Sunday, December 18, 2016

Cats as Carriers of Disease

Peter P. Marra and Chris Santella provide an interesting perspective on our beloved pet cats in their book Cat Wars: The Devastating Consequences of a Cuddly Killer.  They explore the problems that arise because cats are not really domesticated animals; they still can live independently of their human keepers and continue to be effective predators for just about any animal smaller than themselves.  Cats escape from or are abandoned by their owners and become feral.  Given that they breed very rapidly and are prey for few larger animals, they multiply quickly in the wild.  While often providing a useful function in controlling the mice and rat populations near human activities, they are also proficient killers of more valued species such as birds.  The authors are driven mainly by the threat cats present to bird species that are, for a number of reasons, experiencing rapidly falling populations.  The authors conclude that house cats are not a problem if they are kept in the house, but wild cats should be completely eradicated, at least in certain locations.  A review of the authors’ argument can be found here.

As part of their argument for eradication, the authors bring up the topic of disease propagation by feral cats.  Wild cats share the same areas—and feed off of—animals that are reservoirs for serious diseases.  Consequently, they are capable of becoming infected themselves and passing on that infection to humans via bites, scratches, or even by breathing on a human. 

The plague still persists in some regions of the country and transmission to humans via cats does occur, but infrequently.  However, the illness can be fatal if not diagnosed early.

“Cases of plague transmitted from cats to humans are rare in the United States.  From 1977 to 1998 there were twenty-three cases of cat associated human plague in the country.”

A sickness more frequently transmitted is Bartonellosis, more commonly known as cat scratch disease or cat scratch fever.  Infection is likely in cats with estimates of a 40% or greater rate of occurrence.

“….cat scratch fever refers to an infection from a Bartonella bacterium that develops when an infected cat scratches or bites the human skin.  In cats themselves it is usually not a serious problem, and 40% or so of the cats that carry it are asymptomatic.  Humans, similarly, usually are not seriously harmed; a red bump forms, the lymph nodes might swell, and a mild fever may emerge.  However, more serious infections can, and have, occurred, particularly among immune-compromised individuals.”

Rabies is disease that is transmitted by several small to moderate sized mammals that are easily encountered in the wild by cats or dogs.  It is an interesting case of how a virus or bacterium can manipulate a host that has been infected in order to insure its propagation to another specimen.  When infecting an animal the virus travels along nerve fibers as it makes its way to the brain.  Once there it alters the behavior of the host, eventually making it more aggressive.  Meanwhile, the virus is also making its way to the saliva glands so that when the animal bites another animal the virus can be transferred to the next host. 

Dogs have long been the greatest threat to humans when it came to the transmission of the rabies virus.  Prior to the development of vaccines and effective post-exposure treatments, an encounter with the rabies virus was always fatal.  Fear of rabies led to enforced vaccination of pets and the near elimination of wild dogs.  Since there is less awareness of cats as a source of rabies and less popular support for the elimination of wild cats, bites and scratches from cats have become the most serious threat for rabies transmission to humans.

“Since 1988 cats have been the number one domesticated species passing rabies infections to humans.  In 2013, 53 percent of all reported rabid domesticated species were cats, followed by dogs at 19 percent.  The cause of this pattern seems clear—the presence of millions and millions of stray and unvaccinated free-ranging cats on the landscape, many of them sharing feeding stations with wildlife that are susceptible to rabies.”

The animals infected with the disease may not have observable symptoms, and a person infected by the bite or scratch of a rabid animal may go months before symptoms appear.  And once symptoms do appear it is too late and the infected person will die.  Consequently, any such interaction with an unknown or undocumented animal requires anti-rabies treatment.  Since cats can appear to be cuddly animals, especially to children, they can be particularly dangerous.

“….the vast majority of the 38,000 post-exposure rabies treatments conducted annually are the result of people interacting with a suspected rabid cat.  Each of these post-exposure prophylaxis treatments cost public health departments and U.S. taxpayers somewhere in the neighborhood of $5,000 to $8,000, amounting to at least $190 million across the United States each year.”

Cats are also the unique source of a parasite called Toxoplasma gondii.  The life cycle of this parasite requires an infected cat to spread the disease to other animals via cysts (oocysts) that are excreted into the environment where other animals can ingest them and allow the cysts to transform into tachyzoites which then multiply rapidly, and ultimately into something called bradyzoites.  These secondary hosts are then eaten by cats and the infectious cycle begins again.

Unfortunately, humans and other valued species can be infected by these cysts that infected cats distribute.

Toxoplasmosis, the disease caused by Toxoplasma gondii, is one of the most common parasitic infections in humans.  In fact, it is estimated that approximately 30 to 50 percent of the world’s population and up to 22 percent of the U.S. population (more than 60 million Americans) are infected with Toxoplasma gondii….”

The cysts defecated by infected cats are very robust and can last for as long as a year or more in very harsh environments.  Humans can become infected by direct interaction with cat feces or by consuming foods from animals (poorly cooked meat from an infected animal for example) or by eating contaminated vegetables (cats seem to like to defecate in gardens and children’s sand boxes).  Most humans show no symptoms from an infection, but it can generate flu-like symptoms in some individuals.  The consequences of an infection can be quite severe for people with compromised immune systems.

Since so many people are infected by Toxoplasma gondii, and the vast majority of them are asymptomatic, why be concerned?  To begin with, the prime example of a compromised (or nonexistent) immune system exists in the form of a human fetus, and it has long been know that this infection can be transmitted from a pregnant woman to her fetus.

“Pregnant women and their fetuses have been known since the 1920s to be at serious risk.  If infected with toxoplasmosis in the first trimester, one in ten fetuses will be aborted or become malformed—and this is likely an underreported statistic.  Because of this problem, pregnant women have been warned for decades to avoid changing litter boxes and touching cat feces.  Despite these warnings, congenital transfer of Toxoplasma continues to happen across the world.”

Consider what is known about how the disease progresses in infected mice and rats.

“Once in the secondary host, the Toxoplasma oocysts then transform into something called a tachyzoite and multiply asexually rapidly.  Tachyzoites can be as small as one-tenth the size of red blood cells when they invade healthy cells.  There they divide quickly, causing tissue destruction and spreading of the Toxoplasma infection to the new host organism.  Eventually the infection localizes in muscle and nerve tissue—especially in parts of the brain—in the form of cysts called bradyzoites….”

The rabies virus has a strategy for modifying the host’s behavior in order to increase the probability that the host will infect another animal—Toxoplasma gondii does as well.

“Then something odd begins to happen to the newly parasitized host: its normal behavior of fear toward cats turns into attraction.  Specifically, the smell of cat urine—a smell that infected mice and rats were thought to be hardwired to fear and avoid—becomes an attractive aphrodisiac.  This is exactly how the Toxoplasma parasite wants its host to behave, because it turns infected rodents into easy prey.  Once the infected host, along with the parasites infecting its body, is eaten by a new predator, (preferentially a cat or other species of feline), the parasite can begin its sexual reproductive cycle again, infecting a new host, shedding oocysts, and expanding its reach.”

It was generally believed that these bradyzoites were a stable, latent stage that, although it remains in the infected human forever, had little if any effect.  However, if the bradyzoites are capable of altering the brain and changing behavior patterns in rats and mice, why would one assume that they were incapable of doing something similar in humans?

“Then scientists began to look a little deeper and found that the bradyzoites were actually dynamic and replicating.  In fact, one manifestation of toxoplasmosis infection is the development of ocular toxoplasmosis—basically cysts that settle in the eye.  If the cysts burst they can cause a progressive and recurring inflammation of the retina that can result in glaucoma and eventually blindness.  Regrettably, this is not the worst manifestation of toxoplasmosis infection in humans.”

Recent studies have suggested correlations between toxoplasmosis and long-term changes in physical and mental health.  Consider this summary from Wikipedia on the state of knowledge about the long-term effects of toxoplasmosis.

“Some evidence suggests latent infection may subtly influence a range of human behaviors and tendencies, and infection may alter the susceptibility to or intensity of a number of affective, psychiatric, or neurological disorders.   Research has linked toxoplasmosis with schizophrenia.”

“Latent T. gondii infection in humans has been associated with a higher incidence of automobile accidents, potentially due to impaired psychomotor performance or enhanced risk-taking personality profiles.  Moreover, correlations have been found between positive antibody titers [a measure of concentration] to T. gondii and OCD [obsessive-compulsive-disorder], Parkinson's disease, Alzheimer's disease, suicide in people with mood disorders, and bipolar disorder.   Positive antibody titers to T. gondii have been shown to be not correlative with major depression or dysthymia.  Although there is a correlation between T. gondii infection and many psychological disorders, scientists are still trying to find the cause on a cellular level.”

Some researchers are more cautious than others about drawing conclusions from these correlations; some are drawing the direst of conclusions.  The authors describe some of the work and conclusions of Jaroslov Fleger.

“Fleger believes that collectively toxoplasmosis, either through the acute stage of infection or through mental and neurotic illness manifested during the latent phase, has contributed to the deaths of hundreds of thousands of people, if not significantly more, over the last few decades.”

“Jaroslov Fleger….believes that malaria, now considered to be the most devastating protozoan killer of humans, will be ‘dethroned’ by toxoplasmosis.  As long as we continue having outdoor cats, the parasite will spread.”

More on Fleger and related research can be found in an article by Kathleen McAuliffe in The Atlantic: How Your Cat Is Making You Crazy.

For those of you who are not concerned that cats might be killing too many birds, perhaps you might wish to consider the human health consequences detailed for toxoplasmosis.  You can still have cats; you just have to keep them inside.

The interested reader might find this article informative:


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