Haemophilus parasuis infection

M-L PENRITH AND M M HENTON

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS Haemophilus and Histophilus spp. infections

Introduction

Haemophilus parasuis typically causes Glässer’s disease, a peracute to acute disease of 2- to 16-week-old pigs characterized by a serofibrinous polyserositis, polyarthritis and meningitis. In some cases, acute septicaemia results in death before typical lesions develop.^{1, 3, 47} In conventional herds, occurrence is sporadic and morbidity and mortality rates are generally low, although outbreaks may occur. During the present decade, H. parasuis has emerged as a cause of serious disease in specific pathogen-free (SPF) pigs.³⁵ In South Africa, Glässer’s disease is relatively rare, but increasing interest in and importation of SPF pigs could alter the present situation.

A disease syndrome manifested by fibrinous pleuritis, pericarditis and peritonitis, and often accompanied by arthritis, was described by Glässer in 1910.⁹ He was able to demonstrate Gram-negative bacilli in the exudate but could not cultivate them. Some decades later it was observed that meningitis was often an associated lesion in animals suffering from the syndrome.¹³ In 1939, a bacterium thought to be Haemophilus suis, which had first been described in 1931 in association with lesions caused by swine influenza,⁴¹ was isolated from the joints of pigs suffering from arthritis.⁴⁰ This was followed some years later by the isolation of the same bacterium from typical cases of Glässer’s disease and by the experimental production of the disease in pigs by inoculation of H. suis. ¹³

In 1969, a new species of Haemophilus (H. parasuis) isolated from pigs was proposed.⁴ This organism required V-factor or nicotinamide adenine dinucleotide (NAD) for growth, but not X-factor (protoporphyrin IX or protohaeme). During recent years this has been the most frequent bacterium isolated from suspected cases of Glässer’s disease.³⁷ Most strains in culture collections identified as H. suis require only V-factor and therefore belong to H. parasuis, ²¹ because H. suis requires both X- and V-factors. Haemophilus parasuis is therefore considered to be the only cause of Glässer’s disease.

Aetiology

Bacteria of the genus Haemophilus (meaning blood-loving) belong to the family Pasteurellaceae. Haemophilus parasuis (meaning suis-like) is a small, thin, pleomorphic, Gramnegative rod.⁴ The bacterium is V-factor dependent and therefore requires the addition of this factor to the culture medium. Growth on chocolate agar is feeble after 48 to 72 hours; in the absence of the V-factor, the colonies are smooth, greyish and translucent, and reach a diameter of about 0,5 mm. There is, however, marked enhancement of growth on blood agar if the medium is streaked with a Staphylococcus feeder culture (as staphylococci, as well as certain other bacteria, produce the critical growth factor in excess) with colonies attaining a diameter of 1 to 2 mm around the streak.²¹ Chocolate agar and Levinthal media (agar and broth) are satisfactory media for propagation of the bacterium.²¹Growth is also improved if the carbon dioxide level is raised to about 5 per cent.

Haemophilus parasuis is not haemolytic, ferments glucose, sucrose, maltose, galactose, mannose, fructose, ribose and insulin, and is urease- and Camp-negative.²¹ Identification of H. parasuis is based on morphological and colonial characteristics, V-factor dependency and biochemical testing. For this purpose reference should be made to Bergey’s Manual of Systematic Bacteriology. ²²

As has been reported, ⁴⁴ Bakos, Nilsson and Thal in 1952 proposed a serological classification of H. parasuis based on a tube agglutination test of whole cell preparations. Isolates of groups A, B or C are serologically related organisms; all other strains are placed in an ill-defined group D. In an attempt to provide a more comprehensive system, Morozumi and Nicolet in 1986 as cited by Smart et al. ⁴⁵ proposed an alternative typing system based on an immunodiffusion test that used antigens extracted from either the capsule or, in the case of unencapsulated strains, the outer membrane. They described seven serovars. Serovars 2 and 5 correspond to the serovars A and B respectively of Bakos et al.Kielstein and Rapp-Gabrielson in 1992 designated 15 serovars based on immunodiffusion using heat-stable antigens.¹⁷

Using either serotyping system, some strains cannot be typed.^{17, 35, 45} Smart et al. used restriction fragment length polymorphism (RFLP) analysis to group isolates of H. parasuis. ⁴⁵ Thirteen groups were identified by this method. However, 24 of the 69 isolates examined did not correspond with any of the RFLP groups.

In a given herd, many strains of H. parasuis may be isolated, but in most cases, one or two strains predominate.⁴⁵

Epidemiology

Infection with Haemophilus parasuis worldwide results in a sporadic disease affecting young pigs of 2 to 16 weeks of age. Single animals or litters may be affected, and the disease often involves the heaviest animals.²⁹ When outbreaks occur they usually only involve a single farm, i.e. farm to farm spread is unusual. Specific pathogen-free (SPF) herds are more seriously affected by outbreaks and involve pigs of all ages.³⁹ Morbidity rates are generally higher when older pigs are affected.³¹ In South Africa, the disease has been diagnosed more frequently in piglets two to six weeks of...