Zoonotic Importance of Bovine Tuberculosis in Ethiopia: An Overview

INTRODUCTION

The transmission of infectious zoonotic diseases between animals and humans is generally facilitated by close contact between them.^[1] Mycobacterium bovis causes bovine tuberculosis (BTB), a zoonotic disease that affects a wide range of animals and humans.^[2] Tuberculosis in cattle has become a major infectious disease that has spread between species. BTB is found all over the world, and it has a considerable economic impact on the livestock production sector.^[3,4] It has been identified as the most common cause of human zoonotic tuberculosis.^[5]

Human tuberculosis caused by M. bovis has decreased significantly in developed countries as a result of pasteurization of milk and tuberculin skin testing of cattle, followed by culling/ slaughtering of infected cattle.^[6] However, it still poses a public health concern to both humans and animals in developing countries, notably in Africa. This is because 82% of the human population and 85% of the livestock population live in areas, where BTB is widespread.^[7] According to studies, BTB is still prevalent in underdeveloped countries, where routine milk pasteurization is not practiced, and M. bovis is responsible for 10–15% of human tuberculosis cases.^[8,9]

Despite its zoonotic potential, research into M. bovis in both animal and human populations remains minimal in underdeveloped countries, notably in Africa. Several factors, such as social unrest, political uncertainty, the associated cost of testing programs, wars that result in human and animal displacement, and a scarcity of skilled expertise in the field, all had a significant impact on the disease investigation and public health implications.^[10] BTB is highly widespread disease in Ethiopia’s cattle population among African countries. According to study, results in Ethiopia utilizing intradermal tuberculin tests, the prevalence of BTB varies between 0.8% and 78% in free-range rural farming systems with mostly Zebu cattle and intensive systems with exotic and crossbreed cattle, respectively.^[11,12]

Few studies have also shown that because the disease is zoonotic, it can spread from animal to human and vice versa.^[13,14] Many other researchers have shown many potential risk factors contributing to the spread and persistence of BTB in poor countries. Eating habits, educational status, demographics, living style, socioeconomic status, culture, the presence of other immunosuppressive disorders, and sharing a home with animals are just a few of these factors.^[4,13,15] In Ethiopian communities, unpasteurized raw milk is preferably consumed than boiled milk due to its accessibility, convenience, good taste, and reduced cost. The public health concern posed by BTB is mostly linked to the consumption of M. bovis-infected unpasteurized dairy products. Closed human-animal contact is also considered a potential risk factor for the disease.^[16,17]

The distribution of BTB in livestock in Ethiopia is not well known. However, in some of areas, there is a scarcity of information on the current status of the disease. Thus, the objective of this communication is to present an overview on the BTB in Ethiopia.

ETIOLOGY

M. bovis is a facultative intracellular pathogen that can live and grow inside macrophages and other mammalian cells. It is acid-fast, aerobic, non-spore-forming, slow-ogrowing, and non-motile. It is a member of the family Mycobacteriaceae genus Mycobacterium. All pathogenic Mycobacterium, except Mycobacterium avium, are members of the same genetic species, the Mycobacterium tuberculosis complex, according to the phylogenetic analysis of their genomes.^[18]

M. bovis is shown to survive best in the frozen tissue, whereas tissue preservatives, such as sodium tetraborate, have negative impacts on its viability. M. bovis may persist in the environment for several months, especially in cold, dark, and wet situations. The survival period varies from 18 to 332 days at temperatures ranging from 12°C to 24°C, depending on sunlight exposure. From soil or grazing pasture, there is infrequent isolation of this bacterium. It has been discovered that artificially kept samples of the bacterium can be cultured for about two years. The organism’s viability is between fours and eight weeks in 80% shade, whereas it can be killed in either summer or winter.^[19]

HOST AFFECTED

The most common mycobacterial disease, M. bovis, affects a variety of vertebrate animals of all ages, including humans. Cattle, goats, and pigs are the most vulnerable to infection.^[20] In South Africa and Zambia, the African buffalo (Syncerus caffer) and the lechwe antelope (Kobus leche) have both been identified as important BTB wildlife reservoirs. African buffalo is extremely social herding animals that live in groups of several hundred. They graze on grasses or leaves or wallow in mud for most of the day. Each herd has a home range across, in which they travel throughout the year in a clumsy way.^[21]

The lechwe, like buffalo, is ruminants who prefer to graze in swampy areas. In reaction to environmental conditions, they also follow local seasonal migratory patterns.^[22] BTB affected Lechwe live in the Kafue Basin, which is made up of flood plains surrounding the river Kafue in central Zambia.^[23]

EPIDEMIOLOGY

Tuberculosis is an infectious bacterial disease that affects a variety of mammals, including humans, cattle, deer, llamas, and other wild animals. Humans of varying ages, as well as other species, are vulnerable to infection. The disease is present throughout the world, but it is more common in Africa, Asia, and America. Tropical and subtropical countries have a high prevalence of disease.^[24] Based on comparative tuberculin skin tests in different parts of Ethiopia, the prevalence of BTB in smallholder dairy farms was 6.9%, 12.9% in traditionally managed wide production systems, and 24.6% in intensive dairy farms.^[25]

TRANSMISSION

In Ethiopia, animals live in the same home along with their owners, and floors and walls are painted with cow dung, which serves as the source of transmission. The probability of tuberculosis spreading to humans is increased by each of these practices.^[26] Recently, there have been increasing reports of human cases due to M. bovis, especially in patients with HIV/AIDS.^[20]

When cattle stocking density is high, transmission is most likely. M. bovis can be isolated from the nasal, tracheal mucus, and the lungs of most reactors with respiratory lymph node lesions. The infected cattle having the capacity to spread the disease by aerosol also. When possums transmit the disease to deer, it is predicted that M. bovis can travel up to 1.5 meters in aerosols created by them.^[27]

PATHOGENESIS

When bacterium enters a herd of cattle through aerosolized droplets or ingestion, it becomes entrenched. The incubation period might last months or years, depending on the severity of the infection, and the immune system of the individual animal. Typically, bacteria enter a cow’s respiratory system and settle in the lungs. Lung macrophages are then capable of phagocytizing the organism. After being taken up by macrophages, the bacterium replicates intracellularly as the body attempts to wall off infected macrophages with fibrous tissue, granulomas or tubercles formed. The granulomas are 1–3 cm in diameter, yellow or gray in color, spherical, and firm. The granulomas core is made up of dried yellow, caseous, or necrotic cellular debris in a cut portion. The infection can spread hematogenously to lymph nodes and other parts of the body, resulting in smaller tubercles of 2–3 mm in diameter. “Military tuberculosis” refers to the formation of these smaller tubercles. The necrotic cells at the tubercle’s center are surrounded by epitheloid cells and multinucleated giant cells, all of which are encased in collagenous connective tissue. As the tubercle matures, the necrotic core of cells can become calcified.^[28]

CLINICAL SPECTRUM

CONCLUSION AND RECOMMENDATIONS

The transmission of tuberculosis from animals to humans in underdeveloped nations, primarily through infected milk, is a severe and under-appreciated mode of transmission. Antimicrobial treatment has been undertaken, but because long-term treatment is necessary, anti-tubercular medication is not a wise option in eradication-aware countries. The development of BTB diagnostics and vaccinations for cattle has made significant progress along with the test and slaughter policy. In human medicine, the BCG vaccine provides better protection. The following are some recommendations based on the aforesaid conclusion:

• Calves being raised as herd replacements must be fed tuberculosis-free milk, either from known tuberculosis-free animals or pasteurized milk

• Because tuberculosis is a zoonotic disease, it should be vaccinated for both people and animals

• Animals that have been imported from another state or country should be quarantined

• Pasteurization of milk before consumption is imperative.