Question

Anthrax, a bacterial disease caused by Bacillus anthracis, is lethal to most mammalian herbivores. Within a few months during \(1983-1984\) an anthrax epizootic wiped out \(90 \%\) of the impala population in Lake Manyara National Park in Tanzania. How is it possible for an epizootic of this type to suddenly appear in a population and then disappear for decades? Discuss the biological mechanisms that might permit this type of phenomenon. Prins and Weyerhaeuser (1987) discuss this particular impala epizootic.

Short answer

Anthrax spores persist in the environment, initiating sudden outbreaks when disturbed. After host population collapses, the bacterium returns to spore state, appearing to disappear but remaining dormant.

Step-by-step solution

Understanding Anthrax and Bacterial Spores

Anthrax is caused by _Bacillus anthracis_, a bacterium that can form spores. These spores are highly resistant to environmental extremes and can survive in soil for many years, waiting for the right conditions to germinate and become active bacteria again. This spore-forming ability allows anthrax to persist in the environment long after an outbreak has diminished.

Initiation of an Epizootic

An epizootic like the one in Lake Manyara can suddenly occur when animals disturb spore-contaminated soil, such as during dry periods when animals graze at lower levels, or through increased rainfall bringing spores to the surface. The spores germinate under favorable conditions, leading to active infection among susceptible animals such as impalas.

Transmission Dynamics and Peak of Epizootic

Once active, the bacteria spread rapidly through the herd, typically through grazing and contact with contaminated soil. The high lethality rate of anthrax causes a quick but sharp increase in mortality, reducing the population density of the herbivores and thereby the rate of transmission.

Decline and Disappearance

The bacteria primarily spread in environments with high density of susceptible hosts. Once the hosts are significantly reduced or die off, the infection rates decline sharply due to lack of new hosts to infect. Without hosts, the bacterium returns to its spore state, seemingly disappearing while actually remaining dormant in the environment.

Long-term Dormancy and Sporadic Reemergence

Bacillus anthracis spores can remain dormant for decades, allowing them to remain undetected and inactive. Yet, certain environmental changes, human activities, or climatic conditions can disturb these spores, potentially causing future outbreaks when conditions become favorable again.

Key concepts

Bacillus anthracis

_Bacillus anthracis_ is a bacterial species responsible for the disease anthrax. This bacterium is known for its resilience partly due to its ability to form spores. These spores can endure extreme weather conditions, surviving in the soil for prolonged periods.

The lifecycle of anthrax involves two main stages: vegetative cells and spores. Vegetative cells are active and responsible for the disease symptoms. They are short-lived outside a host. However, when environmental conditions become unfavorable, these cells form spores.

These spores have a protective coating allowing them to survive without nutrients. The incredible endurance of these spores is key to their effectiveness in waiting for optimal conditions to infect new hosts. Thus, while the bacterium itself might not be actively causing disease, its potential remains buried in the soil as spores.

Spore Dormancy

Spore dormancy in anthrax plays a major role in the ecology of infectious diseases. Dormant spores are inactive, waiting in the soil for favorable conditions to reanimate and infect a living host. This dormancy provides the bacterium with a clock that can remain paused for years, even decades.

Certain environmental conditions trigger the spores to reactivate. These include increased rainfall, which can wash spores up to the soil surface, or disturbances caused by animals grazing. The moment spores find themselves in a nutrient-rich environment like a warm-blooded animal’s body, they germinate into active bacteria.

Understanding spore dormancy is essential for managing anthrax outbreaks. It explains why anthrax can seem to disappear for a long time but suddenly reappear when conditions switch in its favor, allowing dormant spores to become active and spread.

Epizootic Dynamics

Epizootic dynamics describe the patterns and factors influencing the spread of diseases, like anthrax, among animal populations. An epizootic is essentially an outbreak occurring relatively quickly and affecting a large number of animals.

For anthrax, the dynamics are heavily influenced by the population density of susceptible animals and environmental conditions. When spores become active, as seen in Lake Manyara’s impala population, they cause a rapid increase in infections. The process unfolds as bacteria infect, spread, and wreak havoc on the population.

However, the rapid decline of an epizootic occurs once a significant number of hosts either succumb to the disease or have a reduced density, decreasing transmission rates. With fewer available hosts, bacteria form spores and the disease appears to vanish, only to reemerge under suitable conditions once again. This cycling nature accounts for the seemingly sporadic yet fierce emergence of anthrax in wildlife.