By Simon Coghlan with Dr Megan Pickering. Dr Pickering works as a vet in livestock biosecurity and is currently the Northern Territory E.canis Co-ordinator.
Ehrlichia canis was detected in dogs in northern WA in May 2020. Since then, it has been found in virtually every urban and remote community in the Northern Territory where dogs have been tested. E. canis is now established in northern Australia and the far north of SA.
Acute ehrlichiosis can involve fever, lethargy, joint pain, swollen LNs, ocular discharge, and blood dyscrasias. Some dogs clear the bacteria while others do not. Dogs with chronic ehrlichiosis may or may not have presented with acute illness and/or received treatment.
Problematically, a proportion of infected dogs show no clinical signs. It is not known how many of these dogs will clear the organism through natural immune system defences, and how many will show no signs of acute infection but will subsequently develop chronic (and likely fatal) disease in later months or years.
In the NT, a combination of PCR testing (to detect DNA of the organism in circulation) and ELISA antibody testing are used to determine infection status. Additionally, IFAT testing at the Australian Centre for Disease Preparedness (ACDP) may be undertaken in borderline cases.
The long-term status of a dog that tests positive to any test is difficult to determine. The organism may be successfully cleared from the circulation and be undetectable with PCR testing. However, because the organism can sequester in the spleen and bone marrow, it is not always detectable using conventional testing methods.
The current approach is to regard a positive case as potentially infected for life and at risk of developing chronic disease. This applies to dogs that have been diagnosed and treated with antibiotics in the acute phase, as well as the sub-clinically infected that are diagnosed through screening tests. Treatment includes extended antibiotic therapy (doxycycline). Hospitalisation and supportive care like blood transfusions may be required.
In Australia, the zoonotic potential of this E.canis strain is uncertain, but considered unlikely. A widespread serological survey is proposed for northern Australians who live in close contact with infected dogs, and where effective tick control is limited, to inform this knowledge gap.
When dogs receive effective treatment while acutely infected, they can have a good prognosis. However, end-stage disease is the result of an untreatable immune-mediated pancytopaenia. Such dogs may waste away and die slowly, with or without further veterinary therapy. Unfortunately, many communities in northern Australia lack timely access to veterinary services and resources to treat the disease. No vaccine exists.
Rhipicephalus sanguineus is endemic in northern Australia. Preventing the tick and E. canis becoming established in southern Australia is important, and requires good tick control measures for travelling dogs, including collars or spot-on products that repel ticks and prevent attachment.
Many dogs in northern Australian communities are in a vulnerable environment with no established ownership. An ethical issue arises for organizations looking to rehome dogs from E. canis risk areas. There are a number of rescue and rehoming groups that accept dogs from northern Australia. No nationally agreed regulatory conditions exist for the movement of dogs from areas with confirmed or high-likelihood of infection with E. canis to areas with an unknown or low-likelihood of infection. There is also no obligation for the health status of infected dogs to be disclosed between rehoming groups or from rescue groups to the adopting new owner.
Suppose a dog without a home from an E.canis risk area is tested and is positive or is not tested at all prior to movement and rehoming. Should that dog be rehomed to, say, adopters in Melbourne or Adelaide? How much information around the dog’s history should be communicated to the new owners?
The NT government recommends testing of dogs prior to interstate movement and notifies interstate authorities where there are plans for infected dogs to move to another jurisdiction. The owner or person in charge of the dog at the time of testing is notified about the infection status, and offered advice and support concerning treatment for unwell dogs and possible risks and outcomes for chronic disease.
Not knowing that a dog has tested positive or comes from an endemic area may result in delayed diagnosis or misdiagnoses, especially since many veterinarians in disease-free areas are not on the lookout for ehrlichiosis. The disease, in its later stages, can also be difficult or impossible to treat. In the absence of an accurate diagnosis, this can mean ineffective interventions and suffering. The challenge for veterinarians in disease-free areas is to include ehrlichiosis in exclusion testing, because the disease can mimic a wide variety of other conditions.
What should be done if an unowned dog from endemic areas tests positive for E. canis? If the dog does not have a severe or end-stage form of ehrlichiosis, it may be treated and/or have a good welfare outcome. Furthermore, if prospective owners are informed of the risks of taking on a dog from an area of high likelihood of infection and know whether the dog has tested positive or negative, this may help them decide whether to adopt and how to respond should the dog become ill.
SIMON COGHLAN AND MEGAN PICKERING