Monograph

Thu, 01/27/2011 - 15:52 — Anonyme

definition:

An infectious, virulent, harmful but non-contagious disease caused by a bacteria: Ehrlichia ruminantium, formerly known as Cowdria ruminantium. It affects caprines, ovines and bovines.

This diseased is a tick-borne disease of the Amblyomma genus. In the Caribbean, it is the Amblyomma variegatum species, more commonly known as the "Senegalese tick" which causes the disease.

Amblyomma variegatum male tick (left) and female tick (right)(CIRAD)

Amblyomma variegatum male tick (left) and female tick (right)(CIRAD)

Importance

One of the most significant tick-borne diseases in tropical areas. Mortality, economic loss, cost of fighting the disease and immunisation, obstruction to commercial exchange.

In infested regions in the lesser Antilles, the Amblyomma tick has been made the subject of a regional control programme known as: the "Caribbean Amblyomma Programme".

Situation in America:

Antilles: Martinique, Guadeloupe, Marie Galante, La Désirade, Antigua, Saint Martin, Saint Lucia, Barbados, Dominica, St Kitts and Nevis.

Risk of introduction on the American continent where a potential tick vector is present (Amblyomma maculatum).

Increased risk through the expansion of the cattle egret population (Bubulcus ibis), which transports A. variegatum in the immature stage.

Cattle egret (Bubulcus ibis)

Cattle egret (Bubulcus ibis)

Susceptible species:

Domestic (bovines, ovines, caprines) and wild (zebus, buffaloes, etc.) ruminants.

A. variegatum ticks on a bovine's dewlap (CIRAD)

A. variegatum ticks on a bovine's dewlap (CIRAD)

Receptivity factors

Species: in decreasing order of susceptibility: Caprines, Ovines, Bovines.

Breed: breeds from areas affected by the enzootic are more resistant but it is difficult to distinguish genetic resistance from acquired immunity-induced resistance.

Age: lower mortality rate in young calves with a maximum mortality rate in 1 year old bovines. This suggests that maternal colostrum may play a protective role in areas affected by the enzootic.

Etiological agent:

Ehrlichia ruminantium is an intracellular bacteria from the family Rickettsiaceae, of the Ehrlichia genus. It infests blood vessel endothelial cells and various cells in the reticulohystiocyte tissue of ruminants.

Different strains inducing varying degrees of cross-protection exist. This antigenic diversity makes it difficult to develop an effective vaccine.

Methods of transmission

Source:

Virulent matter: blood

Transmission:

Amblyomma tick life cycle and contamination by Ehrlichia:

The tick undergoes three development stages: larvae, nymph, adult. The tick feeds off the blood of a different host animal at each stage.

Progressive cycle of the "senegalese tick"

Tick infection occurs at the larval or nymphal stage while feeding off the blood of an infected ruminant. Ehrlichia infection is transmitted during the following stages of development (cross-stage transmission): nymph and adult.

After feeding the larvae and nymphs moult and live freely in vegetation before fixing to a new host. The females, following coupling on the host, lay around 15,000 - 25,000 eggs which will produce non-infected larvae (no transovarian transmission).

It is tick saliva which causes contamination of ruminants by Ehrlichia during feeding and after 1 to 3 days' engorging for the nymph and after 2 to 4 days for the female adult tick.

Domestic ruminants having recovered from cowdriosis become asymptomatic carriers and therefore a reservoir for the disease.

Symptoms:

Incubation period: 2 - 3 weeks on average.

Mortality rate from 0% (enzootic stability) to 80% (imported breeds).

Clinical presentation ranging from subacute to benign depending on species, breed, age, immune status, the quantity and strain of Ehrlichia inoculated.

In small ruminants

Severe forms

These affect susceptible animals. Signs are sudden severe hyperthermia (> 41°C), respiratory distress, nervous signs (the animal falls to the ground with opisthotonos and paddling).

- Subacute form: hyperthermia and convulsion, death occurs in a few hours.

- Acute form: evolution over 2 to 5 days with a hyperthermia phase followed by dyspnea and nervous symptoms in the end stages.

Atypical forms

Animals with a certain degree of genetic resistance or acquired immunity or infected by a weakly virulent strain will develop subacute forms over a period of 7 to 10 days. Pulmonary signs are predominant. Recovery is possible but long.

Mild form: fever over 1 to 3 days, spontaneous recovery and life-long immunisation against the Ehrlichia strain encountered. This is the most frequent form of the disease found in stable enzootic areas.

In bovines

• the same forms exist but the nervous symptoms are inconstant.

• digestive signs: profuse diarrhoea.

• the most severe forms occur in taurines imported from disease-free areas (genetic improvement programmes)

Lesions

Macroscopic lesions:

Fluid effusion: hydropericardium, hydrothorax, ascites.

- Hydropericardium: almost constant light yellow exudate in small ruminants.

- Hydrothorax: more frequent in bovines than in small ruminants.

- Intestinal congestion and pulmonary oedema: frequent

Microscopic lesions:

Detected after Ehrlichia staining in brain microvessel endothelial cells.

Diagnostics

Clinical diagnosis:

Evocative signs: severe hyperthermia, nervous disorders progressing to death in 2 to 5 days associated with pericardial effusion in the area infested by ticks of the Amblyomma genus.

Necroscopic diagnosis

Major signs to be detected: hydropericardium +/- hydrothorax and ascites.

Differential diagnosis:

Heartwater must not be confused with:

- Tetanus, rabies, bacterial meningoencephalitis

- Theileriosis and babesiosis

- Anthrax

- Neurotoxicosis and poisoning

- Hypomagnesemia or vitamin B1 deficiency

Laboratory diagnosis:

Detection of the pathogenic agent:

- Bacterioscopic diagnosis on a brain swab (dead animal) to distinguish other possible haemoparasites such as (Babesia bovis)

- Isolation:

In susceptible animals: inoculation, clinical necroscopic and culture diagnosis. Technique used to isolate new strains.

Cell culture: long, requires a specialist laboratory. Fundamental technique in developing vaccines.

- Molecular diagnosis using PCR, ideal for routine testing.

Serological diagnosis:

- Indirect immunofluorescence: risk of false positives through cross-reaction with other Ehrlichia strains.

- More specific ELISA tests based on recombinant antigens (indirect ELISA and competitive ELISA). These methods are recommended by the OIE.

Treatment:

To be effective treatment must start early by antibiotics from the tetracyclin group:

- Oxytetracyclin (10-20mg/kg, IV or IM), very effective and most commonly used. Exists in a "prolonged effect" form.

- Chlortetracyclin (5mg/kg IV)

- Doxycyclin (2mg/kg IV)

+/- diuretic corticoids to reduce cerebral oedema (acute cases).

Prophylaxis:

Sanitary prophylaxis

Disease-free countries may prohibit the importation or transit of ruminants from infected countries and can ask for a certificate stating that the imported ruminants:

- do not present any clinical signs of heartwater on loading

- have undergone screening, approved by the OIE, and have tested negative in the two weeks preceding loading

- are tick-free

Fight against ticks

This is the principal method for controlling heartwater where no vaccine exists on the market. Strategic used of acaricides.

On islands like the Lesser Antilles eradication of A. variegatum is possible. In other areas the aim is simply to reach enzootic stability where subclinical infection and carrying do not affect animal growth or reproduction.

De-ticking: use of a "pour on" product on the animal's back, or spraying while insisting on the most infested parts (belly, udders, underside of the tail). Treatment: every week or once every two weeks depending on product staying power.