Equine infectious disease are commonly caused by bacteria and viruses. However, there are a number of protozoan and fungal diseases of notice amongst working equines.
Diagnosis can be difficult, and knowledge of the typical clinical signs and organs affected can help you to formulate a comprehensive list of differential diagnosis.
While there are thousands of protozoa and fungi that can colonise equines, the following organisms can cause infections of notice:
Protozoa
Besnoitiosis. Affecting the skin, subcutaneous tissue, mucous membranes, blood vessels, and other tissues of cattle, horses and donkeys in tropical and subtropical regions. Caused by Besnoitia bennetti, a protozoa that multiplies asexually in endothelial cells, macrophages, and various other cells in which it produces thick-walled cysts containing what are called bradyzoites.
Cryptosporidiosis. Affecting the intestinal system of horses, ruminants and small animals, as well as humans. It causes self limiting diarrhoea with mild-moderate intestinal damage and scarring. Caused by the coccidian parasite, Cryptosporidium parvum.
Equine babesiosis. Affecting the circulatory system, where it invades red blood cells, replicating within them ahead of causing their breakdown. The disease is therefore characterised by a haemolytic anaemia, malaise, inappetence, fever, depression, jaundice and haemoglobinuria in acute cases. Caused by the intracellular protozoal blood parasites Babesia equi and B. caballi.
Equine protozoal myeloencephalitis. Affecting the nervous system, causing muscular atrophy, blindness, facial paralysis, dysphagia, circling and ataxia. Present in North and South America, it is caused by the protozoan Sarcocystis neurona.
Equine trypanosomiasis. There are 3 distinct forms of the disease (Nagana, Surra and Dourine) which are caused by flagellated protozoa of the genus Trypanosoma, and transmitted by flies (of the tse-tse family). Signs include intermittent fever, anorexia, loss of condition, anaemia, emaciation, edema of the genitalia and limbs.
Neosporosis. A multiorgan infection which is characterised by organ failure. It can therefore manifest as hepatitis, pneumonia, ulcerative dermatitis, blindness, ataxia, myalgia, progressive paralysis, and finally death. The clinical disease is sometimes referred to as protozoal myositis. Caused by the coccidia-like, obligate intracellular protozoan Neospora caninum, it is very rare in equines.
Fungi
Conidiobolus coronatus (conidiobolus). A rare, chronic disease occurring mainly in tropical Africa, South and Central America, and South-East Asia (including India), causing progressive nasal congestion and crusting, sinusitis and granulomatous ethmoid disease.
Cryptococcus neoformans (cryptococcosis). Sporadically causing pneumonia and central nervous disease.
Histoplasma capsulatum (histoplasmosis). Widely spread across Africa and Asia, causing a suppurative, ulcerative disease (epizootic lymphangitis) of the skin, lungs and eyes.
Aspergillus spp. Commonly affecting the upper airway, where it derives nutrition from main arteries in the guttural pouches, causing potential death by arterial rupture.
Fusarium spp (fusarium). Affecting primarily the cornea and conjunctiva of horses, causing serious ocular disease