tick-borne disease

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) new study, 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.