Pox Viruses: Morphology, Cultivation and Classification

In this article we will discuss about Pox Viruses:- 1. Introduction to Pox Viruses 2. Morphology of Pox Viruses 3. Cultivation 4. Classification 5. Small Pox 6. Cow Pox 7. Orf 8. Variola and Vaccinia 9. Prophylaxis.

Contents:

1. Introduction to Pox Viruses
2. Morphology of Pox Viruses
3. Cultivation of Pox Viruses
4. Classification of Pox Viruses
5. Small Pox
6. Cow Pox
7. Orf
8. Variola and Vaccinia
9. Prophylaxis of Pox Viruses

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1. Introduction to Pox Viruses:

Pox viruses are most complex and largest so they can be seen under light microscopes. They replicate in the cytoplasm unlike other DNA viruses. Elementary bodies (Paschen bodies) are found in smears from the lesion.

2. Morphology of Pox Viruses:

Pox virus are brick shaped. They measure 300 x 240 x 100 nm; they are largest and are complex as they do not show any discernible symmetry. This virus consists of double stranded DNA surrounded by double layered membrane and it contains more than 100 different polypep­tides and enzymes which include transcriptase. It affects man, monkeys, calves, sheep and rabbit.

3. Cultivation of Pox Viruses:

They can grow in chorioallantoic membrane (CAM) of chick embryo and tissue culture (Fig. 57.1).

4. Classification of Pox Viruses:

Orthopox virus, Parapox virus and some unclassified species cause human infections. They share a common nucleoprotein antigen in their inner core.

5. Small Pox:

Pathogenesis:

In all pox virus infections, pathogenesis is almost similar. First pox virus enters the respiratory tract by inhalation, settles in the respiratory mucosa, multiplies in the draining lymph nodes and thereby resulting into primary viraemia which favours the dissemination of the virus throughout the body secondary viraemia.

During this stage, the virus settles in the skin and mucosa, produces skin rashes after an incubation period of about 12 days. The rash appears on the buccal mucosa (exanthema), then develops into macules, papules, vesicles and finally pustules which dry up in 10-12 days after the first rash.

6. Cow Pox:

Mainly cow pox virus is the causal agent of ulcer on the teats of cattle. In man, it is acquired from cows or occupational disease. The localised lesions appear in the fingers of man and undergo changes from macules to pustules as in small pox.

Milker’s nodules infections are similar to Cow pox, but the pustules are rare.

7. Orf:

It is a contagious pustular dermatitis or sore-mouth of sheep; man acquires. This infection by contact; single lesion appears on finger, hand or face and takes several weeks for healing.

Molluscum Contagiosum:

It is a benign epidermal tumour-like lesion occurring only in man on the arms, legs, buttocks and genitalia. It is contagious hyaline inclusion bodies (molluscum bodies) are within the proliferated epidermal cells of the nodule.

8. Variola and Vaccinia:

Variola virus causes small pox only in human and monkeys.

In the past, small pox epidemics appeared in two forms:

(1) Florid and fatal disease occurred in India;

(2) Mild, non­fatal disease (alastrim) in Latin America.

The virus causing classical small pox was named variola major; the other one causing alastrim is variola minor. These two viruses are antigeni­cally identical, but differ in certain biological properties only. Vaccinia virus causes vaccinia which is a localised skin infection and infects mice, rabbit.

It is from Jenner’s original cow pox vaccine and is used for small pox vaccination. Now, it is distinct from Orthopox virus and is maintained by arm to arm passage in man. During this passage, it underwent some permanent changes differing from the freshly isolated cow pox. Virus may, be a product of recombination of cow pox and a hybrid of variola.

9. Prophylaxis of Pox Viruses:

Vaccination with vaccinia produces protection against small pox for about 5 years. A natural infection of small pox gives a complete protection against reinfection.