May
25
Written by:
Surgi-Care Center for Horses
5/25/2011 2:36 PM
Rhodococcus
equi is a bacteria that lives in the soil. It is an important cause of disease
in foals between 3 weeks and 6 months of age. All farms are likely to be
infected with the bacteria. Some farms may have a severe problem with the
clinical disease that can be devastating to the farm, while other farms may
have sporadic problems, but most farms will develop no clinical problems with
their foals.
There are
several clinical manifestations of Rhodococcus equi in foals. The most common
is pneumonia with abscessation in the lungs. This form of the disease develops
by inhalation of dust particles containing the organism. Early signs may
include a slight increase in respiratory rate and a mild fever. As the disease progresses, signs
include decreased appetite, lethargy, fever, increased respiratory rate, and
increased effort in breathing. Coughing and nasal discharge are inconsistent findings.
Some foals will develop a more devastating form, in which they are found dead
or found with severe respiratory distress with no previous signs.
Other,
less common manifestations include intestinal disease, which may or may not be
associated with diarrhea. Abdominal abscessation may develop, causing adhesions
of the gastrointestinal tract, leading to colic. Sometimes, a polysynovitis
(inflammation of some or all joints) may develop with the pneumonia, resulting
in a stiff gait. This frequently resolves when the pneumonia resolves. Spread
of the bacteria from the lungs or gastrointestinal tract may occasionally
result in a septic arthritis. In this case, they are quite painful and lame.
Even with aggressive therapy, the prognosis for life is guarded.
The
definitive diagnosis for Rhodococcus equi infection requires bacterial culture
and cytology of a transtracheal
wash. The procedure consists of passage of a tube into the trachea. Sterile
saline is flushed into the trachea and aspirated back immediately. A sample of
the fluid present in the trachea mixes with the saline and provides a sample to
diagnose Rhodococcus equi infection. It is important to also obtain radiographs
of the lungs to visualize the extent of the pneumonia and perform ultrasonography
of the chest to visualize any fluid or lung consolidation. Finally, bloodwork
is useful to identify the degree of infection and inflammation in the
bloodstream.
Rhodococcus
equi infection is most commonly treated with the antibiotics, erythromycin and
rifampin. The combination of these
two drugs together provides a synergistic effect and allows effective penetration
into abscesses. Resolution of clinical signs, normalization of the bloodwork,
and radiographic resolution of the lung lesions are commonly used to guide the
duration of therapy, which typically ranges from 4 to 9 weeks. The long-term
therapy is expensive but essential since relapses can occur if therapy is
discontinued early. Potential side effects of the erythromycin therapy include diarrhea,
mild colic, partial anorexia, increased respiratory rate, and fever. A newer
drug with fewer potential side effects and more effectiveness is clarithromycin
combined with rifampin. Although this combination is superior to the
erythromycin-rifampin, it is significantly more expensive. It is also associated
with diarrhea, but in most cases it is mild and self-limiting.
Prevention
of Rhodococcus equi infection is difficult, but there are important management
factors to help reduce the risk of infection. It is important to house foals in
well-ventilated, dust-free areas, and avoid dirt paddocks and crowding.
Pastures should be rotated and irrigated to reduce dust formation. Finally,
infected foals should be isolated, as they are the major source of
contamination of the environment with the bacteria.
Early
recognition of foals with Rhodococcus equi infection will help reduce foal
losses and prevent the spread of the organism. Careful daily observation of
foals as well as bloodwork performed on all foals on the farm at 2 to 4 week
intervals will help early detection of the disease. It is useful to perform periodic
ultrasonographic examination of the chest of all foals on farms with a known
history of infection.
Farms
that have not previously had problems with Rhodococcus equi infections should
follow preventive measures to avoid future problems. Farms with known history
of infected foals should consider establishing a program with their
veterinarian to monitor foals for detection of problems as soon as they arise.
Contact Surgi-Care Center for
Horses at 813-643-7177 or email lkuebelbeck@surgi-carecenter.com with
any questions regarding this topic.