Effects: Disease

Infectious diseases have long been transmitted via human-animal interactions (Estes 2011).  Many wild animals serve as reservoirs for infection, the preferred environment of bacterial and viral pathogens.  Fluctuations in the abundance of certain animal populations have lead to increased contact with human populations which in tern can result in the transmittance of disease.  Removal of large predators has resulted in the increase abundance and human exposures of animals residing on lower tropic levels which act as infectious agents.  Below are case studies where top-down forcing has resulted in this process occurring.

Vultures in India and Pakistan
Catastrophic declines of vulture populations in India and Pakistan have been attributed to the introduction of a veterinary drug into livestock animals (Oaks 2004).  The anti-inflammatory drug, diclofenac, was widely introduced to livestock animals for health purposes.  When the animal died, vultures would consume the carcasses and be subsequently administered the drug which would result in residues within the bird reaching toxic levels and death from renal failure.  Many species of vulture have experienced massive population declines with the Oriental white-blacked vulture (Gyps bengalensis) showing a decline of 95% since the mid 1990's.

The decline of vulture species have been shown to lead to the increased human exposure of two deadly diseases.

1.  Anthrax is common in the livestock in parts of India (Markandya 2008).  Humans have long been protected from from anthrax exposure as vultures remove animal carcasses quickly and thoroughly.  Without the predatory bird, lack of carcass removal could result in increased human contact with the disease transmitting bodies.

2.  One of the major foreseeable impacts of vulture decline is the increase in dog populations (Prakash 2003).  Feral dogs have replace vultures in removing the carcasses of dead livestock.  Vulture decline has had a direct inverse relationship on the populations of scavenging dogs.  Increased abundance of dogs is feared to result in increased incidence of dog bites among humans, which directly results in the transmittance of the viral infection rabies (Markandya 2008).  Dog bites have shown to be responsible for over 95% of rabies related deaths.