Nontuberculous Mycobacteria

Mycobacterium tuberculosis

Humans are the only reservoir hosts for M. tuberculosis. Dogs and cats are susceptible to infections with M. tuberculosis and M. bovis. Cats are naturally much more resistant to M. tuberculosis than they are to M. bovis. Canine and feline infections with M. tuberculosis are considered an anthropozoonosis—that is, the direction of transmission is from people to animal (Fig. 48-1). Although pets acquire the infection from people, spread back from dogs or cats to people has not been reported. Dogs have had a higher prevalence of infection with M. tuberculosis than have cats. Dogs with tuberculous pneumonitis discharge organisms in the sputum as do infected people. Aerosolized droplets are the primary means of transmission of this disease. Airborne droplet nuclei from respiratory secretions fall to the ground where they temporarily remain viable but stationary and thus relatively noninfectious for other people and pets. Only small (3 to 5 µm) diameter particles can successfully bypass upper respiratory clearance mechanisms and deposit in alveoli. Discharges that are not airborne may potentially be infectious to dogs and cats exposed through close contact. In general, tubercle bacilli are not as transmittable as other bacterial pathogens because prolonged, frequent exposure or large inocula are usually required. Because of measures imposed to control infection in people, the overall prevalence of human and animal M. tuberculosis infections had been decreasing in developed countries. Relative increases have occurred in densely populated urban areas and in economically depressed areas. The interrelated factors of homelessness, illicit drug use, and human immunodeficiency virus (HIV) infection have caused unanticipated increases in its prevalence. Multidrug-resistant tuberculosis has emerged in these affected populations because of irregular compliance with drug therapy. Pets in such environments will have an increased risk of becoming infected. The incidence of M. tuberculosis infections in the United States is highest in Atlantic coast metropolitan regions and in the southeastern regions.

M. tuberculosis infection has become an important emerging anthropozoonosis in free-ranging wildlife. Initially the infection was identified only in wildlife or captive zoo animals such as elephants that had close and prolonged contact with people.219,232 Infections caused by M. tuberculosis have been increasing, especially in underdeveloped countries, as a result of the acquired immunodeficiency syndrome (AIDS) epidemic. In Africa, expansion of ecotourism and changing land use have increased contact between infected people and free-living animals. Wild animals living in close proximity to human habitations and refuse sites have become infected. Certain wildlife populations have become endemically infected, such as the banded mongoose (Mungos mungo) in Botswana and suricates (Suricata suricatta) in South Africa.⁸

Mycobacterium bovis

This organism has a wide host range, including many animals and people, and a worldwide geographic distribution.¹²⁶ In most industrialized countries, bovine tuberculosis has been controlled with surveillance, testing and slaughter, and pasteurization of dairy products. In developing countries, tuberculosis caused by M. bovis has become widespread and a potential risk for exposed people and animals.⁶³ With respect to M. bovis infections, the gastrointestinal (GI) tract is the most common portal of entry. Cats and dogs can be potential disseminators of disease when the organism preferentially localizes in the intestinal or respiratory tracts. Because of localization of infection, cats usually excrete the organism via feces and dogs via sputum. M. bovis does not persist long in the environment, and reservoir hosts are essential for survival of the organism. Outside its hosts, the organism survives for a period varying from 4 days in summer to less than 28 days in winter.¹⁵⁵ However, organisms may persist for several months in organic material such as feces or carcasses.^273,297,297 Dogs and cats acquire infection when they consume contaminated milk or meat. Dogs and cats may be involved in the maintenance of bovine tuberculosis on farms where it is enzootic and may be rarely responsible for transmission of the bovine bacillus to people (see Fig. 48-1).³²⁶ Subclinically, infected dogs and cats sometimes remain on farms after reactor cattle have been identified and removed from the herd, and farms or families with recurrent tuberculous infections should have their pet animal contacts checked periodically. Rarely, spread of M. bovis between people has been reported.

Cats are more commonly infected with the bovine bacillus than dogs are, and on an experimental basis cats appear to be more susceptible to bovine than to human tubercle bacilli. Part of this affinity is related to the frequent ingestion of contaminated, unpasteurized milk or uncooked meat or offal from infected cattle, or from eating M. bovis–infected wild rodents. Milk is an ideal medium for the organism because it buffers the gastric acid that normally prevents colonization of the lower GI tract with tubercle bacilli. As a result of eradication measures, the prevalence of bovine tuberculosis is low in the United States. In Argentina, domestic cats are often fed raw bovine lung, which may be an important source of their infection with M. bovis.³³⁸ Increased use of commercial pet foods and trends to urban living have reduced the prevalence of bovine tuberculosis in dogs and cats.

Despite the reduction of infection in domestic animals, bovine tuberculosis remains a problem worldwide, except in Australia, because it has become established in wildlife hosts that have acquired infection from cattle, and wildlife act as continued reservoirs for infection. As a result, domestic animals such as cattle, cats, and dogs continue to become infected.78,79,164,259,314 For example, in the United States, white-tailed deer (Odocoileus virginianus) in Michigan are infected and provide a reservoir of infection for cattle and other domestic animals including the dog and cat.^164,326 In Great Britain and Ireland, badgers (Meles meles) have become a major wildlife reservoir.⁷⁵ Although transmission between badgers is predominantly by aerosols, M. bovis has also been found in their bite wounds, suggesting that cutaneous inoculation occurs. After an outbreak in cattle from Cornwall, United Kingdom, infection in domestic cats on the farm and local badgers all shared the same spoligotype.²²⁹ Interestingly, the fact that no cattle had been recently kept on the premises suggested that local badgers had somehow infected the resident cat population. Because cats and badgers are unlikely to exhibit the intimate interaction necessary for cross-infection, cats are most likely to be infected by eating secondarily infected small wild mammals, which are commonly found in endemic areas.^75,77 In New Zealand, feral brushtail possums (Trichosurus vulpecula) act as reservoir hosts; wild ferrets, pigs, cats, introduced deer species, and goats serve as amplifier hosts when their carcasses infect carrion-eating animals. Amplifier hosts can become infected from carnivorous ingestion. Sheep, horses, and hedgehogs are spillover hosts that are incidentally infected.⁶¹ Stoats, dogs, hares, rabbits, and many small rodents are occasional dead-end hosts.

Captive and free-living exotic wildlife carnivores, including nondomestic canids and felids, and domestic and feral dogs and cats have been most commonly infected with M. bovis. Ingestion of infected carrion is the most common source of infection. The infection is maintained in gregarious herbivores, while carnivorous predators such as lions (Panthera leo), cheetahs (Acinonyx jubatus), leopards (Panthera pardus), spotted hyenas (Crocuta crocuta), Iberian lynx (Lynx pardina), European lynx (Lynx lynx), Siberian tigers (Panthera tigris longipilis), snow leopards (Uncia uncia), and feral domestic cats have become secondarily infected.* Additional subsequent transmission of infection via aerosol or biting between gregarious carnivores, such as lions, has been suspected.¹⁶⁹

Mycobacterium microti

Infection with M. microti has been seen most frequently in Great Britain, predominantly in rural cats with avid hunting behavior. The source of infection is believed to be prey species.31,122,124,148 The disease is a spillover from infection of wildlife. Some previous reports of this infection may have been mistaken for M. bovis infections.¹²⁴ However, the areas where cats become infected with M. microti and M. bovis are geographically distinct.²⁸⁴ Furthermore the genotypes of M. microti are geographically clustered within the endemic areas. M. microti infection has also been documented in a dog with peritonitis.⁷³ Infection from rodents and South American camelids is considered the source of infection for cats and people. Pulmonary infection was found in the people who had no underlying evidence of immunodeficiency.

Nontuberculous Mycobacteria

Nontuberculous Infections

Infection with MAC organisms usually begins with ingestion of the organism from the environment or contaminated food or uncooked infected viscera. MAC organisms in dogs and cats have often been disseminated throughout lymphoid and many other tissues, without indications of a primary granuloma at the site of entry. These organisms are closely related to M. avium subspecies paratuberculosis, the cause of Johne’s disease, which is a chronic granulomatous enteritis of ruminants and other herbivores. Animals with Johne’s disease may have acquired the infection as neonates, through eating contaminated food or exposure to a contaminated environment. The mycobacteria may be phagocytized by intestinal macrophages, at which time the infection becomes quiescent. Eventually, with stress or acquired or inborn immunosuppression, the organisms replicate and the disease occurs. Similar events are suspected to occur in MAC infections of the predisposed dog or cat breeds, which develop disseminated infections within the first few years of life, likely as the result of unrecognized defects in CMI. Basset hounds and miniature schnauzer dogs, and Siamese and Abyssinian cats, are overrepresented in reports of MAC infection. Siamese cats are similarly predisposed to infections with other persistent intracellular organisms, and Abyssinian cats have immune aberrations resulting in a high prevalence of reactive amyloidosis. Although defects in the immune system of these breeds are likely, the precise nature of these defects have not been completely determined (see Chapter 94). No clear association has been made between feline retroviral infection and mycobacteriosis. In some animals with severe immunosuppression, bacteremia may result and multiple organ dissemination ensues.