Pet Parasites

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A General Discussion on Parasites: Types, Transmission & Life Cycles

 

Holly Nash, DVM, MS

Veterinary Services Department, Drs. Foster & Smith, Inc.

 

 

 

 

A parasite is regarded as an organism that derives nourishment by

feeding on or within another animal. The most noted parasites in dogs

and cats are 'worms' and that is because the majority of all puppies

and kittens acquire intestinal worms either prior to or shortly after

birth. Worms are only part of the parasite picture, however, as many

non-worm parasites exist as well. Common parasites include fleas,

flies, ticks, lice, and mites as well as worms and other intestinal

parasites.

 

All parasites of concern can result in damage to an animal's health.

Similarly, some can be transferred from the dog or cat to other

household animals including humans. No dog or cat that is parasitized

is considered healthy.

 

If you would like a better understanding of parasites, this article is

for you.

 

What are parasites?

 

You have heard of stowaways on ships. They slyly sneak on board, find

a secret place where they can live, steal food, and then eventually

jump ship. Parasites are much like stowaways. They get on or in a

host, move to where they want to be, gain nourishment, warmth, and

other amenities, and then they or their offspring leave the host to

find another. In a broad sense, a parasitic relationship is defined as

one in which one organism (the parasite) lives on or within the other

(host), and may cause the host harm. Before we go on, let us take a

step back and look at how this relationship differs from others in

which different organisms live together.

 

How do different species relate in nature?

 

In nature, we are familiar with all types of relationships between

different species. Some help each other; others harm or even kill the

other.

 

Mutualism: In a relationship called mutualism, both organisms benefit.

It is a win-win situation. Examples include the tickbird in Africa

which rides on the back of elephants and eats the flies or ticks that

are there. The tickbird gets a good meal, and the elephant gets rid of

those pesky flies and ticks. Lichens are another example. Lichens are

really very close relationships between fungi and algae. They need to

be together in order to live.

 

Commensalism: Another type of relationship is called commensalism.

This is a relationship in which one organism benefits, and the other

neither benefits nor is harmed. Think of a hitchhiker. An example in

nature would be the relationship between a remora and a shark. The

remora attaches itself to the underside of the shark. This does not

hurt or benefit the shark, but the remora gets free meals by eating

the scraps of food left over from the shark's dinner. When you think

about it, another benefit is that when you are attached to the shark,

you can hardly become its dinner.

 

Predator-prey: Now we get to the win-lose relationships. In a

predator-prey relationship, one organism quickly (or in the case of a

cat with a mouse, not so quickly) kills the other. This is a win-lose

relationship. Examples would include a cat and mouse, the fisherman

and the trout, or a spider and its captured flies.

 

Parasite: The definition of parasite depends on whom you talk to. As

we stated above, the broad definition of a parasite is any organism

that is dependent on another (the host) for room and board, and does

not necessarily cause the host any harm. Some scientists and

veterinarians prefer to use the term parasite to mean a close

relationship in which one organism harms the other while obtaining

what it needs to live - basically a win-lose situation that takes more

time to develop. In the following discussion, that is the definition

we will be using.

 

Although a parasite may harm a host, it does not usually kill the host

or it would eventually run out of a place to live. Parasites generally

gain food from the host, but they also need a way to reproduce and get

from one host to another. So the next topics we need to talk about are

life cycles and transmission of parasites.

 

Life cycles and transmission of parasites

 

Basically, a life cycle describes how an organism develops from the

immature stage to the adult. Some animals produce live young and

others produce eggs. The same is true of parasites. In addition, there

are some one-celled parasites that reproduce by dividing into two

identical parts.

 

Direct Life Cycle: There are two general types of life cycles. In a

direct life cycle, the immature form of the parasite can infect the

same host it came from. For instance, the adult parasite lives in the

host's intestine, lays eggs which are passed in the stool, and the

eggs can infect the same or similar host. Roundworms have a direct

life cycle.

 

Indirect Life Cycle: In an indirect life cycle, the immature form must

pass through a different type of host before it can re-enter and

infect the host it came from. An example would be heartworms. The

adult worm lives in the dog or cat. The immature form, laid by the

adult heartworm, is taken up by the mosquito. The immature form

develops within the mosquito, and is then reintroduced into another

dog or cat where it develops into the mature adult and the cycle

repeats itself. The host in which the immature form of the parasite

lives is called the intermediate host, in this case, the mosquito. The

host in which the adult form lives is called the definitive host.

 

Transport Hosts: Some life cycles incorporate what is called a

transport host. It is a special kind of intermediate host in which no

development of the immature parasite occurs. The transport host is

really a way for the immature parasite to get from one mammalian host

to another. The parasite does not develop within the transport host,

but is just transferred from one host to another. Rodents can be

transport hosts for the roundworms of dogs and cats.

 

Many parasites have life cycles that include a phase of migrating

through the body, often from the digestive system to the lungs or

other organs such as roundworms. Others, including hookworms and

heartworms migrate from their entrance through the skin to other organs.

 

Aberrant Parasites: Most parasites are host-specific, which means they

only infect and live in certain animals. If they enter the wrong

animal, or host, they may not survive or they may migrate abnormally.

When a parasite enters the wrong host it is said to be aberrant.

Often, aberrant parasites will cause an over-reaction in the body of

this wrong host and serious disease can result.

 

Zoonoses: Infections that can be passed from animals to humans are

called zoonotic diseases, or zoonoses. There are many parasites that

fit this category. For some parasitic zoonoses, humans will have the

same symptoms as a dog or cat. In others, because the parasite is

aberrant, the symptoms may be very different because the parasite

affects different organs or the human body reacts differently.

Roundworms, hookworms, and Toxoplasma are all zoonoses.

 

Prepatent Period: There is one more term related to life cycles that

we would like you to be familiar with. That is the prepatent period.

This is the time interval between when the parasite enters the host

and the subsequent form of the parasite can be recovered from the

host. For example, the prepatent period for heartworms is 6 months. It

takes 6 months from the time the immature forms enter the body through

a mosquito bite, and the presence of heartworms in the heart. For many

parasites, the prepatent period is close to the time between the point

when the animal became infected and when we can diagnose the

infection. The prepatent period for whipworms is three months, i.e.,

it takes three months from the ingestion of the eggs until the adult

worms are in the intestine (and soon laying their own eggs).

 

You may have heard of a time interval called the incubation period.

This represents the time from when the parasite enters the host and

the development of signs or symptoms of disease. The incubation period

is usually longer than the prepatent period. For instance, the

prepatent period for heartworms is 6 months, but infected animals may

not become ill for a year or more after that fateful mosquito bite.

 

Living arrangements of parasites

 

Parasites can get on a host and remain 'on deck.' These are called

ectoparasites (ecto=outside). Examples of ectoparasites are fleas and

ticks. Other parasites move to the inner cabins of the host and are

called endoparasites (endo=in). Roundworms, heartworms, and tapeworms

are examples of endoparasites.

 

Obligatory Parasites: By definition, a parasite must live on a host

sometime during its life or it would not be a parasite. Parasites

differ, however, in how much of their lives they spend on their hosts.

Obligatory parasites live on a host all the time. They cannot live and

reproduce free in the environment. Heartworms are obligatory parasites

– they either have to be in a mammal or a mosquito to live. Most

parasites of dogs and cats are obligatory parasites. The reproductive

forms, e.g., eggs of obligatory parasites can sometimes live for long

periods of time outside of a host, as in the case of roundworms. But

these reproductive forms are dormant, and can do nothing until they

get into a new host.

 

Periodic Parasites: Other parasites are called periodic parasites.

They live part of their lives on a host, and part of it off. These

guys keep jumping on and off ships throughout their lives. Ticks are a

good example of periodic parasites. They get on an animal to eat, but

then they detach themselves when they are done eating – they 'eat and

run (detach, really).'

 

Facultative Parasites: There are other parasites that can actually

eat, sleep, and lay eggs while off of the host. These are facultative

parasites. When they are not on a host we call them 'free-living.'

When a host comes around, they will take advantage of a change of

scenery and live on the host a while. Pelodera strongyloides is an

example of a facultative parasite. It normally lives in the soil,

being especially fond of moist, decaying organic matter such as

bedding. When a dog lies down on the bedding, P. strongyloides invades

the upper layers of skin and can cause a severe inflammation.

 

Classifying parasites

 

Now that we have possibly overwhelmed you with all of the different

ways parasites can be classified, we will add one more. This

classification really takes all the characteristics we described

above, as well as anatomic features and uses them to organize the

parasites by their similarities. This classification system is also

used for plants and larger animals. You probably remember it from

biology class. It is called the Linnaean classification scheme.

 

'Kingdom' is the most general category, and the categories get more

specific until we reach 'Species' (there is actually a more specific

classification after that called 'Variety'). The classifications of

some more well known organisms are shown below:

 

 

Arthropoda

Aschelminths

Aschelminths

Protozoa

Arachnida

Nematode

Zoomastigophorea

 

Acarina

Strongylida

Ascaridida

Trichomonadida

Ixodidae

Ancylostomatidae

Ascarididae

Hexamitidae

 

 

Rhipicephalus sanquineus

Ancylostoma caninum

Toxocara canis

Giardia canis

 

We can see that of those organisms listed, the dog hookworms and the

roundworms are most closely related to each other. They have the same

kingdom, phylum, and class. Giardia canis is the most different from

the rest – it is not even in the same kingdom as the others.

 

It is customary to place the genus and species in italics or quote

marks ( " " ). Once the genus has been specified in a text, it is often

only indicated by its first letter in later text. For instance,

Toxocara canis is referred to as T. canis.

 

Major classes of parasites

 

Helminths: The term 'helminths' is a kind of generic word for 'worm.'

It comes from the Greek 'helmins' which means 'worm.' The term is used

to group certain parasites together and includes the Nematodes, which

are long, cylindrical worms, tapeworms, and flukes. You have maybe

heard of medications to treat infestations with worms called

'antihelmintics.' This group of drugs is literally 'against

helminths.' Let us take a look at the various helminths.

 

Nematodes: In general, Nematodes are round in cross-section, are

longer than they are wide, and are unsegmented. They come in many

sizes, from microscopic to a yard in length. Nematodes are a

significant cause of disease in dogs and cats. For every organ system

in the body, there is probably a nematode that lives there, or at

least passes through. Though they are in the same Class, Nematodes

have extremely varied life cycles. Some require intermediate hosts and

some are facultative parasites. Some lay eggs and others bear live larvae.

 

The Class 'Nematodes' is sometimes referred to as 'Roundworms.' This

can be a bit confusing since we often use the term 'roundworm' when we

are talking about the spaghetti-like intestinal worms of dogs and

cats. The dog and cat roundworms are 'Roundworms' but not all

'Roundworms' are dog and cat roundworms. As a matter of fact, there

are hundreds of 'Roundworms' or Nematodes.

 

Tapeworms: Tapeworms are flat worms that are segmented. They consist

of a head, neck, and then a number of segments. The head usually has

suckers or muscular grooves that enable it to attach itself to the

animal's intestine.

 

Each tapeworm segment has its own reproductive organs. New segments

are continually formed in the neck region while those at the end of

the tapeworm are cast off as they mature. These mature segments are

really egg packets. The segments may often be seen near the anus of

the dog or cat. These segments may move if recently passed, or if

dried, they look like grains of uncooked rice or cucumber seeds.

Tapeworm infections are usually diagnosed by finding these segments on

the animal.

 

Tapeworms of dogs and cats all have life cycles that include an

intermediate host. These hosts include fleas, fish, and domestic

animals such as sheep and pigs. All of these tapeworms live in the

dog's or cat's digestive system. It is interesting that tapeworms have

no digestive systems themselves, but absorb nutrients through their skin.

 

Flukes: Flukes are flat, leaf-shaped, unsegmented worms that look

similar to leeches. Flukes have suckers, hooks, or clamp-like

appendages with which they attach themselves to the host's organs.

Flukes that infest dogs and cats vary in size from ½ millimeters (less

than 2/100 of an inch) to almost ½ inch. Fluke eggs have a unique

characteristic - the shell around the egg has a trap door called an

operculum.

 

The flukes that infect dogs and cats have indirect life cycles. The

intermediate hosts differ quite a bit, from lizards, to fish, to

snails. In general, flukes do not cause serious disease in dogs and

cats unless they are present in very high numbers.

 

Arthropods: The term arthropod means joint-foot and refers to the fact

that all members of this phylum have jointed legs. The two classes

that include parasites of dogs and cats are Insects and Arachnids.

Other classes include the Crustaceans with members such as crayfish

and lobsters, and the class Myriapoda to which millipedes and

centipedes belong. The phylum of Arthropods also includes a group of

organisms that nobody seems to be able to categorize. These are the

pentasomes, one of which is a parasite of dogs and cats. Let us take a

closer look at the parasitic arthropods.

 

Insects: Insects make up 70% of all known species of animals of all

kinds, so it comes as no surprise that some of them are parasites.

When we think of insects, we tend to think of flies and mosquitoes –

pests. But insects can be more than just pests; they can be the cause

of severe disease since the class of Insects also includes lice and fleas.

 

Insects can be a threat to the health of animals through a number of

different mechanisms. Take fleas for example. They are parasites. They

can serve as intermediate hosts for other parasites (the fish

tapeworm). They can also serve as vectors (carriers) of diseases like

bubonic plague. Still other insects have poisonous bites such as bees

and wasps.

 

Insects have antennae, compound eyes, and a body composed of head,

thorax, and abdomen. Many insects have wings, though in some insects

these wings are so small they are more like stabilizers. Fleas are

actually wingless insects, though they do have a mustache.

 

Arachnids: When we think of arachnids we often first think of spiders.

But spiders have some pretty nasty cousins including ticks and mites.

As a Class, Arachnids are carnivorous animals. Some, such as various

species of lice feed on the tissue fluids of other animals through a

sucking apparatus. Some mites eat portions of other animals through

the use of their front legs that may have suckers or hooks. And as we

remember from biology class, arachnids have eight legs as adults, not

six like the insects.

 

Protozoa: Now we come to the last large group of parasites. These are

the one-celled organisms called Protozoa. They are microscopic and

were first discovered by the man who actually invented the microscope,

Anton van Leeuwenhoek. There are over 45,000 species of Protozoa. They

cover the globe and are responsible for millions of cases of disease

in both humans and animals.

 

Protozoa have two main parts – the nucleus and the cytoplasm. They are

surrounded by a non-rigid membrane and have various means of

locomotion. Some Protozoa, such as Giardia canis and Giardia cati,

move about with the assistance of flagella. Flagella are long,

hair-like structures that can whip back and forth rapidly, propelling

the organism. Other Protozoa move about through the use of cilia.

Cilia are much shorter, hair-like structures that often cover the

organism. Balantidium coli has numerous cilia with which it moves.

Some Protozoa, like Toxoplasma gondii, move about through gliding

motions without the aid of appendages.

 

The various types of parasites of dogs and cats are described in

detail in individual articles on our site. If you would like an

alphabetical listing,