Potomac Horse Fever in Mules: Diarrhea, Laminitis Risk, and Treatment

Potomac horse fever, often shortened to PHF, is a serious bacterial disease of equids caused by Neorickettsia risticii. It causes acute inflammation of the intestinal tract, so affected mules may develop fever, depression, reduced appetite, colic signs, and diarrhea. In severe cases, the illness can trigger dehydration, endotoxemia, and laminitis, which is one of the most feared complications.

Although most published research focuses on horses, mules are equids and can be affected in a similar way. That means a mule with sudden fever, loose manure, or signs of foot pain during warm-weather months should be taken seriously. PHF is seen most often in spring through early fall, especially around creeks, rivers, ponds, and other freshwater habitats where aquatic insects are common.

One important point for pet parents: PHF itself is not considered naturally contagious from one equid to another. Still, any mule with diarrhea should be managed carefully until your vet rules out contagious causes such as Salmonella or clostridial disease. Early veterinary care matters because prompt treatment can shorten the course of illness and may reduce the chance of severe laminitis.

PHF is caused by the bacterium Neorickettsia risticii. The organism has a complex life cycle involving freshwater snails, trematodes called flukes, and aquatic insects. Equids are thought to become infected when they accidentally eat infected aquatic insects or ingest infective stages associated with freshwater sources.

Insects linked to PHF include mayflies, caddisflies, damselflies, dragonflies, and stoneflies. Risk tends to be higher in warm months and in areas near streams, rivers, irrigation ditches, ponds, or wet pastures. Barn lights can attract flying aquatic insects at night, which may increase the chance that insects end up in feed tubs, water troughs, or hay.

PHF is not usually spread by routine nose-to-nose contact. However, because diarrhea in equids has several possible causes, your vet may still recommend temporary isolation and manure precautions until testing is complete. In practical terms, the biggest environmental risk is exposure to aquatic insect habitats rather than direct spread from another mule.

Your vet will start with the full picture: season, geography, access to freshwater, fever, diarrhea, appetite changes, and any early signs of laminitis. PHF can look very similar to other causes of equine colitis, including Salmonella and clostridial disease, so diagnosis usually involves both examination and targeted testing.

The most useful confirmatory test is PCR for Neorickettsia risticii. To improve the chance of finding the organism, many labs and clinicians recommend testing both EDTA whole blood and feces, because the bacterium may not be present in both samples at the same time. Bloodwork may also show inflammatory changes, dehydration, electrolyte problems, or changes consistent with endotoxemia, but these findings are supportive rather than specific.

Serology such as IFA can sometimes be performed, but single antibody titers are less helpful because false positives occur and vaccination history can confuse interpretation. For that reason, your vet may begin treatment before test results return if the clinical picture strongly suggests PHF. Rapid improvement after tetracycline therapy can support the diagnosis, but it does not replace proper testing.

Prevention focuses on lowering exposure to infected aquatic insects and discussing vaccination with your vet if you live in an endemic area. Practical steps include keeping feed and water as clean as possible, avoiding placement of bright lights over troughs or near stalls at night, and reducing access to areas where large numbers of aquatic insects collect. If possible, limit grazing or feeding right next to freshwater edges during peak insect seasons.

Clean troughs and buckets regularly, and remove dead insects from water sources when you see them. Good barn lighting choices matter more than many pet parents realize. Lights can attract mayflies and similar insects, which may then fall into water or feed. These steps do not eliminate risk, but they can reduce exposure.

A commercial killed PHF vaccine is available, and Merck lists it as a regional risk-based vaccine rather than a core vaccine. Protection in the field is variable, likely because different strains circulate, so vaccination should be viewed as one layer of prevention rather than a guarantee. In areas where PHF is common, your vet may still recommend vaccination because it may lessen disease severity in some equids.

Even with prevention, warm-season fever or diarrhea in a mule deserves prompt veterinary attention. Early recognition remains one of the most important tools for reducing dehydration, shortening illness, and protecting the feet from laminitis.