• Bacterial Zoonotic Diseases
• Fungal Zoonotic Diseases
• Rickettsial Zoonotic Diseases
• Parasitic Zoonotic Diseases
• Viral Zoonotic Diseases

Viral Zoonotic Diseases
75% of emerging infectious diseases are zoonotic i.e. they are of animal origin. A large part of these emerging diseases are caused by viral pathogens.
The animals which act as reservoirs/intermediate hosts of these pathogens are bats, rodents, pigs, camels, birds, goats, sheep, and primates etc.
However, bats and rodents are considered largest reservoirs of zoonotic pathogens.
We shall consider rodent-borne zoonoses elsewhere. Here, let's take a glimpse into bat-borne zoonotic diseases.
Bat-borne Zoonotic Diseases
       Bats harbor a wide array of viruses including coronaviruses, filoviruses, lyssaviruses and paramyxoviruses. Several bat-borne viruses are considered to cause major emerging viral zoonotic diseases. These zoonotic viruses include rabies virus, Marburg, SARS-COV, MERS-COV, Nipah, Hendra and SARS-COV-2 (causing the currently raging global pandemic, COVID-19) viruses.
We shall have a detailed look at some of these zoonoses later. For now, let's glance at a major zoonotic disease that occurred in Marburg, Germany in 1967. This outbreak was considered to have originated in a kind of fruit bat prevalent in Uganda and jumped into humans through infected African green monkeys handled by laboratory workers who were then using monkey tissues for polio vaccine research.
There are several lessons imparted by this zoonosis (and other zoonoses) that can help us prevent future emerging diseases.
Drivers of Zoonotic Diseases
The factors causing zoonotic diseases are manifold as well as complex. However, the major drivers include:

• Ecological degradation e.g. deforestation & industrialization of agriculture
• Careless handling of animal by-products and wastes
• Wildlife trade
• Population boom
• Urbanization
• Increased mobility of people for trade, work and tourism aided by burgeoning airline networks
• Climate change, (one effect is northward movement of vectors e.g. mosquitoes, increased frequency of malaria in northern latitudes now).

Marburg Virus Disease (MVD)
An outbreak of a mysterious disease took place in 1967, almost simultaneously in 3 cities: Marburg and Frankfurt, Germany; and Belgrade, Yugoslavia (now Serbia). The first case was reported from a laboratory in Marburg. Laboratory workers handling tissues of African green monkeys imported from Uganda developed serious symptoms of an unknown disease, leading to hemorrhagic fever in advanced stage of the disease  (they were using monkey tissues for polio vaccine research).
It was soon found out that the culprit was an emerging zoonotic (animal-borne) virus, whose natural host is possibly a type of fruit bats prevalent in Uganda which can, under favorable circumstances, infect primates (including monkeys) and humans. The viral pathogen was soon called marburgvirus and the disease came to be known as Marburg Virus Disease (MVD). It is possibly transmitted initially through contact with body fluids of infected animals, and, later, through human-to-human transmission.
Marburgvirus is one of the three most deadly viruses infecting humans, besides Ebola and Nipah viruses.
Fatality Rate
The median fatality rate of MVD is around 50% (24-88%), but could be as high as 88%. There is a hidden benefit here. Though the virus is very deadly, its contagiousness is relatively low, largely due to its high fatality, as the virus also dies with the patient!
Causative Agent
MVD is caused by 1 of 2 marburgviruses: Marburg virus (MARV) and Ravn virus (RAVV). RAVV is a close relative of MARV, the emerging zoonotic virus that is usually implicated in Marburg Virus Disease. Both of them along with the deadly Ebola virus belong to filoviruses, viruses in the family filoviridae.
Natural Host
The natural host of this zoonotic virus is now known to be a kind of fruit bat (scientific name: Rousettus aegyptiacus).
The first outbreak in 1967 was through contact with body fluids of green monkeys (scientific name: Cercopithecus aethiops) infected by the virus presumably from the fruit bat.
Incubation Period
The incubation period of MVD, the time from the infection to the time when symptoms appear, ranges from 2-21 days.
Symptoms
The symptoms appear in stages of increasing severity. First, there is fever, chills, headaches and muscle aches; and then, rash, nausea, vomiting, chest pain, abdominal pain, and diarrhea; and lastly, jaundice, pancreatic inflammation, severe weight loss, liver failure and massive hemorrhage with organ dysfunction.
Risk Factors
The major risk factors for MVD include:

• Habitat loss
• Deforestation (so bats are compelled to move to trees growing close to human settlements and domesticated animal farms)
• Poverty leading to bushmeat (African wild animals esp. monkeys) consumption
• Hunting, handling & consumption of bats
• Research work with monkeys
• Contact with infected humans
• Working in mines where bats are prevalent.

MVD: A Past Outbreak or An Impending Threat?
Luckily for us, MVD has had outbreaks in specific areas, or had sporadic cases in limited geographical areas. It has not yet taken pandemic proportions, due partly to the high deadliness of the virus. But we cannot rule out future pandemics caused by this zoonotic virus. This can happen if the virus mutates so that the deadliness is reduced and the contagiousness is enhanced!
The first outbreak occurred in 1967 leading to 7 deaths from 31 infected cases (Marburg, Frankfurt and Belgrade) followed by sporadic cases in 1975/1980/1987/1988/1990/1998/2004/2007/2008/2012/2014 (in areas such as South Africa, Kenya, Kosovo, DRC, Angola, Uganda, Netherlands etc.).
The last outbreak took place in Uganda in 2017 causing 3 deaths from 3 infected cases.
Treatment
As yet, there is no licensed treatment for MVD. Only symptomatic treatments can be provided. No drugs or vaccines exist specifically targeting MARV or RAVV.
Restoring the fluid and electrolyte balance of the patient, maintaining oxygen and blood pressure levels, replacing lost blood and clotting factors, and treating any associated secondary infections (caused by bacteria or fungi) are part of the symptomatic treatments.
Imagine tomorrow we have a pandemic of MVD; what would be the world's response then?
Lessons from the Marburg Virus Outbreak
We cannot blame the hapless bats or monkeys for emerging zoonotic diseases such as Marburg, Ebola, Nipah, SARS, MERS, COVID-19, and Hendra etc. We can prevent future outbreaks only if humans re-learn how to live in harmony with nature. Some instructive lessons are:

• Preserve whatever forests are left
• Regenerate degraded forests
• Stop or reduce wildlife destruction, consumption, and trade
• Close down the wet markets, a la Wuhan market (where first case of COVID-19 was reported in December 2019
• Discourage factory-style domestic animal farms (especially of pigs, chicken, cattle, goat and sheep etc.)
• Invest liberally in public health policies for surveillance, forecasting, prevention and control of zoonotic diseases
• Promote R & D work on zoonotic viruses, bacteria and fungi including intensive research on drug-resistant bacterial superbugs.

Coda
Unless we take proactive measures to protect the forests and the environment, regulate livestock industry and wildlife destruction and consumption, and prevent unnatural man-wildlife encounters, encroachments into wildlife habitats and globalized wildlife trade etc., we shall encounter many more emerging, infectious, deadly and contagious outbreaks such as Nipah, SARS, Ebola, Zika, Marburg, Hendra, Lyme disease and COVID-19 in the remaining decades of the 21st century.
A COVID-21 or another dangerous emerging disease may be already lurking in the hospitals, forests, farms or wet markets, somewhere in the world!