Anaplasmosis

File:Anasmosis.jpg

Rickettsia sp inside tick haemolymph cells (sourced from Wikimedia Commons)

Also Known As – Anaplasma marginale – Anaplasma ovis - Gall sickness – Gallsickness – Tick fever (Australia) – Tristeza (Portugal)

Introduction

Anaplasmosis is a ruminant disease caused by the rickettsial pathogens Anaplasma marginale, A. ovis, A. mesaeterum and Ehrlichia bovis. These rickettsial parasites reside exclusively within the red blood cells of their hosts.

Cattle, sheep, deer, antelope and wild ruminants can all be affected. A. marginale is usually the causative agent in cattle and wild ruminants, while A. ovis is most commonly isolated in sheep and goats.

A. central caused mild disease in cattle and is now the foundation for some vaccines. A. mesaeterum is similar to A. ovis but has a lower pathogenicity.

This disease is notifiable to the World Organisation of Animal Health (OIE)

Signalment

The resistance of Bos indicus cattle to ticks may reduce the rate of transmission in this breed.

No other breed predilection is evident.

Maternal derived immunity in calves is followed by age related immunity, and thus animals do not appear susceptible to anaplasmosis until 9-12 months old. Exposure before this age often results in lifelong, protective immunity providing endemic stability even in the presence of disease and vectors.

No marked age vulnerability appears to exist in sheep and all ages appear to acquire disease equally although it is usually mild and is exacerbated by stress.

Distribution

Anaplasmosis occurs in tropical and subtropical regions, particularly South and Central America, USA, Africa, Australia and Southern Europe. This climate supports the insect and mechanical vectors required for transmission of the parasite.

Transmission and Lifecycle

Anaplasmosis is not directly contagious and most transmission occurs via ticks. Any stage of the tick’s lifecycle can become infected with Anaplasma organisms. After development within the tick, the final infective stage is within the salivary glands and is injected into the bloodstream of the host when the parasite bloodfeeds. Male ticks are more mobile and longer lived therefore may play a bigger role.

See Tick Disease Transmission for more information on ticks as disease vectors.

Infected erythrocytes are disrupted and release bodies which can invade other erythrocytes. These bodies form vacuoles within the cytoplasmic membranes of the red blood cells and then undergo binary fission to form inclusion bodies. This amplifies infection within the host and increases the likelihood of transmission when insects bloodfeed.

Other insects can be important vectors depending upon location, such as dipteran flies in the USA.

Cattle that recover become persistent carriers and although clinical signs do not recrudesce, sequential ricketssial lifecycles are ongoing and therefore reinfection of insect vectors and transmission to other vulnerable animals continues.

Transplacental transmission from infected dams can also occur.

Anaplasmosis is not considered zoonotic.

Clinical Signs

===Cardiovascular=== Profound anaemia resulting in tachycardia and dyspnoea. Congestion of mucous membranes. ===Gastrointestinal=== Anorexia and mucoid diarrhoea, Rumenal atony, Excess salivation. ===Urinary=== Red, brown or pink urine ===Other=== Swelling of the head, face and ears, General paresis/paralysis, Rough hair coat ===Neurological signs=== Shivering, fasciculations, ataxia, seizures, syncope ===Reproductive=== Abortion, infertility, agalactia

Signs are often severely exacerbated by exercise.

Diagnosis

Identification of the organism within the erythrocytes on a thin blood smear with Giemsa staining is both uncomplicated and definitive. A positive result is deemed as >5% red blood cells infected or accompanying anaemic signs. The number of infected cells doubles every 24-48h and may reach 90%.

The degree of parasitaemia is much lower in sheep and goats.

Complement fixation is the current international test but it lacks sensitivity.

On post mortem, carcasses are emaciated and blood is thin and watery. Jaundice is often evident and the spleen may be engorged as a consequence of profound anaemia. Serous effusions may be present in any body cavity. The spleen may be engorged and the kidneys congested.

Urine is dark yellow/brown due to the presence of bilirubin.

Treatment

Oxytetracycline and Imidocarb are both highly effective and prompt treatment usually ensures survival for all but the most severely affected. Note that imidocarb is not approved for use in some countries.

Supportive treatment such as blood transfusions and appetite stimulants in severe cases may be required.

A long combined course of imidocarb and oxytetracycline can also be used to eliminate the carrier state.

Control

Vaccines against anaplasmosis are available, all are derived from the blood of infected cattle. Both live and killed forms are available.

Detection and culling or treatment of carrier animals is essential.

Care when performing veterinary and husbandry procedures to prevent iatrogenic transmission with contaminated instruments.

Control of the tick vector is viable but expensive.

References

Animal Health & Production Compendium, Anaplasmosis datasheet, accessed online 02/06/2011 @ http://www.ahpc.org/