A disease of unknown origin

Potomac horse fever is a potential killer that is best controlled through vaccinationEquine Potomac horse fever (PHF) was first recognized in 1979 near the Potomac River in Montgomery, Maryland, and has now been confirmed in 41 states, Canada, and France. This disease is also suspected in many other countries, and there is serologic evidence that it is now present in Italy and Australia.
The causative agent of PHF is Ehrlichia risticii, a gram-negative bacteria that is highly pleomorphic. It can appear round, oval, or elongated in shape with a highly rippled cell wall. The ability of this bacteria to live within the white blood cell gives it unique status, as having characteristics of both bacteria and viruses. (However, diagnosis of PHF via blood staining is difficult because of the relatively low percentage of affected white blood cells within the peripheral blood.)
The manner in which PHF is transmitted from horse to horse is unknown. The association of PHF to areas of large waterways, its seasonal occurrence (with most cases encountered from May to November, peaking in mid-summer), and its infectious but not contagious nature, has led most researchers to suspect some type of insect vector (carrier). Other rickettsial diseases have proven insect vectors such as Rocky Mountain Spotted Fever, whose vector is Dermacentor variabilis, the American dog tick.

Clinical signs
The most consistent clinical signs seen in cases of PHF are depression, fever, and a decrease in intestinal sounds. PHF is commonly classified into three distinct forms:

• Subclinical disease. This animal is depressed, has a low-grade fever, and an overall decrease in intestinal sounds. Such an animal generally goes unnoticed, makes an uneventful recovery in 1-to-5 days, and is rarely observed or treated by a veterinarian.
  
• Mild disease. This animal is depressed, has a fever (102-107F), cow-like diarrhea (1-to-3 days), and mild colic. He may or may not be observed or treated by a veterinarian, depending upon the concern and horsemanship of the owner.
  
• Severe disease. This animal becomes very depressed, has a fever (102-107F), a profuse amount of watery diarrhea of 5-to-7 days duration, and severe colic. As this form of the disease continues, many of these horses develop severe endotoxemia and can die within hours in spite of extensive therapy.
  

Laminitis can be a serious sequela to PHF and can occur at any stage with each of the three above forms. Some researchers have proposed that this may be the result of a direct infection of the lamina by the Ehrlichia organism. Many of the horses that develop laminitis have to be euthanized, and this contributes to the high mortality rate of PHF (10%-to-30%).
Experimental research has also shown that pregnant mares can abort and develop in utero infections after being infected with the E. risticii organism. Most abortions occur between the sixth and tenth month of pregnancy. These abortions tend to be sporadic and do not appear to be infectious. The aborted fetus shows no significant histo-pathologic changes for one to suspect a PHF-type infection. Many of these PHF-infected mares will become difficult to breed and can remain open for several seasons. Culturing the tissues of aborted fetuses such as the spleen, liver, intestines, and lymph nodes may be necessary in order to make a positive diagnosis of PHF. However, this culturing technique requires special media, is expensive, and can take from 2-to-4 weeks. This procedure also results in a high number of false negative results.

Diagnosis
The clinical signs of PHF are generally too ambiguous to make an accurate diagnosis. Serologia via IFAT (Indirect Fluorescent Antibody Technique) is the most commonly used laboratory test to help confirm a diagnosis. To make a serological diagnosis, a four-fold rise in the titer for PHF must be demonstrated. Antibiotic therapy will not affect serologic testing; however, vaccination might in some cases. Horses with PHF almost always have titers of greater than 1:160. If it has been impossible to obtain paired serum samples, a single sample having a titer of 1:640 or greater, in association with compatible clinical signs, strongly supports a diagnosis of PHF. Experimental DNA gene probe testing has been encouraging and may be the test of the future for this disease.
Laboratory findings with PHF are inconsistent. A mild-to-extreme leukopenia (decrease in white blood cells) is generally observed early in the disease, and this can be followed by a leukocytosis (increase in white blood cells) of greater than 30,000. This high white count is unusual in other enteric diseases that can be confused with PHF, such as Salmonellosis and Colitis X. A mild elevation in the blood fibrinogen is also often found with PHF. This can be associated with the inflammation that occurs as a result of the infiltration of the E. risticii-infected white blood cells within the walls of the colon.

Pathogenesis
After the horse becomes exposed to the E. risticci organism, it is taken up by the white blood cells and they migrate toward the walls of the large colon, causing a localized infection. While no gross pathologic changes are observed upon necropsy, this localized infection within the wall of the colon creates an inflammatory reaction that apparently interferes with the net movement of sodium and chloride. This restricted movement of sodium and chloride within the colon can account for the diarrhea and associated endotoxemia seen in severe cases of PHF.

Treatment
The clinical signs present in each individual case will dictate the degree of therapeutic intervention. Mild cases may only require symptomatic treatment. Severe cases will require more extensive therapy with antimicrobial, antipyretics, and non-steroidal anti-inflammatory medications. Occasional cases may be refractory, relapse, and need additional therapy for 10-to-14 days. In the most severe form of PHF, aggressive fluid therapy is necessary to combat the dehydration that accompanies fluid loss from diarrhea. Animals must also be treated for the severe loss of sodium, chloride, and potassium that occurs with this disease.

Prevention
The insidious nature of this disease, combined with the lack of specific knowledge as to how it is transmitted, gives little hope for eradication. Vaccination is therefore, at present, the best means of controlling it. Intramuscular vaccines are administered prior to the onset of the PHF season, in two doses given three weeks apart. Horses are then given annual boosters to provide for continued protection. In regions where this disease is more endemic, or where there is the potential for year-round infections, booster doses can be given at 4-to-6 month intervals.
Experimental studies have shown vaccination to be very efficacious. However, reports of vaccinated horses developing PHF have caused owners to question this procedure. The intracellular nature of this organism and the possibility of previous exposure may be some of the factors that could interfere with a vaccines effectiveness. However, it can be safely assumed that after horses are vaccinated for PHF and then become exposed to the PHF organism, they will have enhanced resistance to the disease. If they should become ill, their chances of needing intensive fluid therapy, veterinary hospitalization, and developing a fatal case of laminitis will be greatly reduced. Thus, not only will the safety of your horses health be improved, but time and money will be saved.
Veterinary clinicians are faced with a dilemma in that the condition of a colicky, diarrheic horse may worsen with tetracycline treatmentif the causative agent is not the PHF organism, E. risticii.

Summary
Potomac horse fever is an insidious disease of horses, of which the mode of transmission is still unknown. Despite appropriate therapy, many animals can die from PHF because of complications such as laminitis. A positive diagnosis of PHF is generally confirmed by IFA blood-testing or the culturing of the E. risticii organism. Tetracycline therapy is very effective in treating this disease, but vaccination is the safest, most effective, and economical means of controlling this elusive disease.

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Douglass B. Hutchins, MS, DVM, is a veterinarian in the Professional Services Department of Fort Dodge Laboratories.