To the Editor: Mycobacterium avium subsp. hominissuis infection is often detected in pigs and humans (1–3). In most cases, the main sources of this agent are environmental (4,5). During the past few years, 2 hosts infected by this agent, dogs (6) and pet parrots (7), were identified as a possible source of infection for immunocompromised humans who may have close contact with animals. We report massive M. avium subsp. hominissuis infection in 2 sibling riding-type Fjord horses from an amateur-run horse-breeding farm.

The first horse, a 2-year-old colt, was admitted to a veterinary clinic in the Czech Republic in February 2009 with diarrhea and progressive weight loss of 3 weeks’ duration. Multiple diagnostic procedures produced inconclusive results. Cyathostomosis was suspected, so moxidectin was administered twice, and prednisolone was given for 10 consecutive days. The clinical status of the horse initially improved but worsened after 3 weeks. Ultrasonographic examination of the peritoneal cavity showed a nodular mass and a nonperistaltic, thickened portion of the small intestine wall in the left ventrocranial region. Exploratory celiotomy showed enlargement of the mesenteric and colonic lymph nodes and multiple local thickenings of the small intestine wall, large colon, and cecum. The horse was euthanized. Specimens of enlarged lymph nodes and intestinal content were taken during necropsy for histopathologic and microbiologic examination. Microscopically, acid-fast rods (AFR) after Ziehl-Neelsen staining were observed, and quantitative real-time PCR (qPCR) showed 2.89 × 10⁵ and 1.47 × 10⁴ M. avium subsp. hominissuis cells per 1 g of intestinal content and mesenteric lymph node, respectively (8).

The second case, a 1-year-old full sister to the colt described above, was admitted in July 2009 after 1 month of lethargy, weight loss, diarrhea, and nasal discharge. Ultrasonographic examination of the abdominal cavity showed an increased amount of peritoneal fluid and nonperistaltic, corrugated, and thickened parts of the small intestine in the left caudal region. Local thickening of the jejunum and ileum were found during exploratory celiotomy; no lesions on the cecum or colon were observed macroscopically. Mesenteric lymph nodes were enlarged. Microscopically, AFR were observed, and qPCR showed 3.36 × 10⁶ M. avium subsp. hominissuis cells per 1 g of mesenteric lymph node (8). Treatment with clarithromycin and rifampin was begun, but the condition of the filly improved only temporarily. She was euthanized after 4 months because of progressively worsening condition. Postmortem examination showed enlarged colonic lymph nodes with small nodular lesions, hyperemia of the colon mucosa, and corrugation and thickening of the colonic wall. For further examination, samples of feces, colonic lymph nodes and wall, liver, mesenteric lymph nodes, kidney, spleen, and diaphragm were taken. Ziehl-Neelsen staining of tissue smears demonstrated AFR in different tissues. Culture examination following the described method (2) and qPCR confirmed M. avium subsp. hominissuis infection; quantities of this agent were 6.31 × 10⁵ and 2.47 × 10¹¹ in 1 g of feces and mesenteric lymph nodes, respectively (Table).

According to a review (9), infections caused by M. avium subsp. hominissuis have been described in only 6 horses until now. We presume that M. avium subsp. hominissuis infection in both these horses could have been caused by some immunodeficiency related to a genetic predisposition. The shedding of this agent in feces indicates that infected horses can also pose a health risk to humans, particularly immunocompromised persons. M. avium subsp. hominissuis infection is frequently observed in children, in whom it can cause peripheral lymphadenopathy (10). Currently, hippotherapy is a frequently used recreational activity in some countries for various patients, e.g., for children with cerebral palsy. Hippotherapy thus may be associated with a potential risk for humans in contact with clinically ill M. avium subsp. hominissuis–infected horses.

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