I wrote in a previous post about my experience with a rural volunteer veterinary clinic on Ometepe Island, Nicaragua. This initiative was organized by veterinary students from the local International Veterinary Student Association (IVSA) chapter of Oregon State University, and provided great insight on working in rural areas of a developing country for both the students and veterinarians who attended.
One important aspect that working in this environment brings forth is the opportunity to gain experience with diseases rarely seen in other countries. One such disease that was omnipresent on Ometepe Island was Ehrlichiosis, which is definitely of public health importance as it can infect both animals and humans. This is a great example of a vector-borne disease, in which the infection is transmitted from person to person (or animal to animal) by a vector, usually an arthropod like a mosquito or tick.
Our patient was a young female dog that was brought up to the clinic for wellness care and inappetence. On physical examination, the patient was dehydrated, weak, emaciated and infested with ticks, especially around the ears. A more thorough examination revealed that all lymph nodes palpated were enlarged and that the dog was anemic as well.
Diagnosis of Ehrlichia was made by using an antigen SNAP test that allows us to look for other types of infections, such as heartworm and Lyme disease. Every patient seen at the clinic also had blood drawn to look for Ehrlichia microscopically and also assess platelet numbers for potential surgery and, in this dog, visual identification of the organism in a neutrophil confirmed the diagnosis.
Due to the severe emaciation and anemia, our patient did not go to surgery for sterilization and an antibiotic regimen consisting of doxycycline was started. In addition to antibiotics, the owner was given 6 months’ supply of flea and tick medications.
Ehrlichiosis can be caused by a variety of parasites of the Ehrlichia or Anaplasma families. Several of these can cause infection in both animals and people and all of them are transmitted through the bite of a tick vector. The type of ticks that can transmit ehrlichiosis varies depending on geography but most common vectors include the brown dog tick (Rhipicephalus sanguineus) or other ticks from the Amblyomma or Ixodes species.
In dogs, ehrlichiosis is commonly caused by Ehrlichia chaffeensis, E. canis and E. ewingii. These can affect several body systems and clinical signs include fever and enlarged lymph nodes (lymphadenopathy), however, polyarthritis, conjunctivitis, thrombocytopenia and hepatosplenomegaly can also occur. It is important to remember that other animals such as horses, cattle, sheep and even cats can also be infected by other Ehrlichia or Anaplasma.
Human ehrlichiosis can be caused by several of the same species of Erhlichia known to infect animals, including E. canis, E. ewingii and E. chaffeensis. In people, it appears that E. chaffeensis can cause somewhat of a more severe infection. Humans, just like animals, are infected through the bite of a tick and the disease can be characterized by fever, muscle ache, respiratory and neurologic problems such as ataxia and meningitis.
This disease highlights the risk posed by vectors like ticks that can infect animals as well as people. Vector-borne infections give us the chance to look at a true One Health paradigm for disease. Indeed, these infections cause disease in both humans and animals and the distribution of vectors like ticks (but also mosquitoes) is largely dependent on the environment. In fact, simply treating our canine patient for ehrlichiosis on Ometepe Island would have gotten the current infection under control. However, in a setting where ticks are very prevalent, it would be a mistake to forget about environmental factors that lead to infection in the first place. This is the reason why we gave as much topical preventive medication against ticks and fleas as we could with the hopes of reducing the number of ticks climbing on, and thus re-infecting, dogs. In a perfect world, the house and the environment where the animals (and people) live should also be treated to make sure ticks don’t thrive there.
A note on tick-borne diseases
Ehrlichiosis is just one example of a disease transmitted by vectors such as ticks and there are many others. Perhaps the most famous one is Lyme disease, which is present in both North America and Europe, however, different diseases can occur in different parts of the world. The Centers for Disease Control and Prevention (CDC) has a great list of which tick-borne disease are present in the USA as well as in other countries.
One of the great misconceptions about ticks is that people are only exposed when they go into nature, when hiking for example. While it is true that most tick species are usually present in the wild, some species, like the brown dog tick (Rhipicephalus sanguineus) are known to establish themselves in houses and can cause some pretty severe infestations in people’s homes (one female tick can lay at least 2000 eggs in the environment…or in someone’s house).
This is why pets should always be on flea/tick preventives, even if they don’t usually go hiking with their owners. People who notice ticks in their homes should call exterminators to help treat the infestation. Furthermore, it is recommended that those who go hiking wear long pants and sleeves and check themselves for ticks after the hike.
Finally, I would like to, once again, remind readers about the effect of a changing environment on the distribution of vector-borne diseases. Global weather changes can cause higher temperatures and humidity, which leads to an increase in the number of vectors, but also their range. I have touched on that in a previous post, specifically regarding mosquitoes. However, ticks also benefit from these weather changes. A recent study has shown that due to higher average temperatures, Norwegian ticks can now be found in places where they traditionally did not exist.
More information on Ehrlichia and tick-borne diseases: