TRANSMISSION OF MANNHEIMIA HAEMOLYTICA FROM DOMESTIC
SHEEP (OVIS ARIES) TO BIGHORN SHEEP (OVIS CANADENSIS):
UNEQUIVOCAL DEMONSTRATION WITH GREEN FLUORESCENT
Paulraj K. Lawrence,1Sudarvili Shanthalingam,1Rohana P. Dassanayake,1
Renuka Subramaniam,1Caroline N. Herndon,1Donald P. Knowles,2
Fred R. Rurangirwa,1William J. Foreyt,1Gary Wayman,3Ann Marie Marciel,4Sarah K.
Highlander,5and Subramaniam Srikumaran1,6
1Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington
2Animal Disease Research Unit, United States Department of Agriculture, Pullman, Washington 99164-7040, USA
3Department of Comparative Anatomy, Physiology and Pharmacology, Washington State University, Pullman,
Washington 99164-7040, USA
4Department of Radiology, Baylor College of Medicine, One Baylor Plaza, MS280, Houston, Texas 77030-3498, USA
5Department of Molecular Virology and Microbiology, Baylor College of Medicine, One Baylor Plaza, MS280, Houston,
Texas 77030-3498, USA
6Corresponding author (email: email@example.com)
pneumonia when commingled with domestic sheep (Ovis aries) but did not conclusively prove
that the responsible pathogens were transmitted from domestic to bighorn sheep. The objective of
this study was to determine, unambiguously, whether Mannheimia haemolytica can be transmitted
from domestic to bighorn sheep when they commingle. Four isolates of M. haemolytica were
obtained from the pharynx of two of four domestic sheep and tagged with a plasmid carrying the
genes for green fluorescent protein (GFP) and ampicillin resistance (APR). Four domestic sheep,
colonized with the tagged bacteria, were kept about 10 m apart from four bighorn sheep for 1 mo
with no clinical signs of pneumonia observed in the bighorn sheep during that period. The
domestic and bighorn sheep were then allowed to have fence-line contact for 2 mo. During that
period, three bighorn sheep acquired the tagged bacteria from the domestic sheep. At the end of
the 2 mo of fence-line contact, the animals were allowed to commingle. All four bighorn sheep
died 2 days to 9 days following commingling. The lungs from all four bighorn sheep showed gross
and histopathologic lesions characteristic of M. haemolytica pneumonia. Tagged M. haemolytica
were isolated from all four bighorn sheep, as confirmed by growth in ampicillin-containing culture
medium, PCR-amplification of genes encoding GFP and ApR, and immunofluorescent staining of
GFP. These results unequivocally demonstrate transmission of M. haemolytica from domestic to
bighorn sheep, resulting in pneumonia and death of bighorn sheep.
Bighorn sheep, domestic sheep, green fluorescent protein, Mannheimia
haemolytica, Ovis canadensis, pneumonia, transmission.
Previous studies demonstrated that bighorn sheep (Ovis canadensis) died of
The large decline in the bighorn sheep
(Ovis canadensis) population in North
America, from an estimated two million at
the beginning of the 19th century to fewer
than 70,000 now (2009) (Buechner, 1960;
Valdez and Krausman, 1999), has been
attributed in part to diseases, particularly
pneumonia caused by bacteriaof thegenera
Mannheimia, Bibersteinia, and Pasteurella
(Coggins, 1988; Miller, 2001). Bighorn
sheep are much-more susceptible to pneu-
monia than are domestic sheep (Ovis aries;
Foreyt, 1994). Since the early 1980s, there
havebeen anecdotalfieldreportsof bighorn
deaths due to pneumonia following contact
with domestic sheep (Foreyt and Jessup,
1982; Coggins, 1988; George et al., 2008).
Bacteria of the genera Mannheimia,
Bibersteinia, and Pasteurella are commen-
sal bacteria in the pharynx and nasal
cavities of domestic and bighorn sheep
(Ward et al., 1990). Experimental inocu-
lation of some of the isolates from
domestic sheep—isolates which do not
readily cause disease in the domestic
sheep—have resulted in fatal pneumonia
in bighorn sheep (Onderka et al., 1988;
Foreyt et al., 1994). In five experimental
Journal of Wildlife Diseases, 46(3), 2010, pp. 706–717
#Wildlife Disease Association 2010
commingling studies conducted by three
investigators, 41 of 43 bighorn sheep died
following contact with domestic sheep
(Onderka and Wishart, 1988; Foreyt,
1989, 1990; Callan et al., 1991). These
findings appeared to confirm earlier re-
ports of the death of bighorn sheep after
contact with domestic sheep, thus incrim-
inating domestic sheep in the induction of
fatal pneumonia in bighorn sheep. Al-
though Mannheimia (Pasteurella) haemo-
lytica, Bibersteinia (Pasteurella) trehalosi,
and Pasteurella multocida were isolated
from the dead bighorn sheep, these
studies did not demonstrate that these
organisms were transmitted from the
domestic sheep to the bighorn sheep. In
some of these studies, the bacteria that
were isolated from the dead bighorn sheep
were not shown to be present in the
domestic sheep. It is possible that the
bacteria responsible for the death of the
bighorn sheep were not carried by the
domestic sheep. It is also conceivable that
these bacteria were present in the domes-
tic sheep, but were not isolated, because
nasal swabs rather than pharyngeal swabs
were obtained or because adequate num-
bers of bacterial colonies from the initial
isolation were not picked up for further
characterization. Even the isolation of
bacteria belonging to the same species,
serotype, or biotype, from the domestic
sheep and bighorn sheep did not demon-
strate that the organism was transmitted
from domestic sheep.
Our objective was to determine, unam-
biguously, whether a respiratory pathogen
can be transmitted from domestic sheep to
bighorn sheep. Multiple genera, species,
and serotypes of bacteria can colonize the
nasal cavities and the pharynx of a single
animal (Ward et al., 1997). Mannheimia
haemolytica, B. trehalosi, and P. multocida
are commonly isolated from pneumonic
lungs of bighorn sheep, (Jaworski et al.,
1998; Kelley et al., 2007; George et al.,
2008). Mannheimia haemolytica consis-
tently causes severe bronchopneumonia
and the rapid death of bighorn sheep
under experimental conditions (Onderka
et al., 1988; Foreyt et al., 1994; Dassa-
nayake et al., 2009). Therefore, we select-
ed M. haemolytica for this study. We
obtained four M. haemolytica isolates
from the nasopharynx of domestic sheep
and tagged them with a plasmid encoding
genes for green fluorescent protein
(GFP), and for beta-lactamase (Bla),
which confers ampicillin
(ApR). The four domestic sheep were
colonized with the tagged bacteria and
allowed to commingle with bighorn sheep
to determine whether there was transmis-
sion of the GFP-tagged bacteria.
MATERIALS AND METHODS
Screening of animals for respiratory pathogens
Experimental protocols were reviewed and
approved by the Institutional Animal Care and
Use Committee (IACUC) at Washington State
Four, clinically normal domestic sheep from
the same flock were selected for the study.
Nasal and pharyngeal swabs, from two groups
of four domestic sheep and four bighorn
sheep, were collected twice at 1- to 2-wk
intervals. The swabs were collected from the
domestic sheep at the beginning of the study
(61 wk and 63 wk prior to the beginning of the
transmission study) to obtain M. haemolytica
isolates for tagging with GFP and ApR. The
bighorn sheep were sampled 42 days and
35 days prior to the beginning of the
transmission study. The swabs were analyzed
for the presence of ovine respiratory disease
(ORD) pathogens by protocols routinely used
at Washington Animal Disease Diagnostic
Laboratory (WADDL; Pullman, Washington,
USA). The pathogens screened for included
the bacteria M. haemolytica, B. trehalosi, and
Mycoplasma ovipneumoniae and the viruses
respiratory syncytial virus (RSV), parainfluen-
za 3 virus (PI-3), bovine herpesvirus1 (BHV-
1), and bovine viral diarrhea virus (BVDV).
Isolation of viruses from nasopharyngeal swabs
The bovine turbinate (BT) cell line was used
for viral propagation because these cells were
known to support the growth of all the above
viruses. Swabs in universal viral transport
medium (BD Biosciences, Sparks, Maryland,
USA) were vortexed, and the medium was
plated onto BT cells in minimal essential
LAWRENCE ET AL.—INTERSPECIES TRANSMISSION OF MANNHEIMIA HAEMOLYTICA707
medium (MEM) supplemented with 10% fetal
bovine serum (FBS; free of antibodies to
known respiratory viruses) and antibiotics
(penicillin-streptomycin 100 IU/ml; gentami-
cin 50 mg/ml; and fungizone 25 mg/ml).
Inoculated cell cultures were incubated at
37 C in a humidified atmosphere of 5% CO2.
The BT cells were observed daily for cyto-
Isolation of M. ovipneumoniae and M. haemolytica
from nasopharyngeal swabs and lungs
Swabs from each animal were streaked onto
blood agar plates and kept at 37 C overnight
under aerobic and anaerobic growth condi-
tions. The bacterial colony morphology on
brain-heart infusion (BHI) sheep blood agar
and triple sugar iron (TSI) medium; Gram
staining; the ability to hydrolyze arabinose,
trehalose, indole, nitrate, xylose, and catalase;
and oxidase activity were used to differentiate
M. haemolytica from B. trehalosi and P.
multocida isolates. Mycoplasma ovipneumo-
niae was isolated by growth on pleuropneu-
monia-like organism broth and selective agar
plates according to a previously described
protocol (Besser et al., 2008).
Serotyping of M. haemolytica isolates
Mannheimia haemolytica strains were sero-
typed using serotype-specific rabbit antisera
obtained from Glynn Frank (National Animal
Disease Center, Ames, Iowa, USA). Cells from
a single colony of overnight growth on a sheep
blood agar plate were swirled for 30 sec in
30 ml of serum on a glass microscope slide.
Agglutination was observed under a dissecting
microscope. Serotype-specific antisera for the
following serotypes were tested: A1, A2, A5,
A6, A7, A8, A9, A10, A11, A12, A13, A14, and
Polymerase chain reaction (PCR) detection of
The PCR assay specific for M. haemolytica
has been described (Dassanayake et al., 2010).
A portion of the gene encoding M. haemolytica
O-sialoglycoprotein endopeptidase (gcp; Gen-
bank accession number AY83967) was ampli-
fied by PCR using primers MhgcpF: 59-AGA
GGC CAA TCT GCA AAC CTC G-39 and
reverse primer MhgcpR: 59-GTT CGT ATT
GCC CAA CGC CG-39. PCRs were carried
out in a final, 50-ml volume with GoTaqH PCR
SuperMix (Promega Inc., Madison, Wisconsin,
USA) with 0.2 mM each primer and 2 ml
bacterial culture. The PCR cycling conditions
consisted of an initial denaturation at 95 C for
5 min followed by 35 cycles of denaturation at
95 C for 30 sec, annealing at 55 C for 30 sec,
and extension at 72 C for 40 sec, and a final
elongation at 72 C for 5 min. The PCR
products were visualized after electrophoresis
in 1.0% agarose gels run at 7.0 V/cm and
staining with ethidium bromide.
PCR detection of M. ovipneumoniae
Both standard PCR and real-time PCR (RT-
PCR) were used. Standard PCR amplification
conditions were essentially the same as previ-
ously described (Besser et al., 2008). Real-time
PCR was developed in-house at WADDL using
the following primers: Movip F: 59-GGG GTG
CGC AAC ATT AGT TA-39; Movip R: 59-CTT
ACT GCT GCC TCC CGT AG-39; and Movip
(Probe): 59-6-FAM-TTA GCG GGG CCA
AGA GGC TGT A-BHQ-1-39 derived from
GenBank sequences EU290066 and NR_
025989 of M. ovipneumoniae. The RT-PCR
was run in an ABI 7500 Fast Thermocycler
(Applied Biosystems, Carlsbad, California,
USA) with the following cycling parameters:
for 600 sec (optics off); Stage 2: 45 repeat cycles
of 95 C for 15 sec (optics off) to denature and
61 C for 60 sec for annealing and extension
(optics on). Test samples were read on the
FAM wavelength. Those with a cycle threshold
below 40.0 on the FAM channel were classed as
positive for M. ovipneumoniae.
Tagging of M. haemolytica isolates with a plasmid
carrying the genes encoding GFP and ApR
Plasmid pAM2425 was constructed by
cloning the gfp gene from plasmid pAG408
into an M. haemolytica shuttle vector,
pAM2355 (Marciel, 2001). Briefly, the ClaI/
EcoRI fragment of pAG408 was cloned into a
pBluescript KS II+ plasmid carrying the
leukotoxin C promoter, then the PlktC::gfp
fusion was amplified using M13 universal
forward (59-GTA AAA CGA CGG CCA GT-
39) and modified reverse (59-GGG ATA TCT
AGA AGC TTA ACA GCT ATG ACC ATG
ATT ACG-39, HindIII site italicized) primers,
and then cloned as a HindIII/XbaI fragment
into the Bla-resistant vector pAM2355 to
create pAM2425 (Fig. 1). All constructions
were performed in Escherichia coli XL1-Blue
(Stratagene, La Jolla, California, USA) as
described (Fedorova and Highlander, 1997).
Plasmid DNA was purified using the Qiagen
miniprep kit (Qiagen, Valencia, California,
USA), and the four M. haemolytica isolates
from the domestic sheep were transformed
with plasmid pAM2425, by electroporation, as
described by Craig et al. (1989). One-hundred
708JOURNAL OF WILDLIFE DISEASES, VOL. 46, NO. 3, JULY 2010
nanograms of plasmid DNA were added to
each cuvette, which contained 100 ml electro-
competent cells. An electrical pulse of 15–
20 kilovolt, 400 ohm, 25 mfarad was applied
and, immediately, 1 ml BHI/SOC medium
(BHI broth; 2.5 mM KCl; 10 mM MgSO4;
10 mM MgCl2; 20 mM glucose) was added
and the mixture was incubated at 37 C for 3–
4 hr to allow expression of markers. One-
hundred-microliter aliquots were spread onto
sheep blood agar plates containing 20 mg/ml
ampicillin (Bioline, Randolph, Massachusetts,
USA) and plates were incubated overnight at
37 C. Ampicillin-resistant colonies containing
pAM2425 were identified by colony PCR
using gfp and bla gene-specific primers,
respectively (gfp forward 59-ATG AGT AAA
GGA GAA GAA CT-39 and reverse 59-GTA
TAG TTC ATC CAT GCC ATG-39 and bla
forward 59-ATG TTA AAT AAG TTA AAA
ATC-39 and reverse 59-TTA GTT GAG CTG
TAA AGT ATG AAA TAC-39), in a 25-ml
mastermix reaction containing GoTaq, as
directed by the manufacturer (Promega Corp.)
with slight modification. The PCR cycling
conditions consisted of an initial denaturation
at 95 C for 5 min, followed by 30 cycles of
denaturation at 94 C for 30 sec, annealing at
55 C for 30 sec, extension at 72 C for 1 min,
and a final elongation at 72 C for 10 min.
Leukotoxin production by M. haemolytica isolates
before and after tagging with GFP and ApR
Leukotoxin production by the M. haemoly-
tica isolates was confirmed by subjecting
culture supernatant fluid to MTT dye reduc-
tion cytotoxicity assay as described by Gentry
and Srikumaran (1991). The percent cytotox-
icity was calculated as follows: % cytotoxicity
5 [12(OD of toxin-treated cells/OD of toxin-
Colonization of domestic sheep with tagged
Bacteria were cultured overnight at 37 C in
BHI agar supplemented with 5% sheep blood
(Remel, Lenexa, Kansas, USA). Tagged M.
haemolytica was cultured on plates containing
BHI supplemented with 20 mg/ml ampicillin
(Bioline). To prepare the inoculum, the bacte-
ria were cultured in BHI broth at 37 C for 2–
3 hr followed by growth in Roswell Park
Memorial Institute (RPMI) 1640 medium,
without phenol red (GIBCO), under the same
conditions. The bacterial suspension was dilut-
ed in RPMI 1640 to obtain the desired
concentration (colony-forming units [CFU]/
ml; Petras et al., 1995). Using an atomizer,
of phosphate-buffered saline (PBS) were
sprayed intranasally into all four domestic
sheep from which they were originally isolated.
following inoculation to confirm the presence
of tagged bacteria by colony PCR, as described
above. A serotype-2 strain of M. haemolytica,
isolated several years ago from a domestic
sheep (Foreyt et al., 1994), also was tagged with
the plasmid carrying the gfp and bla genes. This
strain failed to colonize the pharynx of the four
domestic sheep and was not used further.
Domestic sheep-bighorn sheep
On day 0, the four domestic sheep and the
four bighorn sheep were placed in two identical
pens (about 2033 m) separated by another pen
(20310 m), and animals were monitored for
clinical signs. After 1 mo, the bighorn sheep
were moved into the middle pen so that they
had fence-line contact with domestic sheep.
For the next 2 mo, the animals were observed
for clinical signs of pneumonia, and nasal and
pharyngeal swabs were collected twice (days 51
and 60) for detection of the presence of tagged
M. haemolytica. After 2 mo in fence-line
contact, the domestic sheep and bighorn sheep
were allowed to commingle in the middle pen
Clinical assessment and necropsy
The bighorn sheep were observed once a
day for clinical signs including anorexia,
mid pAM2425 carrying gfp and bla genes. Plasmid
pAM2425 was constructed by cloning the gfp gene
from plasmid pAG408 into a Mannheimia haemoly-
tica shuttle vector pAM2355, as described in
materials and methods.
Schematic representation of the plas-
LAWRENCE ET AL.—INTERSPECIES TRANSMISSION OF MANNHEIMIA HAEMOLYTICA709
lethargy, cough, dyspnea, and nasal discharge.
When the animals began to show clinical signs
of pneumonia, they were observed more
frequently. Animals that died during the
experiment were necropsied within 6 hr.
Lungs were removed from each animal and
carefully examined for lesions of pneumonia.
The degree of involvement of the lung lobes
was estimated as percent pneumonic scores
(percent of lung that appeared pneumonic on
visual examination). Pleuritis was noted as
present or absent. Representative samples of
pneumonic and normal lung tissue were
prepared for both bacteriologic and histopath-
ologic examination (Odugbo et al., 2004).
Animals that showed severe signs of pneumo-
nia were euthanized by intravenous adminis-
tration of pentobarbital and then necropsied in
the same manner as those found dead.
Detection of tagged M. haemolytica
Colony PCR: Swabs were directly streaked
onto sheep blood agar plates containing 20 mg/
ml ampicillin and the plates were incubated
overnight at 37 C. The following day, 5–10
representative colonies from each plate were
picked and subjected to colony PCR assay,
performed as described above, to confirm the
presence of gfp and bla genes.
Imunoflurorescence labeling of GFP-tagged
M. haemolytica: To detect GFP by immunoflu-
orescence, bacterial cells were fixed in 2%
paraformaldehyde for 10 min, washed with
PBS, and incubated with 100 ml of FITC-
conjugated rabbit polyclonal antibodies spe-
cific for GFP (Abcam, Cambridge, Massachu-
setts, USA) for 30 min at 4 C. The cells were
washed with PBS and mounted onto micro-
scopic slides and visualized using a fluores-
Microbial flora of the upper respiratory tract
Microbial isolation revealed that all four
domestic sheep carried Pasteurellaceae in
the nasopharynx (Table 1). All four also
yielded M. haemolytica from nasopharyn-
geal samples, at least once, prior to
commingling (Table 1). All four domestic
sheep were culture-positive for M. ovip-
neumoniae but were negative for the
respiratory viruses RSV, PI-3, BVDV, and
Prior to beginning the study, the four
bighorn sheep were negative for viruses
and for M. ovipneumoniae by culture
(Table 2). However, three of the bighorn
sheep yielded M. haemolytica from naso-
pharyngeal swabs and all four had B.
trehalosi in their pharynx (Table 2).
Characteristics of the M. haemolytica isolates from
domestic sheep selected for tagging
Four M. haemolytica isolates obtained
from two of the domestic sheep were
designated as numbers 7, 10, 15, and 16.
These isolates were determined to be M.
haemolytica by cultural and biochemical
characteristics and were confirmed by M.
haemolytica-specific PCR assays. Serotype
analysis with antisera specific for all
known serotypes (A1, A2, A5, A6, A7,
A8, A9, A10, A11, A12, A13, A14, and
A16) revealed that isolate 7 belonged to
serotype 9, while the other three were
untypable. All of these isolates produced
leukotoxin in culture (Fig. 2).
Mannheimia haemolytica isolates from domestic
sheep get tagged with the plasmid carrying the gfp
and bla genes
Growth of tagged M. haemolytica iso-
lates on ampicillin plates suggested that
domestic sheep before commingling.
Microbial profile of the nasopharynx of
Bacteria recovered, sample
aSite of sample collection: P 5 pharynx; N 5 nasal cavity.
bSample 1/sample 2 5 Swabs collected at two different
cMh 5 Mannheimia haemolytica.
dBt 5 Bibersteinia trehalosi.
ePast 5 Pasteurella species.
fMovi 5 Mycoplasma ovipneumoniae.
g(2) 5 Absent or not detected; (+) 5 present.
710JOURNAL OF WILDLIFE DISEASES, VOL. 46, NO. 3, JULY 2010
the bacteria were successfully tagged with
GFP and ApR. PCR using gfp- and bla-
specific primers confirmed the presence
of gfp (Fig. 3A) and bla (Fig. 3C) in all
four isolates. Immunofluorescence assays
using FITC-labeled anti-GFP antibodies
further confirmed the expression of GFP
in these isolates (Fig. 4A). Cytotoxicity
assays of the culture supernatant fluid,
before and after the tagging, revealed that
the leukotoxin production was not affected
by the presence of extrachromosomal
plasmid (Fig. 2). In a separate experiment,
two bighorn sheep inoculated intratrache-
ally with 53109CFU of the M. haemoly-
tica isolates tagged with GFP/ApRplasmid
developed pneumonia and died within
2 days postinoculation, indicating that
organisms tagged with the GFP/ApR
plasmid were pathogenic.
GFP- and ApR-tagged M. haemolytica effectively
colonize the nasopharynx of domestic sheep
Three inoculations using a cocktail of all
four, tagged M. haemolytica isolates re-
sulted in colonization of the nasopharynx
of three of the four domestic sheep. The
colonization was detected by analyzing
nasal and pharyngeal swabs for two
consecutive weeks postinoculation (data
not shown). The PCR amplification of gfp
and bla genes confirmed the presence of
the plasmid-tagged M. haemolytica in all
of the three domestic sheep. All of the
four domestic sheep continued to remain
clinically normal after inoculation with
tagged M. haemolytica.
Domestic sheep transmit GFP- and ApR-tagged M.
haemolytica to bighorn sheep
The domestic sheep and bighorn sheep
were separated by about 10 m in individ-
ual pens during the first month. During
that time, no symptoms of respiratory
disease were observed in either domestic
sheep or bighorn sheep. Three bighorn
sheep (Y13, Y15, and Y47) yielded tagged
M. haemolytica from samples collected on
days 51, 60, or both (21 days, 30 days, or
both after fence-line contact began), as
domestic sheep. Bacteria were recovered via culture, except that Mycoplasma ovipneumoniae was also
detected postmortem using polymerase chain reaction (PCR) assay.
Microbial profile of the nasopharynx of bighorn sheep before and after their commingling with
Bacteria recovered before commingling
(sample 1/sample 2)b
Bacteria recovered after commingling
Mh BtMovi (culture)Movi (PCR)
aSite of sample collection: P 5 pharynx; N 5 nasal cavity; L 5 lung.
bSample 1/sample 2 5 Swabs collected on two different dates.
cMh 5 Mannheimia haemolytica.
dBt 5 Bibersteinia trehalosi.
eMovi 5 Mycoplasma ovipneumoniae.
f(2) 5 Absent or not detected; (+) 5 present.
gnd 5 not done.
LAWRENCE ET AL.—INTERSPECIES TRANSMISSION OF MANNHEIMIA HAEMOLYTICA711
revealed by gfp and bla gene-specific
PCR. One of these bighorn sheep (Y15)
developed coughing on day 83, 32 days
following the first evidence of tagged M.
haemolytica infection, but none of the
animals died. On day 92 (2 days post-
commingling), one bighorn sheep (Y15)
died. The remaining animals at this time
were lethargic and showed intermittent
coughing. On day 95 (5 days postcommin-
gling), two more bighorn sheep (Y13 and
Y16) died, and on day 99 (9 days post-
commingling), the remaining bighorn
sheep (Y47) exhibited severe clinical signs
of pneumonia and was euthanized.
Induction of pneumonia in, and death of, bighorn
sheep are caused by M. haemolytica transmitted
by the domestic sheep
Postmortem examinations revealed that
all four bighorn sheep had acute, bilateral,
fibrinohemorrhagic pneumonia that was
equally distributed on both sides (Fig. 5A).
mia haemolytica isolates before and after tagging
with the plasmid carrying gfp and bla. Culture
supernatant fluids from the M. haemolytica isolates
numbers 7, 10, 15, and 16, before and after tagging
with the plasmid carrying gfp and bla, were subjected
to the MTT-dye reduction cytotoxicity assay. The
percent cytotoxicity was calculated as follows: %
cytotoxicity 5 [12(OD of toxin-treated cells/OD of
toxin-untreated cells)]3100. The open and shaded
bars represent % cytotoxicity of culture supernatant
fluids from the respective isolates, before and after
tagging, respectively. Results shown are the means of
three independent experiments. The error bars
indicate standard deviations of the means.
Leukotoxin production by Mannhei-
(PCR) amplification. The M. haemolytica isolates tagged with the plasmid carrying gfp and bla, and the M.
haemolytica isolates recovered from the lungs of the four dead bighorn sheep, were tested for the presence of
gfp and bla by PCR analysis using primers described under materials and methods. Panels A and B represent
PCR amplification of gfp. Panels C and D represent PCR amplification of bla. Ut5the untagged M.
haemolytica (pool of all 4 isolates); Pl5plasmid pAM2425 used as positive control in PCR to indicate the
presence of gfp and bla; numbers 16, 15, 10, and 7 represent the tagged isolates and the numbers Y47, Y16,
Y15, and Y13 represent M. haemolytica isolated from the lungs of bighorn sheep numbers Y47, Y16, Y15, and
Y13 at necropsy. MW5molecular weight markers. Results of one representative experiment out of three
Detection of gfp and bla in Mannheimia haemolytica isolates by polymerase chain reaction
712JOURNAL OF WILDLIFE DISEASES, VOL. 46, NO. 3, JULY 2010
Estimated percent pneumonic involve-
ment ranged from 70–95% in both the
lungs. Fibrinous pleuritis was present in
all four bighorn sheep. Although the lungs
from the different bighorn sheep varied in
severity in gross lesions, they were histo-
logically very similar. In affected areas of
the lungs, alveolar spaces and bronchioles
were filled with edema, fibrin, red blood
cells, and dense collections of primarily
macrophages and neutrophils (Fig. 5B).
The inflammatory cells showed degenera-
tive changes and often had streaming
nuclei (‘oat cells’). Many alveolar walls,
and occasional bronchiolar walls, were
disrupted by necrosis and hemorrhage.
When present, pleuritis was fibrinous.
Re-isolation of tagged M. haemolytica from
pneumonic lungs of bighorn sheep
The swabs taken from lungs during
necropsy were plated on BHI-agar plates
which, upon incubation, showed the
presence of colonies resistant to 20 mg/ml
ampicillin. Further gfp gene- and bla
gene-specific PCR confirmed the pres-
ence of tagged bacteria in the lungs
(Fig. 3B, D). Immunofluorescence assays
using FITC-labeled antiGFP antibodies
further confirmed the expression of GFP
in these isolates (Fig. 4B). None of the
tagged isolates recovered from the lungs
were typable with the antisera specific for
the known serotypes of M. haemolytica
(A1, A2, A5, A6, A7, A8, A9, A10, A11,
A12, A13, A14, and A16).
Several anecdotal reports suggest that
bighorn sheep die from pneumonia fol-
lowing contact with domestic sheep
(Foreyt and Jessup, 1982; Coggins, 1988;
George et al., 2008). Fatal pneumonia in
bighorn sheep following experimental
inoculation of M. haemolytica isolates
from domestic sheep, isolates which did
not cause disease in the domestic sheep,
prompted researchers to perform com-
whether there was transmission of respi-
ratory pathogens from domestic sheep to
bighorn sheep (Onderka and Wishart,
1988; Foreyt, 1989, 1990; Callan et al.,
1991). Although over 95% of the bighorn
sheep in these studies died following
contact with domestic sheep, there was
not clear documentation of transmission of
M. haemolytica, or of any other pathogen,
from domestic sheep to bighorn sheep.
Whole genome sequencing, pulsed field
gel electrophoresis, or amplified fragment
length polymorphism, ribotyping, multi-
locus enzyme electrophoresis, and multi-
locus sequence typing are molecular tools
that are available to compare bacterial
pathogens isolated from domestic sheep
and bighorn sheep. Whole genome se-
immunofluorescence staining. The Mannheimia hae-
molytica isolates tagged with the plasmid carrying gfp
and bla, and the M. haemolytica isolates recovered
from the lungs of the four dead bighorn sheep, were
tested for the expression of GFP by immunofluores-
cence staining with FITC-conjugated rabbit anti-
GFP antibodies. All four tagged isolates (7, 10, 15,
and 16), and isolates recovered from the lungs of all
four dead bighorn sheep (Y13, Y15, Y16, and Y47),
were positive for fluorescence expression. Fluores-
cence exhibited by one representative tagged isolate
(Panel A1), and one representative isolate recovered
from the lungs of the dead bighorn sheep (Panel B1),
are shown. Panel A2 and B2 represent untagged M.
haemolytica used as the negative control.
Detection of expression of GFP by
LAWRENCE ET AL.—INTERSPECIES TRANSMISSION OF MANNHEIMIA HAEMOLYTICA713
quencing is an elaborate and expensive
procedure. The other molecular methods
are time-consuming and cannot identify
bacterial isolates with 100% certainty
(Pitt, 1999; Yakubu et al., 1999). We
reasoned that tagging the bacterial isolates
obtained from domestic sheep, recoloniz-
ing the nasopharynx of these animals with
the tagged bacteria, and commingling
them with bighorn sheep would circum-
vent these problems and provide an
irrefutable method of determining wheth-
er bacterial pathogens can be transmitted
from domestic sheep to bighorn sheep.
We selected M. haemolytica for this study
because of its documented ability to
consistently induce pneumonia in, and
death of, bighorn sheep (Onderka et al.,
1988; Foreyt et al., 1994; Dassanayake et
al., 2009). We employed two markers, the
GFP and ApR, to enhance the validity of
our findings. We also utilized two tests to
detect each marker (PCR and immuno-
fluorescence for GFP and growth on
ampicillin-containing medium and PCR
for ApR). The growth of the tagged M.
haemolytica in the presence of ampicillin,
the PCR amplification of the genes gfp
and bla, and the immunofluorescence
staining with anti-GFP antibodies clearly
indicated that the four isolates of M.
haemolytica obtained from the domestic
sheep were tagged with the markers
(Fig. 3A, C, 4A). These three parameters
were used to clearly document the suc-
cessful colonization of the pharynx of
domestic sheep by the tagged M. haemo-
lytica and, more importantly, to identify
the tagged organisms isolated from the
dead bighorn sheep (Fig. 3B, D, 4B).
Tagged-isolate 7 typed as serotype 9
while the other three (numbers 10, 15,
and 16) were untypable. However, none of
the isolates recovered from the lungs of
the four dead bighorn sheep typed as
serotype 9. This could be because the
tagged-isolate 7 did not colonize the
nasopharynx of domestic sheep; because
it colonized the domestic sheep but was
not shed in adequate amounts to be
Typical gross appearance of the lungs of the dead bighorn sheep. The lungs were removed from the carcass
for examination, and the total area of gross lung consolidation was discerned by visual inspection and by
palpation. In this case, the right cranial and middle, and the left middle lung lobes, are dark red and
consolidated, and additional consolidation was evident from palpation; darkened areas in the photograph were
subsequently determined to be areas of severe hemorrhage. Fibrin strands on the lung surface indicate
pleuritis. (B) The typical histopathologic appearance of the lungs of the dead bighorn sheep. Lung tissue
samples of bighorn sheep were aseptically removed and processed for histopathology. Alveolar septa are
necrotic and replaced by fibrin and debris. Bronchioles and alveoli are filled with streaming mononuclear
cells. H&E stain. 1003.
Representative gross lesions and histopathology of the lungs of the dead bighorn sheep. (A)
714JOURNAL OF WILDLIFE DISEASES, VOL. 46, NO. 3, JULY 2010
acquired by the bighorn sheep; or because
it was acquired by the bighorn sheep but
not recovered by us because it was present
in the lungs in lower numbers than the
other isolates at the time of sampling.
Nevertheless, transmission from domestic
sheep to bighorn sheep clearly occurred
because other tagged isolates of M.
haemolytica were recovered from the
lungs of every bighorn sheep.
Our finding that three out of the four
bighorn sheep acquired the tagged M.
haemolytica within 1 mo of fence-line
contact indicates that such contact was
adequate for transmission of these organ-
isms to occur. Death of the first bighorn
sheep occurred about 1 mo after tagged
M. haemolytica was first detected in that
animal. This lag period may have been
necessary for the transmitted M. haemo-
lytica to colonize and proliferate to the
threshold number of organisms required
to induce pneumonia and death in bighorn
sheep. It is conceivable that the bighorn
sheep that acquired the tagged M. hae-
molytica during the fence-line contact
would have died even without commin-
gling with the domestic sheep. This notion
is supported by the fact that one bighorn
died only 2 days after commingling with
the domestic sheep. However, in order to
determine with certainty whether fence-
line contact is adequate for induction of
pneumonia and death of bighorn sheep,
the experiment would need to be per-
formed with a longer period of fence-line
It is also possible that another patho-
gen(s) was necessary to predispose the
bighorn sheep to pneumonia by M.
haemolytica infection. The bighorn sheep
were not positive for M. ovipneumoniae
before commingling with the domestic
sheep. Lung tissue from one of the dead
bighorn sheep was positive for M. ovip-
neumoniae by standard and RT-PCR
(Table 2), and M. ovipneumoniae was
detected in the nasopharynx of a second
dead bighorn sheep by culture and PCR,
which raises the possibility that these
organisms, along with the tagged M.
haemolytica, were transmitted from the
domestic sheep to the bighorn sheep. It is
possible that during the lag period, M.
ovipneumoniae colonized the upper respi-
ratory tract of at least two bighorn sheep
and predisposed them to the tagged M.
haemolytica, but whether M. ovipneumo-
niae played any role in the other two
bighorn sheep seems even less certain,
based on available data (Table 2). In
domestic sheep, M. ovipneumoniae has
been shown to render the cilia on the
epithelial cells of the upper respiratory
tract dysfunctional (Jones et al., 1985;
Niang et al., 1998). Previous studies have
shown that M. ovipneumoniae does not kill
bighorn sheep (Besser et al., 2008) but can
predispose them to M. haemolytica infec-
tion (Dassanayake et al., 2010). However,
it is not likely that M. ovipneumoniae is a
necessary predisposing factor for fatal
infection of bighorn sheep by every strain
of M. haemolytica because, in an earlier
study, intranasal inoculation with M.
haemolytica resulted in the death of 75%
of inoculated bighorn sheep (n54) within
48 hr (unpubl. data). The M. haemolytica
used in that study was a serotype 2 strain,
which is known to be virulent in bighorn
sheep (Foreyt et al., 1994). Therefore, we
believe that only less-virulent strains of M.
haemolytica may require M. ovipneumo-
niae or another predisposing agent. Stud-
ies are currently underway to elucidate the
role of M. ovipneumoniae in the develop-
ment of pneumonia in bighorn sheep
following contact with domestic sheep. In
summary, this study irrefutably demon-
strated the transmission of M. haemolytica
from domestic sheep to bighorn sheep and
the resulting pneumonia and death of
This study was funded by the Wyoming
Wildlife-Livestock Disease Research Partner-
ship Funds, Foundation for North American
Wild Sheep and its Washington, Oregon,
Idaho, and Eastern Chapters, and the USDA
LAWRENCE ET AL.—INTERSPECIES TRANSMISSION OF MANNHEIMIA HAEMOLYTICA715
Forest Service. We thank Thomas E. Besser
for helpful suggestions and discussion.
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