VetInfo Digest   November 1999

This Month:

Zoonotic Disease Issue:

Bite Wounds
Toxoplasmosis
Ringworm (Dermatophytosis)
Atopy
Rabies
Roundworms and Hookworms
Heartworms
Cat Scratch Fever
Fleas, Ticks and Mange Mites

Diseases that AREN'T Zoonoses

Big Problem, Easy Solution

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Clients ask me every day if it is possible for them to catch the diseases that I have diagnosed in their pets. Most of the time the answer is no, but there are times when humans are susceptible to the diseases their pets are suffering from.  This month we cover the diseases that are most common or that we get many questions about, even if they are not actually a big
threat. In addition, there is a short list of diseases that pets have which people worry over but which are not really transmissible to humans.

Be careful this Thanksgiving. Every year we see a pet (or two) who has managed to eat the whole turkey carcass or a quart of gravy or gets too much ham and is really sick with pancreatitis, intestinal obstruction or some other food related problem. I give my pets treats and special meals when we are celebrating holidays, too,  but this has to be done in moderation.

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Zoonotic Diseases

A zoonosis is a disease that is transmitted from animals to humans. Zoonotic diseases are obviously a concern for pet owners and for people in health professions. Pets are a part of our lives but pet ownership does have some risks. Reducing the risk of disease transmission and working to control dog bites (the number one cause of medical visits related to pets)
is an important goal.

Recently, there have been reports that canine ehrlichiosis has occurred in humans. This is the newest of the recognized zoonotic diseases. Ehrlichiosis is a rickettsial disease, which is the same family of organisms that cause Lyme disease and Rocky Mountain Spotted Fever. There does not appear to be a direct transmission from pets to humans of this condition but people who own dogs who may be infected are more likely to
have the problem since they are more likely to be exposed to infected ticks. This disease does not appear to be common in humans but is another  reason to avoid tick infested areas and to use good tick control products on pets who are likely to be exposed to them. With this recent news making the papers in some areas  it seemed like a good idea to review the diseases
that can be transmitted from pets to people.

Bite Wounds from Pets

The most common "zoonotic" problems are dog bites and to a lesser extent cat bites and cat scratches. Pet owners must participate in the effort to eliminate, or at least minimize, this problem. Dogs and cats that have a tendency to bite should be confined in some manner to prevent them from encountering unwitting victims. Both cats and dogs will bite when they are
injured or frightened. When a pet has been hit by a car or has been in a fight it is important to take a second or two to evaluate the situation prior to reaching for them. It is irresponsible to own a dog that is known to bite and not take adequate precautions to ensure that people will not be
exposed to biting behavior. Take note of warning signs such as obvious fear of humans, growling or barking at people in an aggressive manner and biting in circumstances such as visits to the vet's or groomer's and if these are noted, seek help from a veterinary behaviorist (ideally) or at least take measures to limit the potential for a biting incident to occur, such removing the dog from the room when strangers are present.

The issue of dog and cat bites is pretty complex. Hopefully, in a future issue this can be discussed in more detail.

Toxoplasmosis

Toxoplasmosis is one of the zoonotic diseases that we receive the most questions about. This is a parasitic illness caused by Toxoplasma gondii. Cats are the definitive host of this disease, meaning that they are the only known animal in which the parasite can complete its whole life cycle. Other animals can be infected, though. Pigs and sheep are commonly infected
with toxoplasmosis and eating undercooked meat is the most common way for humans to be infected with this organism. Beef is a less common but occasional source of the infection. Despite the fact that a couple of million cases of toxoplasmosis occur each year due to undercooked meat compared to a few thousand cases linked to direct contamination from cats,
the fact remains that cats are the reason the disease continues to exist and it is possible to be infected due to direct contact with the cat's stools.

A human infected with toxoplasmosis may experience flu-like symptoms or may have an inapparent illness. It is rare for symptoms to be severe in an adult who is not immune compromised. There are situations, such as a person
suffering from acquired immune deficiency syndrome (AIDS), in which this disease does cause serious illness in an adult human, though. The major problems associated with toxoplasmosis are abortion and birth defects when a woman becomes infected during a pregnancy.

It is important to recognize that this illness has to be contracted during the pregnancy for it to cause birth defects. Someone who has been infected prior to becoming pregnant will have sufficient antibodies to prevent reinfection during the pregnancy, which protects the fetus. Knowing this can prevent common mistakes that cat owners make that lead to a greater risk of infection -- sometimes on the advice of human physicians or veterinarians who do not clearly understand the life cycle of Toxoplasma
gondii or do not fully understand how the disease is transmitted.

Cats shed the oocysts (infective stage) of toxoplasmosis in their stool. These oocysts infect small rodents, livestock, dogs and humans who come in contact with them. The infection is normally inapparent. None of the infected animals, except cats, can spread the infection except to their own offspring during a pregnancy. Cats become infected by hunting and eating
small rodents or occasionally by being fed undercooked meat of infected animals such as pigs or sheep.
 
We are frequently asked to do "titers" on cats for toxoplasmosis by pregnant women, worried about the health risks to their babies.  A titer is a measurement of the amount of antibody in the serum to a particular disease. A positive titer would indicate that there has been exposure to the disease but does not indicate that the cat is currently infected. Most cats are infected when they are young and shed the Toxoplasma organism for
about two weeks. They usually develop a titer AFTER they have stopped shedding the Toxoplasma organism. These cats will have titers for a long time. For this reason, testing the titer is most useful when the cat is negative -- meaning that it hasn't been exposed to the parasite. In this case, keeping the cat from being exposed is very important. Most people stop worrying when the titer is reported to be negative. The fault in this
logic is that a cat that has a titer is not very likely to transmit
toxoplasmosis because it won't get a new infection --  but the cat that is negative who gets exposed to the parasite may actually shed the parasite in its stool for several weeks. If this happens during a pregnancy there is a risk that the pregnant woman will become infected.

Toxoplasmosis affects about 3 million adults in the U.S. per year. Most of these people never know that they were infected. Toxoplasmosis  infects about 4000 fetuses a year and most of these babies have complications from the disease at some time during their lifetime.

The most common bad advice we hear is "My doctor told me to have my cat checked for toxoplasmosis."  It just isn't possible to do that during the time the cat can actually cause problems by shedding toxoplasmosis oocysts. The next most common advice is "get rid of the cat". This strikes us as pretty drastic and also pretty ineffective advice for avoiding toxoplasmosis. The last, and potentially worst, bad advice we hear is "put
your cat outside while you're pregnant." Not only is this a huge risk to a house cat's health it also actually increases the risk that a previously unexposed cat will contract toxoplasmosis, making it MORE likely that a pregnant cat owner may be exposed to this disease.

Thinking about the  mode of transmission of toxoplasmosis allows a good plan of attack for avoiding it. Cook all meat to an internal temperature of at least 170 degrees Fahrenheit. Especially pork and lamb. Pregnant women should not dig in the garden without wearing gloves. they shouldn't put
their fingers in their mouths or noses after gardening or handling soil without washing them first. Keep the cat indoors and have someone who is not pregnant change the litter pan. Change the litter pan frequently, preferably every day (or use scoopable litter). Put the waste somewhere that the pregnant member of the household will not be exposed to it. Pregnant women should not help with delivery of piglets, lambs or goat kids. If anyone should have a titer taken to determine if toxoplasmosis is
a threat it should be the pregnant woman, early in the pregnancy. If she has a NEGATIVE titer she must be especially careful about following the above directions. It would be reasonable to do a second titer during the middle of the pregnancy. If it is still negative, all is well. If it becomes positive there is a real need to worry about the baby. A third
titer taken after delivery would confirm that the baby was uninfected if it continued to be negative and would alert the pediatrician of the need to treat the baby if it was positive. This is somewhat expensive but would give a good idea of whether an infection occurred during a pregnancy, if
there is reason to worry over this.

Persons with AIDS and those being treated with chemotherapy are more susceptible to toxoplasmosis, both new infections and severe recurrences of previous infections. It is important for people with these problems to follow the advice for pregnant women very carefully, too.

Dermatophytosis  (Ringworm)

Ringworm infection is a common problem for cat owners. Cats can carry the fungus Microsporum canis and transmit it to people or other pets, even if they have no apparent signs. This is especially true of Persian and Himalayan cats.
 
There are several known fungi that can be involved in ringworm infections. The disease apparently got its name because the lesions it causes in people are often a round sore with a red ring that is raised and can look like there is a worm under the skin that is curled up in a circle or ring shape.

If a new kitten is introduced into the household and family members suddenly develop skin sores, the cat should be suspect. If there are no visible lesions it is best to do a "toothbrush" culture to determine if inapparent ringworm is present. To do this, a new toothbrush is used to brush the cat's haircoat and then the bristles are cultured for the ringworm fungus. This is a pretty good screening test for ringworm. Of
course, kittens aren't the only source of ringworm but this is a common history. An older cat can be a carrier and infect susceptible humans and it is also possible to get ringworm infections from the soil or contaminated surfaces.

When ringworm is identified in a cat it is important to treat them with oral medications such as griseofulvin rather than to try to control the infection with topical medications. Most of the recent well designed studies don't show much effectiveness for topical medications. Sometimes in small kittens it is necessary to use topical medications while waiting for them to get big enough to medicate with oral medicines.

Atopy

Atopy is the name used in medicine to describe inhalant allergies. While not technically a disease, atopy is pretty common and many people with inhalant allergies are allergic to cats or dogs. It is a common problem among pet owners and many do not wish to get rid of their pets. This can be a difficult situation in a household in which one person is atopic and
severely reactive to a household pet. There isn't a  really good solution to this problem, but it can help a great deal to simply wash the pet's fur once or twice a week with distilled water. This helps to keep the dander concentration down, making the pet less irritating to persons with allergies.

Rabies

Rabies is probably the most feared of zoonotic diseases. It is also among the easiest to prevent. Pets should be vaccinated for rabies when they are between 12 and 16 weeks of age using standard vaccination protocols and the newest rabies vaccine, PureVax (TM) from Meriel, may be used as early as 8
weeks of age. The first rabies vaccination must be boostered again in one year and then the manufacturer's directions or state law must be followed to determine the future intervals. In most states it is legal to give rabies vaccinations every three years when a vaccine approved for this interval is used but in some states pets must be vaccinated yearly, by law.
 PureVax and several other rabies vaccinations area only labeled for one year use and the manufacturer's directions must be followed when using these vaccines.

Why would anyone want to use a one year duration vaccine when three year duration is allowed by law? In the case of PureVax the reason is that the vaccine is not adjuvented, which may make it less likely to produce vaccine reactions and vaccine associated cancers in cats.

There is a great deal of misunderstanding about the transmission of rabies, based on the responses our clients have to possible rabies exposures.Except in bats, rabies is usually only transmittable from an infected animal to an animal it bites during the last four or five days the infected animal is alive. This is the reason for the ten day quarantine following bites from animals whose rabies vaccination status is unknown. If the dog
or cat lives ten days it is not likely to be able to transmit rabies. But it certainly could be infected. It can take several months from the time an animal is bitten until the rabies virus travels from the site of the bite to the brain and then to the salivary glands, where it can be passed on. This means that a pet who has not been vaccinated for rabies must be quarantined for at least six months if it is bitten by a known rabid animal
or in situations in which the likelihood of rabies seems high. If clinical signs of rabies do not appear during this time, the pet may be vaccinated and then allowed to resume a normal lifestyle.

We have a number of people who want us to test for rabies when their pets are bitten by wild animals. There is no test that will work prior to the time that clinical signs appear, to the best of my knowledge.  The most reliable tests are still examination of brain tissue for signs of infection, which obviously can not be done unless the pet is euthanized or dies.

Keep your pet's rabies vaccinations current. There has never been a confirmed case of rabies in a properly vaccinated pet using the currently available vaccines.

Toxocara (Roundworm) and Ancyclostoma (Hookworm) Infections

There are a set of conditions, known as visceral larva migrans, cutaneous larva migrans and ocular larva migrans, which occur in human beings as the result of infection with roundworms and hookworms. These conditions occurs because the natural life cycle of roundworms involves migrating out of the intestine and through the body before returning to the intestine. Hookworms
migrate through the skin and to the intestine, leading to the problem with cutaneous larva migrans in areas in which hookworm infestation is common. When the worm finds itself in a host for which it wasn't intended, such as a human, it doesn't usually manage to develop fully.  Instead, its growth is arrested in some other body tissue within the unnatural host. As might
be expected, this can cause serious problems for the human who is infected by the hookworm or roundworm larvae.

Cutaneous larva migrans produces an uncomfortable rash that is persistent if reinfection continues to occur. Visceral larva migrans (VLM) may cause coughing, fever, liver enlargement or lymph node enlargement. Less commonly, there may be neurologic signs, including encephalitis and seizures.  Most affected children have eosinophilia, an increase in the
number of eosinphils in the blood. These are the white blood cells primarily involved in controlling parasite infections in the body. Ocular larva migrans affects the eyes. It usually occurs in children that are a little older than those affected with VLM and sometimes occurs in adults. The affected eye may have decreased vision, redness, inappropriate orientation with the other eye (one eye that looks in a different direction
than the other) or other signs of disease. This is not an especially common disorder but it does occur and should be considered when eye changes occur after exposure to puppies or in a child with a history of eating dirt or other objects that might be contaminated with roundworm eggs.

Roundworms are incredibly common. Someone once said that if the world was suddenly dissolved and everything disappeared except the roundworms that the entire outline of the earth would still be easily discernible. Statistically, about 30% of the soil in parks, sandboxes, gardens and other areas in which animals defecate will be contaminated with roundworm eggs.
Hookworms are more common in the South than they are in more temperate areas but they also have wide distribution in the areas in which they are common.

Protecting against roundworm or hookworm infection involves several approaches. The most important is to deworm all kittens and puppies appropriately. For puppies it is best to start deworming at around 2 weeks of age and continue to deworm on a regular basis until the puppy is sixteen weeks of age. For kittens, it is not necessary to start deworming until about six weeks of age but the deworming should still continue until
sixteen weeks of age. The interval most often recommended for deworming medication is two weeks. The monthly heartworm preventatives Heartgard Plus (Rx) and Interceptor (Rx) do kill roundworms, so they may be included as one of the dewormers if they are being administered. Good sanitation is also important, especially hand washing after touching soil or playing in a
sand box. Covering sand boxes when they are not in use is helpful in keeping cats from defecating in them and spreading roundworm eggs. Discouraging geophagia (eating dirt) is also a good idea, although this can be a difficult process with some children. Using one of the monthly heartworm preventatives that kills roundworms and hookworms is a good idea,
too.

Heartworm Disease (Dirofilaria immitus)

Heartworms infected dogs most commonly but they also can infect cats, sea lions, bears and humans (and probably other species as well).

When humans are infected with heartworms the worm is transmitted to the human from an infected dog through a mosquito bite. The heartworm will only rarely develop fully and make it to the heart or pulmonary circulation as it does in a dog. Usually this is not a cause of clinically apparent disease. The most common finding in humans with heartworm disease are skin sores  containing the wayward larvae or a more mature heartworm. There have been several hundred reported cases of this condition. In other instances the worm causes a cyst in the lung that resembles lung cancer. This usually results in a biopsy to rule out the cancer.  There are new tests being developed to help make this diagnosis easier, at least possible with a
needle biopsy rather than having to surgically remove the cyst. Very rarely heartworms find their way to the inside of the eye and they can actually develop to some degree there.

Cat Scratch Fever ( Bartonella henselae infection)

Cat scratch fever has been recognized as a disease entity for some time. Recently, an organism, Bartonella henselae, has been identified as the causative agent. This bacteria may also be called Rochalimaea henselae in references published more than a couple of years ago. About 22,000 people in the United States suffer from this disease each year. Even though this
sounds like a large problem, it is still a relatively rare occurrence, overall. It is sometimes reported as being most common in immune compromised patients, including people with AIDS and people who are receiving chemotherapy treatments for cancer, but there are reported to be
studies in which this theory did not hold up. Children seem to be infected more commonly than adults.

This disease is still being studied and there are still new revelations on a regular basis, so information about it may change over time. At the present time, it appears that the disease is spread from cat to cat through the bites of infected fleas. This makes good flea control the first preventative measure to take in avoiding this disease. There may be
alternate sources of infection other than cats but it does appear that cats are the major cause of infection. It is unclear exactly how the transmission from cat to human occurs, despite the "cat scratch" name. It is possible that flea bites are the means by which humans are infected as well as cats. But all cat scratches should be washed thoroughly, anyway.

Cats who are infected with this organism can be cleared of the infection using doxycycline for a one week period, followed by Clavamox (Rx) for a one week period (per Alice Wolf, posted on the Veterinary Information Network 4/98). At the present time it appears that cats are immune to reinfection once they are free of the organism. It is a difficult bacteria to culture so cats should be treated if there is presumptive evidence that
they may be infected, such as a member of the household who has been diagnosed with bartonellosis (Cat Scratch Fever).

Ectoparasites (those that live on the pet's skin) and diseases associated
with them

Pets are perfectly willing to share the parasites that inhabit their skin.  Fleas prefer to bite pets but they are not so choosy that they refuse a meal from a human. Ticks have similar feeding habits. These may not technically be zoonotic diseases but good flea and tick control is possible, so why put up with flea bites?

Dogs get sarcoptic mange, which is infection with a small parasite that causes intense itching and hair loss. Humans who have contact with infected pets may experience transient infections with the sarcoptic mange mites, usually on their arms due to holding the pet. Around the waistband is another common site of infection, presumably due to letting the dog sit on a person's lap. These can cause very itchy sores but the parasite can't complete its life cycle on humans and the problem will go away without treatment.

The real problem with ectoparasites is that they carry lots of other diseases. Infected fleas or ticks from pets could theoretically serve as a means of transmission of plague, Lyme disease, cat scratch fever, ehrlichiosis and babesiosis. For all these reasons, use an effective flea control product. The best are Frontline (tm) for fleas and ticks and Advantage (tm) or Program (tm) if fleas are the only problem.  Revolution
(tm), a new product, has not been on the market long enough to evaluate how well it will work but it also carries a claim for protection from fleas for thirty days and it kills at least one species of tick.

This is not the whole list....

There are over 150 known diseases with zoonotic potential. Plus the "not really zoonoses" conditions such as bite wounds and atopy. Pets have consistently been shown to be beneficial to the overall health of humans but there are some risks and it is appropriate to be cautious about them.

Deworm young puppies and kittens, control fleas and ticks, wash hands after handling cat litter or potentially contaminated soil and talk to both your doctor and your veterinarian if you have a medical condition whose diagnosis seems elusive to your physician. Once in a while, your veterinarian can actually help protect your health as well as your pets!

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Diseases that are NOT Zooneses:

There are a number of diseases that people think they can catch from their cat or dog, which are not transmissible to humans. This is a short list of the ones we are most frequently asked about:

Pinworms. Dogs and cats do not get pinworms and can not transmit pinworms to humans.

Feline Leukemia Virus. Despite numerous attempts to prove that FeLV can be transmitted from cats to humans there continues to be no real evidence that a risk of FeLV infection in humans exists.

Feline Immunodeficiency Virus. When human immunodeficiency virus was discovered it was really tempting to assume that FIV might be the same illness. Numerous studies have shown no relationship between FIV and HIV other than that the viruses are in the same viral family.

Streptococcal Infections. There continues to be a persistent rumor among physicians that cats and/or dogs can serve as the reservoir for human streptococcal infections. There are several citations in the literature to this effect. One, written by Craig Green, DVM (Journal of the American Animal Hospital Association, March 1988) says that cats and dogs may be
transiently infected by humans with this condition and could hypothetically serve as source of reinfection in cases of chronic reinfections in humans. Subsequent studies done in cats and dogs have not supported this hypothesis. Dogs do get streptococcal A infections and they even get streptococcal toxic shock syndrome but the isolates from these infections
have been shown to be genetically lacking in the known virulence factors for human infection (Miller et al., Journal of the American Veterinary Medical Association, Oct 15, 1996). It is not possible to be absolutely certain that there is no connection at all between human streptococcal infections and pets but it is very very unlikely. It is reasonably easy to culture or treat family pets if this is a concern but it is probably money that is wasted.

Lice.  Lice are species specific parasites, too. Dog and cat lice might run around on your body for a few minutes but they do not reproduce on humans or infest humans in any major way.  I feel obligated to convey one other message when it comes to lice. We have had two different owners bring us samples of lice that they claimed they got from their pet which turned out
to human pubic lice. Since we send parasites to a parasitologist for examination we get a written report on the identification of the parasite. Think of another way to explain this problem to your spouse!

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A Big Problem with an Easy Solution

If you notice your dog rubbing its nose on the ground,, pawing at its face or lips, not eating meals and just generally not "right", the first place to look for problems is the roof of its mouth. Gently open the mouth and look carefully at the space between the upper rows of teeth. Dogs frequently break sticks or bones they are chewing and the pieces lodge across the roof of their mouth. This is a serious problem because it will quickly lead to ulceration of the hard palate and often will lead to severe
root exposure of the inner aspect of the teeth that are involved. Some dogs resist letting you open their mouth but this is another sign of problems. If you can't look, take your dog to your vet for an examination. If you can see a stick or bone try to remove it. We use a needle nosed pliers to grasp the object and remove it. This can be uncomfortable for the pet but the subsequent relief is great. Just be careful not to get bitten. If your pet
resists at all, let your vet remove the offending object.  We see a dog with this problem once a month or so, on the average.

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If you send us e-mail, remember that Michal Justis answers the e-mail at vetinfo@vetinfo.com. E-mail sent to mervet@inna.net is answered by Dr. Michael Richards.

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The opinions expressed in this newsletter are those of Michael Richards, DVM., author.

Copyright  1999, TierCom, Inc.

This page was last edited  06/21/04

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