Chapter 31: Respiratory

Question 1

Rhodococcus equi bacteria characteristics

Answer 1

Gram-positive, aerobic, facultative intracellular bacteria

Question 2

What age range does R equi affect?

Answer 2

3 weeks to 5 months

Question 3

Can this affect people?

Answer 3

People immunocompromised by HIV can contract R equi.

Question 4

What are the R equi specific virulence-associated protein (Vap) families?

Answer 4

VapA, VapC, VapD, VapE, VapG, Vape (full length).

Question 5

Which is the necessary virulence factor?

Answer 5

VapA. It is necessary, not sufficient.

Question 6

Which virulence factors are required for intracellular growth in macrophages and virulence?

Answer 6

virR and virS

Question 7

What are the clinic forms of R equi?

Answer 7

1. Pulmonary
2. Extrapulmonary
3. Adult

Question 8

Which is the most common clinical form of R equi?

Answer 8

Pulmonary form.

Question 9

What lesions are seen in the pulmonary form of R equi?

Answer 9

Chronic suppurative bronchopneumonia & extensive abscessation.

Question 10

What clinical signs are seen in the pulmonary form of R equi?

Answer 10

Cough, fever, lethargy, increased respiratory rate/effort.
Subacute form: respiratory distress.
Subclinical form: ultrasonographic evidence of peripheral pulmonary consolidation +/- abscessation without clinical signs.

Question 11

Which is the second most common clinical form of R equi?

Answer 11

Extrapulmonary form. Lower survival due to poorer response to treatment compared to the pulmonary form.

Question 12

What lesions are seen with the extrapulmonary form of R equi?

Answer 12

1. Multifocal ulcerative enterocolitis +/- typhlitis - granulomatous/ suppurative lesions in colonnic and mesenteric lymph nodes - Peyer’s patches affected.
2. Polysinovitis - 25 - 30% of cases - Ab-Ag complex deposition in the synovium.
3. Uveitis
4. Osteomyelitis
5. Immune-mediated anaemia, thrombocytopenia, telogen effluvium

Question 13

What is the rarest clinical form of R equi?

Answer 13

Adult infections.
1. Lungs
2. Abdominal lymph nodes
3. Wound infection

Question 14

R equi pathogenesis?

Answer 14

On endemic farms, infection occurs at a younger age (incubation period ~ 2-4 weeks). At a younger age foals have a higher susceptibility to develop clinical disease.
1. Inhalation/ Ingestion
2. Receptor-mediated (CR3/Mac1) phagocytosis into the alveolar macrophages.
3. Immune response
a) increased NO results in iron sequestration and prevention of R equi replication. TNFa and IFNy cytokine action result in reduced growth of R equi.
b) uncontrolled intracellular R equi replication - macrophage necrosis

Question 15

At what age are foals usually diagnosed with R equi?

Answer 15

35-50 days old

Question 16

Main diagnostics used for R equi?

Answer 16

CBC: chronic inflammatory leukogram. hyperfibrinogenaemia
Ultrasound: pulmonary consolidation
Thoracic radiographs: alveolar pattern +/- abscesses

Question 17

What are the immune reponse to R equi?

Answer 17

1. Antibody-mediated: IgGa, IgGb and IgG(T)
2. Cell-mediated (very important): functional Th (CD4+) - cytokine secretion and cytotoxicity.
   Type 1 response - increased IFNy - sufficient to clear the pulmonary form. Young foals, deficient IFNy production.
   Type 2 response - increased IL-4 - detrimental.