We derive a lot of joy and solace from our pets. They are our non-judgmental companions that offer unqualified love. However, on occasion, our pets can unknowingly give us germs and parasites that have the potential to 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 much rarer now in the developed world than they once were. As a general rule, as incomes and education go up, the number of zoonotic diseases in people go down. The following is a list of some of the diseases pets, wildlife and livestock pass on to human beings. They are only the ones that came to mind while I write this article. It is not meant to be all-inclusive. From year to year one or another of them become more or less common. From year to year new ones appear. I do not want you to read this article and become fearful of owning a pet. All the conditions I discuss are quite rare and can be avoided through good hygiene, preventive vaccination, vermifuges and parasite preventative care. Living life to the fullest always has its risks, but a little caution on your part is generally all that is required.

Some Worms That Live Within Cats And Dogs Can Infect You Too:

Read about intestinal parasites including ancylostoma hookworms in cats here in dogs here and when they cause cutaneous larval migrans in people here & here. Hookworms and roundworms (Ancylostoma and Ascaris) are the most common nematode parasites of cats and dogs. When a human accidentally eats something contaminated with roundworm eggs from a pet’s stool, the eggs hatch in our intestines as well and begin migrating throughout a person’s body. You say “well that could never happen to me”. Roundworm eggs are highly resistant to drying. Stool gets trampled to dust. Dust particles float throughout your house and falls on objects. Proximity to an infected pet is sufficient.  Hookworm larva that hatch in damp soil or grime can burrow through intact human skin causing a severe rash, cutaneous larval migrans. Because these parasites were designed to live in dogs and cats, they become lost in the human body – often in the skin liver or eyes. When these lost parasites end up in body organs, the disease is called visceral larval migrans. These disease occurs most often in children due to their poor hygienic practices and inexperienced immune system. When they migrate into the eye, larval nematodes often cause inflammation and even blindness. In the liver they can cause chills, fever, malaise and an elevated white blood cell count (particularly eosinophils). To prevent these issues, have your veterinarian check your dog and cat’s stools yearly for parasites. Popular brands of heartworm/flea/tick preventative (like Heartgard Plus® and Advantage Multi®, and Revolution® ) contain ingredients that should keep your dog or cat free of intestinal parasites as well. Of the three, I believe that Advantage Multi® has been found to kill the largest number of intestinal worms. Be sure that whatever you purchase has a picture on the box that corresponds to your species and size of your pet. I know that will be more expensive for owners with multiple pets. But dog and cat formulations are not always the same. Pyrantel pamoate, as found on the remedy isles of bigbox stores, is quite effective in removing hook and roundworms. Some, like the Hartz® products, are just piperazine and hardly worth giving.  

Raccoons are not pets I recommend. But when they live in proximity to people, their most common round worm presents a serious human health hazard.

Certain Tapeworms Of Dogs And Cats Also Present A Health Hazard to You.

The most common tapeworm of dogs and cats is Dipylidium caninum.  It is not a threat to you or your family. This small tapeworm uses fleas to jump between pets. Read about those fleas and what to do about them here. But another tapeworm of dogs and other carnivores, Echinococcus granulosa, has the ability to form dangerous hydatid cysts in people. These cysts are large, and they contain many immature tapeworms. They can form almost anywhere in your body. Over time, these cysts slowly continue to grow. However, in humans, they never turn into adult tapeworms. When the same cysts form in a prey animal that accidentally ate an echinococcus egg, it is only when that animal is eaten by a wolf or dog or another predator that a new adult intestinal tapeworm develops. In humans, the cysts are commonly mistaken for tumors. Dogs involved in spreading these parasites to people are only those that eat fresh raw meat that contains the encysted juvenile forms of the parasite. Meat that has at one point been frozen is probably safe. Cats that range outdoors might also potential carry this parasite. (read here)

Dog Heartworms

On occasions, mosquitoes will transfer larval dog heartworms (Dirofilaria immitis) from infected dogs to people. (read here)  It happens frequently. However, in most of us, our body’s immune system quickly recognizes these parasite larva (microfilaria) and destroys them. But in rare instances, the larval heartworm(s) manage to migrate into a person’s lungs. There, the dying parasite can stimulate the formation of an inflammatory mass sometimes called a “coin lesion”. These lesions are not that serious in themselves; but they become very significant when a physician misinterpreted one or more of them as being lung tumors. This is not a very common occurrence, within the last twenty years in the United States about eighty cases were reported in the State of Florida. Read about dog and cat heartworms in detail here & here A second heartworm relative, D. repens, that also affects dogs has occasionally been found in the human eye. (read here)

PROTOZOAN DISEASES

Protozoa are microscopic single-celled organisms. The vast majority of protozoa live free in the environment or as harmless inhabitants of the body, but a few of them are capable of causing disease. Here are a few of the more common ones:

Giardia

Giardia lamblia aka G. duodenalis is a small motile protozoan that inhabits the intestines of dogs, cats many other mammals and birds. There are a very large number of strains of giardia (about 40 have been described) and it is unclear how many and under what circumstances they cause diarrhea and intestinal inflammation in dogs, cats or people. Giardia is the most common form of non-bacterial diarrhea in people in the United States. Children are most commonly affected. Among our pets, kittens and puppies are the most likely to be affected.  Most cases are silent with no overt (visible) symptoms because the normal resident microbes of the intestinal tract keep giardia numbers very low. (read here & here) When diarrhea does occur, the illness normally lasts only one to two weeks, but chronic cases in frail or immunosuppressed people have lasted for years. There are even thoughts that a giardia infection might be one trigger for subsequent inflammatory bowel disease. How frequently a dog or cat might spread giardia to its owners remains unknown. (read here) Metronidazole (Flagyl®) is usually a very effective treatment for giardia in animals and people.

Cryptosporidium

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. The most common signs of the disease are diarrhea, and cramps. In unusually severe cases in humans, flu-like symptoms can accompany infection and last up to six weeks. The disease tends to be more severe in very old and very young animals and people. In immunocompromized individuals cryptosporidium can cause chronic inflammation of the digestive tract. The disease is known to have through fecal contamination from animals to man. These organism is quite resistant to drying and disinfectants. So, it can survive for a long time in contaminated waste and grime. Besides loose or watery stool and cramps, some patients develop fever. In people with healthy immune systems, symptoms often last no more than two weeks. In one study in children, less than half that were infected with the parasite showed any symptoms at all. (read here) In infants and the immunocompromized, cryptosporidium can also infect the lungs. (read here) 

Toxoplasmosis

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. Read about that here. The complete life cycle of toxoplasmosis occurs only in cats. Felines become infected by preying on infected birds and rodents. Most cats show no symptoms of disease. In those cats, the toxoplasmosis organism live within some of the cells that line your cat’s small intestine. Periodically, some of those cats shed toxoplasma oocysts (the infective stage of the parasite) in their stool. Felines are the only animals that shed toxoplasma oocysts. Most exposures of humans to these oocysts cause no overt (visible) disease. In a small percentage of people, however, the oocysts proliferate 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. Besides, exposure to the stools of infected cats, eating raw or poorly cooked meat of an infected intermediate host animal is another way toxoplasmosis passes on to humans. If a woman should become infected during the later two thirds of pregnancy, toxoplasmosis may cause severe fetal abnormalities. This is why obstetricians suggest pregnant women not change their cat’s litter boxes.

VIRUS

Rabies

The flagship virus that we all associate with wildlife and occasionally dogs and cats is rabies. But any species of warm-blooded animal is susceptible to rabies. The most common rabies carriers in the United States are bats, foxes, raccoons and skunks. At one time, rabid dogs were also a likely threat. However, the widespread use of dog and cat rabies vaccines and availability of post-bite (post exposure) rabies vaccines for bitten people have made pet-to-human transmission of rabies quite rare. Rabies is passed through a saliva-contaminated bite. Read about rabies in dogs, cats and people in detail here. 

BACTERIAL DISEASE

Salmonellosis

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 can also carry and spread the disease without showing symptoms. Common animal carriers of these bacteria are reptiles, rats and mice – not dogs and cats. People with a robust immune system rarely experience more than severe cramps and diarrhea when exposed to salmonella. However, in infants and people with weak immune systems the disease can be life-threatening. Read more about salmonella in dogs, cats, people and pet food here.

Pasteurellosis

Many rabbit farms harbor Pasteurella multocida in their breeding stock. Most rabbits that carry Pasteurella do not appear ill. Only a few have eye infections, snuffles-like signs and/or enlarged lymph nodes that surround the head and neck. Poultry can also develop pasteurellosis. In birds the disease is called hemorrhagic septicemia.

Pasteurella bacteria are also carried in the mouth of many healthy cats. That can lead to contaminated cat bites and abscesses when outdoor cats fight for territory or mates. Dog bites also occasionally transmit this bacteria. (read here) The most common form of pasteurellosis in people is a skin and soft tissue infection at the site of the bite or scratch. (read here) However, some people exposed to Pasteurella have developed severe eye infections. (read here) On rare occasions, Pasteurella causes pneumonia in humans. (read here)

Streptococcus and Staphylococci

Staphylococcus and streptococcus bacteria are found on the body surfaces of all animals and humans. They are part of the normal skin flora. In the great majority of cases they cause no disease when they remain on the skin surface. However, when they find their way deeper into the body such as through a wound or in pets and humans with weakened immune systems, itchy allergies or fleas, they are capable of causing abscesses and organ damage. In dogs, cats and humans, these bacteria occasionally cause infections. Both bacteria can spread from pets to humans on contaminated hands and objects. Infections are generally limited to your skin and eyes. Different strains of staphylococcus vary greatly in their ability to produce disease. (read here) Some strains of staphylococcus secrete toxins that can result in food poisoning. (read here) 

Veterinarians treat many non-life-threatening dog and cat health issues with antibiotics. Staphylococci have the unfortunate ability to rapidly become immune to these antibiotics. One result is the emergence of antibiotic-resistant staphylococcus bacteria strains that have become immune to almost every available antibiotic. (read here)  They are called MRSA staph. When your dog or your cat suffered from repeated skin or ear infections and antibiotics are repeatedly dispensed, the possibility exists that your pet will eventually introduce MRSA or MRSP drug-resistant staphylococcus bacteria into your household and you. (read here  &  here) That is one reason why I often suggest canine ear surgery and anal sac surgery to cure chronic ear and rear infections once and for all, rather than repeated, shortly lived improvements provided by antibiotic treatment. 

Plague 

This infection, caused by a bacteria, Yersinia pestis, occurs naturally in a number of wild rodents including prairie dogs in well-defined areas throughout the world. Why and when this bacteria becomes common and responsible for the scourge of the Middle Ages, bubonic plague is still unclear. (read here) The disease is transmitted from rodent to rodent and rodent to man through the bite of a flea. Occasionally, a domestic cat will become infected with this disease through the capture of prey rodents. These cats have the potential to pass the disease on to their owners. (read here) The common cat flea, does not appear to be a good spreader of this plague bacteria. So, you are unlikely to catch plague from the bite of a cat flea. But well-fed outdoor-roaming cats have a tendency to bring home the wild rodents they catch alive and present them to their owners. Any fleas on those rodents are certainly a threat to you when you live in areas of the world in which plague is found. That is another good reason not to allow your cat to roam. Another very serious virus that these wild rodents can carry into your house is hantavirus. (read here)

Cat Scratch Fever (Bartonellosis)

This infection, caused by Bartonella henselae, Cat Scratch Fever, is commonly acquired by people from an asymptomatic (clinically normal) cat. Various studies over the years have reported that 20 – 45% of free roaming North American cats carry the disease organism. Worldwide, the statistics are similar. (read here) Besides claw scratches and bites, cat fleas can transmit the disease. Ticks feeding on cats also carry the bartonella organism. (read here)  People infected with the disease often experience fever, malaise and enlarged painful lymph nodes as well as a local inflammation at the site of the wound. A good reason to use monthly flea/tick control products on your cat. (read here & here)

TICK BORNE DISEASES

Lyme Disease

First seen in Lyme, Connecticut, Lyme disease is an illness caused by a bacteria, Borrelia burgdorferi. This organism is in a group of bacteria called spirochetes due to their spring-like shape. Lyme disease commonly affects dogs. The most common way and humans become infected is through the bite of a woodland tick – not a tick that previously fed on a domestic dog. But ticks on Lyme disease carrier dogs are thought by some to also pose a threat to their human owners. (read here)  In people, Lyme disease can cause a wide variety of signs including rash, painful, swollen joints, fever, enlarged tender lymph nodes and a variety of neurological signs. Over the last few years a number of products have come onto the market that are quite good at keeping ticks off of your pets. Three of these products are Frontline® spray, Revolution® and Preventic® tick collars, but there are many more. Read about some newer effective flea/tick control products here. Dogs can also be vaccinated against Lyme disease. The vaccine does not prevent infection but the manufactures claim it reduces the severity of the disease if they catch it. Read more about Lyme disease here 

FUNGI

Ringworm

Ringworm is not a worm and is not always ring-shaped. Read more about ringworm in pets here. Ringworm is actually 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 an infected pet. If these spores come in contact with abraded human skin, the fungal infection can be transferred to that person.

SKIN PARASITES

Sarcoptic Mange Or Scabies

Sarcoptic mange mites are common on many species of domestic and wild animals. Mange mites are not particular as to the species of animal they attack. One of the dog and cat mange mites is Sarcoptes scabei. When your pet first becomes infected, it is most common on their ears, face and extremities. Cats have their own mange mite, notoedric mites. With time the entire body can become affected. These mites are passed from animal to animal, animal to human and human to human by direct contact. They do not survive long off a host. Close contact is required. 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®) and a number of other products that kill fleas and ticks.

 Read more about sarcoptic mange here

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