Signs of Fall 5: Roundworms in the Park

Toxocara canis (nematode) adults. Photo by A. Walker. Wikimedia Commons

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“Roundworm” is another name for “nematode.” When you consider that every pinch of topsoil contains dozens to hundreds of individual nematodes, and that every living vertebrate and invertebrate animal and also every plant contains hundreds to thousands to many millions of nematodes, it is very easy to see why many biologists rank nematodes as the most numerous animal on Earth (only the tiny crustaceans that make up the oceanic clouds of “krill” come close to the numerical abundance of free-living and parasitic nematodes).

One of my agronomy professors at Ohio State once said that if all of the mineral and humic components of soil were magically removed you could still see the outline and depth of the original soil because of all of the nematodes. You could even walk around on them, he added, but they might be a bit slippery! (That may be the only joke ever told in one of my agronomy classes!)

So, we all agree that there are a lot of nematodes on Earth!

Some of the nematode parasites found in vertebrates live in the food-packed section of the digestive system called the small intestine. These “intestinal roundworms” feed on the digesting food flowing through this tubular organ and then grow and reproduce prolifically making hundreds of thousands of eggs per day! Sometimes these nematodes live in equilibrium with their host, sometimes they grow out of control and inhibit digestion and absorption of nutrients from the small intestine. Sometimes they can even cause serious blockages of this part of the digestive tract!

A basic concept in parasitology involves the distinction between a “definitive host” and a “non-definitive host.” A parasite can only reach adulthood and be able to reproduce inside the body of one of its “definitive hosts.” In a “non-definitive host” (which could be almost any other type of animal that has inadvertently picked up some other species’ definitive parasite), the parasite often gets stalled in some pre-adult, “larval” stage and is never able to finish its life cycle. These non-definitive host infections, though, may have some serious health impacts on the host organism!

Photo by M. Hamilton

Dogs and cats are the definitive hosts for some species of intestinal round worms in genus Toxocara (the very logically named species are Toxocara canis and Toxocara cati).  Part of the regular veterinary care we give our pet dogs and cats involves examination of the animal’s feces for Toxocara eggs and the administration of some very effective treatments to both eliminate any Toxocara that might be living in the pet’s intestine and prevent their re-occurrence. Puppies are often loaded with Toxocara nematodes. The newest addition to the Colorado branch of my extended family (“Gedi”) is pictured to the left. He has had to have numerous treatments before his “birth-load” of nematodes came under control!

The good news is, then, that most well cared for dogs and cats do not have intestinal nematodes and are not depositing nematode eggs in the spots where they have voided their feces. The bad news is, though, stray dogs and cats that do not receive these regular nematode assessments and treatments do often have active Toxocara populations in their intestines, and these animals shed an incredible number of potentially infectious, microscopic eggs each time they void their feces.

Humans can be a “non-definitive host” for either dog or cat Toxocara nematodes. This of course means that the nematodes are not able to reproduce inside of a person, but the larval forms of the roundworms can impact a number of internal organ systems and, occasionally, do some considerable damage to a human host.

Toxocara embryonated eggs. Photo by Flukeman, Wikimedia Commons

The Toxocara eggs in an infected animal’s feces must mature out in the soil/feces environment for two to four weeks before they are potentially infectious. The nematode embryo in the fertilized eggs needs to develop to a point that it is able to actively move about inside of whatever host it enters. Once the egg/embryo is in its infectious life form, though, it may be able to persist in the soil/feces external environment for up to a year. Considerable numbers of these infectious life stages, then, can accumulate in a soil system that regularly receives contaminated dog or cat feces.

Most often people are exposed to these nematode eggs via the inadvertent ingestion of contaminated soil. Unwashed or poorly washed hands are the most common infection pathway. Flies that have walked about on contaminated surfaces may also spread the nematode eggs to human foods. Children because of poor hygiene habits and their tendency to play in sand boxes and dirt piles of their yards and playgrounds are frequently exposed to infectious Toxocara eggs. These eggs and their infectious larval life forms can also enter a body via cuts or abrasions in the skin and via the ingestion of uncooked or poorly cooked meats or organs (especially livers) of other non-definitive host species (rabbits, chickens, snails, earthworms, etc.). Cultures with traditions of consuming uncooked internal organs or raw, soil dwelling invertebrates may have very high levels of Toxocara infections.

Most human Toxocara infections are short-lived and mild (general fever, malaise etc.). Sometimes, though, Toxocara larvae can move about inside a person’s body and accumulate in the liver, lungs or eyes causing significant, and very persistent inflammatory damage. There is also some evidence that higher brain development in children and intelligence can be negatively affected by chronic Toxocara infections.

Anywhere between five and ten percent of the population of the United States carry antibodies to Toxocara nematodes (different sources cite different overall numbers). The presence of these antibodies means that these individuals have had a Toxocara infection sometime in their past. According to the Center for Disease Control (CDC) poor and minority populations have a significantly higher Toxocara infection rate than the rest of U.S. population.

Photo by Bru-no. PIxabay

A recent study mentioned in a New York Times article about the ubiquity and potential impacts of Toxocara infections in New York (“The Parasite on the Playground,” January 16, 2018) found that 75% of the playgrounds in the lower socio-economic sections of the city tested positively for infectious Toxocara eggs while no eggs were found in playgrounds located in the more affluent sections of the city. The vigorous control of stray dogs and the enforcement of dog waste cleanup and removal in the more affluent neighborhoods are two logical reasons for this discrepancy.

In all of this information are a couple of key points for all pet owners to recognize: 1. Make sure your pet is free of intestinal nematodes, and 2. Pick up and dispose of their feces as quickly as possible (remember, the eggs in the feces take several week to grow into their infectious state!).

So, enjoy every minute with your dog and cat, but take the time to be a biologically aware owner!

 

 

 

 

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