By Dr. Luke D. Peterson, DVM, MS
Let me start by saying this article is an attempt to put together information from multiple resources to better educate ourselves about this disease. Hopefully, through education we can make the best decision for our hounds, our clubs, and our sport overall. The recent epidemic in the rabbit population at the UBGF grounds has brought this disease to the forefront of our minds but know this article is in no way a criticism of them or any club or federation that operates in conjunction with them. What happened there could happen anywhere with a running pen full of cottontails.
I think some of the biggest questions with the hardest answers to find in the scientific literature are the following:
1. Can infected dogs transmit it to people and if so, how?
2. Can dogs become carriers and introduce it into other clubs running grounds/rabbit population?
To be honest, there is no way I can fully answer these questions because the situation has not been fully studied before. We can only evaluate information from other studies and try to apply it to our own situations.
The best piece of information to tell you is all of the science currently available indicates that as a species, dogs are naturally more resistant to Tularemia than the majority of other mammals.
The bacterium that causes Tularemia is naturally distributed throughout the Northern Hemisphere including North America. It is able to cause disease in a variety of animals but is most often affiliated with hares, rabbits, and other small rodents. However, it is the arthropod vectors that are the most important in the transmission and maintenance of this bacterium to mammals. Ticks are the most common but fleas, biting flies and mosquitos have also been documented as transmitters. In the Eastern United States, the predominant reservoirs are cottontails and the American dog tick and Lonestar tick. Affected rabbits are often found dead but before that, they endure weakness, fever, skin ulcers, abscessed, and enlarged lymph nodes. It is likely in natural settings that affected rabbits are predated upon before they have the opportunity to die. However, in rabbit enclosures with limited predators, it is more likely they would be found dead. Other signs of affected rabbits are easy capture by running slowly and rubbing nose and feet on the ground. The incubation period from the time of exposure/inoculation to the time of disease onset is fairly similar for all affected species: 3-5 days is the average with 1-21 days being the overall range.
Disease in Humans
It’s important to understand that anytime an infectious agent is capable of transmission from animal to human, the understanding of human disease takes precedence over that of animal. Data from the CDC indicates there are about 100 human cases per year throughout the US. So, it is still a relatively uncommon disease. Infections peak in summer during peak tick and fly season and again in winter, presumably due to rabbit hunting exposure. Humans can experience six different forms of the disease:
Ulceroglandular – the most common, causes skin ulcers and swelling of the lymph nodes in the region of the ulcers
Glandular – second most common – lymph node swelling but no skin ulcers. These two encompass 75-85% of human cases.
Oculoglandular – conjunctivitis, regional lymph node enlargement, in severe cases corneal ulceration and large amounts of pussy eye discharge
Oropharyngeal – by ingestion – throat swelling, diarrhea, abdominal pain, vomiting, nausea
Typhoidal – this has rapid onset of septicemia throughout the entire body
Pulmonary – by inhalation – pneumonia and possible spread to the rest of body through blood stream. Typhoidal and Pulmonary are the most severe forms and have a 30-60% fatality rate if untreated.
Disease in Dogs
Dogs have been documented to have the same six forms as people. Most of the initial information you find while doing internet searches will tell you that infected dogs will surely die without treatment. There are some severe cases that do require aggressive treatment but the reality is most infected dogs will have very mild symptoms and recover without any treatment at all. Experimentally infected dogs that were fed the bacterium developed a self-limited illness with fever and mucous and pus discharge from the nose and eyes. Dogs inoculated in the skin developed pustules at the inoculation site and regional lymph nodes. Dogs recovered without treatment. Symptoms are dictated by the route of infection. For example, ingestion often leads to gastrointestinal symptoms, tick transmission causes skin abscesses at the site of the bite, and inhalation causes nasal and eye discharge or pneumonia. The disease can be confirmed on diagnostic tests in one of three ways. (1.) Antibody test – this test can sometimes cross react with other infectious diseases so more than one type of test is often done to confirm it. Additionally, presence of antibodies alone does not confirm active disease. We often use what we call “acute” and “convalescent” antibody levels to confirm a true infection. During the acute phase, antibodies are low, but after the immune system has had time to respond the number of antibodies increases substantially during the convalescent phase. As of now, there are few laboratories in the US that perform this test. (2.) PCR – tests for the presence of genetic material from the bacterium and can only be positive if the bacterium is actively present in the sample taken from the animal. This test is also performed in limited places in the US. (3.) The bacterium can sometimes be identified by culturing it from infected wounds although this is not easy to do as the bacterium is very picky about growing in lab conditions.
I suspect myself and many other veterinarians have treated Tularemia in dogs without knowing it. I can think of many cases in which a dog came in with a history of a tick removed 1-2 weeks prior, has a mild fever, a little lethargic, maybe a little painful but nothing specific. Screening for the more common tick-borne disease Lyme, Anaplasma, and Ehrlichia all come back negative. But, it looks like tick borne illness so we treat it as such and voila, they get better. Much like these other tick-borne diseases, Tularemia is susceptible to doxycycline and also to fluoroquinolones like enrofloxacin. The more severe cases require more than just antibiotics as these dogs can become quite ill. As a general recommendation, I recommend letting your veterinarian know you run rabbits and that you are concerned about Tularemia. It should be on their radar but it doesn’t hurt to express your concern more directly.
The Unanswered Questions
Can dogs transmit this disease to other dogs or humans? It has been suggested that direct contact with ulcers or pus from lesions could potentially inoculate open wounds on others though this has not been attempted experimentally to confirm the possibility. Experiments in laboratory voles have shown that during clinical illness, they shed the bacterium in their urine and feces. The few experiments in which dogs were infected did not evaluate shedding of the bacterium in bodily fluids. A study published earlier in the year titled “Seroprevalence of Francisella tularensis in Austrian Hunting Dogs” found that 6.25% of 80 dogs tested positive, leading the authors to speculate that dogs may act as reservoirs for the bacterium. A study conducted in dogs from Slovakia and Moravia found antibody positive ranges of 16-37% based on region and lifestyle of dog. Farm dogs and roaming dogs tested highest. What you need to know though, about both these studies, is the dogs had antibodies indicating exposure to the bacterium, but none of the studies attempted to isolate the bacterium from antibody positive dogs. Generally speaking, reservoir species either actively shed and/or harbor live bacterium somewhere in their body. No studies have yet demonstrated that non-ill dogs act as reservoirs of live bacterium.
A second consideration is can dogs transmit the bacterium by licking. Again, no studies have been done to determine if this can happen. It has been suggested that if a dog were to step on the carcass of a dead animal or ingest a dead animal, the bacterium could colonize in the toenails or the mouth of the dog which could then transmit the bacterium by licking or scratching. This hypothesis is based on the established transmission of cat scratch fever in which the bacterium Bartonella henselae colonizes in the claws or mouth of cats and is transmitted through scratches and bites.
The best prevention to reducing risk of infection to both dog and man is tick control. There is a myriad of products on the market to prevent tick bites in dog and man (I will be reviewing all the tick preventative products available to dogs in an upcoming article). A product that not only kills but also repels ticks and biting flies and mosquitos is preferred. Likewise, a repellent for yourself is important. Even with the best repellents there are still some ticks able to overcome them so it is always wise to check yourself and your dogs over after being in the field.
Secondly, it would be prudent to wear latex or vinyl gloves when handling dead rabbits and especially when butchering them. Likewise, thoroughly cooking the meat is important to kill the bacterium so it is not transmitted by ingestion (it also prevents other food borne diseases rabbits can carry which are more likely such as toxoplasma).
For assisting your rabbit population, ensuring adequate food supplementation is important as well as implementing a tick control program for the grounds. Providing dusting areas treated with rabbit safe insecticides allows rabbits to express natural behavior and to have some defense against tick infestations. If you are releasing or relocating rabbits into an enclosure, try to allow ample time for them to adjust to their new habitat prior to running dogs. (I realize this step is not often practical or possible but some places may have the ability to do this.)
It would be interesting to see what the seroprevalence in hunting/trialing beagles are compared to non-rabbit dogs in the US. I think the data would show what other studies already have, that 20-30% of rabbit dogs would be seropositive compared to maybe 10% of non-rabbit dogs. For those who are concerned that dogs exposed to Tularemia may bring it into another running ground at a trial it is likely that your rabbits already have it and that many of the beagles running at your grounds have already been exposed regardless of where they have been. I think running pens are more likely to experience epidemics in their rabbit populations due to the higher density of rabbits that we strive for and the stress that higher population density places on those rabbits for social status and competition of resources and territory. The decrease in predation that naturally removes the sick rabbits more likely to act as reservoirs also contributes to higher rates of transmission in pens. I suspect many of the rabbits found dead in running pens with no signs of trauma are due to Tularemia but because it is typically sporadic we often do not investigate why they have died.
I also realize there is some concern because Tularemia is listed as a potential bioterrorism agent, but so are botulism, the plague, and anthrax, all of which are also naturally occurring in nature. The reason Tularemia has been identified as a potential bioterrorism agent is because it could be aerosolized into the environment and cause infection through inhalation. This is an uncommon route of natural infection and would require a very sophisticated device to facilitate. The CDC also states that although many people could be infected by such a device, it would be readily treatable with antibiotics.
Tularemia. The Center for Food Security & Public Health. Iowa State University College of Veterinary Medicine. 2017. http://www.cfsph.iastate.edu/Factsheets/pdfs/tularemia.pdf
Posautz A. et al. Seroprevalence of Francisella tularensis in Austrian Hunting Dogs. Vector Borne Zoonotic Diseases. 2018 Feb;18(2):117-119
Gurycová D. and Kopcok M. Surveillance of Francisella tularensis infection in dogs in Bratislava. Vet Med (Praha). 1992 Mar;37(3):169-76
Feldman K. Tularemia: Zoonosis Update. JAVMA. 2003 March; 22 (6): 725-3
Tularemia. 2016 Sept. https://www.cdc.gov/tularemia/