Rocky Mountain Spotted Fever: Rickettsia rickettsii
Despite its name, Rocky Mountain Spotted Fever is found mainly in the Eastern US and usually makes itself known in spring and summer (March to October) but cases of human RMSF have been reported in almost every state, Canada, South America and Mexico. Since it infects both species, then, logically, if it's there for us, it's there for our dogs, who are at greater risk of contracting it.
Dogs most at risk are young, generally large breeds that spend a lot of time outdoors. Dogs over 3 or 4 years old aren't as likely to get RMSF, possibly having acquired an immunity from other dogs.
The onset of the disease is rapid. Four or five days after being bitten, the dog will become feverish (up to 105 degrees). Symptoms may include depression, loss of appetite, swollen lymph nodes, bruising on the skin or gums, tiny hemorrhages under the skin (petechiae), muscle pain, retinal hemorrage, swollen or painful joints. A staggering gait, difficulty keeping his balance, is the dog's most common neurological sign. Seizures are possible. A decrease in the platelets responsible for clotting (thrombocytopenia), is the most consistent finding in blood tests.
If left untreated, RMSF either kills quickly (the disease runs its course in approximately two weeks) or the dog gets over it and is then immune. Symptoms may range from slight through severe to fatal.
It takes two to three weeks for antibodies to be detectable but then their numbers rise sharply for several weeks before they level off and begin a long, slow, gradual decline over many months. Obviously the timeline is very tight here, so, since research indicates that death from untreated RMSF is more common than previously thought, a dog that appears to have it should be treated immediately without waiting for test results to come back.
Tetracycline and doxycycline are the drugs of choice. According to Dr. Ed Breitschwerdt, one of the most prominent of the research scientists concerned with tick-borne disease in dogs, fluid therapy has to be used with caution if RMSF is suspected because this disease makes the blood vessels become more permeable (sponge-like, porous). A rapid response to doxy or tetracycline in dogs without neurological signs should occur. If a fever hangs on, he says that another diagnosis should be pursued.
"Neurologic signs including pain, loss of balance, tilting of the head, stupor, seizures, and coma may occur in dogs with RMSF. Similar to ehrlichiosis, this presentation can mimic canine distemper in the young dog."
If a low titer for RMSF is found on a tick panel, it is not necessarily conclusive evidence of the disease.
According to Dr. Tom Beckett: "The cause of Rocky Mountain spotted fever (RMSF) is Rickettsia rickettsii. Dogs can, and frequently do, harbor Rickettsia of species closely related to R. rickettsii that are generally considered to be harmless (i.e., causing no disease.), but which do tend to cause the dog to produce antibodies so similar to R. rickettsii antibodies that they "cross-react" and cause low to middling titers on tests for R. rickettsii antibodies. Therefore a single low-to-middling RMSF titer cannot be unequivocally interpreted.
"Such a low-end RMSF titer might be found in a dog...
...that is in the very early stages of an acute RMSF attack, OR
...that harbors one of the several non-pathogenic spotted fever group (non-pathogenic = not capable of causing disease) Rickettsial species, OR
...that had an acute episode of RMSF at some time in the past, and although recovered, still continues to produce R. rickettsii antibodies, OR
...there is also some emerging information which suggests there may be cases where presence of other, non-Rickettsial, pathogens (e.g., Bartonella vinsonii subspecies berkhoffii) can cause (for unknown reasons) low positive reactions on the RMSF IFA tests.
"Diagnostic confirmation of RMSF by serology (antibody testing) requires
comparison of two blood samples: an "acute sample" taken early in the acute attack (hopefully immediately the disease is suspected) compared with a "convalescent sample" taken 2 to 3 weeks later. The criteria for
confirmation is that the antibody levels show a four-fold or greater
increase from first sample to second. A mere two-fold increase from first
to second sample has to be regarded as equivocal--could be a "real"
increase; could be cyclic variation."
From the AVMA: "Dogs recovering from infection with R rickettsii have been immune to reinfection, with dramatic seroconversion when challenged 6 to 12 months later. None of the naturally infected dogs studied by the author has been known to have a second episode. It also appears that dogs in enzootic areas that are infected naturally at an early age become immune, as most infections have been in dogs ≤ 3 years old." http://www.avma.org/reference/zoonosis/znrmsfvr.asp
RMSF in Arizona: a good, readable overview of Rocky Mountain Spotted Fever.
Remember, as soon as RMSF is suspected your dog should be treated without waiting for the test results to come back.Cross infection with other tick-borne diseases is common.
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"Workers at Auburn University reported severely debilitating hepatozoonosis in dogs from Alabama and western Georgia. After diagnosing a large number of cases and recognizing that the parasite was more pathogenic than the Old World form, additional studies led the authors to propose a new species, which they assigned the specific epithet "americanum," a decision that has been endorsed by others. Among others, a major difference between H. canis and H. americanum that justified the definition of a new species included failure to infect R. sanguineus with the organism."
As so often happens with a tick-borne disease, signs can be vague and not point in any one direction: fever, lethargy, a lack of interest in food, weight loss, hind end weakness, sometimes mild anemia, a runny nose or bloody diarrhea . As things get worse, the dog might go lame, have muscle pain or be unable to get up.
The Perdue University Newsletter, Spring 2003, states that H. americanum "typically presents as severe clinical disease. A majority of the clinical syndrome is composed of clinical signs related to chronic inflammatory disease. Many patients present with recurrent fever, lethargy, depression, and weight loss. Muscular disease is also apparent on presentation. ...stiffness, lameness, hyperesthesia (heightened or altered sensitivity to stimuli), and muscle atrophy (wasting). Clinical signs fail to resolve with antibiotics. Bloody diarrhea, related to intestinal penetration by the sporozoites, may be documented soon after exposure. A generalized lymphadenomegaly may also be present."
According to a study published in Clinical Microbiology Reviews, "Diseased dogs are often febrile (feverish), stiff, lethargic, and depressed. Gait abnormalities and muscle wasting are usually obvious, as is copious mucopurulent ocular discharge (mucous and pus containing white blood cells, bacteria and cell debris coming from the eye). Atrophy of head muscles is especially noticeable. Dogs may eat readily when food is placed immediately in front of them, but they often refuse to move to food and water, presumably owing to intense pain, which derives in part from periosteal bone proliferation (the rapid multiplication of cells in the membrane enclosing the bone) and inflamed muscles."
When an infected tick is eaten, the disease organism migrates through the dog’s intestine and, it's suspected, is carried by leukocytes to skeletal and heart muscles. In a fairly complicated process, the parasite reproduces by dividing and eventually ruptures the cells of these muscles. Then it moves on and does it again. It's the rupturing of these muscle cells that causes the severe pain.
Imizol used with other drugs might limit the number of organisms but it's implied in most of the information I've read that there is no cure for this disease. The study in Clinical Microbiology Reviews is somewhat more hopeful in stating that, "It is likely that some dogs with ACH (American Canine Hepatozoonosis) that is never diagnosed recover as a result of good care by owners." The study also discusses a treatment protocol.
The vector is Amblyomma maculatum, the Gulf Coast tick, and the area in which H. americanum is found centers on Texas and the Gulf Coast region. A. maculatum does not infect the dog by attaching and injecting a disease-laden saliva. The disease doesn't go into the tick's salivary glands. The dog must swallow the tick to be infected.
