Dogs are in close interaction with human and they are the most popular companion animals kept worldwide. The large number of dogs, the fact that they are co-habitating with people and can also be in contact with wild animals makes them potential risk factors, as the source and transmitter of zoonotic diseases, like rabies or rotavirus.
Zoonotic diseases now account for approximately 58% of all human pathogens. In the United States, the top five neglected parasitic infections include four zoonotic diseases, namely Chagas disease (infection with Trypanosoma brucei protozoa), cysticercosis (infection with tapeworm Taenia solium), toxocariasis (infection with immature roundworms), and (infection with Toxoplasma gondii protozoa). All of these parasites infect domestic dogs and cats.
A zoonosis (pl. zoonoses) is a disease occurring primarily in animals (usually mammals), but that can occasionally be transmitted to humans. Many zoonoses are caused by viruses. Some are extremely dangerous and even fatal, whereas others can resolve with little or no therapy. One of the most dangerous zoonosis in the world is rabies, a viral disease causing acute infection of the central nervous system. Five general stages are recognized in humans: incubation, prodrome, acute neurologic period, coma, and death. Rabies occurs in nearly all countries. The disease in humans is almost always due to a bite by an infected mammal. Nonbite exposures (e.g., mucosal contact) rarely cause rabies in humans. Early diagnosis is difficult. Rabies should be suspected in human cases of unexplained viral encephalitis (inflammation of the brain) with a history of animal bite. Unvaccinated persons are often negative for virus-neutralizing antibodies until late in the course of disease. Virus isolation from saliva, positive immunofluorescent skin biopsies or virus neutralizing antibody from cerebrospinal fluid, or serum of a non-vaccinated patient establish a diagnosis.
Lymphocytic choriomeningitis is a flu-like disease caused by lymphocytic choriomeningitis virus that can be transmitted through bites. Hidden infection has been found both in dogs and humans. The disease has two stages. In the first stage, headaches, sneezing, bronchitis, and fever are generally seen. After a short recovery, central nervous system signs may appear, which include stiffness of the neck, severe headaches, nausea, and confusion. Meningencephalitis or encephalomyelitis may follow with or without paralysis. Recovery make take weeks. Nipah virus is a newly recognized zoonotic virus that was discovered in 1999. It has caused disease in animals and in humans and is transmitted through contact with infectious animals. The virus is named after the location where it was first detected in Malaysia. It is believed that certain species of fruit bats are the natural hosts of Nipah virus. The onset of the infection is usually with "influenza-like" symptoms, with high fever and muscle pains. The disease may progress to encephalitis with drowsiness, disorientation, convulsions and coma. Fifty percent of clinically apparent cases die.
Dogs also harbor kobuviruses, close relatives of human Aichi virus (causes gastroenteritis in humans) and Sapoviruses (cause diarrhea in humans).
Another group of zoonoses is caused by bacteria. Rickettsiae are small microorganisms that have evolved in such close association with arthropod hosts that they are adapted to survive within their host cells. Rickettsioses are zoonoses that, except for Q fever, are usually transmitted to humans by arthropods, such as tick, mite, flea, louse, and chigger. Therefore, the geographic distribution of rickettsial diseases is determined by that of the infected arthropod, which for most rickettsial species is the reservoir host. Rickettsiae are important causes of human diseases in the United States (Rocky Mountain spotted fever, Q fever, murine typhus, sylvatic typhus, human monocytic ehrlichiosis, human granulocytic ehrlichiosis, and rickettsial pox) and around the world (Q fever, murine typhus, scrub typhus, epidemic typhus, boutonneuse fever, and other spotted fevers). The tick-borne relapsing fevers and Lyme disease are zoonoses with rodents as the major reservoir. Their incidence and distribution depend mainly on the biology of the tick vectors. Areas known to harbor infected ticks and lice should be avoided. No vaccines are available. Tetracycline is an effective treatment.
A number of bacterial zoonoses are transmitted to people by dogs. Bordetellosis is caused by Bordetella bronchiseptica bacterium. B. bronchiseptica is a frequent cause of bronchopneumonia in animals that may generate infectious aerosols. Fever, abdominal pain, watery or bloody diarrhea, occasionally chronic colitis, and arthritis are seen. The disease can be transmitted from infected animals to humans by infected droplets. In dogs the bacterium causes kennel cough (tracheobronchitis), whereas the human illness can range in severity from mild respiratory symptoms to pneumonia; carrier state may exist.
Brucellosis is a true zoonosis in that virtually all human infections are acquired from animals. It is caused by Brucella bacterium. Human brucellosis is either an acute febrile disease or a persistent disease with a wide variety of symptoms. In the simplest case, the onset is influenza-like illness with fever reaching 101 to 103 degrees F. Limb and back pains are unusually severe, however, and sweating and fatigue are common symptoms. If the disease is not treated, the symptoms may continue for 2 to 4 weeks. True relapses may occur months after the initial episode, even after apparently successful treatment.
Campylobacter enteritis is a diarrheal disease caused by Campylobacter bacteria. Dogs frequently become carriers of this organism and show no signs of the disease. Dogs recently adopted from shelters often serve as sources of human infection. Intermittent diarrhea may persists for months.
Leptospirosis is a potentially fatal bacterial disease that damages the liver and kidneys of dogs and humans. It is caused by Leptospira bacteria. It occurs worldwide and affects humans and animals. The bacteria are spread through the infected urine and can survive in water and soil for weeks to months. Humans and animals can become infected through contact with contaminated body fluids (except saliva), water, or soil. Get your pet vaccinated against leptospirosis.
Disseminated infections with Pasteurella multocida can lead to septic shock, meningitis, endocarditis, and other severe illnesses. Most human infections with Pasteurella multocida occur as localized abscesses of the extremities or face as a result of cat or dog bites. These abscesses require surgical drainage.
Salmonellosis occurs as (1) gastroenteritis, (2) septicemia, or (3) enteric fever. The severity of the infection and whether it remains localized in the intestine or disseminates to the bloodstream may depend on the effectiveness of the patient's immune system and the virulence of the Salmonella bacteria. Signs may include fever, loss of appetite, headache, muscle pain, and constipation. Gastroenteritis (food poisoning) depends on the number of bacteria. Symptoms usually begin 6 to 48 hours after ingestion of contaminated food or water and usually take the form of nausea, vomiting, diarrhea, and abdominal pain. Enteric fevers are severe infections and may be fatal, if antibiotics are not promptly administered.
Dogs have been found to be carriers of Staphylococcus pseudintermedius (MRSP) and Staphylococcus aureus (MRSA), common multi-drug resistant (MDR) bacteria in dogs. The bacterium can cause infections in dogs and is also capable of causing infections in humans. Methicillin resistant S. aureus (MRSA), is a common hospital pathogen of humans.6
Capnocytophaga canimorsus bacteria is a bacterium that lives as in the dog mouth and causes rare but severe infections in humans who are in contact with dogs with mortality in the range of 50%.7
Dogs can be infected and transmit protozoan parasitic diseases. Chagas disease (American trypanosomiasis) is caused by the Trypanosoma cruzi parasite. The disease is transmitted by cone-nosed bugs and may also be transmitted congenitally and by blood transfusion. Dogs are reservoirs of the infective parasite. Symptoms of acute disease may include fever, inflammation of lymph nodes, increased heart rate, heart enlargement, and myocarditis. Megaesophagus may develop in some cases. Giardiasis is a long-lasting, intestinal protozoal infection caused by species of Giardia. The infection can cause a variety of intestinal symptoms, which include diarrhea, gas or flatulence, greasy stools that tend to float, stomach cramps, upset stomach, and nausea. Leishmaniasis is a disease that is transmitted by sandflies and caused by intracellular protozoa of the genus Leishmania. Dogs act as parasite reservoir for humans. Human infection is caused by about 21 of 30 species that infect mammals, including dogs. There is no vaccine for this disease, which can rapidly lead to death if no treatment is given.
Domestic and wild animals are important reservoirs for parasitic worms. Coenurus cerebralis infection has been observed as a common and worldwide problem of small ruminants. Dogs being definitive host of Taenia multiceps play an important role in spreading the disease. Occurrence of coenurosis cysts in brain, spinal cord and in other tissues have been noticed in a wide range of animals such as sheep, goats, cattle, horse, buffalo, camel and yak. Diphyllobothriasis (fish tapeworm infection) is a disease caused by the cestode worm Diphyllobothrium latum, the largest cestode worm of humans measuring 5 to 10 m in length. Signs may include megaloblastic anemia as the parasite competes with the host for B group vitamins, and neurologic disorders. The adult parasite can survive in its host for 35 years. Echinococcosis is a zoonotic infection caused by the larval stage of the tapeworm Echinococcus. Two of the recognised species, E. granulosus and E. multilocularis, are of importance for humans. Infection is acquired through ingestion of parasite eggs released in feces of the host (a carnivore or a rodent) bearing the adult worm in its gut. Dirofilaria immitis (the dog heartworm) is a worldwide filarial parasite of dogs. Adult worms (up to 30 cm long) usually are located in the dog's heart. In heavy infections, or when adult worms die, the parasites may be carried to the pulmonary vessels where they may produce clogs.
Gnathostoma spinigerum is a nematode that resides in the stomach wall of dogs and cats. Most human infections occur in Thailand and other Asian countries. Infective larvae develop in copepods and are transferred through the food chain. Human infection results from consumption of improperly cooked fish or other food containing infective larvae. The larvae migrate in the tissues and may invade the eyes, brain, and other organs. The larvae may cause eosinophilic meningitis. The immature worm may be recovered from subcutaneous nodules. Cutaneous larva migrans (creeping eruption) is a dermatitis caused by the larvae of Ancylostoma braziliense, the dog and cat hookworm, which penetrate human skin and migrate to the underlying tissue. Ancylostoma caninum and other species of hookworms also can cause this infection. Strongyloidiasis is caused by Strongyloides stercoralis worm. Signs include pneumonitis, epigastric pain, and diarrhea. In immunocompromised individuals worms may spread to other organs and cause autoinfection. Infection can range from asymptomatic to multiorgan failure. The mortality rate for patients requiring hospitalization with Strongyloides infection is 16 percent. In disseminated strongyloidiasis, the mortality rate can be as high as 70-90 percent. Infection may most frequently initially occur in childhood, as children are most likely to play outdoors in contaminated soil. Pentastomid infection is caused by Linguatula serrata (tongue worms). It is transmitted through eggs in feces or nasal discharge from infected dogs. Signs of infection in humans include irritation of throat and nose, vomiting, tearing, difficulty breathing, and headache. Signs in dogs are excessive nasal discharge and sneezing.
Other zoonotic helminths which are transmitted from dogs to humans and affect the human eye include:
- Baylisascaris procyonis infection is caused by a roundworm found in raccoons. This roundworm can infect people as well as a variety of other animals, including dogs. Undetected Baylisascaris species infections in dogs is particularly troubling because larva of some Baylisascaris species migrate in the central nervous system and can cause severe neurologic disease. A number of cases of severe Baylisascaris species-induced neurologic disease and death have been reported in children who consumed eggs containing infective larva from a fecal-contaminated environment.
- Dirofilaria repens is a common parasite of dogs and other canids that can infect conjunctiva. Acanthocheilonema reconditum, a subcutaneous filarial infection in dogs worldwide, may infect human eyes.
- Coenurus cerebralis (the larval stage of the tapeworm Taenia multiceps) which develops in the small intestine of dogs, foxes, and other canids, causes severe anterior uveitis, orbital cystic tumor-like masses, and subretinal damage.
- There is an increasing number of reports of zoonotic Onchocerca infections, and several of these have been either within the eye or associated with the conjunctiva or connective tissue of the orbit. Onchocerca lupi affects dogs and it induces acute or chronic ocular disease characterized by conjunctivitis, photophobia, lacrimation, ocular discharge and marked protrusion of the eyeballs (exophthalmia).4
Halitosis or bad breath is a common finding in dogs of different breeds and ages. Bacteria and yeast that cause halitosis can be transmitted from animal companions to humans. More than 80% of patients belonging to the halitosis group reported having pets in childhood and over 70% still owned a pet. Bacteria and yeast residing in dog mouth can be transmitted from dogs to humans and cause periodontal disease.