Potomac Horse Fever
Encyclopedia
Potomac Horse Fever is a potentially-fatal febrile illness affecting horses caused by the intracellular bacterium Neorickettsia risticii. PHF is also known as Shasta River Crud and Equine Monocytic Ehrlichiosis. It was first described in areas surrounding the Potomac River
northwest of Washington, D.C.
, in the 1980s, but cases have been described in many other parts of the United States, such as Minnesota
, California
, and Pennsylvania
. Currently, it is found in more than 40 states within the US, as well as in Canada.
l bacterium.
The vector of Neorickettsia risticii is believed to be a trematode (fluke). The life cycle of the fluke takes it through freshwater snail
s and back into water, where it is ingested by the larval stages of several aquatic insects, including caddis flies and mayflies. It is thought that the main mode of infection is by accidental ingestion of infected adult insects, who may fly into barns and die in stalls or on pastures after eclosure. Experimental infection has been produced with oral administration of infected insects and subcutaneous inoculation of N. risticii. All attempts to transmit the disease using ticks have failed.
Several outbreaks of PHF have been found to coincide with mass emergences of burrowing mayflies of the genus Hexagenia; these insects hatch en masse and may be found littering the ground in nearby stables, where they are attracted by light.
The entire natural history and life cycle of N. risticii has yet to be elucidated, but bats and birds may be wild reservoirs of infection.
Unlike other causes of acute colitis
in horses, such as Salmonella
and Clostridium
, PHF is not spread directly from horse to horse.
-like symptoms, decreased manure production, profuse watery non-fetid diarrhea endotoxemia, edema
due to protein imbalances, abortion by pregnant mares, and acute laminitis
(20-40% of cases). Infected horses founder usually within 3 days of the initial symptoms, thought to be secondary to endotoxemia. Death may occur and is usually due to severe laminitis leading to founder.
Horses may not always display any other symptoms beyond a fever.
s or PCR
testing to look for the bacterium in the blood and feces. However, most veterinarian
s opt to initiate treatment right away, as the disease can progress quite quickly. Veterinarians may also run complete blood count
s and chemistry and electrolyte
panels to determine the course of care. Radiographs may be taken to track the progress of laminitic horses.
. Mild cases may be treated with oral doxycycline
, while severe cases are usually treated with intravenous oxytetracycline
.
Supportive care for severe cases is aimed at minimizing the effects of endotoxemia and preventing laminitis. This may include intravenous fluids
and electrolytes to counteract the diarrhea; NSAIDs such as Banamine (flunixin meglumine
); intravenous dimethyl sulfoxide
; administration of products such as Biosponge or activated charcoal via nasogastric tube to bind endotoxin
s; polymyxin B
or plasma for endotoxemia; supportive shoeing; low doses of intramuscular acepromazine
; and pentoxifylline.
is available for PHF, it does not cover all strains of the bacterium, and recent vaccine failures seem to be on the rise. Additionally, the vaccine usually produces a very weak immune response, which may only lessen the severity of the disease rather than prevent it.The vaccine is administered twice a year, in early spring and in early summer, with the first one inoculation given before the mayflies emerge and the second administered as a booster.
Some veterinarians have started making recommendations for farm management to try to prevent this disease:
Potomac River
The Potomac River flows into the Chesapeake Bay, located along the mid-Atlantic coast of the United States. The river is approximately long, with a drainage area of about 14,700 square miles...
northwest of Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....
, in the 1980s, but cases have been described in many other parts of the United States, such as Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, and Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
. Currently, it is found in more than 40 states within the US, as well as in Canada.
Cause
The causative agent of PHF is Neorickettsia risticii (formerly Ehrlichia risticii), an intracellular rickettsiaRickettsia
Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...
l bacterium.
Transmission
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s and back into water, where it is ingested by the larval stages of several aquatic insects, including caddis flies and mayflies. It is thought that the main mode of infection is by accidental ingestion of infected adult insects, who may fly into barns and die in stalls or on pastures after eclosure. Experimental infection has been produced with oral administration of infected insects and subcutaneous inoculation of N. risticii. All attempts to transmit the disease using ticks have failed.
Several outbreaks of PHF have been found to coincide with mass emergences of burrowing mayflies of the genus Hexagenia; these insects hatch en masse and may be found littering the ground in nearby stables, where they are attracted by light.
The entire natural history and life cycle of N. risticii has yet to be elucidated, but bats and birds may be wild reservoirs of infection.
Unlike other causes of acute colitis
Colitis
In medicine, colitis refers to an inflammation of the colon and is often used to describe an inflammation of the large intestine .Colitides may be acute and self-limited or chronic, i.e...
in horses, such as Salmonella
Salmonella
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...
and Clostridium
Clostridium
Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
, PHF is not spread directly from horse to horse.
Signs and Symptoms
Signs and symptoms of PHF include acute-onset fever, depression (sometimes profound), inappetance, mild colicHorse colic
Colic in horses is defined as abdominal pain, but it is a clinical sign rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. The most common forms of colic...
-like symptoms, decreased manure production, profuse watery non-fetid diarrhea endotoxemia, edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
due to protein imbalances, abortion by pregnant mares, and acute laminitis
Laminitis
Laminitis is a disease that affects the feet of ungulates. It is best known in horses and cattle. Symptoms include lameness, and increased temperature in the hooves...
(20-40% of cases). Infected horses founder usually within 3 days of the initial symptoms, thought to be secondary to endotoxemia. Death may occur and is usually due to severe laminitis leading to founder.
Horses may not always display any other symptoms beyond a fever.
Diagnosis
Diagnosis of PHF is accomplished by measuring antibody titerAntibody titer
An antibody titer is a measurement of how much antibody an organism has produced that recognizes a particular epitope, expressed as the greatest dilution that still gives a positive result. ELISA is a common means of determining antibody titers....
s or PCR
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
testing to look for the bacterium in the blood and feces. However, most veterinarian
Veterinarian
A veterinary physician, colloquially called a vet, shortened from veterinarian or veterinary surgeon , is a professional who treats disease, disorder and injury in animals....
s opt to initiate treatment right away, as the disease can progress quite quickly. Veterinarians may also run complete blood count
Complete blood count
A complete blood count , also known as full blood count or full blood exam or blood panel, is a test panel requested by a doctor or other medical professional that gives information about the cells in a patient's blood...
s and chemistry and electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
panels to determine the course of care. Radiographs may be taken to track the progress of laminitic horses.
Treatment
N. risticii responds well to tetracycline antibioticsTetracycline antibiotics
Tetracyclines are a group of broad-spectrum antibiotics whose general usefulness has been reduced with the onset of bacterial resistance. Despite this, they remain the treatment of choice for some specific indications....
. Mild cases may be treated with oral doxycycline
Doxycycline
Doxycycline INN is a member of the tetracycline antibiotics group, and is commonly used to treat a variety of infections. Doxycycline is a semisynthetic tetracycline invented and clinically developed in the early 1960s by Pfizer Inc. and marketed under the brand name Vibramycin. Vibramycin...
, while severe cases are usually treated with intravenous oxytetracycline
Oxytetracycline
Oxytetracycline was the second of the broad-spectrum tetracycline group of antibiotics to be discovered.Oxytetracycline works by interfering with the ability of bacteria to produce proteins that are essential to them. Without these proteins the bacteria cannot grow, multiply and increase in numbers...
.
Supportive care for severe cases is aimed at minimizing the effects of endotoxemia and preventing laminitis. This may include intravenous fluids
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...
and electrolytes to counteract the diarrhea; NSAIDs such as Banamine (flunixin meglumine
Flunixin Meglumine
Flunixin is a non-steroidal anti-inflammatory drug , analgesic, and antipyretic used in horses. It is often formulated as the meglumine salt. In the United States, it is regulated by the U.S...
); intravenous dimethyl sulfoxide
Dimethyl sulfoxide
Dimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
; administration of products such as Biosponge or activated charcoal via nasogastric tube to bind endotoxin
Endotoxin
Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...
s; polymyxin B
Polymyxin B
Polymyxin B is an antibiotic primarily used for resistant gram-negative infections. It is derived from the bacterium Bacillus polymyxa. Polymyxin B is a mixture of two closely related compounds, polymyxin B1 and polymyxin B2. It has a bactericidal action against almost all gram-negative bacilli...
or plasma for endotoxemia; supportive shoeing; low doses of intramuscular acepromazine
Acepromazine
Acepromazine or acetylpromazine is a phenothiazine derivative antipsychotic drug. It was first used in humans in the 1950s, but is now little used in humans...
; and pentoxifylline.
Prevention
While a vaccineVaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
is available for PHF, it does not cover all strains of the bacterium, and recent vaccine failures seem to be on the rise. Additionally, the vaccine usually produces a very weak immune response, which may only lessen the severity of the disease rather than prevent it.The vaccine is administered twice a year, in early spring and in early summer, with the first one inoculation given before the mayflies emerge and the second administered as a booster.
Some veterinarians have started making recommendations for farm management to try to prevent this disease:
- Maintaining riparianRiparian zoneA riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the fifteen terrestrial biomes of the earth. Plant habitats and communities along the river margins and banks are called riparian vegetation, characterized by...
barriers along bodies of water may encourage aquatic insects to stay near their places of origin. - Turning off outside lights around the barn will prevent insects from being attracted.
- Cleaning water buckets and feed areas frequently and keeping food covered will reduce the chance that the horse will accidentally ingest infected insects.