Camelpox

Camelpox

R P KITCHING

Introduction

Camelpox is a systemic infection of dromedary camels (Camelus dromedarius) characterized by fever and the development of papules, vesicles, pustules and scabs in the skin particularly of the nostrils, lips and eyelids, caused by a species of orthopoxvirus. It has not been reported in bactrian camels (Camelus bactrianus). The camelpox virus is specific to dromedary camels and is found throughout the camel’s distribution in Africa and Asia,4, 9, 10, 11 but is absent from the wild dromedary population of Australia. The dromedary camel is found in Kenya, Somalia, Ethiopia and the Saharan countries, throughout the Arabian Peninsula, Iraq, Syria, Turkey, Iran, Afghanistan, Pakistan, southern countries of the former USSR, north India, west China and Mongolia.

Both bactrian and dromedary camels are susceptible to infection with a parapoxvirus which causes a very localized infection, with lesions similar to those of orf in sheep and goats; it is known as camel ecthyma.

Aetiology

There was considerable concern towards the end of the human smallpox eradication programme because of the fact that camelpox virus appeared indistinguishable from smallpox virus, and that it might cause disease in humans.1 However, it was subsequently shown to produce a different cytopathic effect in HeLa and certain other cell lines in which it produced the formation of multinucleate giant cells, unlike smallpox virus.2 When grown on the chorioallantoic membrane of embryonated chicken eggs at 37 °C the lesions it induces are dense white pocks which are very similar in appearance to those produced by smallpox virus, but when cultivated at 34,5 °C the lesions can be distinguished from those of smallpox virus in that pocks with a haemorrhagic centre are produced. Camelpox virus also grows on Vero cells, BHK cells, lamb testis cells, and lamb and calf kidney cells. It haemagglutinates cockerel erythrocytes at low dilutions.5 Field investigations have shown that it only rarely causes a mild skin rash in humans, and when inoculated into the skin of rhesus monkeys (Macaca mulatta) it causes only a localized pustule.

Strains of camelpox virus can now be identified using restriction endonucleases, and shown to be different from other species of orthopoxvirus.6 Under the electron microscope camelpox virus is morphologically typical of other orthopoxviruses and distinct from parapoxviruses.

Epidemiology

In endemic situations camelpox is a disease of young camels, affecting them as they lose their colostrum-derived immunity. It possesses many of the epidemiological characteristics of smallpox and sheep- and goatpox, depending for its survival on a constant supply of susceptible camels, as recovery from infection probably gives life-long sterile immunity, there being no carrier state. Where a population of camels is unable to maintain the virus, the susceptible population will also include adult animals, so that when the virus is re-introduced, all age groups will develop disease. However, the movement of camels over long distances usually ensures the spread of virus. Transmission is by contact between infected and susceptible camels and the virus is probably spread in aerosols, but the involvement of arthropod vectors such as the camel tick, Hyalomma dromedarii, is also suspected to play a role in its transmission. 11 Disease is reported to occur more frequently and in a more severe form during the rainy season.

There have been no reported investigations on the ability of the virus to persist in scab material in the environment, but its survival is likely to be similar to that of capripoxvirus.

Pathogenesis, clinical signs and pathology

In experimental cases of the disease induced by intradermal or subcutaneous inoculation of the virus the incubation period is five days. At the site of inoculation a papule develops which, over a 9- to 10-day period progresses to form a vesicle, pustule and scab. Generalization, characterized by fever and the development of multiple skin lesions at other sites, occurs 9 to 11 days after inoculation.

The natural incubation period following contact exposure is approximately 13 days. During an outbreak, affected camels first develop a mild fever, followed by the appearance of multiple lesions in the skin, particularly of the head, neck and limbs. The skin of the mammary glands and external genitalia is also frequently affected, and in severe cases, the lips and nasal mucosa become very swollen. Death may occur at this stage (which may possibly be related to the observation that the virus grows in myocardial cells). The duration of the disease is between 10 and 30 days, unless secondary bacterial infections develop. Case mortality rates of up to 28,5 per cent have been reported. 9, 10, 11

At necropsy, apart from the skin lesions, red pustular lesions may be present in the mucosa of the trachea and oesophagus, and nodular lesions up to 50 mm in diameter may occur in the lungs.

Diagnosis and differential diagnosis

A presumptive diagnosis can usually be made on the basis of clinical signs which, when generalized, can be confused with little else. Localized lesions may be mistaken for a papilloma or parapox virus infection. Samples of scab or biopsy material contain typical orthopox virions which can be identified and differentiated from parapox virions by electron microscopy. Virus in a...

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