Etiology

• Ehrlichia ruminantium of the order Rickettsiales which infects endothelial cells, macrophages and neutrophils in its host and is spread by Amblyomma ticks in endemic areas Ticks: overview.
• Vertical transmission can also occur naturally, whilst horizontal transmission through infected colostrum has been postulated Pathogen transmission: overview.

Predisposing factors

General

• The presence of the infected biological vector, the Amblyomma tick in a naïve cattle population.
• The movement of infected ticks and cattle (including subclinically infected) from endemic to non endemic areas.
• Migrating birds and wild ungulates have been implicated in aiding the transmission of infected.

Pathophysiology

• E. ruminantium multiplies in vascular endothelial cells throughout the body and hence causes severe vascular compromise which may manifest as hydrothorax Thoracocentesis or hydroperitoneum Abdominocentesis.

Timecourse

• The average incubation period is 2 to 3 weeks but can vary from 10 days to 1 month.

Epidemiology

• Amblyomma ticks are biological vectors of heartwater and the pathogen is transmitted transstadially and intrastadially, but not transovarially.
• Amblyomma ticks have a three host cycle which may take from 5 months to 4 years to complete and as such, infection may remain for an extended period of time within the resident tick population.
• Infected wild ungulates may also serve as a reservoir of infection to ticks and naïve cattle.
• Endemic stability is often considered an important factor in epidemiology and control of ticks and tick borne disease whereby a high level of challenge with hemoparasite-infected ticks exists with a concurrent low incidence of clinical disease
• It has been questioned whether the concept of endemic stability should be applied to infections involving Ehrlichia ruminantium due to the lack of clarity surrounding innate immunity, passive immunity and infection dynamics exist.
• Colostral antibodies have been shown to be protective in calves, but there is less documentary support for inverse age immunity in heartwater caused by E. ruminantium, as is commonly seen, in other tick-borne diseases.
• Vertical transmission of disease from the dam to the calf, either in-utero, or via colostral transfer has been shown in the absence of the tick vector. This infection is controlled by maternally derived antibody transfer and indicates that the previously reported inverse age-related immunity is actually dependent upon the dams immune status against E. ruminantium.
• Vertebrate host can remain as long-term carriers of disease and will therefore serve as an important source of infection for Amblyomma ticks and thereby play an important role in the maintenance of infection.