what it is, how it's transmitted, the vectors that carry it
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What is TBD? The simple answer is that it is any disease carried by a tick (the vector) from an infected animal on which it has fed to another one when it feeds again. Fortunately, the Ixodidae (hard-bodied ticks) which carry these diseases feed only 3 times in their lives which means that each tick can normally spread infection only twice, though it is possible for some to be born infected. Unfortunately, while it must be remembered that not every tick is going to carry disease, there are a lot of them and that's how the danger of infection multiplies. One thing must be mentioned here as many people don't understand this: tick-borne diseases cannot be passed by contact. If your dog has one or more than one, you're perfectly safe touching him, holding him, playing with him. He cannot infect you or your other animals. TBDs are carried only by vectors independent of the dog. How is tick disease transmitted? Tick-borne diseases, known as TBDs, are usually transmitted by ticks after they have latched onto a dog and begun to feed. (Neorickettsia risticii and Hepatozoon canis are the exceptions.) The disease organisms in the tick's gut begin to move up as it sucks, keeping the wound open with the anticoagulants in its saliva. Mixing into the saliva, the organisms are injected into the dog. Watch it happen by clicking on this link to the Australian Paralysis Tick. As if we needed to hear this, there is evidence now that some erhlichia are able to stay hidden from the immune system's defenses at least long enough to establish infection. "In (a) study in 2006, scientists uncovered a clue to how ehrlichiosis-causing bacteria infect such diverse animals. One of the three primary bacteria sequenced, A. phagocytophilum (E. equi), contains roughly 1,400 genes--including more than 100 variations of a single gene that codes for a protein allowing the bacteria to evade the immune system of the organism it has infected. This protein sits on the bacteria's outer membrane surface. When the bacteria, through tick bites, transfers to (a dog), the bacteria chooses the protein variation needed to stay hidden from that particular host." Lyme disease seems to have a similar defense against detection and I wouldn't be surprised to learn that other TBDs do, too. How long does it take to infect a dog? While it's generally said that it takes a minimum of 24 hours for disease to be transmitted to the dog it can happen sooner. According to Dr. Ed Breitschwerdt of North Carolina State University, referenced by this University of Georgia website, transmission of Rocky Mountain Spotted Fever (Rickettsia rickettsii) may be accomplished in as little as 5 hours. And it is theorized that on occasion, probably rare, that a tick which has been interrupted while it is feeding on an infected animal may finish its meal on another and pass a TBD almost immediately when it does because the saliva is already primed with disease organisms. Bottom line, use a proven tick preventive and if you see ticks on your dog, get them off! There are right and wrong ways of doing this, so be sure to read the page on tick removal. The hard-bodied ticks Most dog owners are familiar with ticks, the ugly, engorged things that fall off, sated, when they've drunk their fill, large as grapes and creating a real mess when you step on them. But the tick nymph may be more dangerous than the adult because it is quite capable of transmitting disease yet it can be as small as a pencil dot. To give you an idea of how small they can be, here's an image of deer ticks from the Old English Sheepdog.org webpage on tick identification. From left to right, you see the female, the male, the nymph and, almost invisible, the larva. Many thanks to the webmaster for allowing me to use the image, originally from a Rhode Island.gov webpage, which he reduced to show the ticks in their actual size.
It should be pretty obvious after looking at that photo that picking ticks off by hand is not an effective way to prevent tick-borne disease. It can help...but only if you can see them. A good preventive is a must. A Tick's Life A three-host Ixodid tick begins life as one of thousands hatched from the eggs of a single female. Once the six-legged larval tick has fed, it molts into an eight-legged nymph and is then capable of transmitting TBD if the first meal it had was on an infected host. Feeding a second time, it molts again and becomes an adult. Ticks find their hosts by "questing". Crawling up grass stems, heading for the edges of leaves on the ground, on shrubs or even up in trees, they wait for a potential meal to come by. As far as I know, they have no sense of smell but react to heat, motion and the carbon dioxide exhaled by mammals like humans, deer and dogs. When a host is detected, they raise their front legs and wait until the animal brushes against them, then latch on...all but the Lone Star tick. This one is a lot more aggressive about finding a host and will actually pursue a potential meal a relatively long distance. So, if you see a tick running after you, it's probably a Lone Star tick.
Depending on the climate, ticks can live more than three years, longer lives being more common in colder climates. After the last feeding, after the male has had his chance at her and the female has laid her eggs, the ticks die.
Hard-bodied (Ixodid) ticks and the diseases they carry.
The American Dog Tick (Dermacentor variabilis) and the wood tick (Dermacentor Andersoni) are the primary vectors of Rocky Mountain Spotted Fever (RMSF). Images of Dermacentor variabilis feeding on a dog. The brown dog tick (Rhipicephalus sanguineus) is the vector for E. canis (Canine Monocytic Ehrlichiosis or CME), E. chaffeensis and E. platys (Anaplasma platys). It has also been known to carry RMSF. The black-legged tick or deer tick, Ixodes scapularis, transmits Lyme disease and babesiosis. The Lone Star Tick, Amblyomma americanum, carries granulocytic Ehrlichiosis (E. ewingii) and a small percentage have been found to carry Lyme disease. The relatively small number of cases of Lyme in Texas were transmitted by the Lone Star Tick. Ixodes ticks (scapularis and pacificus) carry E. equi (now Anaplasma phagocytophillum). Amblyomma maculatum is the tick host for Hepatozoon americanum, a disease caused by "ingesting" ticks. These ticks are found in warm, humid regions, generally around the Gulf Coast from South Carolina to Texas and north as far as Kansas. Tick Paralysis is caused by quite a few species of ticks belonging to several genera and is characterized by spinal paralysis that comes on rapidly and severely, progressing from the rear toward the head. Once the tick is removed, the dog usually recovers completely if he's diagnosed in time. Further information can be found here or for anyone reading in Australia, here or here. Soft-bodied (Argasid) ticks I've read of one Argasid tick, Ornithodorus hermsi, that is common in wild animals and thought to be the primary vector for human relapsing fever but as far as I can tell neither this tick nor any other soft-bodied tick causes disease in dogs. |
Use your judgment when reading these maps. What appears to be high risk for a given disease in one state may actually be a very low risk when compared to the incidence in another. The risk of a dog contracting Lyme disease in Georgia, for instance, is very low when compared to the risk in a state like Massachusetts. Certainly, when reading these maps, population should be taken into account. Four cases in a highly populated state probably mean a lot less than four cases in a state with a small population.
Don't let the statistics scare or lull you, let them inform you. _______________
For fuller information on the Ixodidae and the diseases they carry, see the Tick-L Links Page. The recent renaming of diseases that used to be lumped together under the heading "Ehrlichiosis" can be confusing. Dr. John Burchard, our resident acarologist on Tick-L, has added links to Classification and New Taxonomy on the
Links Page which should help you sort that out better than I can.
Tick Management Handbook from the Connecticut Agricultural Experiment Station, New Haven, is now available for download. This is an excellent resource.
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