Toxocara canis larva emerging from its egg. |
Toxocara canis is found all over the
world and is most prevalent in areas populated by domesticated dogs and other
canids. This is because dogs are the
definitive hosts for this parasite. In the U.S., about 98% of puppies and about
20% of adult dogs are infected. Pigs, mice, foxes, and even birds can serve as
paratenic hosts for Toxocara canis. They
live in and feed on intestinal contents when in the definitive host. *Fun side note*: The adults undergo a
specialized type of anaerobic metabolism that results in the production of an
extra ATP!!!
Posterior end of a male T. canis. |
These squirmy little dudes are
dioecious (meaning that they have separate male and female individuals) with
the males being smaller than the females and possessing a ventrally curved
posterior end sporting simple spicules for a direct transfer of sperm. The females possess large, extensive ovaries
and can hold up to 27 million eggs at a time in their uteri. *Another fun side note*: nematode sperm are
not flagellated like human sperm, they are actually amoeboid.
Anterior end of a female T. canis. |
Taxonomy
Toxocara canis is a nematode belonging in the class secernentea. Members of this class have caudal papillae
as well as lateral canals in the excretory system. This class includes the harmless nematode Caenorhabditis elegans, which is
commonly used in genetic and soil ecology studies. Toxocara
canis also belongs in the order ascaridida, which includes many families of
roundworms with three “lips” on the anterior portion of the body. In fact, this species is one of the smallest
members of this group. It also belongs to the family toxocaridae. Members of
this family infect mostly felids and canids.
Anterior end of a T. canis specimen. |
Life
Cycle
When
an infected dog defecates, the eggs of T.
canis can live in the feces for up to three weeks as they become
embryonated with larvae. If a new host
ingests eggs that have developed into the L3 stage (after second molt, but
still inside the egg), the larvae break out in the gut and move to the lungs
through the circulatory system. Then they migrate to the bronchial tubes and on
into the esophagus. Eventually they make their way back to the gut. When the
larvae make their way to the lumen of the intestine they develop into adults,
mate, and lay eggs that exit the body via the feces. Eggs don’t start appearing
in the feces until about 5 days post-infection. Studies have shown that eggs
can survive in the external environment for 10-20 days. One British study
demonstrated that under the right climatic conditions, some eggs can persist in
the soil for up to three years!!! Also, pregnant dogs that are infected can
pass these parasites on to their offspring.
Life Cycle of T. canis |
Toxocariasis
Dogs infected with these parasites
are said to have toxocariasis. Symptoms may include emaciation, anemia, diarrhea,
constipation, roughness of the pelage, and/or pale mucous membrane. Dogs tend
to be reluctant to moving or being moved and may display either a pot-bellied
appearance or a tucked abdomen appearance due to the damage caused to the
intestinal lining. There will often be vomiting, coughing, nasal discharge, and
noisy breathing in puppies as they worms migrate through the respiratory tract. Puppies may also develop nervous system
problems known collectively as “ascaris toxaemia”. If the infection is severe enough for long
enough, death can be caused due to intestinal obstruction, ulceration, or
perforation of the intestinal wall.
If the parasites enter a non-canid
host, such as a human, they will wander throughout the body. These wandering larvae are termed larva
migrans. If they wander to the lungs,
brain, heart, muscles, liver, or other organs, they are called visceral larva
migrans. This wandering causes
inflammation and can result in a myriad of syptoms such as fatigue, anorexia,
pneumonia, fever, coughing, abdominal pain, headaches, rashes, and even
occasionally seizures.
A child with ocularis larva migrans. |
If the parasites wander to the eyes,
the resulting infection is called ocularis larva migrans. This tends to happen in older children and
young adults. Symptoms for this form
include red eyes, decreased visual clarity, a whitening of the pupil
(leukokoria), granulomas, chorioretinitis (inflammation of the choriod and the
retina), and even retinal detachment.
Children can also develop a form
known as covert toxocariasis, which can result in coughing, abdominal pain,
difficulty sleeping, wheezing, or headaches. This form develops due to chronic exposure.
Because infection is so prevalent in
canids in the U.S., human exposure risks are very high here. Most cases go unrecognized or
unreported. However, about 10,000 cases
occur annually. Somewhere between 4% and
8% of children test positive in serological tests for toxocariasis. Toxocariasis
is most prevalent in children ages 1-3, and boys are at a higher risk of infection
that girls. (Probably due to an increased probability of fecal-oral
contamination since kids this age aren’t exactly well-known for their impeccable
hygiene.)
Diagnosis,
Treatment, and Prevention of Toxocariasis
Diagnosis
includes identifying clinical symptoms and demonstrating the presence of adult
worms in feces or vomit. Often times, a
fecal examination is needed to confirm a diagnosis in dogs. For humans, ELISA, PCR, and serological tests
are most commonly used for diagnostics.
Ivermectin in paste form. |
Dogs
are treated using an antihelminthic drug such as piperazine adipate, diethyl carbamazine,
pyrantel, benzimidazole compounds, or ivermectin.
Human infections typically resolve
themselves, but where treatment is required, it typically includes the
administration of antihelminthic drugs such as albendazole or mebendazole. In severe cases, corticosteroids may be given
or surgery may be required.
Infection can be prevented by giving
pets regular de-wormers such as fenbendazole, dichlorvos, and piperazine. (Good
for your pooch, and reduces your risk of infecting yourself or your dirty
children.) This is especially important in
females because dormant larvae may persist in the tissues for years and can be
passed on to pups. Outdoor kennels should have floors that are impervious and
easy to clean. (In other words, no dirt floors.) Keep food/water containers and
bedding clean. Make sure that the rodent
populations are controlled as rodents can be paratenic hosts for the
parasites. Finally, try to keep your dog
from eating feces or soil that may have been contaminated with feces. Infections can also be prevented by owners
who take the initiative to clean up after their pets, and by always washing
hands thoroughly with soap and water following the handling of dog feces…especially
before handling food. Also, try not to
let your children eat dog poop or dirt that could have come into contact with
dog poop.
Moral
of the Story
Don’t
be the d-bag who is too good to clean up after his/her Toxocara-infested mutt! Keep your pups and kiddos from eating poop
and/or dirt and keep your kennels cleaned! (Dog and kid-kennels alike! J/k…Scrubs
reference? Anyone?) Also keep Fluffy up to date with de-wormers, and wash your
hands if you are playing with his poop. So, watch your children, watch your
pets, and wash, wash, wash your hands!
This blog is beautiful! I especially like the material on _Demodex_ (a former student contributed one [a personal one, that is, from an undescribed site] that ended up in a textbook. Now, for Friday coffee, maybe a conversation about the parasites of woolly mammoths or dinosaurs?
ReplyDeleteThanks! :) We really need to get together for Friday coffee...it's been too long! I know nothing of woolly mammoth or dino-parasites...looks like I'm going to have to do some research! Yay! :)
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