Neorickettsia risticii is a bacterium classified within the order Rickettsiales and family Anaplasmataceae. It is an obligate intracellular, gram-negative bacterium, meaning it must live and reproduce inside the cells of other organisms. This bacterium causes a significant disease in horses.
Potomac Horse Fever Explained
The disease caused by Neorickettsia risticii is known as Potomac Horse Fever (PHF). This condition primarily affects the horse’s gastrointestinal system, particularly the large colon, leading to inflammation called enterocolitis. Clinical signs in affected horses can vary widely, ranging from mild to severe. Common symptoms include a high fever, which can reach up to 107°F (41.6°C), depression, and a loss of appetite.
Horses may also experience colic and reduced gastrointestinal sounds. Diarrhea, which can be watery and profuse, is a frequent sign, though it only develops in about 10% to 30% of cases. A serious complication of PHF is laminitis, an inflammation of the laminae within the hoof, which can occur with or without diarrhea and may progress to a severe state. Pregnant mares infected with Neorickettsia risticii are at risk of abortion or giving birth to compromised foals due to the bacteria’s ability to cross the placenta. PHF is not considered directly contagious between horses through casual contact, although horses in the same barn may be exposed to a common source of infection.
Understanding Its Transmission
The transmission pathway of Neorickettsia risticii involves several intermediate hosts in an aquatic environment. The bacterium lives within parasitic flatworms, or flukes. These flukes utilize freshwater snails as their first intermediate host.
The flukes then infect various aquatic insects, such as caddisflies, mayflies, damselflies, and dragonflies, which carry the Neorickettsia risticii bacteria. Horses become infected by accidentally ingesting these infected aquatic insects or their larval stages. This often happens when the insects, attracted to barn lights at night, fall into water troughs or feed, or when horses graze near contaminated water sources.
Infection can also occur if horses ingest water containing free-living trematode stages. The incubation period after a horse ingests the infected insects ranges from one to three weeks before clinical signs appear. The disease is seasonal, occurring during the warmer months from late spring to early fall, peaking in mid-to-late summer, when these aquatic insects are most active and abundant.
Diagnosis and Treatment
Diagnosing Potomac Horse Fever involves observing clinical signs, considering the seasonal and geographical occurrence of the disease, and conducting specific laboratory tests. Veterinarians often suspect PHF based on symptoms like fever, depression, and diarrhea, especially if the horse has been near freshwater sources. Definitive diagnosis relies on identifying Neorickettsia risticii DNA in samples from the infected horse.
Polymerase chain reaction (PCR) testing of blood or fecal samples is considered the most reliable method for detecting the bacterial DNA. While serologic tests, such as the indirect fluorescent antibody (IFA) test, measure antibody titers, their diagnostic value is limited due to potential false positives or interference from vaccination.
The primary treatment for PHF involves the administration of specific antibiotics, with oxytetracycline being the most common choice, typically given intravenously for 5 to 7 days. Many horses show significant improvement within 12 to 72 hours of starting antibiotic therapy. Supportive care is also provided to manage symptoms, including intravenous fluids to manage dehydration and electrolyte imbalances, and non-steroidal anti-inflammatory drugs (NSAIDs) like flunixin meglumine to manage fever, pain, and inflammation. For horses that develop laminitis, additional care such as cryotherapy (icing the hooves) is necessary.
Preventing Potomac Horse Fever
Preventing Potomac Horse Fever involves vaccination as a primary strategy. Vaccines are available for horses at risk of exposure, typically administered in the spring before the peak season for the disease. While vaccination may not offer complete protection, it can significantly reduce the severity of clinical signs if a horse contracts the disease, leading to quicker recovery and reduced risk of complications.
Environmental management practices also help reduce exposure. Limiting horses’ access to stagnant water sources, such as ponds, creeks, and rivers, minimizes contact with infected aquatic insects and snails. Turning off barn and stable lights at night is a practical measure, as these lights attract the aquatic insects that carry Neorickettsia risticii. Covering feed and water sources can prevent contaminated insects from falling into them. Regularly cleaning water troughs and feeders, especially in pastures near waterways, reduces the risk of accidental ingestion.