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Potomac Horse Fever
The first major outbreak of Potomac Horse Fever (PHF) was in Maryland in 1979. At first, farmers and scientist did not know why their horses were acting strangely, and why they were experiencing symptoms such as poor appetite, depression, and diarrhea. It was soon determined that these horses were suffering from Potomac Horse Fever, a serious illness that is caused by bacteria and is fatal to 30 percent of its victims. In order to do research on PHF, scientist had to determine the common location of the illness, the life cycle of the insect that carries PHF, different traits of this insect, and how the illness is transmitted from one horse to another.

Neorickettsia risticii, formerly known as Ehrlichia risticii, is the tiny bacteria that causes Potomac Horse Fever. Also related to the organism that causes salmon poisoning in dogs, Neorickettsia risticii is usually seen in the early stages of PHF, but is digested before the later stages occur. Once ingested by the horse, this tiny bacterium multiplies in the animal’s intestinal tract, which causes marked inflammation and clinical signs of fever and diarrhea. In a few unique cases, if molecular diagnostics is involved, Neorickettsia risticii can go into the monocyte (type of white blood cell) of the horse. If this happens, not only will the horse suffer from severe depression, but, in addition, the horse will have a low blood count.

Most reports of PHF have been in the United States. For the Neorickettsia risticii to survive, the bacteria need to start out by some type of a body of water. The type of water varies from stream to a lake and beyond that, it even includes a small water puddle. However, it is mainly found in river and streams. This is known because it infects a trematode, which is then eaten by the caddisfly when it is still a larva. When it has not been found near water, it has been thought to travel by the insects that carried it. Another way that it can move to a different location is to be left behind by an animal that has been infected. Also, another main factor in the survival of the Neorickettsia risticii depends on the amount of rainfall in the environment where it lives.

The insect that transmits Potomac Horse fever is the Caddisfly. They are classified as followed: Kingdom Animalia, Phylum Arthropoda, Class Insecta, Order Trichoptera, and Family Limnephilidae. Trichoptera means “hair wings.” The adult caddisfly have silk like hair called setae, which is a distinct characteristic of that order. They are also mostly nocturnal and attracted to light. The caddis fly does not live a very long life. They can be herbivores, scavengers, or predators. Unlike the adult caddisflies, the larvae live in aquatic environments. There are many other names for the caddisfly such as sedge, shadfly, periwinkle, and hellgrammite.

This particular parasite breeds specifically on the eastern coast of the United States. Unfortunately, researchers are discovering this horse directed parasite in other places in the United States and also Canada. The parasite latches onto ticks and caddisflies usually and this is the major way that the disease is transmitted. PHF is seasonal but it generally occurs at high rates between late spring and fall. The peak of this infectious disease is almost always during the summer when tick and mosquitoes are roaming freely and are more active, thus enabling them to attack and infect more horses. Immediate symptoms are high fevers, diarrhea, and extreme decrease in appetite. A vaccination can be given to horses in order to protect them against PHF, but if a horse owner is not careful or they do not take the proper precautions, there is nothing that can truly prevent a mosquito or tick bite from occurring.