Arthropod-borne infectious diseases of the dog and cat

Arthropod borne infectious diseases of the dog and cat
Hepatozoonosis

 H. americanum cyst. The round-to-oval ‘onion skin’-shaped

cysts are 250–500 µm in diameter. The outer portion of the

cyst is made up of concentric layers of fine, pale blue-staining

laminar membranes. A developing parasite may sometimes be

observed at the centre of the cyst.

80 H. canis gamonts on the edge of a blood smear from a

naturally infected dog with extreme leucocytosis and a high

parasitaemia approaching 100% of the neutrophils.

 Miniature Schnauzer naturally infected with H. americanum

exhibiting ‘His Master’s Voice’ stance due to severe

musculoskeletal pain and excessive stiffness.

 Rottweiler naturally infected with

H. americanum, with typical mucopurulent

ocular discharge and facial muscle atrophy.

Arthropod-borne infectious diseases of the dog and cat

Laboratory findings

Most dogs with HCI have white blood cell counts within

the reference range. However, dogs with a high

parasitaemia frequently have extreme neutrophilia (up to

150 × 109/l), although it is less common than in dogs with

HAI. Normocytic, normochromic non-regenerative

anaemia is the most common haematological abnormality

reported in HCI. Less frequently, a regenerative anaemia,

sometimes severe, may be seen. In a case-controlled study

of dogs with H. canis parasitaemia admitted to a

veterinary teaching hospital in Israel, dogs with

hepatozoonosis were significantly more anaemic than the

control hospital population admitted with other diseases,

and dogs with high parasitaemia were more anaemic and

had higher leucocyte counts than both the controls and

the dogs with low parasitaemia. Thrombocytopenia and

proteinuria have also been reported in HCI.

In dogs with HAI, the most outstanding

haematological abnormality is marked leucocytosis,

characterized by neutrophilia. The white blood cell count

typically ranges from 20–200 × 109/l, with reported

means of 76.8 and 85.7 × 109/l. A mild to moderate

normocytic, normochromic, non-regenerative anaemia is

typical. Thrombocytosis, with platelet counts of 422–916

× 109/l, occurs in a considerable number of dogs.

Thrombocytopenia is rare unless there is concurrent

infection with Ehrlichia canis, Anaplasma platys,

Rickettsia rickettsii or other tick-borne organisms.

Abnormalities in serum biochemistry in highly

parasitaemic dogs with HCI include hyperproteinaemia

with hyperglobulinaemia and hypoalbuminaemia, and

increased creatine kinase (CK) and alkaline phosphatase

(AP) activities. In dogs with HAI the most common

biochemical changes are a mild elevation in AP and

decreased albumin. Artefactual hypoglycaemia (in the

range of 2.22–3.33 mmol/l and occasionally as low as

0.28 mmol/l) due to increased in vitro metabolism by the

elevated number of white blood cells may be seen if

sodium fluoride is not used for sample collection. The low

albumin has been attributed to decreased protein intake,

chronic inflammation or renal loss. Blood urea nitrogen

(BUN) is also frequently decreased below the reference

range. Surprisingly, CK activity is typically normal despite

the myositis caused by H. americanum. Although the

decreases in albumin and BUN are suggestive of hepatic

failure, both fasting and postprandial bile acids are

usually within the reference range or only slightly

elevated.

Radiographic findings

Dogs with HAI commonly develop osteoproliferative

lesions. Periosteal new bone formation is typically

disseminated and symmetric and is usually most frequent

and severe on the diaphysis of the long bones. The

radiographic appearance of the bony lesions ranges from

subtle bone irregularity to a dramatic smooth laminar

thickening (83). A study of the formation of the bone

lesions after experimental infection with H. americanum

 Radiograph showing periosteal proliferation of the femurs

and pelvic bones in H. americanum infection. Bone lesions

range from rough irregularity to smooth laminar thickening

such as in this case.

revealed that the stages of morphologic development of

the lesions very closely resemble those of hypertrophic

osteopathy.

Diagnosis

HCI is usually diagnosed by microscopic detection of

H. canis gamonts in the cytoplasm of neutrophils, and

rarely monocytes, on Giemsa- or Wright’s-stained blood

smears. They have an ellipsoidal shape, are about 11 ×

4 µm and are enveloped in a thick membrane (84).

Between 0.5% and 5% of the neutrophils are commonly

infected, although this may reach as high as 100% in

heavy infections. A case-controlled study of dogs with H.

canis parasitaemia admitted to a veterinary hospital in

Israel indicated that 15% had a high number of

circulating parasites (>800 gamonts/mm3).

In contrast to H. canis, gamonts are infrequently found

on blood smears from dogs infected with H. americanum.

When they are identified, it is usually in very low

Hepatozoonosis

numbers, rarely exceeding 0.1% of the leucocytes

examined (85). Gamonts may exit the leucocytes rapidly

after blood is drawn, leaving behind an empty capsule

that is difficult to identify. Consequently, blood smears

should be made rapidly after sampling to enhance

identification. Buffy coat smears will also increase the

chance of gamont detection. Bone marrow aspirates

usually show granulocytic hyperplasia with an increased

myeloid:erythroid ratio, and lymph node aspirates often

reveal lymphoid hyperplasia. However, neither procedure

is useful in making a definitive diagnosis as organisms are

rarely seen in these samples.

Radiography of the limbs or pelvis can be used for

screening a suspected animal because many dogs with

HAI will show periosteal proliferation. However, muscle

biopsy is a more consistent method of diagnosis of HAI as

it typically reveals the unique cyst and pyogranuloma

formation associated with H. americanum (77–79).

Myositis with muscle atrophy, necrosis and infiltration of

inflammatory cells between muscle fibres is a frequent

finding. The parasites are widely distributed in the muscle

tissue but multiple biopsies are recommended to increase

the chances of detecting the organism, especially in early

or low level infections. The biceps femoris, semitendinosus or epaxial muscles are recommended sites

for biopsy.

An indirect fluorescent antibody test (IFAT) and

western blot for the detection of anti-H. canis antibodies

were developed using gamont antigens (86). The IFAT has

been used for epidemiological studies in Israel and Japan.

A survey of dogs from Israel showed that 33% had been

exposed to the parasite as indicated by the presence of

anti-H. canis antibodies. Only 3% of the seropositive

dogs had detectable blood gamonts and only 1% had

severe clinical signs associated with the infection. This

indicates that although there is a wide exposure to

 Single H. canis gamont on a blood smear from a naturally

infected dog. Note the ellipsoidal shape of the gamont

compressing the lobulated neutrophil nucleus towards the cell

membrane.

 Indirect fluorescent antibody test (IFAT) for the detection of

antibodies against H. canis. Note the specific fluorescence of

the gamont membranes in the positive reaction shown.

 Blood smear showing a gamont of H. americanum in a

neutrophil. Although similar in appearance, the gamonts of

H. americanum are slightly smaller in size than those of

H. canis (8.8 × 3.9 µm compared to 11 × 4 µm).

Arthropod-borne infectious diseases of the dog and cat

H. canis, most infections are probably subclinical. IgM

and IgG class antibodies to H. canis were detected by

IFAT in experimentally infected dogs as early as 16 and

22 days post infection, respectively, well in advance of

gamont detection by microscopy at 28 days post

infection. Antibodies detected by IFAT may be formed

against conserved antigens found in earlier life cycle

stages of H. canis.

Sera from dogs infected with H. americanum showed

only a low degree of cross-reactivity to H. canis antigens

by IFAT. However, an ELISA for H. americanum, using

sporozoites as antigen, was reported to have a sensitivity

of 93% and a specificity of 96% when compared with

muscle biopsy.

Necropsy findings

HCI may be found as an incidental finding in

histopathological specimens from dogs from endemic

areas. In dogs with a low parasitaemia, few tissue lesions

may be identified. However, necropsies of dogs with a

high parasitaemia reveal hepatitis (87), pneumonia and

glomerulonephritis associated with numerous H. canis

meronts. Meronts and developing gamonts are also found

in the lymph nodes, spleen and bone marrow (88).

H. canis meronts are usually round to oval, about 30 µm

in diameter, and include elongated micromerozoites with

defined nuclei. A cross-section of the meront through the

midshaft of the micromerozoites reveals a form with a

central core mass surrounded by a circle of micromerozoite nuclei, which is often referred to as a ‘wheel

spoke’ (76). This form is typical for HCI but is not found

in HAI. Meronts of H. canis can sometimes be detected in

tissues with little or no apparent host inflammatory

response (89). This is possibly associated with the ability

of the parasite to cause chronic subclinical infections and

avoid an extreme immune response.

87 Hepatitis associated with H. canis meronts in a section of

liver from a dog with a high parasitaemia. Arrows indicate the

location of H. canis meronts.

Cachexia and muscle atrophy are consistent gross

findings on necropsy of dogs chronically infected with

H. americanum. Roughening and thickening of bone

surfaces may be apparent. Grossly, pyogranulomas may

appear as multiple, 1–2 mm diameter, white-to-tan foci

diffusely scattered throughout muscle and various other

tissues. Microscopically, the cysts, meronts and

pyogranulomas are found predominantly in skeletal and

cardiac muscle but they may also be found sporadically in

other tissues including adipose tissue, lymph node,

intestinal smooth muscle, spleen, skin, kidney, salivary

gland, liver, pancreas and lung. Vascular changes in

various organs include fibrinoid degeneration of vessel

walls, mineralization and proliferation of vascular intima

and pyogranulomatous vasculitis. Renal lesions are

frequently present and include focal pyogranulomatous

inflammation with mild glomerulonephritis, lymphoplasmacytic interstitial nephritis, mesangioproliferative

glomerulonephritis and, occasionally, amyloidosis. Amyloid deposits may also be found in spleen, lymph nodes,

small intestines and liver. Occasional findings include

pulmonary congestion, splenic coagulative necrosis,

lymphadenopathy and congestion of the gastric mucosa.

Treatment and control

H. canis infection is treated with imidocarb dipropionate

(5–6 mg/kg i/m every 14 days) until gamonts are no

longer present in blood smears. Doxycyline (10 mg/kg p/o

q24h for 21 days) is also used in combination with

imidocarb dipropionate for treatment of HCI. The

elimination of H. canis gamonts from the peripheral

blood may require eight weeks, and a haematological

evaluation every two weeks is indicated. Treatment is

recommended for all infected dogs, including those with a

mild disease, because parasitaemia may increase over time

and develop into a severe infection. Generally, the survival

 H. canis meront in kidney tissue. An arrow indicates the

location of the meront. Meronts are often found with little or no

surrounding inflammatory response.

rate of dogs with a low H. canis parasitaemia is good. It

is often dependent on the prognosis of any concurrent

disease conditions. The prognosis for dogs with a high

parasitaemia is less favourable. Seven of 15 dogs (47%)

with a high parasitaemia included in the case-controlled

study survived only two months after presentation despite

specific treatment.

Both specific therapy using antiprotozoal drugs and

palliative therapy using a non-steroidal anti-inflammatory

drug (NSAID) have been used in the treatment of HAI.

The best results occur when both are used together

initially. An NSAID at standard doses can provide

immediate relief from fever and pain during the first days

of therapy before the effects of the antiprotozoal drug

become evident.

Currently, it appears there is no drug capable of

eliminating all stages of the organism. Remission of

clinical signs can usually be obtained quickly by

administering a combination of trimethoprimsulphadiazine, clindamycin and pyrimethamine (TCP) for

14 days (Table 20). Although the clinical response is

dramatic, it is often short-lived and most dogs relapse

within 2–6 months following treatment.

The anticoccidial drug decoquinate helps prevent

relapses when given daily to H. americanum-infected dogs

after completion of TCP therapy. It likely arrests

development of parasites released from mature meronts,

thereby interrupting the repeated cycles of asexual

reproduction. Decoquinate does not clear gamonts from

the dog’s circulation nor is it effective in reducing clinical

signs associated with acute relapse. This drug must be

given every day to be effective. Although not approved for

use in dogs, decoquinate has been proven to be safe in

dogs at both high dosages and prolonged administration.

The drug is available in the USA as a cornmeal-based

premix for livestock at a concentration of 27.2 grams of

decoquinate per pound of premix (Deccox, Alpharma Inc.,

Fort Lee, NJ). The powder is given at a rate of 0.5–1.0

teaspoonful per 10 kg body weight, mixed with moist dog

food and fed twice daily. This amount corresponds to a

decoquinate dosage of 10–20 mg/kg every 12 hours. It

appears that the drug must be given long term (1–2 years

and possibly longer) to prevent relapses.

In a study comparing treatment protocols, the twoyear survival rate for dogs receiving only TCP was 12.5%

compared to a two-year survival rate of greater than 84%

when TCP was followed by long-term daily decoquinate

therapy. Most dogs that received TCP alone had a very

good initial response, followed by periodic relapses,

resulting in chronic wasting and debilitation and ending

in renal failure, euthanasia or death with a median

survival time of approximately 12 months.

Control and prevention

Prevention of both HCI and HAI consists primarily of

good tick control using an effective acaricide and close

examination of dogs after hunting or outdoor activity.

Dogs must be prevented from ingesting ticks. Until more

is known about whether HCI or HAI can be transmitted

through ingestion of infected tissues, dogs should also be

prevented from eating raw meat or organs from wildlife

and prevented from scavenging.

Arthropod-borne infectious diseases of the dog and cat

FELINE HEPATOZOONOSIS

Feline hepatozoonosis was first described in a domestic

cat in 1908 in India, and has since been reported from

several countries including France, Israel, South Africa,

Brazil and Nigeria. The species of Hepatozoon that infect

cats have not been identified and the vector is unknown.

Gamonts of Hepatozoon are found in peripheral blood

neutrophils (90) and histopathological studies have

identified meronts in the myocardium and skeletal

muscles of infected cats. In addition, in one retrospective

study elevated CK levels have been found in the majority

of cats with hepatozoonosis, indicating the importance of

muscle as a target tissue for this infection. Feline

hepatozoonosis is commonly associated with

immunosuppressive viral disease caused by feline

immunodeficiency virus or feline leukaemia virus.

ZOONOTIC POTENTIAL/PUBLIC

HEALTH SIGNIFICANCE

There is only one report of human infection with a

Hepatozoon species. Gamonts were found in the blood of

a male patient from the Philippines on two different

occasions but liver and bone marrow biopsies failed to

reveal any parasites. Because canine hepatozoonosis

occurs as a result of ingestion of a tick, transmission of

H. canis or H. americanum to humans is unlikely except

perhaps in small children inclined to put foreign objects

into their mouths. Since other tick-borne diseases may be

transmitted through the bite of a tick, all ticks should be

promptly removed from any human or animal.

Disclaimer

The views expressed in this chapter are those of the

authors and do not reflect the official policy or

position of the Department of the Army, the

Department of Defense, or the US Government.

Gamont of an Hepatozoon species in a neutrophil of a

naturally infected domestic cat.

Leishmaniosis

Gad Baneth, Michael Day,

Xavier Roura and Susan Shaw

BACKGROUND, AETIOLOGY AND

EPIDEMIOLOGY

Canine leishmaniosis is an important, potentially fatal

disease that is also infectious to people. It is a part of a

broad spectrum of diseases caused in humans and animals

by several species of the intracellular protozoan genus

Leishmania and is transmitted by sandflies. The disease

syndromes caused by Leishmania species in people are

cutaneous, mucocutaneous and visceral leishmaniosis, the

last being the most severe form. Visceral leishmaniosis is

further divided into zoonotic, in which dogs are reservoirs

of the disease for people, and anthroponotic, in which

man is the reservoir of infection for other humans and

transmission by sandflies occurs without apparent

involvement of an animal reservoir. L. infantum and

L. chagasi cause zoonotic visceral leishmaniosis (ZVL),

while anthroponotic infection is caused by L. donovani,

mostly in India and East Africa. The two main groups of

human patients at risk for ZVL are young children and

HIV-positive patients. The domestic dog is considered the

main reservoir for human ZVL infection and, more

recently, dogs have also been incriminated as reservoirs

for Leishmania species causing cutaneous and

mucocutaneous leishmaniosis in South America. Infection

among populations of wild canines such as foxes and

jackals has been reported in the Mediterranean basin and

South America and may also play a role in the epidemiology of ZVL in these regions.

 The global distribution of

canine leishmaniosis.

ZVL transmission occurs in tropical, subtropical and

temperate regions of the world including southern Europe,

North and Central Africa, the Middle East, China and

South and Central America (91). In the Mediterranean

region and the Middle East, ZVL is caused by L. infantum

and in South America by L. chagasi, which is thought to

be synonymous with L. infantum. Canine leishmaniosis

caused by L. infantum has also been recently reported

from multiple kennels in the eastern USA, where the

patterns of transmission are currently unknown. An

additional Leishmania species, L. tropica, which is an

agent of cutaneous leishmaniosis in the Old World that

can visceralize in people, has been reported as a rare cause

of canine visceral leishmaniosis in North Africa.

The prevalence rates of canine leishmaniosis in endemic

areas vary depending on the environmental conditions

required for transmission and the methods used for

detecting infection. Seroprevalence rates in the

Mediterranean basin range between 10% and 37% of the

dogs in endemic foci. Surveys employing methods for the

detection of leishmanial DNA in canine tissues, or

combining serology and DNA detection, have revealed

even higher infection rates approaching 70% in some foci.

It is probable that all dogs living in endemic foci of leishmaniosis are exposed to infection and will develop either

disease or subclinical infection, or resistance to infection.

91

89

90

Arthropod-borne infectious diseases of the dog and cat

Leishmaniosis is now frequently diagnosed in countries

where no sandfly transmission occurs in dogs; it is related

to increased mobility of dogs and their owners. In Europe,

many dogs travel to and from Leishmania-endemic areas

and re-homing of stray animals from endemic areas by

welfare groups is increasing the number of clinical cases

seen in non-endemic areas. In addition, leishmaniosis is

seen in non-travelled animals resident in non-endemic

areas of both Europe and the USA. The mechanisms of

transmission in these cases are currently unknown.

In contrast to dogs, natural infection and clinical

disease in domestic cats caused by Leishmania species

appear to be rare. Whether the low prevalence of infection/disease in endemic areas is due to under-reporting or

to the fact that cats have a high degree of natural

resistance is unknown. It has been shown that cats are

relatively resistant to experimental infection with

L. chagasi and L. donovani. However, cases of systemic

clinical disease and asymptomatic infection due to

L. infantum and other species are reported, and wild cats

have been incriminated as reservoirs for leishmaniosis in

endemic Mediterranean countries. Cutaneous lesions

alone have been reported in association with

L. venezuelensis, L. mexicana, L. braziliensis and unspecified Leishmania species in Europe and South

America and in the southern USA.

Transmission and life cycle

Leishmania are diphasic parasites that complete their life

cycle in two hosts: a vertebrate where the intracellular

amastigote parasite forms are found, and a sandfly that

harbours the flagellated extracellular promastigotes.

Sandflies of the genus Phlebotomus are vectors in the Old

World, whereas the vectors in the New World are

sandflies of the genus Lutzomyia. The life cycle in both

the reservoir host and vector is illustrated (92).

Although transmission of L. infantum occurs naturally

by the bite of sandflies, vertical transmission in utero

from dam to its offspring has been documented. Direct

transmission without involvement of a haematophagous

vector has been suspected in some cases of infection in

areas where vectors of the disease are absent.

Transmission of L. infantum by infected blood transfusion has been reported in dogs in North America and in

human intravenous drug users sharing syringes in Spain.