Picture of roundworms ( Toxocara canis )

We will look at worm that can infest dogs, the roundworm, scientific nameToxocara canis. This is an important worm to be aware of, not only becauseof its effects on the health and wellbeing of dogs, but also because it posesa risk to human health.We call an infectious disease like this, one that canjump from an animal to a human (or vice versa), a zoonosis. Understandingwhich parasites are zoonotic is important to allow the implementation ofstrategies to minimise the risk of their transmission to pet owners or othersthat may come into contact with the pet or its environment.

Infectious Toxocara egg

Five weeks after infestation, a dog will start shedding roundworm eggsin their faeces

The time period that elapses from when an animal is infested with a parasiteto when eggs or larvae are detectable in body fluids or excretions is known asthe prepatent period. It is important to understand prepatent periods whenconsidering optimal parasite control programs.

Dogs are infested with Toxocara canis through ingestion of infective eggs orthrough eating an infested host animal such as a rodent (parasitologists callthese paratenic hosts). Newborn pups may have already acquired a roundworminfection prior to birth in the womb (the worm larvae can cross the placenta)or soon after birth via their mother’s milk. The prepatent period forToxocara canis varies a little depending on how the larvae are acquired,being five weeks for infestations acquired from ingesting eggs or paratenichosts, and a little shorter (three to four weeks) if acquired in the womb orvia their mother’s milk.

200 , 000 is the number of eggs a female roundworm can layin a day

Roundworms are world class at reproducing, with a single adult femaleroundworm laying up to 200,000 eggs per day. The eggs that are shed are notimmediately infectious, however they can develop to the infectious stage in aslittle as 2 to 3 weeks under the right environmental conditions, and mayremain infectious for several years. Roundworm eggs are also resistant to manycommon chemicals and disinfectants and have a sticky outer coating that canmake them hard to remove. Little wonder then that high rates of contaminationwith Toxocara eggs have been documented in soil samples taken from parks,backyards, playgrounds and sandpits around the world.

7% of people in Australia are estimated to have been infected with Toxocara

With so many eggs shed, it is not surprising that human exposure and infectionoccurs. A recent published study estimated that 7% of Australians haveantibodies against Toxocara , indicating they are currently or havepreviously been infected.

Humans are infected through ingesting infectious eggs in soil on unwashedfoodstuffs or unclean hands, or through ingestion of undercooked paratenichosts (e.g. chickens). As eggs in freshly passed dog faeces are non-infectious, it poses little risk, however due to their sticky outer coat,infectious eggs may be found stuck to the animal’s coat if they have rolled incontaminated soil. However, transmission in this way is not thought to be asignificant risk.

__Four different clinical syndromes are recognised in humans dueto infestation with Toxocara

Human infection with roundworms can result in four different significantclinical syndromes, however in many cases it causes no obvious clinical signs.Patent infestations, that is infestations that result in worms reaching theintestine and shedding eggs, do not occur in humans, with disease being causedby larvae migrating through organs and the resultant immune response. The wormlarvae can migrate through various organs in the body, and depending on wherethey roam, different signs may be seen. For example, worm larvae migratinginto the eye or optic nerve can cause vision loss or even complete blindness;worm larvae migrating in the brain can cause neurological signs. Even whenobvious signs are not present, scientific studies have shown a link betweenhaving been infected with Toxocara and reduced cognitive function children.A similar link has been reported for asthma in children.

Why do these numbers matter?

These numbers highlight a key challenge in the control of this parasite. Tofully appreciate this it needs to be remembered that most dewormers for dogsand cats, while highly effective at killing worms present at the time ofdosing, provide no ongoing protection. This means that animals in contaminatedenvironments can become rapidly re-infested. With deworming every threemonths, as historically recommended, this would mean dogs may be sheddingroundworm eggs for 6 of the 12 weeks between doses – not an ideal scenariogiven the potential consequences of roundworm infection in humans.

The key to minimising shedding of gastrointestinal worms is to deworm at afrequency that is less than the prepatent period. In this scenario, although adog can be re-infested soon after a treatment, these newly acquired worms arekilled by the next dose, before the worms start shedding eggs. For Toxocaracanis , this means monthly treatment, as recommended by the AustralianCompanion Animal Zoonoses Advisory Panel.

Convenient monthly treatment with an endectoparasiticide that controlsroundworm, like NexGard Spectra, is a simple solution to help keep dogshealthy, and minimise the risk of zoonotic roundworm infections.

It is important to remember that regular deworming is only part of thesolution. Other key considerations in controlling roundworms include:

  • Avoiding faecal contamination of playgrounds/sandpits and garden areas
  • Ensuring good food and hand-hygiene practices
  • Preventing children from ingesting soil and faeces
  • Avoid feeding raw meat or allowing dogs to hunt prey animals that may be infested

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