyonis have not been commonly associated with clinical disease in dogs and cats. However, treatment is still warranted, particularly in light of the severe zoonotic disease associated with the latter ascarid. There are limited reports of neurologic disease in dogs attributed to B. procyonis larvae migrating in the central nervous system.
- Toxocara spp. are extremely common parasites of dogs and cats throughout the world. A survey conducted in 1996 using samples collected from across the U.S. found that more than 30% of dogs younger than 6 months of age were shedding T. canis eggs, and some studies have shown that virtually all pups are born infected with T. canis.
- Toxocara cati infections are present in more than 25% of cats in some surveys.
- The ability of Toxocara spp. to be transmitted from the dam to the fetal or nursing offspring, together with the hardy nature of larvated eggs in a contaminated environment, contribute to the high prevalence of infection even among pets that are well cared for and routinely treated for intestinal parasites. Because somatic infection is very common in adult dogs and cats (even those on routine monthly intestinal parasite control), veterinarians should assume that essentially every pup or kitten is likely to harbor developing ascarids in the small intestine, and thus young animals should be routinely dewormed for Toxocara spp. (see Treatment below).
- Although infection rates are likely to be higher in pets kept outdoors or allowed to consume prey species that may harbor larvae in their tissues, infection with Toxocara spp. is common in all dogs and cats.
- The geographic distribution of Toxascaris leonina is more focal in nature than that of Toxocara spp., and infection with Toxascaris leonina occurs less frequently. However, this parasite is still widespread in dog and cat populations.
- Baylisascaris procyonis occurs in raccoons across the U.S. and Canada, with the highest prevalence in the midwestern and northeastern U.S. and along the west coast. In some studies, up to 90% of adult raccoons from a given geographic region were found to be infected with B. procyonis. Recent reports have also documented this parasite in raccoons in parts of the southern U.S. where it had not previously been found, suggesting that the range of this parasite may be expanding or that it may occur in localized populations in some areas. Although comprehensive surveys to determine the prevalence of B. procyonis in dogs are lacking, more than two dozen cases of confirmed canine infection with adult raccoon ascarids have been reported, primarily from the midwestern U.S. Because of the greater zoonotic threat posed by this species, veterinarians and technicians should properly diagnose intestinal Baylisascaris infections in dogs and make sure they receive appropriate treatment.
Host Associations and Transmission Between Hosts
- Dogs and cats become infected with ascarids upon ingestion of larvated eggs from a contaminated environment and ingestion of larvae in tissues of vertebrate hosts.
- Earthworms and possibly other invertebrate paratenic hosts can harbor larvae from eggs in the soil that can be passed by their ingestion to vertebrate hosts both paratenic (birds and rodents) and perhaps to the final host, i.e., the dog or cat.
A wide variety of vertebrate hosts can harbor ascarid infections, and infection is common in dogs and cats that consume prey species.
- Hosts infected with larvae, e.g., primates, rabbits, cats, and birds, can develop signs of infection similar to those seen in children with visceral larva migrans.
- Baylisascaris procyonis larvae have caused visceral disease and death in more than 100 species of vertebrate hosts.
- Transmission of Toxocara canis occurs directly from mother to puppies through the placenta. Many of the T. canis larvae ingested by adult dogs become arrested in somatic tissues. If arrested T. canis larvae are present in an intact female that becomes pregnant, the larvae are activated to resume migration late in pregnancy, making their way across the placenta to infect pups. Similar transmission has also been suggested to occur with Baylisascaris in dogs. In dogs (T. canis) and cats (T. cati), if eggs are ingested near the end of pregnancy or during lactation, the larvae can enter the puppies or kittens in the milk; unlike with hookworms (Ancylostoma caninum) in the dog, this is not considered a major means of transmission.
Prepatent Period and Environmental Factors
- The prepatent period of T. canis varies from 2 to 4 weeks, depending on how larvae are acquired. Pups infected in utero will not shed eggs before 2.5 to 3 weeks of age. Worms acquired after birth from ingestion of larvated eggs in the environment will not become adults and begin passing eggs into the environment for approximately 4 weeks. However, larvae acquired via ingestion of infected vertebrate hosts may develop into adults in as little as 2 weeks.
- Similar variation is seen in the other ascarid species, but in general, T. cati has an approximately 8-week prepatent period, and the prepatent period of Toxascaris leonina is approximately 8 to 10 weeks. Baylisascaris procyonis has a 7- to 10-week prepatent period in raccoons following ingestion of larvated eggs, but patent infections may develop in as little as 4 to 5 weeks upon ingestion of larvae in a vertebrate host.
- Most ascarid eggs require 2 to 4 weeks in the environment to larvate and develop to the infective stage. Toxascaris leonina is the exception; eggs of Toxascaris leonina become infective as soon as 1 week after being shed. Because of the time required, fecal material has often broken down before the eggs are infective, and thus there is often no gross evidence that the environment is contaminated with ascarid eggs. However, once present, ascarid eggs are hardy and can survive and remain infective for years.
- Removing eggs from a contaminated environment is difficult, and common disinfectants are not effective at killing them. Strict adherence to leash laws and prompt removal of feces from environments in which pets defecate are essential aids in the prevention of ascarid infections. Preventing environmental contamination by routinely deworming infected animals before the infections become patent is another key component of achieving effective control (see Control and Prevention below).
Site of Infection and Pathogenesis
Toxocara canis in the duodenum
- Upon infection, larvae of Toxocara spp. undergo an extensive migration through the liver and lung before making their way back to the small intestine to develop into adult worms.
- Focal scarring associated with larval ascarid migration may be evident in the liver at necropsy, but clinical disease because of this hepatic migration rarely, if ever, occurs.
- In contrast, the hemorrhage and inflammation that occur in response to the traumatic migration of larvae across the pulmonary alveoli may result in overt pulmonary disease, particularly in young pups that acquired a large number of larvae from the bitch in utero.
- Adult ascarids in the small intestine of dogs and cats may cause a mucoid enteritis and occasionally, mild diarrhea. Although dramatic cases of intestinal obstruction and intussusception associated with large numbers of ascarids in the small intestine have been reported, such sequelae are relatively rare.
- Adult ascarids that migrate to the stomach may cause irritation of the gastric mucosa that results in vomiting.
- Because ascarids are prolific egg producers and the eggs float readily in most flotation solutions, diagnosis of patent ascarid infections via fecal flotation is straightforward. A single adult T. canis can produce as many as 85,000 eggs per day, making detection of eggs in feces unambiguous.
- Eggs of Toxocara spp. can be readily differentiated from those of Toxascaris leonina (see image under Life Cycle).
- Eggs of B. procyonis are more difficult to distinguish from those of the more common Toxocara spp., although morphologic differences exist (see image under Life Cycle) and differentiation becomes easier with experience.
- There are no available clinical assays to allow detection of prepatent infections. In light of the high prevalence of infection in young pups and kittens and the inability to detect infections in animals before eggs are passed in the feces, presumptive, routine deworming of all animals is recommended (see Treatment).
- Adult nematodes recovered from the vomitus of an infected dog or cat can be definitively identified as ascarids by their large size, light tan color, and the presence of three prominent lips on the anterior end. T. cati also has distinct cervical alae, giving it an arrowhead appearance (See image under Life Cycle).
- Fenbendazole, milbemycin oxime, moxidectin, and pyrantel pamoate are approved for the treatment of ascarid (T. canis, T. cati, and/or Toxascaris leonina) infections in dogs and cats. Selamectin is also approved for treating T. cati in cats. Pyrantel is approved, in combinations with ivermectin or ivermectin and praziquantel, for treatment of T. canis and Toxascaris leonina infections in dogs. Febantel is approved, in combination with pyrantel and praziquantel, for treatment of T. canis and Toxascaris leonina infections in dogs. Pyrantel and febantel are approved for treating T. cati in cats. Emodepside is approved for treating T. cati in cats.
- Most of the drugs known to remove T. canis from dogs will also remove Baylisascaris spp. although none are approved for this use.
- Piperazine is also approved for treatment of ascarids in dogs and cats but may have a lower efficacy than other available products.
- Pyrantel is available in a highly palatable liquid formulation that is readily administered to nursing animals and thus may be considered the preferred treatment for very young pups.
- To prevent environmental contamination, all pups should be routinely treated with pyrantel pamoate at 2, 4, 6, and 8 weeks of age and then placed on a monthly heartworm preventative with efficacy against Toxocara spp.
- Because the prepatent period of T. cati is 8 weeks, kittens do not need to be treated for ascarids until 6 weeks of age. However, given concern about hookworm infection (see Hookworm Recommendations), all kittens should be routinely treated with pyrantel pamoate beginning at 2 weeks of age and then placed on a monthly heartworm preventative with efficacy against Toxocara spp.
- Nursing dams should be treated for ascarids at the same time as their litters.
- Pregnant bitches may be treated during pregnancy with daily fenbendazole or 2 to 4 times with a high dose of ivermectin to prevent transplacental and transmammary transmission of T. canis larvae to the pups. Both protocols involve off-label use of anthelmintics.
Control and Prevention
- Puppies and kittens should be routinely dewormed beginning at 2 weeks of age, with deworming repeated every 2 weeks, until the animals are placed on a monthly control product with efficacy against ascarids at 4 to 8 weeks of age.
- To treat potential newly acquired infections, dogs and cats should be maintained on monthly intestinal parasite-control products with efficacy against ascarids.
- Efficacy of the initial dewormings, monthly control product, and client compliance should be monitored by performing a fecal examination 2 to 4 times in the first year and 1 to 2 times per year thereafter, depending on the age of the animal and its prior history of infection.
- Prevention of predation and scavenging activity by keeping cats indoors and dogs confined to a leash or in a fenced yard will limit the opportunity for cats and dogs to acquire infection with ascarids via ingestion of vertebrate hosts or from an environment contaminated with feces from untreated animals.
- Prompt removal of feces from the yard or the litterbox will also help prevent ascarid eggs from remaining as the fecal material decomposes or is dispersed into the environment.
- Enforcing leash laws and requiring owners to remove feces deposited by their dogs can protect public areas from contamination with ascarid eggs.
- To avoid contamination with eggs of B. procyonis, raccoons should not be kept as pets and should be discouraged from defecating in areas frequented by people or dogs.
Public Health Considerations
- Toxocara spp. are well-documented, important zoonotic disease agents. Infection with Toxocara spp. is most common in children and occurs upon ingestion of larvated eggs from a contaminated environment following geophagy or other forms of pica. Although all children are susceptible to infection, some studies have shown that toxocariasis is more common in rural or inner-city areas, and is associated with both poverty and contact with breeding and/or untreated, free-roaming dogs.
- Larvated eggs of Toxocara spp. are commonly found in soil collected from playgrounds or parks, and the eggs survive and remain infective for many years. When these eggs are ingested, the larvae they contain migrate internally in the child, resulting in disease. Syndromes of toxocariasis include visceral larva migrans, which is usually characterized by hepatomegaly, pulmonary disease, and eosinophilia; neural larva migrants, characterized by progressive neurologic disease; ocular larva migrans, characterized by a unilateral granulomatous retinitis; and covert toxocariasis, in which chronic abdominal pain or other nonspecific symptoms develop.
- Baylisascaris procyonis also causes disease in children following ingestion of larvated eggs from a contaminated environment. The larvae of B. procyonis migrate extensively in the central nervous system, commonly resulting in severe neurologic disease in affected individuals; B. procyonis can also produce visceral, ocular, and covert forms of disease.
- Prevention of disease caused by infection with zoonotic ascarids requires preventing the ingestion of eggs from the environment. Young children should be closely monitored so that geophagy and other forms of pica can be discouraged, particularly in public areas known to be frequented by dogs and cats or populated with raccoons.
- Early and regular deworming is essential in preventing contamination of the environment with Toxocara eggs. Treating pets to prevent egg shedding is critical because the eggs are very hardy and long-lived in the environment. Once present, the eggs can be removed or destroyed only through extreme measures such as entombment of kennel areas or areas of pet defecation in concrete or asphalt, complete removal of topsoil, prescribed burns, or treatment with steam.