Listeriosis, also referred to as Circling
Disease or Silage Sickness, is a sporadic bacterial infection
caused by Listeria monocytogenes. Listeriosis
is a worldwide disease, and affects a wide variety of mammalian
and avian species, including man. Encephalitis is the most
frequently recognized form of listeriosis of animals. The
infection most commonly occurs in adult ruminants that are
being fed contaminated silage.
Listeria monocytogenes is a
small, motile, gram-positive nonspore-forming cocco-bacillus.
This ubiquitous saprophyte lives in a plant-soil environment
and can be found in soil, vegetables, sewage, genital secretions
and nasal mucous of apparently healthy animals. The organism
is very resistant to drying and can survive up to two years
in dry soil and feces. It is also capable of growing well
under a wide range of temperatures, 4 - 44°
There are four common manifestations
of listeriosis: encephalitis in adult ruminants, septicemia
in monogastrics and neonatal ruminants, abortion and perinatal
deaths in all species, and mastitis in ruminants. It is uncommon
for all forms of listeric infections to occur at one time
within a flock/herd and for the majority, the infections are
subclinical. However, when animals become stressed, immunocompromised
or pregnant, clinical listeriosis often develops.
The route of infection seems to vary
in accordance with the different clinical syndromes: encephalitis
by small wounds in the buccal mucosa, while septicemia and
abortions come from ingestion and inhalation.
Encephalitis is most prevelant in late
winter and early spring when animals are confined and silage
feeding is greatest. Silage fed from trenches or pits is most
often the source of infection. The sides and lower, damper
layers of silage from these pits seem to be the most contaminated
because these sites are often exposed to air. This results
in an aerobic decomposition of silage and an increase in pH
(>5) which enhances the multiplication of L. monocytogenes.
L. monocytogenes commonly enters abrasions
in the oral mucosa or at sites where teeth have fallen out
and ascends via the trigeminal nerve to the brainstem. Here
a unilateral, localized lesion is produced within the medulla
oblongata and pons.
The number of animals affected clinically
in an outbreak of listeriosis is usually low but mortality
is extremely high. Sheep and goats are most susceptible to
Listeria infections and are overcome by an acute disease
with death occurring 4-48 hours after the onset of clinical
signs. In cattle, infections are sporadic, less acute and
most survive for 4-14 days. Spontaneous recovery may occur,
but permanent CNS injury is frequent in these animals.
The clinical signs of affected animals include
depression, fever, disorientation, and an indifference to
their surroundings. They often separate themselves and crowd
into corners and head press. They stumble and circle continuously.
An associated head tilt is also commonly seen in these animals.
Facial paralysis characterized by a drooping ear, dilated
nostril and lowered eyelid (ptosis) on the same side as the
lesion often develops. Intermittent twitching and paralysis
of facial, throat and tongue muscles are usually present resulting
in tongue protrusion, excessive salivation and dysphasia.
A progressive paralysis develops throughout the course of
the disease and in the terminal stages, the animal often falls
and is unable to rise. Exhaustion followed by coma and death
rapidly occurs once the animal has become recumbent.
Histological examination of brain tissue,
preferably the brainstem, is necessary to demonstrate the
monocytic perivascular cuffing and microabscesses that are
characteristic of the disease.
Septicemic or visceral listeriosis is commonly
observed among monogastric animals, including pigs, dogs,
cats, rabbits and chinchillas, as well as neonatal ruminants.
The clinical signs include an acute onset of depression, fever,
anorexia, coughing and respiratory distress, diarrhea, prostration
and death. The principle lesion is focal hepatic necrosis.
For the septicemic form, the finding of multiple necrotic
foci in any organ, especially the liver, is often highly suggestive
All pregnant domestic animals are susceptible
to Listeria and an infection at this time often results
in placentitis, fetal deaths, abortions, stillbirths, and
neonatal deaths. Abortions in cattle are sporadic and occur
within the last third of pregnancy. Abortion storms are more
common in sheep and most abortions occur after the 12th week
of pregnancy. When the placental tissues are retained, a secondary
metritis often develops. This metritis is long lasting but
has little or no effect on the animal's reproductive future.
These animals commonly shed the organism in milk and vaginal
secretions for a period of two months following an abortion
Several suggestive gross lesions can be demonstrated
in aborted fetuses. These include small yellow foci of necrosis
in the liver, shallow abomasal erosions and a yellow-orange
meconium. The fetus is often edematous and autolyzed masking
these lesions. Dams and ewes should also be examined for placentitis
Mastitis is a rare manifestation of listeriosis.
It affects only a single quarter and is unresponsive to antibiotics.
Although uncommon, it does occur, and should be considered
when dealing with chronic cases of mastitis in cattle.
The definitive diagnosis can only be made
by the isolation and identification of L. monocytogenes.
Brain tissue, aborted placenta and fetus are the preferred
specimens for culture. The organism is often difficult to
grow and is commonly missed if not specifically requested.
In the past, cold enrichment procedure incubated at 4°
C was used for the isolation of L. monocytogenes. Recently,
a more selective media established by USDA scientists, were
adopted by the ADDL bacteriology lab. This method will decrease
the long incubation time (1-2 months) to the cold enrichment
protocol of 1-2 weeks. In addition, a PCR for L. monocytogenes
has been developed by the ADDL bacteriology lab to detect
L. monocytogenes. Other tests like immunohistochemical
testing, using the perioxidase-antiperoxidase (PAP) method,
has been used for the detection of L. monocytogenes
antigen within brain tissue of infected animals. Both PCR
and immuno-histochemistry methods are rapid and confirmatory,
especially when inappropriate materials are submitted for
other diagnostic methods.
Due to its ubiquitous nature, minimizing
the opportunity for exposure seems to be the best preventive
alternative against listeriosis. When an outbreak occurs,
the affected animals should be immediately treated and isolated
and those that have died should be destroyed or removed from
the premises. Buildings should be thoroughly disinfected and
cleaned and all bedding and feed should be burned. Silage
feeding should be reduced and if spoiled, should be avoided.
Other recommendations for silage fed herds/flocks may include:
minimize soil contamination when making silage and filling
the trench, perform routine silage testing and use additives
to improve the fermentation process. It is extremely hard
to totally eliminate L. monocytogenes from a
surrounding environ-ment but the incidence of disease within
a herd or flock can be significantly reduced if these recommendations
Listeriosis is also an important zoonotic
disease. Aborted fetuses and necropsies of septicemic animals
present the greatest hazard for veterinarians and agricultural
workers. People have been reported to develop meningitis,
septicemia, and a papular exanthema on their hands and arms
after handling such tissues. Pregnant women are susceptible
and at great risk of aborting if they are not properly protected
from infection. The young, the elderly, and the immuno-compromised
are most susceptible to infection and need to take precaution
as well. The sources of infection include milk that was improperly
pasteurized or contaminated after pasteurization, cheese by-products,
and raw vegetation. Just as in animals, the only reasonable
alternative for prevention is to minimize exposure and ensure
the use of good personal and food hygiene to reduce the incidence
of listeriosis within the human population.
- by Julie Stone, Class of 1998
- edited by Ching Ching Wu, DVM, PhD