The bacteria Ehrlichia canis

Canine ehrlichiosis is a tick-borne disease of dogs caused by infection with the bacteria Ehrlichia canis (E. canis). Dogs become infected after being bitten by an infected tick, primarily the brown dog tick.

Ehrlichiosis is a bacterial infection that primarily infects the cells of the immune system. Infected dogs can show a range of clinical signs including fever, lethargy, anorexia and bleeding, or in some cases, show no signs.

E. canis is found in most of the world and follows the distribution of the vector: brown dog tick Rhipicephalus sanguineus.

The first detection of canine ehrlichiosis in Australia was in May 2020. Since then, the disease has become established in the Northern Territory and has been detected across northern Queensland, Western Australia and South Australia.

Queensland local government areas known to be infected (as of February 2024) include Mount Isa, Cloncurry, Carpentaria, Doomadgee, Kowanyama, Burke, Mornington, Townsville, Palm Island, Mareeba and Tablelands.

As more cases of canine ehrlichiosis occur and the known distribution of E. canis continues to expand, all dogs that live in or travel through brown dog tick-infested areas of Australia may be at risk of infection.

Domesticated or wild animals in the family Canidae including dogs and foxes are considered to be the reservoir hosts.

Infected ticks, primarily the brown dog tick (Rhipicephalus sanguineus), are vectors for E. canis infection between dogs.

E. canis is maintained in cycles between ticks and either domesticated or wild animals in the family Canidae, including dogs and foxes.

Ticks acquire E. canis by feeding, as larvae, on infected canids. Once infected, the larvae maintain their infection and may transmit the infection to other canids when feeding as nymphs or adults. Adults may then produce eggs which are not infected with E. canis.

Dogs do not transmit the disease to each other, however the organism can be transmitted directly from dog to dog through blood transfusions.

Disease phases

Canine ehrlichiosis has 3 disease phases:

• acute
• subclinical
• chronic.

The severity of clinical signs can vary considerably among dogs.

The incubation period for the development of acute disease is about 1–3 weeks, although the chronic form may not show any clinical signs until months or years after infection.

Acute phase

This phase is characterised by non-specific clinical signs, including:

• fever
• lethargy
• enlarged lymph nodes
• anorexia
• weight loss
• discharge from the nose and eyes
• bleeding disorders, such as nosebleeds or bleeding under the skin that looks like small spots, patches or bruising.

There are usually abnormalities on blood tests including low platelets and mild anaemia. This phase typically lasts for 2–4 weeks.

In countries where ehrlichiosis is established and endemic, death is rare during this phase. Most dogs recover after 1–2 weeks without treatment, however some may remain persistent subclinical carriers for months or years.

In Australia, where the disease is infecting a naïve population, very high morbidity with severe illness and mortality occurs in the acute phase.

Subclinical phase

Some dogs may progress from the acute phase and become subclinical carriers for months or years. These dogs present no clinical signs and therefore may not appear to need veterinary attention.

Subclinical dogs either:

• clear (get rid of) the organism
• remain asymptomatically infected
• progress to the chronic form of ehrlichiosis.

The likelihood that subclinically-infected dogs will go on to develop chronic, end-stage ehrlichiosis is unknown.

Chronic phase

Clinical signs are similar to the acute phase but are more severe. They can include:

• fever
• weakness
• weight loss
• bleeding disorders
• pale mucous membranes
• eye abnormalities
• neurological abnormalities.

Infected dogs may be more susceptible to secondary infections. Blood tests often show severely low platelets, low white blood cells and anaemia. This form of the disease is usually fatal and treatment efforts are likely to be futile.

Transmission between dogs and other canids

Transmission between canids requires the tick vector. Transmission of E. canis from infected ticks to uninfected canids can occur within a few hours of attachment. Canids may also become infected through blood transfusions.

As E. canis does not transmit naturally between dogs, possible pathways for its spread include movement of:

• infected ticks that may infect naïve dogs (dogs that haven't been exposed to the bacteria before) in new locations
• infected dogs (acute, subclinical and chronic phases) that infect naïve tick populations in new locations.

Transmission to humans

Infected canids do not transmit E. canis to people. In rare cases, people may become infected with E. canis after being bitten by an infected tick.

Diagnosis

Ehrlichiosis is diagnosed through a combination of:

• consistent clinical signs
• blood tests.

If you see clinical signs consistent with ehrlichiosis, you should consult your private veterinarian and seek assessment as soon as possible. Early diagnosis and treatment provide the best chance of recovery.

Veterinarians should refer to the information for veterinarians on Ehrlichia canis for advice on diagnostic testing.

Prevention

There is no vaccine for ehrlichiosis. To help protect your dog from infection:

• maintain an effective tick prevention and control program — this includes:
  • using external tick control that kills ticks on contact (tick collars and spot-ons). Products that allow ticks to feed before killing (tablets and chews) may aid in control of ticks but do not prevent infection with E. canis
  • treatment of bedding, kennels and yards to prevent reinfection from the environment
• avoid taking dogs into tick-infested areas (such as heavily-vegetated areas) as much as possible
• inspect dogs regularly for ticks, paying particular attention to the head and neck, inside their ears, on their chest, between their toes and around their mouths — keeping long or fluffy coats clipped short can make finding ticks easier.

Veterinarians can provide advice on tick prevention and control programs, effective products and tick removal.

People who adopt rescue dogs should always ask questions about where the animals came from, their history, their health status and what tick prevention they have been on.

If your dog has not been on a tick prevention program, is unwell, or you are unsure, seek veterinary advice.

Treatment

In addition to removal of ticks and applying an appropriate tick control program, E. canis infection in dogs may be treated by a veterinarian with antibiotics. In some cases, supportive therapy and hospitalisation may be necessary. Early treatment provides the best chance of recovery.