Hines DVM PhD
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derive a lot of joy and solace from our pets. They are our non-judgmental
companions that give unqualified love. However, on occasion, they
give us germs and parasites that make us ill. Diseases passed from
animals to man are called zoonoses or zoonotic illnesses. Because
of good hygiene and veterinary care, animal-transmitted diseases
are rarer in the United States than they once were. The following
is a list of some of the diseases pets and livestock pass on to
human beings. They are ones that come to my mind as I write this
article. It is not meant to be all-inclusive. I do not want you
to read this article and become fearful of owning animals. All the
conditions I discuss are quite rare. Living life to the fullest
has its risks but a little caution on your part is all that is required.
WORMS THAT LIVE IN YOUR PET:
Hookworms and roundworms
Hookworms and roundworms (Ancylostoma and Ascaris
) are common nematodes of dogs and cats. When a human accidentally
eats something contaminated with worm eggs from a pet’s stool,
the eggs hatch in the intestines and begin migrating throughout
that person’s body. Worm larva can also burrow through intact
skin. Because these parasites were designed to live in dogs and
cats, they become lost in the human body – often in the liver
or eyes. When this occurs, the disease is called visceral larval
migrans. This disease occurs most often in children due to their
poor hygienic practices. In the eye the larval nematodes cause inflammation
and blindness. In the liver they can cause chills, fever, malaise
and an elevated white blood cell count. To prevent this disease,
have your pet’s stool checked yearly for parasites and feed
a monthly heartworm preventative that also kills nematodes. Both
popular brands of heartworm preventative for dogs contain ingredients
that keep your pet free of intestinal worms. Pyrantel pamoate is
a very effective medicine to remove hook and roundworms. It is sold
on the pet isle at WalMart.
eggs of the roundworm of raccoons, Balisascaris, are particularly
dangerous when ingested by people. If you keep pet raccoons or raise
orphans, worm them frequently with pyrantel pamoate and milbemycin
oxime. If you have neighborhood raccoons, do not leave dog or cat
food outside where it will attract them. Keep your trash cans well
covered and seal up attic crawl spaces where these animals can nest.
Certain tapeworms that encyst in the muscles of livestock and fish
can also infect humans. Taenia solium is carried through
pork, T. saginata by beef and Diphyllobothrium latum
by fish. The common pigmy tapeworm of dogs and cats (dipylidium)
that I see frequently in dogs and cats is never infectious to people.
Besides the three tapeworms previously mentioned, Echinococcus
granulosa, can infect people. The first three develop in the
human intestine while the last can cause major damage to the human
brain and body organs. Praziquantel and fenbendazole destroy these
On rare occasions, dog heartworms will infect human beings. This
parasite, Dirofilaria immitus, is spread dog to dog by
mosquitoes. When a mosquito obtains a blood meal from an infected
dog and then goes on to bite the owner it is common for some of
the heartworm larva to be transferred to the human. In a normal
person, the body’s immune system quickly recognizes the parasite
and destroys it. In rare instances, however, the larval heartworm
manages to migrate to the person’s lungs where the dying parasite
stimulates an inflammatory lesion called a “coin lesion”.
These lesions are not serious in themselves but they become very
significant when they are misinterpreted as being lung tumors. This
is not a very common occurrence for example, within the last twenty
years in the United States about eighty cases were reported in the
State of Florida.
Protozoa are microscopic single-celled organisms. The vast majority
of protozoa live free in the environment or as harmless inhabitants
of the body but some are capable of causing disease.
Giardia lamblia is a small motile protozoan that inhabits the intestines
of mammals and birds. There are many strains of giardia and it is
unclear how many are infectious to people. Giardia is the most common
form of non-bacterial diarrhea in people in the United States. Children
are most commonly affected. Many cases are silent with no overt
symptoms. When diarrhea does occur, the illness normally lasts one
or two weeks but chronic cases in frail people have lasted for years.
Outbreaks due to contaminated water supply occur from time to time
in humans. Metronidazole (Flagyl) is an effective treatment in animals
Cryptosporidium parvum causes diarrhea in dogs, cats, rodents,
young calves and people. It is found throughout the World. It is
passed from individual to individual through fecal contamination.
The disease in animals and man is usually mild and self-limiting.
Signs of the disease include diarrhea, abdominal pain and flu-like
symptoms that can last up to six weeks. The disease is more severe
in very old and very young animals and people. In immunocompromized
individuals it may cause chronic inflammation of the digestive tract.
The disease is passed through fecal contamination from animals or
man. The organism is quite resistant to drying and disinfectants
so, it can survive a long time in contaminated waste. Signs in people
and animals besides loose watery diarrhea include stomach cramps
and mild fever. In healthy people, symptoms last about two weeks.
Toxoplasmosis is caused by Toxoplasma gondii, a small single-celled
protozoan. About forty percent of the people in the United States
have been exposed to the disease at some point in their lives. The
complete life cycle of toxoplasmosis occurs only in cats. These
felines become infected by preying on infected birds and rodents.
Most cats show no symptoms of disease. In these cats, the organism
lives within the cells that line the small intestine. Cats are the
only animals that shed this ineffective stage of this protozoan
called an oocyst. Most exposures of humans to oocysts cause no overt
disease. In a small percentage, however, the oocysts proliferates
in many organs of the body causing fever, malaise, enlarged lymph
nodes, headache, sore throat and muscle pain. In severe cases the
central nervous system, eyes and liver become inflamed. Eating raw
or poorly cooked meat of an infected animal is another way this
disease is passed on to man. If a
woman becomes infected during the later two thirds of pregnancy
toxoplasmosis may cause severe fetal abnormalities. This is why
obstetricians suggest pregnant women not change cat litter boxes.
Eastern, Western and St. Louis encephalitis are passed to humans
through the bite of an infected mosquito. Encephalitis is an inflammation
of the brain. Horses also suffer from these diseases but they are
dead end hosts that do not pass the infection on. The reservoir
animals for these viruses are migratory water birds.
West Nile virus generally affects humans birds and horses. The disease
causes an inflammation of the brain or encephalitis. It is transmitted
from animal to animal and animal to person by the bite of an infected
mosquito. In 2002 the disease sickened about 2000 people and resulted
in 94 deaths. In horses, the mortality rate is about 30 percent.
Dogs are resistant to the disease but cases have occurred in cats,
goats, chipmunks rabbits, skunks, bats, llamas and domestic birds.
The parapoxvirus that causes this disease is found in goats and
sheep throughout the United States. In livestock, the virus causes
inflammation and scabs on the lips, nostrils, mouth and around the
penis or vulva. Transmission to humans occurs thorough infected
wool or contaminated sharp objects used on the animals. In people,
single lesions develop on the hands, arm or face. These lesions
are easily mistaken for abscessed. They heal in six to eight weeks
The flagship virus that we all associate with wildlife is rabies.
Any species of warm-blooded animal is susceptible to this disease
but the most common carriers in the United States are bats, foxes,
raccoons and skunks. The disease is passed by a saliva-contaminated
bite. Occasionally the disease will leave its wildlife reservoirs
and infect dogs and cattle. Excellent vaccines exist to protect
your pets from this disease. People like me who work with wildlife
can, themselves, be immunized against rabies.
Herpesvirus simiae or B-virus is a normal inhabitant of
the mouth of macaque monkeys. Approximately 80-90% of adult macaques
are infected. Many species of macaques are offered as pets in the
United States. This dangerous disease is mild to asymptomatic in
monkeys. It causes a lifelong infection with intermittent shedding
of the virus in saliva and genital secretions – particularly
during periods of stress. Among monkeys it is transmitted sexually
and by bites. This virus can cause fatal meningoencephalitis (the
veil-like covering of the brain) in people. It is usually transmitted
by a bite. In these patients treatment with acyclovir can be life
saving. These species of Old World monkeys should never be kept
as pets because of the threat of B-virus and tuberculosis (see my
article in this series on diseases transmissible from monkeys to
Hantavirus of Rodents
Hantavirus pulmonary syndrome is a rare viral disease associated
with wild deer mice. It occurs primarily in the fall when rodents
move indoors to escape the cold. In the process of the human body
attacking the virus in the linings of blood vessels throughout the
body, the capillaries are damaged and leak (increased permeability).
In humans this causes life-threatening pneumonia, edema, bleeding,
fever and kidney failure. Infected mice pass the virus in urine,
saliva and feces. Although not a serious disease in rodent the disease
in humans is often fatal. People contract the disease by breathing
in aerosolized or pulverized virus in rodent urine and feces. Rodent
control around the home is the best way to protect against this
Salmonella are a group of intestinal bacteria that can
cause disease in animals and man. In birds and small mammals salmonella
causes diarrhea, septicemia (blood infections) and asymptomatic
carrier states. People too, like “Typhoid Mary” can
carry and spread the disease without signs of disease. Common animal
carriers of these bacteria are reptiles, rats and mice. People with
a robust immune system rarely experience more than severe cramps
and diarrhea. However in infants and people with weak immune systems
the disease can be life-threatening.
Shigella flexneri is an intestinal bacterium responsible
for severe diarrhea in people and non-human primates (monkeys).
About 25,000 cases are reported in people in the United States every
year. Many monkeys carry this bacteria without symptoms. People
are also common carriers. Small children at child care facilities
and people who handle monkeys are most at risk. The diarrhea produced
in people exposed to human or monkey waste is never life threatening
but during the two to three weeks the diarrhea lasts, the victim
is quite ill. The chief danger from this disease is dehydration.
Many rabbit farms harbor Pasteurella multocida in their
stock. Most rabbits that carry this disease show no symptoms. Some
have eye infections and enlarged lymph nodes of the head and neck.
A respiratory disease in rabbits, caused by these bacteria is called
snuffles. Poultry also develop Pasteurellosis. In birds the disease
is called hemorrhagic septicemia. The bacteria is also carried in
the mouth of many cats leading to contaminated cat bites. The most
common form of Pasteurellosis in people is a skin and soft tissue
infection at the site of a bite or scratch. When people’s
eyes are exposed to Pasteurella they may develop a severe eye infections.
On rare occasions it will cause pneumonia.
Campylobacteriosis is a bacterial disease caused by Campylobacter
jejuni, an organism that lives in the intestinal tract of many
animals. The signs of this disease are diarrhea, cramping, abdominal
pain and fever. The illness typically lasts one week. It is worse
in the very young and the very old. Many infections are silent with
no symptoms at all. Most people become infected by handling or eating
raw poultry. Occasionally it is spread through contact with the
wastes of pet dogs and cats.
Streptococcus and Staphylococci
These bacteria are found on all animals that typically associate
with humans. In the great majority of cases they cause no disease
in the animal. However, in pets, eye infections are occasionally
associated with Streptococci and skin infections with Staphylococci.
Both bacteria can spread from pets to humans on contaminated hands
and objects. Infections are generally limited to the skin and eyes.
Tuberculosis is a chronic infection of the lungs and lymph nodes
of many species of animals. Dogs and cats are quite resistant to
this disease but cattle, deer and monkeys are quite susceptible.
The organism responsible for tuberculosis is Mycobacterium tuberculosis.
There are specific strains that affect each type of animal but all
of them on occasion infect humans. The human strain is passed from
victim to victim by a cough. Before the advent of pasteurization,
raw milk was the major source of the bovine strain. Pet African
and Asian monkeys are now a common source of exposure. Unfortunately,
new strains of tuberculosis are now often immune to the most common
anti-tuberculosis drugs, isoniazide and rifampin.
This infection, caused by a bacteria, Yersinia pestis,
occurs naturally in a number of wild rodents including prairie dogs
in well defined areas of the southwestern United States. It is transmitted
from rodent to rodent and rodent to man through the bite of a flea.
Occasionally a domestic cat will obtain the disease from prey rodents
and pass it on to their owners. Yersinia was responsible for the
scourge of the Middle Ages, bubonic plague.
Fever, Psittacosis or Ornithosis
Parrot fever or chlamydiosis is caused by a small intracellular
bacteria, Chlamydia psittaci, that lives within the respiratory
system of birds. A similar organism, which doesn’t seem to
affect humans, is found in cats. Transmission is through inhalation
of dust, dander and nasal secretions of infected birds – especially
parrots and turkeys. The flu-like illness that develops ranges from
very mild to life-threatening. In rare instances the heart and liver
become involved. The disease is often misdiagnosed as influenza.
When a correct diagnosis is made psittacosis responds well to antibiotics
of the tetracycline class.
Anthrax, caused by Bacillus anthracis, is primarily a disease
of cloven-footed animals. The disease is often fatal to animals
and man. This bacteria produces spores or “seeds” that
remain infectious for years in the soil under the right conditions.
It kills cattle very rapidly – the first sign of the disease
being death. It is sometimes confused with lightening strike or
snake bite. Prior to death the cows are weak and have difficulty
breathing. Their blood may refuse to clot. In humans the lung or
pulmonic form of the disease is the most fatal. It is spread through
contact with the carcasses of infected animals. It can also occur
on the hands and arms as small pus-filled lesions called carbuncles.
If caught early the disease is curable with penicillin or tetracycline.
There are several species of Leptospira that can transfer
from animals to man. The organisms are often associated with rats
and swine. In mammals they cause a generalized infection that often
localizes in the kidneys. Urine from these animals late in the disease
is highly infectious. It is pass through contaminated water. In
people, signs of leptospirosis include headache, vomiting, muscle
pain and ,occasionally, hepatitis, meningitis and kidney failure.
When brucellosis occurs in people it is called undulant fever or
Malta fever. In cattle it is caused by a bacterium, Brucella
abortus. In cattle, deer, elk, swine and goats and dogs different
species of the bacteria attack the reproductive organs or cause
generalized malaise and fever. In people brucellosis causes long-term
malaise, joint pain, intermittent fevers and flu-like signs and
fatigue. Brucellosis has almost been eradicated from cattle in the
This spiral bacteria is capable of forming ulcers in the stomachs
of animals and people. It resides in the stomach and the upper area
of the small intestine called the duodenum. We suspect that on occasion,
it is spread to humans from cats dogs and ferrets through poor sanitation.
In dogs, cats and people the most frequent sign of Helicobacter
is intermittent vomiting. Affected individuals may also become nauseous,
loose their appetite, and lose weight. Amoxicillin, metronidazole
and H2 antacids such as famotidine or cimetidine cure the disease
in animals and man.
Scratch Fever (Bartonellosis)
This infection, caused by Bartonella henselae, is commonly
acquired from asymptomatic (clinically normal) carrier cats. About
half of the outdoor cats in the southern United States have been
exposed at one time or another to the disease (please read my article
on Cat Scratch Fever). We think it is spread by the cat flea. People
infected with this disease by a contaminated cat scratch or bite
experience fever, malaise and enlarged painful lymph nodes as well
as a local inflammation at the site of the wound.
Q fever is a zoonotic disease caused by Coxiella burnetti,
a bacteria found world wide. Cattle, sheep and goats are the principle
reservoirs of the disease. Most human cases occur in veterinarians,
meat plant workers and farmers that raise sheep and cattle. The
organisms are excreted in milk, urine and feces. These bacteria
are tough, they resist heat, drying and common disinfectants and
they live for long periods of time in the environment. They can
also be transferred to humans by ticks. Only one half of the people
exposed to Coxiella burnetti develop disease. When they do, fevers
up to 105F are common along with severe headaches, malaise, muscle
aches, sore throat, chills, sweats, cough, nausea, vomiting, diarrhea,
abdominal and chest pain and confusion. These symptoms last for
1-2 weeks. During this time some patients develop hepatitis. One
or two percent of infected people die of the disease due to secondary
heart inflammation (endocarditis) and liver failure.
Doxycycline is the treatment of choice for acute Q fever. Quinolone
antibiotics such as ciprofloxacin also work well.
Or Rabbit Fever
This disease, caused by Francisella tularensis is associated
with rabbits and rodents. It was first formally recognized in 1911
in Tulare County, California in ground squirrels. It occurs in two
forms, a glandular form affecting the lymph nodes and a more acute
and severe typhoidal form affecting the entire body. In the first
instance the organism enters through a scratch. In the second it
is inhaled. The disease in man is characterized by high fever, painful,
enlarged lymph nodes, chills, myalgia and malaise. On rare occasions
it attacks the brain, heart and bones. Ticks are the most common
method of spread of the disease from animals to man. When it is
passed from direct handling of wild rabbit carcasses inflamed lesions
are primarily on the hands. It can also be transmitted in undercooked
meat consumed from infected animals.
TICK BORNE DISEASES
Ticks that feed on wildlife reservoirs of diseases will sometimes
transfer them to your pets. If the ticks later leave the pet and
bite the owner for their next blood meal they may transfer a number
of diseases caused by bacteria, rickettsia and spirochetes.
First seen in Lyme, Connecticut, lyme disease is an illness caused
by a bacteria, Borrelia burgdorferi. This group of bacteria
are called spirochetes due to their spring-like shape. The most
common source of infected ticks are household dogs. In humans, this
disease causes a wide variety of signs including rash, painful,
swollen joints, fever, enlarged tender lymph nodes and a variety
of neurological signs (please read the article in this series on
Lyme disease in pets and man). Over the last few years a number
of products have come onto the market that are quite good at keeping
ticks off your pets. Three of these products are Frontline spray,
Revolution and Preventic tick collars. Dogs can also be vaccinated
to prevent this disease.
Mountain Spotted Fever
Rocky Mountain spotted fever is the most severe and most common
rickettsial illness in the United States. It is caused by Rickettsia
rickettsii, a bacteria that is spread to humans by ticks that
have fed on infected wildlife. The signs of this disease are fever,
headache, muscle pain and spotted rash. The rash is very dark –
hence its nickname, Black Measles.
Ringworm is not a worm and is not always ring-shaped. It is a slow
growing fungus that feeds on dead skin cells and hair of all species
of mammal. The most common one, Microsporum canis, is common on
juvenile cats and dogs where it appears as a dry, oval, scurfy patch
of broken off hair. Many of these lesions glow brightly under ultraviolet
light. The spores of these fungi often contaminate brushes and cloth
that have been in touch with the pet. If these spores come in contact
with abraded skin, the fungal infection may transfer to the pet
These organisms which are more primitive than virus occur naturally
in a number of animals. The current large reservoir of prions are
cattle that were feed meat and bone meal derived from infected ruminants.
When the disease appears in cows it is called Mad Cow Disease or
Bovine Spongiform Encephalopathy (BSE). When it occurs in sheep
it is called scrapie. In cats it is called feline spongiform encephalitis.
Prions are unique pathogens in that they are very slow to cause
disease and can not be destroyed by ordinary methods of sanitation.
Humans become infected by eating contaminated meat products. When
the disease occurs in humans it is called Creutzfeldt-Jakob disease.
The signs of this disease in humans mimic Alzheimer’s Disease.
Fleas and ticks are parasites that transfer readily from pets to
people. Because they do not actually “infect” people
or pets I do not discuss them in this article. You can read about
them in other articles in this series.
Mange Or Scabies
Mange mites are common on large and small, wild and domestic animals.
Mange mites are not particular as to the species of animal they
attack. Dog and cat mange mites are Sarcoptes
scabei. Initially they are most common on the ears, face and
extremities but with time the entire body becomes affected. These
mites are passed from animal to animal by direct contact. They do
not survive long off the host. The mites burrow through the deeper
layers of the skin causing intense itching and a red rash. From
this the term “seven year itch” was derived. They are
easily killed with ivermectin, dips or Selamectin (Revolution).