The transmissibility of tuberculous infection to other than natural hosts constitutes one of the important problems in the control of T.B.
The ability of the respective pathogenic species of mycobacterium tuberculosis to infect naturally one or more heterologous hosts makes tuberculosis in any species of animals a potential threat to others, including human beings.
Dr. Provet will address How can T.B be transmitted from animals to man in detail through this article, including:
Cross infection of T.B species.
Epidemiology and infectious agent of T.B in man.
Risk factors of T.B in man and animals.
Factors affecting the frequency of bovine tuberculosis in man.
Sources and reservoirs of infection of T.B in man.
HOW CAN T.B transmitted from animals to man?
Susceptibility of infection of T.B in man.
The incubation period of T.B in man.
Cross infection of T.B species
The fact that Mycobacterium bovis is capable of infecting a diversity of species including human beings emphasizes the necessity of a comprehensive plan if tuberculosis is to be successfully controlled.
Likewise, the human type (Mycobacterium Tuberculosis) is capable of inducing tuberculosis in cattle, swine, dogs, and parrots.
The organism of avian tuberculosis, in addition to being pathogenic for most birds, is capable of causing widespread destruction of tuberculosis in swine and sheep.
These facts make it evident that eliminating tuberculosis in one species and ignoring the disease in others is not likely to solve the larger problem.
because unless tuberculosis is eliminated in all species, the disease can exist, heterologous hosts, in which the presence of tubercle bacilli constitutes a potential source of a new infection, perhaps for natural hosts.
Epidemiology and infectious agent of T.B in man.
Tuberculosis is passing into history as a major zoonotic disease in man.
The disease in cattle presented(constituted) a major source of human infection in many countries until the milk was pasteurized and regulatory control measures are taken to eradicate the disease among these animals.
These measures have proved so highly effective that many countries are now entirely free from the disease.
Tuberculosis in man is mainly caused by human tubercle bacilli (Mycobacterium tuberculosis).
However, many cases of human infection do occur in which the bovine strain (Mycobacterium bovis) is the causal agent.
On the other hand, avian tubercle bacilli (Mycobacterium avium) rarely cause human infection; although many authors proved the presence of avian infection in man.
Risk factors of T.B in man and animals
Tuberculosis in man may be either chronic, subacute, or very acute disease which most commonly affects the respiratory tract (Usually the lungs).
However, the infection may involve parts of other systems, such as the lymphatic, osseous, urogenital, nervous, and gastrointestinal tract.
The disease is characterized by a destructive process and the replacement of normal tissues with tubercles, which may produce both local and constitutional reactions.
In animals, Tuberculosis affects all species of animals, but cattle, poultry, and pigs are most frequently affected.
Dogs and cats are less common while horses and sheep are very rarely affected.
Factors affecting the frequency of bovine tuberculosis in man
The frequency of the bovine type of tuberculous infection in human beings is dependent upon:
The incidence of the disease in cattle.
The degree of consumption of contaminated food (food of tuberculous animal origin).
Sources and reservoirs of infection of T.B:
It may be one or a combination of the following:
Raw or improperly pasteurized milk and milk products from tuberculous cows.
Meat and meat products from infected animals.
Contaminated air (droplet and dust infections) expired from the lungs of either tuberculous animals.
Human patients affected with Mycobacterium bovis.
All tuberculous animals with the bovine strain( cattle, dogs, cats, and pigs) may act as reservoirs of infection to man.
Certain species of wild animals (badgers) were found on P.M examination to be infected with a bovine strain of mycobacteria.
The organism was also found in badger feces.
The incidence of T.B in cattle in different countries varies greatly depending on :
Type of animal husbandry practiced.
The commercial exchange of cattle from one country to the other without proper regulatory supervision.
The presence or absence of an effective veterinary program of eradication and control.
Mode of transmission of T.B to man
T.B can be transmitted to man through:
ingestion of raw or improperly pasteurized milk or its products(butter, cream, or cheese), or improperly cooked meat or meat products containing M.bovis.
This occurs mostly in children because:
The main food for children is milk or its products.
They frequently put contaminated objects in their mouths.
Inhalation of air contaminated with the discharges of tuberculous animals. or human beings infected with Mycobacterium bovis (droplet or dust infection).
Through the skin: Direct inoculation of the skin with Mycobacterium bovis inspectors, occurs mostly in butchers and meat whose duties require the exposure of unprotected skin of the arms and hands to tuberculous carcasses.
Transmission of Avian T.B to man through ingestion of:
Eggs of tuberculous hens( in experimentally infected hens Mycobacterium avium could be isolated from their eggs after 11 days from the infection).
Organs from tuberculous hens, rabbits, and pigs(esp. the liver).
Milk of cows and goats infected with Mycobacterium avium, or contaminated with the droppings of tuberculous hens.
Vegetables used as a green salad are contaminated with the droppings of tuberculous hens which are usually used as fertilizers.
Dust contamination with the droppings of infected hens.
Reservoirs of avian tuberculosis infection are tuberculous birds, rabbits, and other animals infected with M.avium especially pigs.
Susceptibility of infection of T.B in man
Many factors affect the susceptibility of T.B in man, including:
Age
Young infants are quite susceptible and before 2 years of age, an acute form of the disease may result.
Between 5-15 years of age, there are relatively fewer cases and deaths.
But after 15 years, both cases and deaths increase with advancing age.
The largest number of deaths occur between people between 20-40 years of age. The mortality rate is highest among older people.
Sex
Females succumb about twice as frequently as the age of the group 15-30 years.
Race
certain races seem to be more susceptible than others.
Negros and Indians were found to be more susceptible than the white population; probably because of their mode of life.
Economic state
Generally, tuberculosis is a disease of poverty conditions that favor overcrowding and undernutrition.
So, they have poor body resistance to infection, and consequently, large numbers of tubercle bacilli are readily transmitted.
Occupation
All trades and professions in which persons are exposed to the inhalation of particles of metallic or dust containing silica have higher susceptibility than others.
Nurses, medical students, veterinarians, laboratory technicians, and physicians are all exposed to a greater risk of infection than the general population. mode al molest
Diet or, any faulty constitutional state may lower the resistance to tuberculosis.
Such as the metabolic disorders diabetes, prolonged state of fatigue, or overexertion.
The strain of pregnancy may predispose to more cases of tuberculosis than in women who were not pregnant.
Period of communicability:
The period of communicability remains or extends as long as tubercle bacilli are being discharged by the affected host.
It begins when a lesion becomes open and continues until healed or death occurs(some patients remain sputum positive intermittently for several years.
The period of communicability may extend up to several years.
The degree of communicability depends upon:
The number of bacilli discharged during coughing.
The hygienic practice of the patients. Antimicrobial therapy commonly shortens the period of communicability.
The incubation period of T.B in man
From infection to the formation of demonstrable lesions it is about 4-6 weeks.
From infection to the production of primary progressive tuberculosis may extend for many years.
From the incubation period to the clinical signs of T.B in man and how to prevent this disease, that's what Dr.provet will address in the next article, just follow dr. provet.
Written by: Dr. Mai Foda
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